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MOSFET selection for boost converter problem
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John Nagle
Nov 17, 2016, 2:34:00 AM11/17/16
to
I've been working on a power supply for antique Teletype
selector magnets. These need 120VDC 60mA for the first
2ms or so of each bit time, but the sustain voltage needed
is only 3-4V. So I've built an isolated boost supply and
interface board, all powered from a USB port.
The whole project is on Github:
https://github.com/John-Nagle/ttyloopdriver
where there's an explanation of how it works which
doesn't require downloading the whole KiCAD project.
Schematic as image:
http://www.aetherltd.com/public/misc/loopdriverv1.0.png
LTSpice model:
http://www.aetherltd.com/public/misc/ttydriver22.asc
I've had a board made, and it produced 120VDC
briefly, until the IRLB8721 MOSFET blew out. I just
used that because it's a common MOSFET.
Not surprising - the circuit shows 40V across source
and drain, and that's a 30V part.
So what to put in there? I've been trying various
parts in LTSpice. The AP9465GEM works great in LTSpice,
but it's a surface mount part and only 40V, so there's
no headroom on voltage. Almost nothing with a rating
of 50V or better works in simulation. Turn-off is too
slow, or capacitance is too high, and the resulting
output voltage peak on the primary is too low. Try
loading up the LTSpice model and using different MOSFETs
to see this.
I don't really know what I'm doing here. I'm
aware that this MOSFET application depends strongly
on the transient properties of the device. Turning
off that transformer primary to get the inductive kick
is tricky. I'm trying to struggle through the Art
of Electronics chapter on MOSFETs, while hammering
on the problem with LTSpice.
(I'd like to use a TO-220 form factor, or at least
a through-hole part, and get this first board working.
After that, I can get a new board made.)
(I was asking previously about low duty cycle
555 timer circuits. That part is working fine now.
Thanks.)
John Nagle
selector magnets. These need 120VDC 60mA for the first
2ms or so of each bit time, but the sustain voltage needed
is only 3-4V. So I've built an isolated boost supply and
interface board, all powered from a USB port.
The whole project is on Github:
https://github.com/John-Nagle/ttyloopdriver
where there's an explanation of how it works which
doesn't require downloading the whole KiCAD project.
Schematic as image:
http://www.aetherltd.com/public/misc/loopdriverv1.0.png
LTSpice model:
http://www.aetherltd.com/public/misc/ttydriver22.asc
I've had a board made, and it produced 120VDC
briefly, until the IRLB8721 MOSFET blew out. I just
used that because it's a common MOSFET.
Not surprising - the circuit shows 40V across source
and drain, and that's a 30V part.
So what to put in there? I've been trying various
parts in LTSpice. The AP9465GEM works great in LTSpice,
but it's a surface mount part and only 40V, so there's
no headroom on voltage. Almost nothing with a rating
of 50V or better works in simulation. Turn-off is too
slow, or capacitance is too high, and the resulting
output voltage peak on the primary is too low. Try
loading up the LTSpice model and using different MOSFETs
to see this.
I don't really know what I'm doing here. I'm
aware that this MOSFET application depends strongly
on the transient properties of the device. Turning
off that transformer primary to get the inductive kick
is tricky. I'm trying to struggle through the Art
of Electronics chapter on MOSFETs, while hammering
on the problem with LTSpice.
(I'd like to use a TO-220 form factor, or at least
a through-hole part, and get this first board working.
After that, I can get a new board made.)
(I was asking previously about low duty cycle
555 timer circuits. That part is working fine now.
Thanks.)
John Nagle
bitrex
Nov 17, 2016, 8:05:32 AM11/17/16
to
a 5 volt supply to drive the '8721, which has a max Vth of ~2.4 volts.
However the bipolar 555's output high voltage is going to be quite a bit
lower than the supply voltage - I've seen it as much as 2 volts lower
than Vcc in practice. And maybe I'm reading the schematic wrong, but it
looks like you've got a 680 ohm resistor in the 555's power line. That's
not going to help things.
Unfortunately I think that using a 555 as a gate driver from a 5v supply
is a poor choice here. Needs to be driven harder.
bitrex
Nov 17, 2016, 8:10:12 AM11/17/16
to
and expect it to source/sink much current from a load during the
switching cycles (which needs to be done.) The output BJTs don't turn on
and off too quick.
John Larkin
Nov 17, 2016, 11:36:22 AM11/17/16
to
On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <na...@animats.com>
wrote:
There's no flyback clamping or snubbing on that fet. The drain
probably spikes up a hundred volts or more. Your sim has no leakage
inductance, so doesn't model the drain spikes.
Use a higher-voltage fet and add some sort of flyback absorber, an R-C
or a zener or something.
And isn't the fet switching, basically, into a shorted load most of
the time?
Why all that isolation? Aren't the keyboard and magnet isolated
already?
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
wrote:
probably spikes up a hundred volts or more. Your sim has no leakage
inductance, so doesn't model the drain spikes.
Use a higher-voltage fet and add some sort of flyback absorber, an R-C
or a zener or something.
And isn't the fet switching, basically, into a shorted load most of
the time?
Why all that isolation? Aren't the keyboard and magnet isolated
already?
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
bitrex
Nov 17, 2016, 12:09:39 PM11/17/16
to
switch, OP says he needs 120V @ 60 mA for a few mS, but from the '2030
data sheet it says that the switch current-limits at 300mA. Could be a
problem
bitrex
Nov 17, 2016, 12:13:01 PM11/17/16
to
John Larkin
Nov 17, 2016, 12:50:17 PM11/17/16
to
bitrex
Nov 17, 2016, 2:14:09 PM11/17/16
to
the wrong choice for whatever application you have in mind.
Phil Hobbs
Nov 17, 2016, 2:25:09 PM11/17/16
to
wrong for this one, sure. But CMOS 555s are useful sometimes, e.g. for
chopping LEDs. I use them all the time in protos, though not recently
in a finished design.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
P E Schoen
Nov 17, 2016, 8:00:54 PM11/17/16
to
"John Nagle" wrote in message news:o0jmg3$gj6$1...@dont-email.me...
I made some suggestions in your previous post, so I am familiar with what
you want to do. So, where to begin?
As a reference, here is the LTSpice simulation you provided:
http://enginuitysystems.com/pix/electronics/ttydriver22.png
Some issues:
(1) D1, D2, and D3 have no model defined (probably not a problem)
(2) Your source is 4.6V with 5 ohms resistance - that severely limits
performance
(3) As mentioned, no leakage inductance for L1-L2 is specified.
(4) Also as mentioned, 100 kHz is very high, and the 555 is not a very good
gate driver under best circumstances
(5) The AP9465GEM is not commonly available.
(6) The IRLB8721 is only a 30V part, and it is really characterized for gate
drive of at least 3 volts and preferably 4.5V. Your power supply is already
crippled with 4.6 volts, and the NE555 has a typical output high voltage of
only 3.3V at 100mA. Even at 1 mA the output will be 1 volt below the rail,
so just barely 3.6 volts with a 4.6 volt supply.
(7) I don't see any bypass capacitors on your actual schematic on the
nominal 5V supply.
(8) You have a TPS2030 USB switch for the enable. This part may be obsolete,
but probably not a problem.
(9) There is U5 that looks like "IEQ524S" but I can't find it. Looks like an
interface to the optos.
Enough for now.
Paul
you want to do. So, where to begin?
As a reference, here is the LTSpice simulation you provided:
http://enginuitysystems.com/pix/electronics/ttydriver22.png
Some issues:
(1) D1, D2, and D3 have no model defined (probably not a problem)
(2) Your source is 4.6V with 5 ohms resistance - that severely limits
performance
(3) As mentioned, no leakage inductance for L1-L2 is specified.
(4) Also as mentioned, 100 kHz is very high, and the 555 is not a very good
gate driver under best circumstances
(5) The AP9465GEM is not commonly available.
(6) The IRLB8721 is only a 30V part, and it is really characterized for gate
drive of at least 3 volts and preferably 4.5V. Your power supply is already
crippled with 4.6 volts, and the NE555 has a typical output high voltage of
only 3.3V at 100mA. Even at 1 mA the output will be 1 volt below the rail,
so just barely 3.6 volts with a 4.6 volt supply.
(7) I don't see any bypass capacitors on your actual schematic on the
nominal 5V supply.
(8) You have a TPS2030 USB switch for the enable. This part may be obsolete,
but probably not a problem.
(9) There is U5 that looks like "IEQ524S" but I can't find it. Looks like an
interface to the optos.
Enough for now.
Paul
Joerg
Nov 17, 2016, 9:04:00 PM11/17/16
to
On 2016-11-16 23:33, John Nagle wrote:
> I've been working on a power supply for antique Teletype
> selector magnets. These need 120VDC 60mA for the first
> 2ms or so of each bit time, but the sustain voltage needed
> is only 3-4V. So I've built an isolated boost supply and
> interface board, all powered from a USB port.
> The whole project is on Github:
>
> https://github.com/John-Nagle/ttyloopdriver
>
> where there's an explanation of how it works which
> doesn't require downloading the whole KiCAD project.
>
> Schematic as image:
> http://www.aetherltd.com/public/misc/loopdriverv1.0.png
>
> LTSpice model:
> http://www.aetherltd.com/public/misc/ttydriver22.asc
>
> I've had a board made, and it produced 120VDC
> briefly, until the IRLB8721 MOSFET blew out. I just
> used that because it's a common MOSFET.
> Not surprising - the circuit shows 40V across source
> and drain, and that's a 30V part.
>
Others have pointed out potential problems, including the lack of a
> I've been working on a power supply for antique Teletype
> selector magnets. These need 120VDC 60mA for the first
> 2ms or so of each bit time, but the sustain voltage needed
> is only 3-4V. So I've built an isolated boost supply and
> interface board, all powered from a USB port.
> The whole project is on Github:
>
> https://github.com/John-Nagle/ttyloopdriver
>
> where there's an explanation of how it works which
> doesn't require downloading the whole KiCAD project.
>
> Schematic as image:
> http://www.aetherltd.com/public/misc/loopdriverv1.0.png
>
> LTSpice model:
> http://www.aetherltd.com/public/misc/ttydriver22.asc
>
> I've had a board made, and it produced 120VDC
> briefly, until the IRLB8721 MOSFET blew out. I just
> used that because it's a common MOSFET.
> Not surprising - the circuit shows 40V across source
> and drain, and that's a 30V part.
>
snubber. In the interest of getting this going on the bench fast put a
zener-based snubber in there, like here:
http://www.eetimes.com/author.asp?doc_id=1280601
A 7-12V zener to your 5V supply, fast Schottky diode in series, from the
drain. Use a big zener of at least 1/2 watt if you have one. That should
prevent it from blowing up. Of course, a 555 is too wimpy to drive this
FET but this should essentially get the circuit going. I never use 555
but maybe you can find a CMOS version that will at least swing all the
way to 5V? Longterm I'd get rid of the 555 as was already suggested.
> So what to put in there? I've been trying various
> parts in LTSpice. The AP9465GEM works great in LTSpice,
> but it's a surface mount part and only 40V, so there's
> no headroom on voltage. Almost nothing with a rating
> of 50V or better works in simulation. Turn-off is too
> slow, or capacitance is too high, and the resulting
> output voltage peak on the primary is too low. Try
> loading up the LTSpice model and using different MOSFETs
> to see this.
>
> I don't really know what I'm doing here.
> ... I'm
> aware that this MOSFET application depends strongly
> on the transient properties of the device. Turning
> off that transformer primary to get the inductive kick
> is tricky.
When you set the coupling coefficient in SPICE to, say, 0.98 you'll get
> on the transient properties of the device. Turning
> off that transformer primary to get the inductive kick
> is tricky.
the leakage inductance. Or just put a small non-couple inductance in
series with the primary. The sim will now slow way down but you'll see
the ugly stuff brewing at the drain node. Lightning, peals of thunder,
some smoke.
[...]
--
Regards, Joerg
http://www.analogconsultants.com/
P E Schoen
Nov 17, 2016, 9:29:35 PM11/17/16
to
I found a major error in your simulation. The Coilcraft DA3022 has a ratio
of 10:1, so with a primary inductance of 10 uH, the secondary is 100 times
that, or 1000 uH. I also replaced your MOSFET with a commonly available
TO-220 logic level device rated 100V and 15A IRL530NS_L
http://www.mouser.com/ds/2/427/91299-106665.pdf
So here is the simulation:
http://enginuitysystems.com/pix/electronics/ttydriver22b.png
Note that the peak drain voltage on the MOSFET is less than 20V, and the
output now easily reaches 120V. But something else that may be a problem is
that the voltage on the selector coil drops to -120V when turned off. This
might be corrected by using a standard diode in place of D7 (but this will
cause current to flow for a longer time). Other methods as discussed in the
previous thread may point to a better solution.
Thru-hole components are becoming scarce for some devices, especially newer
ones. But it is really quite easy to solder larger packages such as the
power flat pack, and even SOIC-8 and SOT parts are not bad, even on the
usual proto-boards. Don't let SMT keep you from choosing better components.
Paul
of 10:1, so with a primary inductance of 10 uH, the secondary is 100 times
that, or 1000 uH. I also replaced your MOSFET with a commonly available
TO-220 logic level device rated 100V and 15A IRL530NS_L
http://www.mouser.com/ds/2/427/91299-106665.pdf
So here is the simulation:
http://enginuitysystems.com/pix/electronics/ttydriver22b.png
Note that the peak drain voltage on the MOSFET is less than 20V, and the
output now easily reaches 120V. But something else that may be a problem is
that the voltage on the selector coil drops to -120V when turned off. This
might be corrected by using a standard diode in place of D7 (but this will
cause current to flow for a longer time). Other methods as discussed in the
previous thread may point to a better solution.
Thru-hole components are becoming scarce for some devices, especially newer
ones. But it is really quite easy to solder larger packages such as the
power flat pack, and even SOIC-8 and SOT parts are not bad, even on the
usual proto-boards. Don't let SMT keep you from choosing better components.
Paul
Michael A. Terrell
Nov 18, 2016, 12:16:02 AM11/18/16
to
John Nagle wrote:
> I've been working on a power supply for antique Teletype
> selector magnets. These need 120VDC 60mA for the first
> 2ms or so of each bit time, but the sustain voltage needed
> is only 3-4V. So I've built an isolated boost supply and
> interface board, all powered from a USB port.
> The whole project is on Github:
Why not dig up a '70s edition of the ARRL Radio Amateur's Handbook,
> I've been working on a power supply for antique Teletype
> selector magnets. These need 120VDC 60mA for the first
> 2ms or so of each bit time, but the sustain voltage needed
> is only 3-4V. So I've built an isolated boost supply and
> interface board, all powered from a USB port.
> The whole project is on Github:
and see what they did for an interface?
--
Never piss off an Engineer!
They don't get mad.
They don't get even.
They go for over unity! ;-)
Joerg Niggemeyer
Nov 18, 2016, 2:10:44 AM11/18/16
to
In message <qgmr2ctl7k3m0hnan...@4ax.com>
> There's no flyback clamping or snubbing on that fet. The drain
> probably spikes up a hundred volts or more. Your sim has no leakage
> inductance, so doesn't model the drain spikes.
> Use a higher-voltage fet and add some sort of flyback absorber, an R-C
> or a zener or something.
> And isn't the fet switching, basically, into a shorted load most of
> the time?
Good point, that's why first initial chops on an empty
cap load should be very short.......
The snubber alone will not be enough, the selection of the transformer
is OK. What is missing is gate drive and THE-control.
When MOSFETs had been poor some spikes above the max drain voltage
instantly killed them - like the old BUK Philips ones, what a crap
this was....
You can sense and regulate by the flyback pulse the output and limit
thereby also the flyback pulse down to non dangerous amplitudes.
Used this method in my TL594 controlled analogue ballasts
many years ago.
Now even available in one chip from LT:
http://www.linear.com/docs/10239
Regards
Joerg Ni
--
Joerg Niggemeyer on RISCOS Raspberry Pi2
http://www.led-temperature-protection.com
http://www.nucon.de
John Larkin <jjla...@highlandtechnology.com> wrote:
> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <na...@animats.com>
> wrote:
>>
>>(I was asking previously about low duty cycle
>>555 timer circuits. That part is working fine now.
>>Thanks.)
>>
>> John Nagle
555 reasons to not use this anymore ;-)
> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <na...@animats.com>
> wrote:
>>
>>(I was asking previously about low duty cycle
>>555 timer circuits. That part is working fine now.
>>Thanks.)
>>
>> John Nagle
> There's no flyback clamping or snubbing on that fet. The drain
> probably spikes up a hundred volts or more. Your sim has no leakage
> inductance, so doesn't model the drain spikes.
> Use a higher-voltage fet and add some sort of flyback absorber, an R-C
> or a zener or something.
> And isn't the fet switching, basically, into a shorted load most of
> the time?
cap load should be very short.......
The snubber alone will not be enough, the selection of the transformer
is OK. What is missing is gate drive and THE-control.
When MOSFETs had been poor some spikes above the max drain voltage
instantly killed them - like the old BUK Philips ones, what a crap
this was....
You can sense and regulate by the flyback pulse the output and limit
thereby also the flyback pulse down to non dangerous amplitudes.
Used this method in my TL594 controlled analogue ballasts
many years ago.
Now even available in one chip from LT:
http://www.linear.com/docs/10239
Regards
Joerg Ni
--
Joerg Niggemeyer on RISCOS Raspberry Pi2
http://www.led-temperature-protection.com
http://www.nucon.de
John Nagle
Nov 18, 2016, 2:22:21 AM11/18/16
to
On 11/17/2016 6:29 PM, P E Schoen wrote:
> I found a major error in your simulation. The Coilcraft DA3022 has a
> ratio of 10:1, so with a primary inductance of 10 uH, the secondary is
> 100 times that, or 1000 uH. I also replaced your MOSFET with a commonly
> available TO-220 logic level device rated 100V and 15A IRL530NS_L
>
> http://www.mouser.com/ds/2/427/91299-106665.pdf
>
> So here is the simulation:
>
> http://enginuitysystems.com/pix/electronics/ttydriver22b.png
>
> Note that the peak drain voltage on the MOSFET is less than 20V, and the
> output now easily reaches 120V. But something else that may be a problem
> is that the voltage on the selector coil drops to -120V when turned off.
> This might be corrected by using a standard diode in place of D7 (but
> this will cause current to flow for a longer time). Other methods as
> discussed in the previous thread may point to a better solution.
I tried that. Then I put in a tiny resistor (0.01 ohm) between
> I found a major error in your simulation. The Coilcraft DA3022 has a
> ratio of 10:1, so with a primary inductance of 10 uH, the secondary is
> 100 times that, or 1000 uH. I also replaced your MOSFET with a commonly
> available TO-220 logic level device rated 100V and 15A IRL530NS_L
>
> http://www.mouser.com/ds/2/427/91299-106665.pdf
>
> So here is the simulation:
>
> http://enginuitysystems.com/pix/electronics/ttydriver22b.png
>
> Note that the peak drain voltage on the MOSFET is less than 20V, and the
> output now easily reaches 120V. But something else that may be a problem
> is that the voltage on the selector coil drops to -120V when turned off.
> This might be corrected by using a standard diode in place of D7 (but
> this will cause current to flow for a longer time). Other methods as
> discussed in the previous thread may point to a better solution.
the output of the 555 timer and the gate of the MOSFET so I
could get LTSpice to measure current. That
shows a peak current of 350mA for about 200ns. The 555 timer
can output a maximum of 200mA. I'm using the "idealized 555
timer" model, and it can apparently deliver more current
with no voltage drop.
In the real world, the output from the 555 timer drops
to about 2V and stays there for the entire ON portion of
the cycle. I'm not sure how the gate of a MOSFET behaves
when you underdrive it. I thought the high current draw
was just during turn-on. Is the problem that, without
enough drive, the gate current remains high? Or
is something else wrong?
Here's the scope trace again:
http://www.aetherltd.com/public/misc/mosfetgate1.jpg
Incidentally, the other waveform
http://www.aetherltd.com/public/misc/unloadedoutput.jpg
shows data being sent through the board at 45 baud.
Functionally, roughly the right stuff is happening.
Someone suggested using an MC34063 instead of a
555 timer, and I'll have to look at that option.
But it's a lower switching frequency. More later.
John Nagle
John Nagle
Nov 18, 2016, 2:22:54 AM11/18/16
to
nominally 5V.
> (3) As mentioned, no leakage inductance for L1-L2 is specified.
> (4) Also as mentioned, 100 kHz is very high, and the 555 is not a very
> good gate driver under best circumstances
> (5) The AP9465GEM is not commonly available.
> (6) The IRLB8721 is only a 30V part, and it is really characterized for
> gate drive of at least 3 volts and preferably 4.5V. Your power supply is
> already crippled with 4.6 volts, and the NE555 has a typical output high
> voltage of only 3.3V at 100mA. Even at 1 mA the output will be 1 volt
> below the rail, so just barely 3.6 volts with a 4.6 volt supply.
doesn't have enough output power to drive it properly. Here's the
scope trace at the gate of the IRL530.
http://www.aetherltd.com/public/misc/mosfetgate1.jpg
It's only reaching 2V. The overall result is that the circuit runs
but the final output is only 45V instead of 120V.
http://www.aetherltd.com/public/misc/unloadedoutput.jpg
So the big problem is getting enough drive for the power MOSFET's gate.
> (7) I don't see any bypass capacitors on your actual schematic on the
> nominal 5V supply.
> (8) You have a TPS2030 USB switch for the enable. This part may be
> obsolete, but probably not a problem.
> (9) There is U5 that looks like "IEQ524S" but I can't find it. Looks
> like an interface to the optos.
http://www.digikey.com/product-detail/en/xp-power/IE0524S/1470-1407-5-ND/4487788
That's a 5VDC to 24VDC converter used to provide voltage for the
keyboard side of the system. That's independent of all the custom
switching stuff.
Thanks.
John Nagle
P E Schoen
Nov 18, 2016, 4:11:49 AM11/18/16
to
"John Nagle" wrote in message news:o0ma68$jd6$1...@dont-email.me...
> I tried that. Then I put in a tiny resistor (0.01 ohm) between the
> output of the 555 timer and the gate of the MOSFET so I could get LTSpice
> to measure current. That shows a peak current of 350mA for about 200ns.
> The 555 timer can output a maximum of 200mA. I'm using the "idealized 555
> timer" model, and it can apparently deliver more current with no voltage
> drop.
> In the real world, the output from the 555 timer drops to about 2V and
> stays there for the entire ON portion of
the cycle. I'm not sure how the gate of a MOSFET behaves when you
underdrive it. I thought the high current draw
was just during turn-on. Is the problem that, without enough drive, the
gate current remains high? Or is something else wrong?
> Here's the scope trace again:
> http://www.aetherltd.com/public/misc/mosfetgate1.jpg
> Incidentally, the other waveform
> http://www.aetherltd.com/public/misc/unloadedoutput.jpg
> shows data being sent through the board at 45 baud.
> Functionally, roughly the right stuff is happening.
> Someone suggested using an MC34063 instead of a 555 timer, and I'll have
> to look at that option. But it's a lower switching frequency. More later.
I could try to analyze what's going on with your most recent efforts, but I
found more problems that are more serious. First off, it's not going to work
with a 5 ohm power source resistance. A USB will supply close to 5VDC (maybe
as low as 4.8) and it will do so until it approaches the standard 500 mA
limit, at which it may throttle back or shut down completely. So I used your
4.6V with a 0.5 ohm resistance. That will still mean a drop to about 4.3
volts at 500 mA, but still workable.
BTW, you can read current without adding a resistor. Hover the cursor on the
pin (such as the gate of M2). It will show an icon for a black clamp-on
meter and in this case it will measure Ig(M2). You may also choose it from a
list of available data by right-clicking on the plot, choosing "Add Trace
(Ctrl-A)", and a list will appear. You may also manually define what you
want to plot, such as a voltage ratio, or voltage times current (power).
Next, I changed the input diode to a Schottky, which drops only about 0.3
volts rather than almost 1 volt. That was a huge improvement. I also found a
better MOSFET, the IRL3915. It seems that it is actually an IRLR3915. There
are probably even better ones available, but this is only a dollar and is
easily available:
http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as 2V on
the gate. I also cleaned up your simulation, getting rid of parts not
needed, and added a commutating snubber that totally eliminates negative
spikes and ringing. I dropped the frequency to about 40 kHz. Here is the
result:
http://enginuitysystems.com/pix/electronics/ttydriver22c.png
Note that this seems to draw only about 300 mA from the supply, or about 1.6
watts. Fine for a USB source.
Here is the netlist:
http://enginuitysystems.com/pix/electronics/ttydriver22c.asc
Hope this helps.
Paul
> I tried that. Then I put in a tiny resistor (0.01 ohm) between the
> output of the 555 timer and the gate of the MOSFET so I could get LTSpice
> to measure current. That shows a peak current of 350mA for about 200ns.
> The 555 timer can output a maximum of 200mA. I'm using the "idealized 555
> timer" model, and it can apparently deliver more current with no voltage
> drop.
> In the real world, the output from the 555 timer drops to about 2V and
> stays there for the entire ON portion of
the cycle. I'm not sure how the gate of a MOSFET behaves when you
underdrive it. I thought the high current draw
was just during turn-on. Is the problem that, without enough drive, the
gate current remains high? Or is something else wrong?
> Here's the scope trace again:
> http://www.aetherltd.com/public/misc/mosfetgate1.jpg
> Incidentally, the other waveform
> http://www.aetherltd.com/public/misc/unloadedoutput.jpg
> shows data being sent through the board at 45 baud.
> Functionally, roughly the right stuff is happening.
> Someone suggested using an MC34063 instead of a 555 timer, and I'll have
> to look at that option. But it's a lower switching frequency. More later.
found more problems that are more serious. First off, it's not going to work
with a 5 ohm power source resistance. A USB will supply close to 5VDC (maybe
as low as 4.8) and it will do so until it approaches the standard 500 mA
limit, at which it may throttle back or shut down completely. So I used your
4.6V with a 0.5 ohm resistance. That will still mean a drop to about 4.3
volts at 500 mA, but still workable.
BTW, you can read current without adding a resistor. Hover the cursor on the
pin (such as the gate of M2). It will show an icon for a black clamp-on
meter and in this case it will measure Ig(M2). You may also choose it from a
list of available data by right-clicking on the plot, choosing "Add Trace
(Ctrl-A)", and a list will appear. You may also manually define what you
want to plot, such as a voltage ratio, or voltage times current (power).
Next, I changed the input diode to a Schottky, which drops only about 0.3
volts rather than almost 1 volt. That was a huge improvement. I also found a
better MOSFET, the IRL3915. It seems that it is actually an IRLR3915. There
are probably even better ones available, but this is only a dollar and is
easily available:
http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as 2V on
the gate. I also cleaned up your simulation, getting rid of parts not
needed, and added a commutating snubber that totally eliminates negative
spikes and ringing. I dropped the frequency to about 40 kHz. Here is the
result:
http://enginuitysystems.com/pix/electronics/ttydriver22c.png
Note that this seems to draw only about 300 mA from the supply, or about 1.6
watts. Fine for a USB source.
Here is the netlist:
http://enginuitysystems.com/pix/electronics/ttydriver22c.asc
Hope this helps.
Paul
P E Schoen
Nov 18, 2016, 4:32:56 AM11/18/16
to
I thought you might like to se the details of the waveform, particularly the
gate current Ig(M2), drain current Id(M2), and the drain voltage Vd,
actually V(n004).
http://enginuitysystems.com/pix/electronics/ttydriver22c-Ig-Id-Vd.png
Paul
gate current Ig(M2), drain current Id(M2), and the drain voltage Vd,
actually V(n004).
http://enginuitysystems.com/pix/electronics/ttydriver22c-Ig-Id-Vd.png
Paul
John S
Nov 18, 2016, 8:42:29 AM11/18/16
to
bitrex
Nov 18, 2016, 11:41:22 AM11/18/16
to
On 11/18/2016 02:22 AM, John Nagle wrote:
>> (8) You have a TPS2030 USB switch for the enable. This part may be
>> obsolete, but probably not a problem.
>
> It's dated, but available in DIP.
>
If I'm reading the schematic correctly it looks like you're feeding the
>> (8) You have a TPS2030 USB switch for the enable. This part may be
>> obsolete, but probably not a problem.
>
> It's dated, but available in DIP.
>
flyback from the '2030 high-side switch, which the datasheet has a
continuous current limit of 300mA.
Looking at the waveforms here:
http://enginuitysystems.com/pix/electronics/ttydriver22c-Ig-Id-Vd.png
it looks like the mean MOSFET drain current may be over that.
Tim Wescott
Nov 18, 2016, 12:57:13 PM11/18/16
to
you'll have a much better time of it if you buy DPAK-2 parts and solder
pigtails onto the lands to make them into sorta-TO-220 parts. Or just
bite the bullet and start using surface-mount parts throughout.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
I'm looking for work -- see my website!
Joerg
Nov 18, 2016, 1:15:55 PM11/18/16
to
> ... I suspect
> you'll have a much better time of it if you buy DPAK-2 parts and solder
> pigtails onto the lands to make them into sorta-TO-220 parts. Or just
> bite the bullet and start using surface-mount parts throughout.
>
In this case no problem but I think John wants to have the board
> pigtails onto the lands to make them into sorta-TO-220 parts. Or just
> bite the bullet and start using surface-mount parts throughout.
>
through-hole. TO220 are still widely available and will be for a long
time because bigger supplies than this need heat sinks. You can't
reliably sink more than low single-digit watts into a PCB. I design in
SMT since 1986, was an early adopter but had to use many TO220 parts for
power stages.
John Nagle
Nov 18, 2016, 3:33:26 PM11/18/16
to
On 11/18/2016 1:11 AM, P E Schoen wrote:
> "John Nagle" wrote in message news:o0ma68$jd6$1...@dont-email.me...
>
....
> "John Nagle" wrote in message news:o0ma68$jd6$1...@dont-email.me...
>
>
> I could try to analyze what's going on with your most recent efforts,
> but I found more problems that are more serious. First off, it's not
> going to work with a 5 ohm power source resistance. A USB will supply
> close to 5VDC (maybe as low as 4.8) and it will do so until it
> approaches the standard 500 mA limit, at which it may throttle back or
> shut down completely. So I used your 4.6V with a 0.5 ohm resistance.
> That will still mean a drop to about 4.3 volts at 500 mA, but still
> workable.
Makes sense. I'd changed that in some of my own simulations.
> I could try to analyze what's going on with your most recent efforts,
> but I found more problems that are more serious. First off, it's not
> going to work with a 5 ohm power source resistance. A USB will supply
> close to 5VDC (maybe as low as 4.8) and it will do so until it
> approaches the standard 500 mA limit, at which it may throttle back or
> shut down completely. So I used your 4.6V with a 0.5 ohm resistance.
> That will still mean a drop to about 4.3 volts at 500 mA, but still
> workable.
> Next, I changed the input diode to a Schottky, which drops only about
> 0.3 volts rather than almost 1 volt. That was a huge improvement. I also
> found a better MOSFET, the IRL3915. It seems that it is actually an
> IRLR3915. There are probably even better ones available, but this is
> only a dollar and is easily available:
>
> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
That simulation is still using the idealized 555 timer, which
seems to have unlimited output current. The MOSFET gate current
spikes to about 350mA, which won't happen in the real world.
So I added a 10 ohm resistor to the sim between the 555 output and
the MOSFET input. That limits the current to about 200mA
(the real 555 limit) and the switcher still works. Probably
don't need that resistor in the real world.
> It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as 2V
> on the gate.
> and added a commutating snubber that totally eliminates negative
> spikes and ringing.
Look at the current through I3. It's above 60mA for the whole
OFF period. The energy in the magnet needs to be dumped in
about 2ms or the mechanism won't work. The idea is to use
just enough snubbing to avoid component damage. The
original 1926-1930 equipment used mechanical contacts which arced when
the circuit opened. Around 1940, RF suppression was added,
as a small RC circuit.
> I dropped the frequency to about 40 kHz. Here is the
> result:
>
> http://enginuitysystems.com/pix/electronics/ttydriver22c.png
>
> Note that this seems to draw only about 300 mA from the supply, or about
> 1.6 watts. Fine for a USB source.
What's with the note "Draws about 1.7 amps or 6.2 watts?"
I'm going to try substituting a IRLU3915 for now and take
measurements, then do a new board design. Thanks.
John Nagle
bitrex
Nov 18, 2016, 3:40:10 PM11/18/16
to
synthesizers, etc. it's all through-hole except for one or two big chips
with many pins, like a custom gate array or display controller, which
look surprisingly modern by comparison.
P E Schoen
Nov 18, 2016, 8:19:58 PM11/18/16
to
"John Nagle" wrote in message news:o0nohf$oo9$1...@dont-email.me...
> On 11/18/2016 1:11 AM, P E Schoen wrote:
>
>> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
> That's a big help. It's IRLU3915 in through-hole form.
> That simulation is still using the idealized 555 timer, which seems to
> have unlimited output current. The MOSFET gate current spikes to about
> 350mA, which won't happen in the real world. So I added a 10 ohm resistor
> to the sim between the 555 output and the MOSFET input. That limits the
> current to about 200mA (the real 555 limit) and the switcher still works.
> Probably don't need that resistor in the real world.
Don't worry too much about the MOSFET gate current spikes. Driving a
capacitance with a square wave does that. The 555 is not an ideal gate
driver, but probably adequate. What is important is for the ON voltage to be
well above the linear threshold. This was not the case previous to fixing
the 5V supply problems.
>> It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as 2V on
>> the gate.
> Sim says 25V, 2.7A max at the MOSFET, so that's within limits.
>> and added a commutating snubber that totally eliminates negative spikes
>> and ringing.
> Too much snubbing - the magnet won't release fast enough. Look at the
> current through I3. It's above 60mA for the whole OFF period. The energy
> in the magnet needs to be dumped in about 2ms or the mechanism won't work.
> The idea is to use just enough snubbing to avoid component damage. The
> original 1926-1930 equipment used mechanical contacts which arced when the
> circuit opened. Around 1940, RF suppression was added, as a small RC
> circuit.
That's my error, and today I saw that the single Zener was acting as a
commutating diode and my snubber was actually doing nothing. I was looking
at Vselector and not I(L6). I added a silicon diode in series to block the
commutating action and now the snubber works as expected. Actually, D6 and
the added D8 are probably not needed at all and can be removed. My
simulation seems to run awfully slow so I'm not going to run again to
verify, but it looks much better now!
http://enginuitysystems.com/pix/electronics/ttydriver22d.png
I also changed the PWM duty cycle to about 10%. Now the input draws only
about 150 mA or 800 mW.
I also updated the ASCII file:
http://enginuitysystems.com/pix/electronics/ttydriver22c.asc
> I can tell the USB interface to ask for 300mA. No problem.
About half that now ;)
> What's with the note "Draws about 1.7 amps or 6.2 watts?"
That was for one of the previous simulations. Probably when the MOSFET was
running in linear mode due to the supply voltage crashing.
> I'm going to try substituting a IRLU3915 for now and take measurements,
> then do a new board design. Thanks.
I'd be happy to review your new design before you send off for new boards.
This has been an interesting project. I am not particularly "into" antique
electronics but I do find them interesting, and I also enjoy working on
electro-mechanical contrivances. I have lots of old electronic components
and equipment, and I recently toured a local computer museum that has old
mechanical computers and various examples of the transition from iron core
to solid state memory, relays and vacuum tubes to transistors and ICs, and
other technology. They are even restoring a big old linotype machine, and
they probably have some teletypes as well.
https://museum.syssrc.com/
They may benefit from your work and you are of course welcome to visit. Let
me know. I also have a friend who specializes in old electronic stuff,
mostly ham radio gear, as well as juke boxes, pinball machines, and scopes
and such. He also volunteers at the National Electronics Museum at BWI:
https://en.wikipedia.org/wiki/National_Electronics_Museum
http://nationalelectronicsmuseum.org/
Paul
> On 11/18/2016 1:11 AM, P E Schoen wrote:
>
>> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
> That's a big help. It's IRLU3915 in through-hole form.
> That simulation is still using the idealized 555 timer, which seems to
> have unlimited output current. The MOSFET gate current spikes to about
> 350mA, which won't happen in the real world. So I added a 10 ohm resistor
> to the sim between the 555 output and the MOSFET input. That limits the
> current to about 200mA (the real 555 limit) and the switcher still works.
> Probably don't need that resistor in the real world.
capacitance with a square wave does that. The 555 is not an ideal gate
driver, but probably adequate. What is important is for the ON voltage to be
well above the linear threshold. This was not the case previous to fixing
the 5V supply problems.
>> It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as 2V on
>> the gate.
> Sim says 25V, 2.7A max at the MOSFET, so that's within limits.
>> and added a commutating snubber that totally eliminates negative spikes
>> and ringing.
> Too much snubbing - the magnet won't release fast enough. Look at the
> current through I3. It's above 60mA for the whole OFF period. The energy
> in the magnet needs to be dumped in about 2ms or the mechanism won't work.
> The idea is to use just enough snubbing to avoid component damage. The
> original 1926-1930 equipment used mechanical contacts which arced when the
> circuit opened. Around 1940, RF suppression was added, as a small RC
> circuit.
commutating diode and my snubber was actually doing nothing. I was looking
at Vselector and not I(L6). I added a silicon diode in series to block the
commutating action and now the snubber works as expected. Actually, D6 and
the added D8 are probably not needed at all and can be removed. My
simulation seems to run awfully slow so I'm not going to run again to
verify, but it looks much better now!
http://enginuitysystems.com/pix/electronics/ttydriver22d.png
I also changed the PWM duty cycle to about 10%. Now the input draws only
about 150 mA or 800 mW.
I also updated the ASCII file:
http://enginuitysystems.com/pix/electronics/ttydriver22c.asc
> I can tell the USB interface to ask for 300mA. No problem.
> What's with the note "Draws about 1.7 amps or 6.2 watts?"
running in linear mode due to the supply voltage crashing.
> I'm going to try substituting a IRLU3915 for now and take measurements,
> then do a new board design. Thanks.
This has been an interesting project. I am not particularly "into" antique
electronics but I do find them interesting, and I also enjoy working on
electro-mechanical contrivances. I have lots of old electronic components
and equipment, and I recently toured a local computer museum that has old
mechanical computers and various examples of the transition from iron core
to solid state memory, relays and vacuum tubes to transistors and ICs, and
other technology. They are even restoring a big old linotype machine, and
they probably have some teletypes as well.
https://museum.syssrc.com/
They may benefit from your work and you are of course welcome to visit. Let
me know. I also have a friend who specializes in old electronic stuff,
mostly ham radio gear, as well as juke boxes, pinball machines, and scopes
and such. He also volunteers at the National Electronics Museum at BWI:
https://en.wikipedia.org/wiki/National_Electronics_Museum
http://nationalelectronicsmuseum.org/
Paul
P E Schoen
Nov 18, 2016, 11:23:08 PM11/18/16
to
I did a bit more on this. I removed the 555 and replaced it with a simple
pulse generator with 4V output. I replaced the MOSFET with a simple 2N4401
BJT. I added some parallel capacitance to the transformer, and I tweaked the
output snubber. Results seem pretty good:
http://enginuitysystems.com/pix/electronics/ttydriver_PES_schematic.png
http://enginuitysystems.com/pix/electronics/ttydriver_PES_waveform.png
http://enginuitysystems.com/pix/electronics/ttydriver22c.asc
I'm sure many more improvements can be made. There is still a lot of high
frequency ringing on the transformer that perhaps could be reduced. I might
use a PIC for the PWM, and possibly also for the USB-serial conversion
rather than the CP2102 module.
Paul
pulse generator with 4V output. I replaced the MOSFET with a simple 2N4401
BJT. I added some parallel capacitance to the transformer, and I tweaked the
output snubber. Results seem pretty good:
http://enginuitysystems.com/pix/electronics/ttydriver_PES_schematic.png
http://enginuitysystems.com/pix/electronics/ttydriver_PES_waveform.png
http://enginuitysystems.com/pix/electronics/ttydriver22c.asc
I'm sure many more improvements can be made. There is still a lot of high
frequency ringing on the transformer that perhaps could be reduced. I might
use a PIC for the PWM, and possibly also for the USB-serial conversion
rather than the CP2102 module.
Paul
John Nagle
Nov 27, 2016, 9:40:45 PM11/27/16
to
On 11/18/2016 5:19 PM, P E Schoen wrote:
> "John Nagle" wrote in message news:o0nohf$oo9$1...@dont-email.me...
>
>> On 11/18/2016 1:11 AM, P E Schoen wrote:
>>
>>> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
>
>> That's a big help. It's IRLU3915 in through-hole form.
>
>> That simulation is still using the idealized 555 timer, which seems
>> to have unlimited output current. The MOSFET gate current spikes to
>> about 350mA, which won't happen in the real world. So I added a 10 ohm
>> resistor to the sim between the 555 output and the MOSFET input. That
>> limits the current to about 200mA (the real 555 limit) and the
>> switcher still works. Probably don't need that resistor in the real
>> world.
>
> Don't worry too much about the MOSFET gate current spikes. Driving a
> capacitance with a square wave does that. The 555 is not an ideal gate
> driver, but probably adequate. What is important is for the ON voltage
> to be well above the linear threshold. This was not the case previous to
> fixing the 5V supply problems.
>
>>> It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as
>>> 2V on the gate.
Well, I substituted that MOSFET, and it doesn't help much.
> "John Nagle" wrote in message news:o0nohf$oo9$1...@dont-email.me...
>
>> On 11/18/2016 1:11 AM, P E Schoen wrote:
>>
>>> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
>
>> That's a big help. It's IRLU3915 in through-hole form.
>
>> That simulation is still using the idealized 555 timer, which seems
>> to have unlimited output current. The MOSFET gate current spikes to
>> about 350mA, which won't happen in the real world. So I added a 10 ohm
>> resistor to the sim between the 555 output and the MOSFET input. That
>> limits the current to about 200mA (the real 555 limit) and the
>> switcher still works. Probably don't need that resistor in the real
>> world.
>
> Don't worry too much about the MOSFET gate current spikes. Driving a
> capacitance with a square wave does that. The 555 is not an ideal gate
> driver, but probably adequate. What is important is for the ON voltage
> to be well above the linear threshold. This was not the case previous to
> fixing the 5V supply problems.
>
>>> It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as
>>> 2V on the gate.
It will not work with 2V on the gate. It takes 3V in the real
world, and that's consistent with the data sheet. See
Fig. 3 on
http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
and note that at 2V, it's starting to turn on, at
3V it's passing 10X as much current, and at 7V
on the gate, it's fully turned on and passing
100X as much current as at 2V. This is not a 2V
device.
As a test, I replaced the 555 timer signal with
a bench waveform generator. This can go to 3V,
but not to 5V, and at 3V, things sort of start to
work. But not very well. There's no inductive kick
from the transformer at turn-off. See this waveform,
which is MOSFET drain to ground, with a 10x probe.
This is with gate drive at 100KHz, 25% duty cycle.
http://www.aetherltd.com/public/misc/drainvoltage.jpg
The comparable simulation works much better than the
real world. Much faster rise time at MOSFET turn-off,
and to a higher voltage.
http://www.aetherltd.com/public/misc/ttydrivermosfettest1.png (screen)
http://www.aetherltd.com/public/misc/ttydrivermosfettest1.asc (LTspice)
The blue line is the drain of the MOSFET; the green line is the
gate. The output voltage in the sim ramps up much faster than
in the real world. Both make it to 120VDC eventually, but it's
about 12ms in sim and about 500ms real world (which is too slow
for the application.) I don't understand why it's so much
less effective in the sim.
The model of the Coilcraft DA2032-AL transformer is probably
inadequate. Here's its data sheet:
http://www.coilcraft.com/da2032.cfm
Suggestions?
John Nagle
P E Schoen
Nov 28, 2016, 1:32:40 AM11/28/16
to
"John Nagle" wrote in message news:o1g5ct$stv$1...@dont-email.me...
> On 11/18/2016 5:19 PM, P E Schoen wrote:
>> Don't worry too much about the MOSFET gate current spikes. Driving a
>> capacitance with a square wave does that. The 555 is not an ideal gate
>> driver, but probably adequate. What is important is for the ON voltage to
>> be well above the linear threshold. This was not the case previous to
>> fixing the 5V supply problems.
4.5 volts on the 555 and that should provide at least 2.5-3 volts gate
drive. You might be able to tweak the output a bit higher with a pull-up
resistor to the 5V supply.
> and note that at 2V, it's starting to turn on, at 3V it's passing 10X as
> much current, and at 7V on the gate, it's fully turned on and passing 100X
> as much current as at 2V. This is not a 2V device.
It's probably about as good as you're going to get in a TO-220 package.
There are many devices in SOIC-8 and SOT-23 packages with 1.8, 1.5, and even
lower gate voltages, although the Vce may not be more than 30V. Try
IRLML6344, which will conduct 3A with 0.2V drop using a gate voltage of 1.7
volts.
> As a test, I replaced the 555 timer signal with a bench waveform
> generator. This can go to 3V, but not to 5V, and at 3V, things sort of
> start to work. But not very well. There's no inductive kick from the
> transformer at turn-off. See this waveform, which is MOSFET drain to
> ground, with a 10x probe. This is with gate drive at 100KHz, 25% duty
> cycle.
> http://www.aetherltd.com/public/misc/drainvoltage.jpg
It appears that the supply voltage is only about 3 volts. That is much too
low for the circuit to work. It also appears that the signal generator does
not have much drive current which is indicated by the slow ON time and the
very visible Miller plateau. The delay provides a much reduced effective
duty cycle.
> The comparable simulation works much better than the real world. Much
> faster rise time at MOSFET turn-off, and to a higher voltage.
> http://www.aetherltd.com/public/misc/ttydrivermosfettest1.png (screen)
> http://www.aetherltd.com/public/misc/ttydrivermosfettest1.asc (LTspice)
> The blue line is the drain of the MOSFET; the green line is the gate. The
> output voltage in the sim ramps up much faster than in the real world.
> Both make it to 120VDC eventually, but it's about 12ms in sim and about
> 500ms real world (which is too slow for the application.) I don't
> understand why it's so much less effective in the sim.
> The model of the Coilcraft DA2032-AL transformer is probably inadequate.
> Here's its data sheet:
> http://www.coilcraft.com/da2032.cfm
> Suggestions?
The simulation does not adequately model your real-world circuit. The
transformer model is probably OK. You should use actual component modules
for the diodes. Also, the TPS2030 shown in your original schematic is
current-limited to 300 mA. It would help if you could update the schematic,
and also perhaps add a proper low-side MOSFET driver. It is also possible to
use a BJT such as a MPSA06 or 2N4401. With a 50 ohm base resistor, you will
get about 25 mA and with hfe >= 100 you can easily drive 500 mA.
Paul
> On 11/18/2016 5:19 PM, P E Schoen wrote:
>> Don't worry too much about the MOSFET gate current spikes. Driving a
>> capacitance with a square wave does that. The 555 is not an ideal gate
>> driver, but probably adequate. What is important is for the ON voltage to
>> be well above the linear threshold. This was not the case previous to
>> fixing the 5V supply problems.
> Well, I substituted that MOSFET, and it doesn't help much.
> It will not work with 2V on the gate. It takes 3V in the real world, and
> that's consistent with the data sheet. See Fig. 3 on
> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
Have you fixed the power supply voltage problem? You should be able to get
> It will not work with 2V on the gate. It takes 3V in the real world, and
> that's consistent with the data sheet. See Fig. 3 on
> http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf
4.5 volts on the 555 and that should provide at least 2.5-3 volts gate
drive. You might be able to tweak the output a bit higher with a pull-up
resistor to the 5V supply.
> and note that at 2V, it's starting to turn on, at 3V it's passing 10X as
> much current, and at 7V on the gate, it's fully turned on and passing 100X
> as much current as at 2V. This is not a 2V device.
There are many devices in SOIC-8 and SOT-23 packages with 1.8, 1.5, and even
lower gate voltages, although the Vce may not be more than 30V. Try
IRLML6344, which will conduct 3A with 0.2V drop using a gate voltage of 1.7
volts.
> As a test, I replaced the 555 timer signal with a bench waveform
> generator. This can go to 3V, but not to 5V, and at 3V, things sort of
> start to work. But not very well. There's no inductive kick from the
> transformer at turn-off. See this waveform, which is MOSFET drain to
> ground, with a 10x probe. This is with gate drive at 100KHz, 25% duty
> cycle.
> http://www.aetherltd.com/public/misc/drainvoltage.jpg
low for the circuit to work. It also appears that the signal generator does
not have much drive current which is indicated by the slow ON time and the
very visible Miller plateau. The delay provides a much reduced effective
duty cycle.
> The comparable simulation works much better than the real world. Much
> faster rise time at MOSFET turn-off, and to a higher voltage.
> http://www.aetherltd.com/public/misc/ttydrivermosfettest1.png (screen)
> http://www.aetherltd.com/public/misc/ttydrivermosfettest1.asc (LTspice)
> The blue line is the drain of the MOSFET; the green line is the gate. The
> output voltage in the sim ramps up much faster than in the real world.
> Both make it to 120VDC eventually, but it's about 12ms in sim and about
> 500ms real world (which is too slow for the application.) I don't
> understand why it's so much less effective in the sim.
> The model of the Coilcraft DA2032-AL transformer is probably inadequate.
> Here's its data sheet:
> http://www.coilcraft.com/da2032.cfm
> Suggestions?
transformer model is probably OK. You should use actual component modules
for the diodes. Also, the TPS2030 shown in your original schematic is
current-limited to 300 mA. It would help if you could update the schematic,
and also perhaps add a proper low-side MOSFET driver. It is also possible to
use a BJT such as a MPSA06 or 2N4401. With a 50 ohm base resistor, you will
get about 25 mA and with hfe >= 100 you can easily drive 500 mA.
Paul
Joerg
Nov 28, 2016, 1:52:30 PM11/28/16
to
(check first though) but would have to be ordered.
For a quick fix hang a bus driver between 555 and FET gate. Maybe with
several sections paralleled. That is usually something that can be found
in the parts bins.
P E Schoen
Nov 28, 2016, 8:04:46 PM11/28/16
to
"Joerg" wrote in message news:ea3cn9...@mid.individual.net...
> As suggested before a CMOS version of the 555 will probably go higher
> (check first though) but would have to be ordered.
> For a quick fix hang a bus driver between 555 and FET gate. Maybe with
> several sections paralleled. That is usually something that can be found
> in the parts bins.
The real problem appears to be the low voltage from the USB power lines. The
drain voltage on the MOSFET when it is OFF is only about 3.5 volts. I think
the USB switch chosen for this circuit limits current to 300 mA by becoming
a current source when overloaded.
A CMOS 555 might provide a voltage close to the rail, but the output current
drive is much less than the ~200 mA of the standard part. The ICM7555 CMOS
part has a typical output high of 4.3 volts with 5 volt supply, at 0.8 mA.
It is actually a CMOS inverter with a rated load of 2 TTL gates. For an 8 mA
load, the voltage drops to 2 volts below supply rail. This is equivalent to
a 250 ohm source impedance. The IRL3915 MOSFET has an input capacitance of
almost 2000 pF, and this corresponds to a TC of about 500 nSec. With a 100
kHz PWM and about 10% duty cycle, the ON pulse is 1000 nSec, so the TC
effectively cuts that in half. This is also seen in the scope trace.
Paul
> As suggested before a CMOS version of the 555 will probably go higher
> (check first though) but would have to be ordered.
> For a quick fix hang a bus driver between 555 and FET gate. Maybe with
> several sections paralleled. That is usually something that can be found
> in the parts bins.
drain voltage on the MOSFET when it is OFF is only about 3.5 volts. I think
the USB switch chosen for this circuit limits current to 300 mA by becoming
a current source when overloaded.
A CMOS 555 might provide a voltage close to the rail, but the output current
drive is much less than the ~200 mA of the standard part. The ICM7555 CMOS
part has a typical output high of 4.3 volts with 5 volt supply, at 0.8 mA.
It is actually a CMOS inverter with a rated load of 2 TTL gates. For an 8 mA
load, the voltage drops to 2 volts below supply rail. This is equivalent to
a 250 ohm source impedance. The IRL3915 MOSFET has an input capacitance of
almost 2000 pF, and this corresponds to a TC of about 500 nSec. With a 100
kHz PWM and about 10% duty cycle, the ON pulse is 1000 nSec, so the TC
effectively cuts that in half. This is also seen in the scope trace.
Paul
Joerg
Nov 28, 2016, 8:11:10 PM11/28/16
to
you can usually parallel sections inside the same IC.
Also, you don't need a FET with 2000pF gate capacitance for this job.
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