Feedback for low frequency PWM regulator
JMini
described in a thread called "RMS Approximation of PWM/Square wave". In any
case. Since there is no inductor/diode/cepacitor in the output stage I'll be
using an RMS converter (LTC1968). For the PWM section I'm using the MIC1557
(SOT-23 size 555 equiv) for a R-C sawtooth to a comparator (TLV7211a)
inverting input. I can choose the frequency (probably in the 200-800Hz
range). The feedback is sent through the LTC1968 RMS converter to the FB pin
(0.8V) of a tiny (SC-70) 5mA voltage regulator (OnSemi NCP102). It's really
just a powerful error amplifier. The Output of that is sent to the
non-inverting input of the comparator. So if the feedback voltage drops, the
NCP102 increases voltage ot the non-inverting input of the TLV7211, thus
increasing duty cycle. I've tested this type of layout on breadboard using
different components. I got to thinking though.
Question:
Would it be possible to use a resistor divider between the MIC1557 and
comparator to reduce the voltage of the sawtooth and feed it to the
NON-inverting comparator input and send the RMS converter output directly to
the INVERTING input of the comparator? That way, a falling output voltage
would cause a reduction in voltage to the INVERTING input and increase duty
cycle? I could reduce the component count.
I realize there is no true reference voltage in the system, but since the
1557 is fed from a fixed 5V source, the sawtooth would be a constant 5*1/3 to
5*2/3 V. Thoughts guys?
Martin Griffith
Phew,
Any chance of posting a circuit somewhere?
martin
JMini
OK. I've uploaded a quick drawing to imageshack. PLEASE forgive the hand
drawing. I'm awful at getting this stuff into schematic software.
http://img386.imageshack.us/img386/9920/pwmschemajx8.jpg
Mook Johnson
> OK. I've uploaded a quick drawing to imageshack. PLEASE forgive the hand
> drawing. I'm awful at getting this stuff into schematic software.
> http://img386.imageshack.us/img386/9920/pwmschemajx8.jpg
Couple of questions.
1) Why are you driving the high side of the bulb instead of the ground side.
Ground side is much easier becaust eh source is at ground so the gate driver
can be referenced to ground. Unless it is a P-channel and VIN is less than
15V you're probabbly better off with a low side N-channel (they are more
robust).
2) Why use Comverter. Average should be fine as you are only going to
create a DC average. for average its as simple as a R-C if the pwm
frequency is high enough or a multi pole active filter. if it is lower.
If you comare average voltage in with a variable reference, you can have a
adjustable brightness curcuit that is linar with your pot adjustment
andfixed from external changes in VIN or temperature.
John Popelish
> OK. I've uploaded a quick drawing to imageshack. PLEASE forgive the hand
> drawing. I'm awful at getting this stuff into schematic software.
> http://img386.imageshack.us/img386/9920/pwmschemajx8.jpg
I'm not so sure about the error amplifier part, but I see
why you are using an RMS converter, so you are controlling
the "effective" voltage to the lamp. I take it you are
after constant brightness. Of course, you could also
measure the lamp light output so you compensate for it aging
(and for the initial tolerance).
Are you very cramped for space, or is there some other good
reason you don't want to average the voltage of the pulses
with an inductor, so you can eliminate the RMS converter?
Having the inductor would lower the current ripple from the
power source and also the losses in the switch.
--
Regards,
John Popelish
Phil Allison
"Mook Johnson"
>
> 2) Why use Comverter. Average should be fine as you are only going to
> create a DC average.
** Nonsense.
Lamps are rated for DC or rms AC voltage.
The average value of a PWM wave can easily be way under the rms equivalent
value - hence you will wind up with a blown lamp.
...... Phil
John Larkin
I guess the real question is, why? The light output will still be very
nonlinear on any control input. Why not feedback on the light?
John
Martin Griffith
legg
<j.min...@minihane.tzo.com> wrote:
>I'm building a PWM regulator for an incandescent bulb. Some of thos was
>described in a thread called "RMS Approximation of PWM/Square wave". In any
>case. Since there is no inductor/diode/cepacitor in the output stage I'll be
>using an RMS converter (LTC1968). For the PWM section I'm using the MIC1557
>(SOT-23 size 555 equiv) for a R-C sawtooth to a comparator (TLV7211a)
>inverting input. I can choose the frequency (probably in the 200-800Hz
>range). The feedback is sent through the LTC1968 RMS converter to the FB pin
>(0.8V) of a tiny (SC-70) 5mA voltage regulator (OnSemi NCP102). It's really
>just a powerful error amplifier. The Output of that is sent to the
>non-inverting input of the comparator. So if the feedback voltage drops, the
>NCP102 increases voltage ot the non-inverting input of the TLV7211, thus
>increasing duty cycle. I've tested this type of layout on breadboard using
>different components.
Why ?
This sort of arrangement will hit the lamp with the mother of all
turn-on surges.
Once stable (tee hee), you've got an rms voltage comparison to a
buried reference that bears no constant relationship to anything else
in the circuit, save the NCP102's reference voltage.
What are you trying to do?
RL
JMini
that I illustrate the voltage dividers feeding the differential inputs of the
RMS converter. That's all.
JMini
I am extremely cramped for space. the RMS converter is an 8-MSOP package. The
inductor would be HUGE. This regulator will carry about 10-11 amps RMS. I'm
also looking at keeping the frequency rather low to avoid a ton of switching
noise an reduce the phantoms that pop up when using high frequency PWM in
close proximity to other sentitive bits. I don't imagine 40+ kHz 10 Amps RMS
would play bery nicely just millimeters away from my RMS converter.
JMini
The input voltage will only be a few volts higher than the regulated output.
However, since the Vin is from batteries, the input voltage will be falling
the whole time, but I want constant RMS voltage to the bulb. This is for a
regulator for obscenely powerful flash lights. I recently built a
non-regulated version (PWM soft-start) that was 220W in a 3D Maglite size. It
has to be small (30mm round x 6mm high). The NCP102 has a built in
programmable softstart. I can stretch out the start-up over a full second or
more if I need to. I thought that the known min/max of the sawtooth would
provide a sort of reference. But the NCP102 looks like it might just be a
requirement That inrush current is monsterous. You're right. But
soft-starting will save the bulb. I plan on using the International rectifier
IRLR7843 for lower power applications and the IRF2804S for higher power ones.
legg
The output of the rms-dc converter has a very slow response time -
measured in the 100s of milliseconds. In order to get the NCP102 to
work with this inside the feedback loop, you're going to have to slow
it's regulator down considerably. There's a model available if you
want to see what a pspice-type simulator shows.
Although the converter has differential inputs, which will simplify
interface to the low-side driver actually being employed, it's linear
output is in the 0-400mV range. How are you matching this to the
regulator's 800mV internal reference?
Fast-rising and falling current transients will radiate, even in an
800hZ pwm cicuit. The resonant frequency is determined by your
battery, lamp and switch wiring loops reacting with the fet's output
capacitance. Check it out with a scope. At this low frequency, you can
probably be generous with snubbers.
RL
JMini
RMS output using a resistor divider by +750mV because the Feedback voltage of
the TL5001 is 1V. I can do the same with this design. I can just level shift
the output by 600 mV. That puts the RMS converter output (in regulation) at
200mV. Right in the middle of that linear range.
Good catch on the linear range. I'll make a note.
Please give a little more detail regarding the use of snubbers. Snubbers
around the FET? In a previous design of a simple PWM soft-start, scope trace
of the output under a 12 Amp load was pretty clean. It runs at 175 Hz.
HarryD
"JMini" <j.min...@minihane.tzo.com> wrote in message
news:VYvbk.450$4a3.312@trnddc04...
> I'm building a PWM regulator for an incandescent bulb. Some of thos was
Cut lots of bad ideas!
You are trying to control a resistive load so a low side N-MOSFET to ground
is the simplest configuration. No need to look at the current waveform it is
just Vin/Rl. You just have to control the pulse width as the input voltage
changes, that is "Voltage Feed Forward Control". A simple comparator whose
output is beefed up to drive the FET. One input has a cap to ground and
charged thru a resistor by Vin. The cap is reset in the Toff time. The other
comparator input is your DC control voltage. This voltage must have a soft
start at power up. You might preheat the bulb with a resistor in parallel
with the FET prior to soft start. Most of the above can be found in one
controller chip or the ubiquitous UC42XX controllers can easily be
configured into Voltage Feed Forward control.
Cheers,
Harry
nospam
>
>"JMini" <j.min...@minihane.tzo.com> wrote in message
>news:VYvbk.450$4a3.312@trnddc04...
>> I'm building a PWM regulator for an incandescent bulb. Some of thos was
>
>Cut lots of bad ideas!
Cut another bad idea.
He needs to regulate the power in a resistive load. The required PWM duty
cycle is inversely proportional to the square of the supply voltage. You
don't square anything with an RC network.
You could get a square law by making the PWM on time inversely proportional
to the supply voltage *and* the PWM period proportional to the supply
voltage but there are simpler solutions.
--
JMini
I'm going to readup on "Voltgae Feed Forward" control, but could you
elaborate on how the cap resets during Toff. If the cap is charged through
the resistor by Vin, its voltage will rise until it's at Vin. Or is the
resistor connected between the bulb and the FET drain?
Are there some shematic/examples on-line I can reference. Maybe I've just
been over complicating things. Regarding the laready available PWM contriller
(UC42XX included). They operate at very high frequencies. I don't need high
frequencies. Thanks for your input.
JMini
I'm all ears. I'm really looking for all input here.
If someone has simple solution to keep the RMS voltage constant into a
resistive load using PWM while the input voltage drops, I'd love to hear
them. I showed you mine, now you show me yours. TIA, guys.
nospam
>> You could get a square law by making the PWM on time inversely
>> proportional to the supply voltage *and* the PWM period proportional to
>> the supply voltage but there are simpler solutions.
>
>I'm all ears. I'm really looking for all input here.
>If someone has simple solution to keep the RMS voltage constant into a
>resistive load using PWM while the input voltage drops, I'd love to hear
>them. I showed you mine, now you show me yours. TIA, guys.
For less that $1 you can get a PIC in SO8 (and smaller) which has built in
5v regulator, built in voltage reference, built in oscillator, 10 bit ADC,
10 bit PWM generator, and enough processing power to measure the supply
voltage, calculate and apply the required PWM duty at several hundred Hz
(and still have 4 pins left over).
If you can't do software the lamp filament *is* a thermistor providing
direct feedback of what you are actually trying to control (the filament
temperature). You can connect the lamp as one leg of a wheatstone bridge
and make a bistable circuit controlling the MOSFET from a comparator which
flips off when the filament resistance exceeds a value set by the rest of
the bridge resistors. A second comparator can make a timer to flip the
bistable on again after a fixed dead time or an oscillator which flips the
bistable on at constant frequency.
Both are one chip (+ maybe something to drive the MOSFET hard) solutions.
--
HarryD
"nospam" <nos...@please.invalid> wrote in message
news:s6h4749vgrq39dd7d...@4ax.com...
A better way may be controlling current thru the lamp. VFF is a simple
current control. We need a plot of lamp current and power vises luminous
intensity. I'm betting on current being the more linear and best to control
intensity. Where is Don Klipstein when you need him? Sorry if this is a
rehash.
Cheers,
Harry
JMini
knowledge with regard to uCs. This would be a simple thing if I had the first
idea what I was doing with those things. I've been pointed in the direction
of the ATTiny85 AVR as well as the AVRFreaks forum, but I need a lot more
learning. I don't even know what to buy to connect my PC to the board to
program the chip.
Also, the circuit really needs to control RMS current or RMS voltage. The
bulb mary vary by application, so it's resistance would be different from
bulb to bulb.
HarryD
"nospam" <nos...@please.invalid> wrote in message
Harry
JMini
constant ambient luminosity level, however it almost definitely won't.
nospam
>Also, the circuit really needs to control RMS current or RMS voltage. The
>bulb mary vary by application, so it's resistance would be different from
>bulb to bulb.
The required current changes just as much as the filament resistance and
controlling current would be a terrible thing to do anyway.
Yes the target filament resistance would have to be adjusted for each type
of lamp and controlling filament resistance is a bit more sensitive to
lamp to lamp variation than controlling voltage.
--
HarryD
"HarryD" <har...@tdsystems.org> wrote in message
news:E9tck.166176$3p2....@fe10.news.easynews.com...
5W to 300W and the efficacy changes from 5 to 20.7. Power dissipated in the
lamp is not good intensity control, maybe current control is just as good or
better? It sure is simpler!
Cheers,
Harry
JMini
Regulating RMS current would eliminate the need for a soft-start. The bulb
pulls a lot of amps during startup. For example, a 100W bulb on 12V pulls a
50+ amp spike then settles in around 8.3A. If the circuit only allowed an RMS
current of 8.3A max, the bulb would soft-start itself. Good thoughts.
But how would controlling RMS current be easier than controlling RMS voltage?
Spehro Pefhany
wrote:
>"JMini" <j.min...@minihane.tzo.com> wrote:
>
>>> You could get a square law by making the PWM on time inversely
>>> proportional to the supply voltage *and* the PWM period proportional to
>>> the supply voltage but there are simpler solutions.
>>
>>I'm all ears. I'm really looking for all input here.
>>If someone has simple solution to keep the RMS voltage constant into a
>>resistive load using PWM while the input voltage drops, I'd love to hear
>>them. I showed you mine, now you show me yours. TIA, guys.
>
>For less that $1 you can get a PIC in SO8 (and smaller) which has built in
>5v regulator, built in voltage reference, built in oscillator, 10 bit ADC,
>10 bit PWM generator, and enough processing power to measure the supply
>voltage, calculate and apply the required PWM duty at several hundred Hz
>(and still have 4 pins left over).
Which SO-8 one(s) have an internal voltage reference (ie. not Vdd) for
the ADC?
>If you can't do software the lamp filament *is* a thermistor providing
>direct feedback of what you are actually trying to control (the filament
>temperature). You can connect the lamp as one leg of a wheatstone bridge
>and make a bistable circuit controlling the MOSFET from a comparator which
>flips off when the filament resistance exceeds a value set by the rest of
>the bridge resistors. A second comparator can make a timer to flip the
>bistable on again after a fixed dead time or an oscillator which flips the
>bistable on at constant frequency.
>
>Both are one chip (+ maybe something to drive the MOSFET hard) solutions.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
HarryD
After further review, VFF (Voltage Feed Forward) would keep the voltage
across the lamp constant with changes of input voltage. The current would
vary with filament resistance. Now, is controlled voltage adequate for your
application? If yes, a simple VFF, one 8 pin IC, will do the trick. We still
need the curves of lamp Voltage, Current and Power vs. Luminous intensity
output to determine the best variable to control.
Cheers,
Harry
Spehro Pefhany
If you measure the voltage v(t), then the required PWM output %
is proportional to 1/v^2(t).
You'd need to calculate a voltage divider to give you the reference
voltage at maximum input, and specify the output % at that input.
Eg. 10V maximum input with 25% output at 10V in (say it's a 25 ohm 1W
bulb). Then at 6V in you'd have about 69.5% pwm %, for that same 1W
output. It would hit the end stop at 50% of the maximum input voltage
in this example, below which you could extinguish the light, flash it
or whatever, or simply allow it to drop naturally below that level.
nospam
>>For less that $1 you can get a PIC in SO8 (and smaller) which has built in
>>5v regulator, built in voltage reference, built in oscillator, 10 bit ADC,
>>10 bit PWM generator, and enough processing power to measure the supply
>>voltage, calculate and apply the required PWM duty at several hundred Hz
>>(and still have 4 pins left over).
>
>Which SO-8 one(s) have an internal voltage reference (ie. not Vdd) for
>the ADC?
12F615 has a 0.6 and 1.2v bandgap reference but looking closer you can
measure it (to calibrate) but not use it as an ADC reference. The HV615
shunt regulator is derived from the bandgap so maybe Vdd is good enough
anyway.
--
Spehro Pefhany
<spef...@interlogDOTyou.knowwhat> wrote:
<hmm... repeated since my laptop clock seems to have been off by a
couple of days>
Spehro Pefhany
wrote:
Too bad, I was hoping I might have missed something. A built-in 2.50V
reference would be nice.
Jamie
I don't why you're making it so hard on your self..
Simply place a photo detector diode in the path of the light and use
that as a feed back to the PWM circuit..
the circuit can be operating on a low voltage fixed regulator ..
etc..
legg
<j.min...@minihane.tzo.com> wrote:
If you've checked it out with a scope and seen no funny stuff at
turn-on and turn-off of the switch, then there's no issue. Dedicated
gate rivers tend to be pretty quick, though ~ it's their job.
RL
JMini
hesitant to try controlling luminous intensity or something other than a
basic electrical property. Voltage or current. But would it be possible to
use VFF in a low frequency design and limit RMS current to the bulb?
JMini
that.
legg
<j.min...@minihane.tzo.com> wrote:
>On 7/5/2008 8:50:29 PM, legg wrote:
>> On Fri, 04 Jul 2008 20:44:37 GMT, "JMini"
>> <j.min...@minihane.tzo.com> wrote:
>>
>>>I'm building a PWM regulator for an incandescent bulb. Some of thos was
>>>described in a thread called "RMS Approximation of PWM/Square wave".
<snip>
>>
>> Why ?
>>
>> This sort of arrangement will hit the lamp with the mother of all
>> turn-on surges.
>>
<snip>
>>
>> What are you trying to do?
>>
>The input voltage will only be a few volts higher than the regulated output.
>However, since the Vin is from batteries, the input voltage will be falling
>the whole time, but I want constant RMS voltage to the bulb. This is for a
>regulator for obscenely powerful flash lights. I recently built a
>non-regulated version (PWM soft-start) that was 220W in a 3D Maglite size. It
>has to be small (30mm round x 6mm high). The NCP102 has a built in
>programmable softstart. I can stretch out the start-up over a full second or
>more if I need to. I thought that the known min/max of the sawtooth would
>provide a sort of reference. But the NCP102 looks like it might just be a
>requirement That inrush current is monsterous. You're right. But
>soft-starting will save the bulb. I plan on using the International rectifier
>IRLR7843 for lower power applications and the IRF2804S for higher power ones.
By regulating the rms voltage across the lamp, you're not regulating
luminous intensity or power any more effectively than by regulating
the average voltage. For a resistor and pwm, these two voltage
quantities are the same.
Neither has a predictable relationship to power consumption in a load
with a positive temperature coefficient of resistance. PTC's with low
mass are strongly self-regulated for power. In the E^2 / R
relationship, as E increases, then so does R.
The use of the rms converter is simply complicating this circuit
unnecessarily. The use of PWM to reduce power loss in a control
element IS effective...you should be satisfied with this and
concentrate on simplifying the interface to a low-side switch and to
additionally reducing inrush current to values that will improve lamp
and switch operating life.
RL
HarryD
> On 7/7/2008 4:35:54 PM, "HarryD" wrote:
>>
>> "JMini" <j.min...@minihane.tzo.com> wrote in message
>> news:TOuck.637$HY.46@trnddc01...
>>> On 7/7/2008 4:08:49 PM, "HarryD" wrote:
>>>>
>>>> "HarryD" <har...@tdsystems.org> wrote in message
>>>> news:E9tck.166176$3p2....@fe10.news.easynews.com...
>>>>>
>>>>> simple current control. We need a plot of lamp current and power vises
>>>>> luminous intensity. I'm betting on current being the more linear and
>>>>> best
>>>>> to control intensity. Where is Don Klipstein when you need him? Sorry
>>>>> if
>>>>> this is a rehash.
>>>>>
>>>>> Cheers,
>>>>> Harry
>>>> I see a chart on Wikipedia (SP?) of efficacy (Lm/W) for 120W lamps
>>>> from
>>>> 5W to 300W and the efficacy changes from 5 to 20.7. Power dissipated in
>>>> the lamp is not good intensity control, maybe current control is just
>>>> as
>>>> good or better? It sure is simpler!
>>>> Cheers,
>>>> Harry
>>>>
>>>>
>>>
>>
>> across the lamp constant with changes of input voltage. The current would
>> vary with filament resistance. Now, is controlled voltage adequate for
>> your application? If yes, a simple VFF, one 8 pin IC, will do the trick.
>> We still need the curves of lamp Voltage, Current and Power vs. Luminous
>> intensity output to determine the best variable to control.
>> Cheers,
>> Harry
>>
>>
>>
> There are at least 20 different bulbs that could be used. That's why I'm
> hesitant to try controlling luminous intensity or something other than a
> basic electrical property. Voltage or current. But would it be possible to
> use VFF in a low frequency design and limit RMS current to the bulb?
VFF will control average voltage across the lamp. Since the waveforms are
almost constant, the RMS voltage across the lamp is just a constant factor.
This is the first circuit you should try and then see what else is needed.
Tell me your requirements, input voltage range, switching frequency,
dimming control voltage, lamp current, lamp control voltage range and any
other info necessary. If you have parts already selected, I will try to use
them. A simple pdf design on "alt.binaries.schematic.electronic" will be
posted for your viewing pleasure.
Cheers,
Harry
Jim Thompson
On Tue, 08 Jul 2008 15:01:22 GMT, "HarryD" <har...@tdsystems.org>
wrote:
Brings to mind, why not just regulate the bulb current?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
It's what you learn, after you know it all, that counts.
Due to excessive spam, gmail, googlegroups, UAR, AIOE are blocked!
HarryD
"Jim Thompson" <To-Email-Use-Th...@My-Web-Site.com> wrote in
message news:b75774tr39mbc1bc7...@4ax.com...
sensing. IMHO the OP has not tried the simple things to determine what is
adequate for his application. Sometimes you have to put the pencil down and
grab the soldering iron. If the human eye is in the loop, it is very
insensitive to light intensity changes and control methods must be tried. So
the order of progression from simple to difficult control, voltage, current
and then power. In most lighting applications I have seen, current is the
best parameter to control.
Cheers,
Harry
HarryD
Schematic posted on "alt.binaries.schematics.electronics" as "VFF 10A
Lamp Driver".
Just a starting point. Need lots of added features the OP must specify.
Cheers,
Harry
nospam
> Schematic posted on "alt.binaries.schematics.electronics" as "VFF 10A
>Lamp Driver".
> Just a starting point. Need lots of added features the OP must specify.
Over the supply range of 9 to 14v the lamp light output will vary by a
factor of about 3 and the lamp life vary by a factor of about 300.
So it isn't much of a starting point.
--
HarryD
"nospam" <nos...@please.invalid> wrote in message
as input changes from 9VDC to 15VDC.
Different component values would control RMS power in a similar manner. What
the OP needs may be a different story.
Cheers,
Harry
John Popelish
> Bitch, bitch, bitch, see updated driver at ABSE that controls Vrms to
> +/-3% as input changes from 9VDC to 15VDC.
> Different component values would control RMS power in a similar manner.
> What the OP needs may be a different story.
How badly is this design upset by a non zero source
impedance? I ask because you have included no supply bypass.
--
Regards,
John Popelish
HarryD
"John Popelish" <jpop...@rica.net> wrote in message
news:pOadnRgYhpNIBerV...@comcast.com...
I believed that the OP stated batteries. The circuit will correct for noise
inside the 9V to 15V range on a pulse by pulse basis. If filtering is needed
it would be wise to increase the 260Hz switching frequency for filter size
reduction.
Cheers,
Harry
nospam
>> Over the supply range of 9 to 14v the lamp light output will vary by a
>> factor of about 3 and the lamp life vary by a factor of about 300.
>>
>> So it isn't much of a starting point.
> Bitch, bitch, bitch, see updated driver at ABSE that controls Vrms to +/-3%
>as input changes from 9VDC to 15VDC.
Good, a circuit which tries to do what is required.
--
HarryD
"nospam" <nos...@please.invalid> wrote in message
driven by a discharging battery. The circuit shown can be trimmed to +/-1%
of a fixed RMS voltage for a battery input range of 15V to 9.0V. The +/-3%
above was just a fast cut.What else do you want?
Cheers,
Harry
JMini
the same. I've tried regulating Average voltage. It doesn't work.
JMini
resource on-line to get this pdf?
JMini
I'm trying to find this schematic. Is there an web based tool to download
this schematic. F*cking Verizon.
JMini
Found it. Looking at things now.
legg
<j.min...@minihane.tzo.com> wrote:
This is one of the few applications where the average and rms voltage
are identical.If you averaged the voltage before it hit the resistive
load, there would be an issue. You're not doing this.
In both cases regulation is possible.
If you can't get the simpler arrangement working using an RC filter in
the feedback path, adding an rms converter will not improve the
situation.
RL
John Popelish
> This is one of the few applications where the average and rms voltage
> are identical.If you averaged the voltage before it hit the resistive
> load, there would be an issue. You're not doing this.
I think you are mistaken, if you are saying that the average
voltage of a unipolar rectangular pulse is the same as the
RMS voltage of that pulse. It is not. I think you are
misremembering something else (a bipolar square wave, taking
the average of the absolute value, for example). Please try
running through the math.
For example, a 2 volt peak, 50% duty cycle, rectangular
pulse has an average voltage of 1 volt, but and RMS value of
1.414 volts. It will produce a 2 watt dissipation with a 1
watt load (half of the 4 watt dissipation of a 100% duty
cycle 2 volt 'pulse'), while 1VDC will produce a 1 watt
dissipation, even though the average voltage across the
resistor in each case is 1 volt.
--
Regards,
John Popelish
nospam
>>As the input voltage falls, the PWM needs to increase to keep the RMS voltage
>>the same. I've tried regulating Average voltage. It doesn't work.
>
>This is one of the few applications where the average and rms voltage
>are identical.If you averaged the voltage before it hit the resistive
>load, there would be an issue. You're not doing this.
This is rubbish.
Average and RMS values are directly proportional to PWM duty cycle with the
same supply voltage.
The PWM duty cycles required to maintain a constant average or constant RMS
voltage with varying supply voltage are completely different.
20v with 25% duty and 10v with 50% duty give the same average voltage but a
2:1 difference in RMS voltage.
--
legg
It's funny, considering the number of times I've gone through the
procedure of explaining RMS and average relationships, over the years,
to have to re-train myself on this matter. Sort of refreshing.
The formula Epk x rootD is so automatic that I seldom give it a second
thought. Just set me down with a pencil and paper, however, and I seem
to be able to prove to myself that the moon is made of green cheese.
Wonder why wasn't there wasn't more response, reacting to this
(temporary....?) lunacy? I suspect that spending too much time typoing
large databases must atrophy regular thought processes.
Sorry for the diversion.
RL
jpop...@rica.net
legg wrote:
> It's funny, considering the number of times I've gone through the
> procedure of explaining RMS and average relationships, over the years,
> to have to re-train myself on this matter. Sort of refreshing.
>
> The formula Epk x rootD is so automatic that I seldom give it a second
> thought. Just set me down with a pencil and paper, however, and I seem
> to be able to prove to myself that the moon is made of green cheese.
>
> Wonder why wasn't there wasn't more response, reacting to this
> (temporary....?) lunacy? I suspect that spending too much time typoing
> large databases must atrophy regular thought processes.
>
> Sorry for the diversion.
If this is the dumbest thing you have done in the last month, you
should break out the champagne and congratulate yourself. ;-)
--
Regards,
John popelish
Jamie
Well now is the time to bail out of Verizon and go somewhere else with
out penalties. THey broke the contract of being a full internet
provider, so now its up to you!.
JMini
>
This looks nice and simple.
What is the switching frequency? It looks like 47kHz is as low as this thing
can go. It looks like the UVLO would kick in at ~8.5V. Is there another IC
with a lower UVLO threshold? I'd like to just design a single circuit that
could operate from 6V to ~38-40V and just set the output voltage from
~0.6*Vin up to Vin. I would warn the user to avoid using battery voltages
that exceed desired RMS output by too much.
I could use an LDO between Vin and Vcc to limit Vcc voltage to something like
6V and adjust R5 to maintain voltage at the Comp pin to the desired range. A
Zener from Comp to GND would limit voltage for safety purposes. The output
(pin 6) I see is driving the gates of the power FET (Q1) , but also a P-ch
FET Q3 which grounds VR through R7 when Pin 6 goes low. What is the reason to
pull Vref low during Toff? Sorry if some of these questions seem naive, but
this is quite new to me.
JMini
Sorry for the reply to myself...
How about the UCC1803? Vcc-max = 12V
UVLO at 4.1V Pin compatible with the UC3842/A.
It has a lower Vcc-max, but if I use an LDO between the battery and Vcc, it
should be OK at battery voltages higher than 12V.
JMini
>
If I substitute the UCC1803/3803, how would a softstart be implemented? I
need to start this thing slowly. 500mS is in the right range.
HarryD
Many solutions, here is simplest; insert an 'Inrush Current Limiter"
(#MS320R536 at DK) in series with the load, 0.50 ohms cold and 25mR running.
Other more elegant solutions upon request.
If you are going to operate down to 4.5VDC, select a power MOSFET which
has low Ron @ 4.5Vgs. The operating frequency of 260Hz was stated on the
schematic.
Cheers,
Harry
JMini
The voltage in use will be no lower than 6V. For the greatest majority of
applications it will be over 10V. An FET driver like the TPS2819 can have a
Max input of 40V with an internal regulator to reduce voltage to 14V max to
the gate. Below 14V, the onboard regulator would just be in dropout. For
battery voltages lower than 10V I would select an FET like the IRLR7843. Good
Rds(on) at Vgs = 4.5V. The IRF2804 would be used for more powerful
applications. I've seen folks use the kind of Inrush Current Limiter you've
mentioned. Personally I'd rather do soft-starting on a time basis rather than
a current limit based on series resistance. Also, at 30mm x 8mm, it's about
the same size as the space I have for the entire circuit.
Can I request one of the "more elegant solutions"?
I'm still trying to wrap my brain around the workings of your schematic.
I see that with Rt=20K, the maximum duty cycle is limited to ~97%, would it
be possible to increase the size of Rt to 100K and reduce Ct to 68n? That
would keep the frequency roughly in the range you had (253Hz) Also, if the
UCC2803 is substituted the frequency is determined by 1/RC, But specs
recommend a Max Ct=1n and Rt=200K. That works out to 5kHz. Is a lower
frequency just not possible? Or is increasing Ct to 22n possible even though
it's way outside the recommended range?
I know this is a lot to throw at you, but you seem to have SPICE software and
the brains to use it.
Thanks, Harry.
HarryD
You design like a TI apps. engineer, lots of silicon. Let's see a
schematic so we can remove parts.
The UCC2803 is a good choice. At your switching frequency, 250Hz, only
1.0mA is needed for supply current. So with input voltage from 30VDC to
50VDC, a 18K/0.25W resistor will suffice, no regulator is needed. 10K<
Rt<150K will work just fine, so Rt=120K and Ct =33nF will yield 250Hz and D
=~ 0.995. The internal soft start of 4mS will be of no help, you need
>100mS.
What happened to the other members of this NG? Do I have to do a complete
design here? This is turning into my day job with no benefits. JF could do
this with some 555s. Any more help will require good Tequila.
Cheers,
Harry
JMini
incompetence. Thus far I've just been Looking for uc3843 chips like the 2803
that have the lower voltage range I 've been looking for.
The ucc2803 has the lower UVLO that I need and you say it should be fine.
Great. All I have are hand drawn schematics at the moment. I was going to
order some components and build your design and start fiddling with it. For a
softstart, using linear regulators at least, you can produce a softstart but
holding down the Vref with a capacitor. But your design has the Vref switched
by the BSS84. I'd have to figure out what's going on there. From your design,
it looks like I can adjust output voltage by varying the values of the R5s
(46.4K and 13.3K)
HarryD
Ok, ok, updated schematic posted on ABSE with the UCC2803 and soft start
added.
Q2 (BSS84) is not controlling VR, it is controlling Q3. That is how we
generate the voltage ramp for VFF control.
Cheers,
Harry