On 1/31/23 9:52 PM, boB wrote:
> On Tue, 31 Jan 2023 13:51:52 -0800, Joerg <ne...@analogconsultants.com
>> On 1/30/23 7:10 PM, boB wrote:
>>> Yes, the input voltage 'shouldn't' go high but the battery side
>>> certainly does go high. I think you are right about the input voltage
>>> not going high, but having fixed this many years ago, I can't remember
>>> the failure mechanism exactly. But it can go at least to the PV's Voc
>>> voltage, but much above that, (just a few volts) the PV array should
>>> clamp that.
>> It will go to Voc if the series FET shorts out. So in my case around
>> 75V, two residential panels in series. I can also connect them in
>> parallel but that requires much beefier cables and extra diodes in case
>> one is shaded (they already have bypass diodes).
> What I meant was that if it is a bi-directional (synchronous) buck,
> then it is possible for the PV input voltage to be raised above Voc
> from the battery. The PV array, being a series of diodes, will clamp
> the voltage at approximately Voc, or what would be Voc, even at night.
> This assumes something is not right of course.
MPPT charger controllers are mostly just buck converters. Very rarely
boost or SEPIC, for small stuff. Sometimes bucks are synchronous but
usually not intended to be used bidirectionally. What for?
>>> What was the nominal PV input voltage of this controller ? Was it set
>>> up as a 24V array or 48V array or maybe higher voltage ? That can
>>> also make a difference. What kind was it ? I am familiar with a lot
>>> of the commercial ones.
>> I don't know yet. My friend has jury duty out of town and is gone.
>>> It might be also that the battery voltage flying up is shoving current
>>> the other way, too fast and even with the indutor in there can over
>>> current the FETs. The input capacitors could make a good load for
>>> that backwards current and hurt things as well.
>> I don't quite follow. How can there be a problem if the capacitors on
>> the PV side are large enough?
> When the battery breaker trips, the energy in the battery side caps
> might not be enough to charge the PV input caps very much because of E
> = 1/2 CV^2 depending on the capacitance in the controlle
Those caps will never have more than battery voltage so they won't dump
anything large into the front side. When the battery vanishes those caps
are the only energy storage there is on the output side and preciously
little of that since the manufacturers don't want to waste money.
>>> Is the controller designed to work backwards reliably, too ?
>> Very unlikely. The one I designed wasn't either but it was in this
>> regard bullet-proof. You could not destroy it by randomly disconnecting
>> things. The same should go for commercial ones but obviously they can't
>> get it done.
> Non-synchronous is good in that respect. Just a bit less efficient
> because of the diode drop on the low side of the 1/2 bridge.
On a good design that doesn't matter.
>>> What I find is that the PV input caps, with much higher PV voltage
>>> than battery voltage, is a great source of blow-up energy for the
>>> controller but that is usually from HV to LV battery direction.
>>> Ultimately it is the high current from the battery and shorted FETs
>>> that can cause the PCB to catch fire if things go very wrong.
>> That sounds like a seriously flawed design.
> Any design can have this problem if the bottom FETs short AND the
> relay contacts weld shorted to the battery. Double fault so that
> would be a rare occurrance.
Which relay contacts?
The problem with this design flaw is that it is caused by a single fault
that might not even be a fault: The BMS opens. This could simply happen
because the battery isn't happy about the temperature. Or the amount of
peak load. Or as in one case I know of a simple screw terminal
connection had come loose. Or whatever. The result in all these cases
can be and usually is that the battery "goes away". That fact alone
triggering a MPPT charger meltdown is a serious design flaw in my book.
>>> One thing to do is to catch the Vbattery side going too high *FAST*
>>> and immediately shut off the converter(s)
>> Well, I am going to add in a crowbar anyhow to protect my electronics.
>> An SCR the size of a gold ball that is capable to reliably blow a 50A
>> car fuse if needed. If I put that sans fuse on the MPPT output it would
>> present a dead short to the MPPT, like an empty battery. If it doesn't
>> survive that then the MPPT is real junk.
> Crowbar should work as long as nothing breaks before the breaker
> Electronics stuff breaks sometimes. Just have to do our best so if it
> does, the damage is low as possible.
Yes, but first we have to ponder and anticipate normal and frequenctly
occurring scenarios such as a battery vanishing in the electrical sense.
Not doing so is poor engineering. Even if the engineers are younger and
may be not yet competent enough, that's what we have design reviews for.
Or should have ...