mosfet strings

On a sunny day (Wed, 20 Apr 2022 10:09:53 -0700) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in snipped-for-privacy@4ax.com:

Just wondering is it not simpler to use them all in parallel and connect the drains directly to the metal of the heatsink? Washers decrease thermal resistance

On a sunny day (Wed, 20 Apr 2022 17:41:17 GMT) it happened Jan Panteltje snipped-for-privacy@yahoo.com wrote in <t3pgoe$el$ snipped-for-privacy@dont-email.me:

oops increase I mean, and then insulate the heatsink.

MOSFET strings are prone to problems. Dynamic switching can be solved with adding capacitors in the divider string

I just want to burn watts to test the thermals. I do want the dissipations to be about equal among the fets.

I've seen the zener version used in a HV power supply about 20 years ago.

Cheers

In this case, I just want the fets to get hot, to test a heat sink.

On a sunny day (Wed, 20 Apr 2022 13:51:50 -0700) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in snipped-for-privacy@4ax.com:

Use power resistirs? Much simpler.

The question is, how hot do these fets get on this heat sink?

One issue with heat sinks is that the theta is usually/probably measured with heat uniformly applied to the baseplate. Real parts don't do that... they are usually a small number of hot spots. So footprints matter.

My tests will use the real fets and the real insulators and we'll measure real fet temperatures.

On a sunny day (Thu, 21 Apr 2022 07:01:19 -0700) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:

OK, if that is what you want to do, looks like a nice application for my FLIR camera recording.

We have a big old E45 Flir with the gorgeous wide-angle germanium lens. That should be pretty good for snooping the tops of the mosfets and estimating junction temp.

The option is to squash some of the thin foil thermocouples between the neoprene washer and the tops of the fets.

We don't need extreme temp accuracy because the maximum power dissipation spec, and the max allowable Tj, will be mostly guesses.

There has to be one element in the string to determine the string's function, before they can share the voltage stress.

Rgs >> Rdiv and should not be in series with the divider string.

Zeners can only zener. connecting drain to gate just gets a zener at Vgson.

RL

We will power the string from a big bench power supply, and we can set its voltage and current limit to get whatever power dissipation we want. We'd like the power per fet to be about the same, namely identical Vds, but we can measure the drop across each fet and calculate its actual dissipation.

I wish we had software tools to simulate thermal situations like this, but lacking that we measure.

onsdag den 20. april 2022 kl. 19.10.06 UTC+2 skrev John Larkin:

run them in reverse (using the body diode) from a current limited supply ?

you might even be able to periodically get the actual die temperature, applying a small current and looking at the voltage drop

fredag den 22. april 2022 kl. 00.43.31 UTC+2 skrev John Larkin:

I imagine something like a resistor in series with a CC/CV supply CV and resistor set measurement current, short the resistor CC sets the power level scope or multimeter across the mosfet stack measure voltage drop calibrating at two known temperatures will probably do

On Thu, 21 Apr 2022 12:00:14 -0700, John Larkin <jlarkin@highland_atwork_technology.com> wrote:

watch for text wrap on PWL

Version 4 SHEET 1 1440 768 WIRE 64 -16 -80 -16 WIRE 320 -16 64 -16 WIRE 432 -16 320 -16 WIRE 560 -16 432 -16 WIRE 64 16 64 -16 WIRE 320 16 320 -16 WIRE -80 64 -80 48 WIRE 272 96 112 96 WIRE 112 112 112 96 WIRE 176 112 112 112 WIRE 288 112 240 112 WIRE -80 160 -80 144 WIRE -80 160 -112 160 WIRE 64 160 64 96 WIRE 64 160 -80 160 WIRE 112 160 112 112 WIRE 112 160 64 160 WIRE 160 160 112 160 WIRE 288 160 288 112 WIRE 288 160 240 160 WIRE 320 160 320 112 WIRE 320 160 288 160 WIRE 352 160 320 160 WIRE 64 176 64 160 WIRE 320 176 320 160 WIRE -80 240 -80 224 WIRE 272 256 112 256 WIRE 112 272 112 256 WIRE 176 272 112 272 WIRE 288 272 240 272 WIRE -80 320 -112 320 WIRE 64 320 64 256 WIRE 64 320 -80 320 WIRE 112 320 112 272 WIRE 112 320 64 320 WIRE 160 320 112 320 WIRE 288 320 288 272 WIRE 288 320 240 320 WIRE 320 320 320 272 WIRE 320 320 288 320 WIRE 352 320 320 320 WIRE 320 336 320 320 WIRE -80 400 -80 384 WIRE 272 416 112 416 WIRE 112 432 112 416 WIRE 176 432 112 432 WIRE 288 432 240 432 WIRE -80 480 -112 480 WIRE 64 480 64 400 WIRE 64 480 -80 480 WIRE 112 480 112 432 WIRE 112 480 64 480 WIRE 160 480 112 480 WIRE 288 480 288 432 WIRE 288 480 240 480 WIRE 320 480 320 432 WIRE 320 480 288 480 WIRE 352 480 320 480 WIRE -80 496 -80 480 WIRE 64 496 64 480 WIRE 560 528 560 -16 WIRE 320 544 320 480 WIRE -80 576 -80 560 WIRE -80 688 -80 656 WIRE 64 688 64 576 WIRE 64 688 -80 688 WIRE 320 688 320 624 WIRE 320 688 64 688 WIRE 560 688 560 608 WIRE 560 688 320 688 WIRE 560 704 560 688 FLAG 352 480 VS1 FLAG 352 320 VS2 FLAG 352 160 VS3 FLAG 432 -16 V1 FLAG -112 160 VG3 FLAG -112 320 VG2 FLAG -112 480 VG1 FLAG 560 704 0 SYMBOL res 48 0 R0 SYMATTR InstName R1 SYMATTR Value 100K SYMBOL res 48 160 R0 SYMATTR InstName R2 SYMATTR Value 100K SYMBOL res 48 304 R0 SYMATTR InstName R3 SYMATTR Value 100K SYMBOL res 48 480 R0 SYMATTR InstName R4 SYMATTR Value 100K SYMBOL res 256 144 R90 WINDOW 0 55 95 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1E6 SYMBOL res 256 304 R90 WINDOW 0 57 103 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 1E6 SYMBOL res 256 464 R90 WINDOW 0 59 103 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 1E6 SYMBOL nmos 272 16 R0 SYMATTR InstName M1 SYMATTR Value BSC340N08NS3 SYMBOL nmos 272 176 R0 SYMATTR InstName M2 SYMATTR Value BSC340N08NS3 SYMBOL nmos 272 336 R0 SYMATTR InstName M3 SYMATTR Value BSC340N08NS3 SYMBOL zener 240 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D1 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL zener 240 256 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL zener 240 416 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D3 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL current 320 544 R0 WINDOW 3 -388 186 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value PWL(0 0 1E-3 0 1.001E-3 1 2E-3 1 2.001E-3 0.1 3E-3 0.1

3.001E-3 10 4E-3 10 4.001E-3 0) SYMBOL voltage 560 512 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 100 SYMBOL cap -96 -16 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL cap -96 160 R0 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL cap -96 320 R0 SYMATTR InstName C3 SYMATTR Value 10n SYMBOL cap -96 496 R0 SYMATTR InstName C4 SYMATTR Value 10n SYMBOL res -96 48 R0 SYMATTR InstName R8 SYMATTR Value 1K SYMBOL res -96 224 R0 SYMATTR InstName R9 SYMATTR Value 1K SYMBOL res -96 384 R0 SYMATTR InstName R10 SYMATTR Value 1K SYMBOL res -96 560 R0 SYMATTR InstName R11 SYMATTR Value 1K TEXT -182 750 Left 2 !.tran 5E-3

On Thu, 21 Apr 2022 12:00:14 -0700, John Larkin <jlarkin@highland_atwork_technology.com> wrote:

Check out the voltage compliance of the control element for constant current

Version 4 SHEET 1 1784 768 WIRE 64 -16 -80 -16 WIRE 320 -16 64 -16 WIRE 432 -16 320 -16 WIRE 560 -16 432 -16 WIRE 64 16 64 -16 WIRE 320 16 320 -16 WIRE -80 64 -80 48 WIRE 272 96 112 96 WIRE 112 112 112 96 WIRE 176 112 112 112 WIRE 288 112 240 112 WIRE -80 160 -80 144 WIRE -80 160 -112 160 WIRE 64 160 64 96 WIRE 64 160 -80 160 WIRE 112 160 112 112 WIRE 112 160 64 160 WIRE 160 160 112 160 WIRE 288 160 288 112 WIRE 288 160 240 160 WIRE 320 160 320 112 WIRE 320 160 288 160 WIRE 352 160 320 160 WIRE 64 176 64 160 WIRE 320 176 320 160 WIRE -80 240 -80 224 WIRE 272 256 112 256 WIRE 112 272 112 256 WIRE 176 272 112 272 WIRE 288 272 240 272 WIRE -80 320 -112 320 WIRE 64 320 64 256 WIRE 64 320 -80 320 WIRE 112 320 112 272 WIRE 112 320 64 320 WIRE 160 320 112 320 WIRE 288 320 288 272 WIRE 288 320 240 320 WIRE 320 320 320 272 WIRE 320 320 288 320 WIRE 352 320 320 320 WIRE 320 336 320 320 WIRE -80 400 -80 384 WIRE 272 416 112 416 WIRE 112 432 112 416 WIRE 176 432 112 432 WIRE 288 432 240 432 WIRE -80 480 -112 480 WIRE 64 480 64 400 WIRE 64 480 -80 480 WIRE 112 480 112 432 WIRE 112 480 64 480 WIRE 160 480 112 480 WIRE 288 480 288 432 WIRE 288 480 240 480 WIRE 320 480 320 432 WIRE 320 480 288 480 WIRE 352 480 320 480 WIRE -80 496 -80 480 WIRE 64 496 64 480 WIRE 560 528 560 -16 WIRE 320 544 320 480 WIRE -80 576 -80 560 WIRE -80 688 -80 656 WIRE 64 688 64 576 WIRE 64 688 -80 688 WIRE 320 688 320 624 WIRE 320 688 64 688 WIRE 560 688 560 608 WIRE 560 688 320 688 WIRE 560 704 560 688 FLAG 352 480 VS1 FLAG 352 320 VS2 FLAG 352 160 VS3 FLAG 432 -16 V1 FLAG -112 160 VG3 FLAG -112 320 VG2 FLAG -112 480 VG1 FLAG 560 704 0 SYMBOL res 48 0 R0 SYMATTR InstName R1 SYMATTR Value 100K SYMBOL res 48 160 R0 SYMATTR InstName R2 SYMATTR Value 100K SYMBOL res 48 304 R0 SYMATTR InstName R3 SYMATTR Value 100K SYMBOL res 48 480 R0 SYMATTR InstName R4 SYMATTR Value 100K SYMBOL res 256 144 R90 WINDOW 0 55 95 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1E6 SYMBOL res 256 304 R90 WINDOW 0 57 103 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 1E6 SYMBOL res 256 464 R90 WINDOW 0 59 103 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 1E6 SYMBOL nmos 272 16 R0 SYMATTR InstName M1 SYMATTR Value BSC340N08NS3 SYMBOL nmos 272 176 R0 SYMATTR InstName M2 SYMATTR Value BSC340N08NS3 SYMBOL nmos 272 336 R0 SYMATTR InstName M3 SYMATTR Value BSC340N08NS3 SYMBOL zener 240 96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D1 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL zener 240 256 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL zener 240 416 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D3 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode SYMBOL current 320 544 R0 WINDOW 3 29 80 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value 1 SYMBOL voltage 560 512 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PWL(0 10 1E-3 10 1.001E-3 100 2E-3 100 2.001E-3 40 3E-3

40 3.001E-3 140 4E-3 140 4.001E-3 0) SYMBOL cap -96 -16 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL cap -96 160 R0 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL cap -96 320 R0 SYMATTR InstName C3 SYMATTR Value 10n SYMBOL cap -96 496 R0 SYMATTR InstName C4 SYMATTR Value 10n SYMBOL res -96 48 R0 SYMATTR InstName R8 SYMATTR Value 1K SYMBOL res -96 224 R0 SYMATTR InstName R9 SYMATTR Value 1K SYMBOL res -96 384 R0 SYMATTR InstName R10 SYMATTR Value 1K SYMBOL res -96 560 R0 SYMATTR InstName R11 SYMATTR Value 1K TEXT -182 750 Left 2 !.tran 5E-3

I was recently scribbling something similar for a high voltage power supply, namely a stack of fets. In my case, your I1 becomes my optocoupler. M3 could be a depletion fet.

On a sunny day (Thu, 21 Apr 2022 08:16:01 -0700) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:

The nice thing about being able to record temperature is that you can fast forward or slow motion it later in playback to see how heat spreads over time etc. should not be that hard to add current or voltage or some other parameter as subtitle. (idea!).