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irf9640 p-chan mosfet switching buck converter blows without load, whats wrong

magtec

Jul 29, 2020
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no load 60vdc in adjusted for 30vdc out only load is led with 11k ohm, dc signal very clean only
transistor fails (shorts) for no apparent reason, using 10k ohm source to gate resistance. gate
pulled down with irfp250n n-chan using 2.6k ohm to ground. irfp250n biased with 5v regulated source
through 100k ohm on gate pulled down with d526-y npn through 1k ohm. what am I missing?
 

Harald Kapp

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Circuit diagram? A picture is worth a 1000 words.
 

magtec

Jul 29, 2020
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schematic attached
 

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  • DC Circuit.pdf
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Harald Kapp

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This doesn't look like a buck converter at all. There is no switching involved - at least none I can see.
The circuit looks more like a rather strange form of linear regulator. The IRF9640 is a switching transistor. This type of transistor can be operated in a linear fashion only within a limited range, the so called SOA (Safe Operating Area). See figure 8 in the datasheet. This application note discusses linear operation of MOSFETs.

Where is this design from? Do you really want to use a linear regulator? Why not use a proven design?
 

magtec

Jul 29, 2020
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This doesn't look like a buck converter at all. There is no switching involved - at least none I can see.
The circuit looks more like a rather strange form of linear regulator. The IRF9640 is a switching transistor. This type of transistor can be operated in a linear fashion only within a limited range, the so called SOA (Safe Operating Area). See figure 8 in the datasheet. This application note discusses linear operation of MOSFETs.

Where is this design from? Do you really want to use a linear regulator? Why not use a proven design?
 

magtec

Jul 29, 2020
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I was using the output voltage divider (130k, 5k pot, 100) to turn on Q3 at the desired output voltage. Q3 on = Q1 off, this would act to
automatically set the switching frequency according to the load requirements via the voltage drop. This actually works as far as being able to set an output voltage (30-60vdc IN to 10-60vdc OUT) except for some reason source drain will short without a load. I was wondering if the gate control circuitry had something to do with it. My purpose of this circuit was to simplify function and parts replacement as I am working on experimental systems
 

magtec

Jul 29, 2020
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As voltage builds at C1, Q3 is forward biased for buck desired output from voltage divider 130k, 5kpot, 100(back-up protection for pot)
pulling gate voltage of Q2 to ground turning Q1 off. Q1 starts on with bias by the 5 volt regulator divider 1k, 10k pot,100
applied at Q2 gate pulling Q1 p-channel gate lower than source (on), again I verify this actually works as I am able to adjust the
output voltage with the 5k pot and since the only factor for switching off Q1 is the output side divider voltage to Q3 which is
totally dependent, not relying on a set frequency or pulse width, this makes the frequency and pulse width set according to the
charging of C1. Note: I have an led w/10k across output and have successfully temporarily connected a 3ohm load @ 20v out so I am
not sure why the irf9640 melts down. I am wondering if the gate circuit is amiss somehow or maybe a current spike?, also notable is
the 15v zener which was an add on in hopes of a fix, this was not in original design which worked the same without it
 

Alec_t

Jul 7, 2015
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Q2 won't switch on reliably. The IRFP250N has a Vgs(thr) which can be as high as 4V.
As noted above, this is a linear regulator design, not a switch-mode one. LTspice simulation confirms no oscillation occurs.
There is no accurate reference voltage source to establish the output voltage, so the output will vary considerably with changes in MOSFET properties and load.
A 3Ω Load @ 20V implies a ~ 7A current. If the input is 60V then the P-MOSFET will be dissipating (60V-20V)*7A = 280W (which is way above its 190W rating)! Even half that would require a pretty big heatsink to prevent the MOSFET self-destructing.
this makes the frequency and pulse width set according to the
charging of C1.
The purpose of C1 should be to suppress pulses. The output is presumably supposed to be DC (not pulsed).
 
Last edited:

bertus

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Hello,

As @Alec_t calculated, the power for the mosfet is way to high.
The datasheet shows a maximum dissipation of 125 W.

Bertus
 

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  • IRF9640.pdf
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magtec

Jul 29, 2020
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Thanks alot to everyone involved, looks like I will stick to frequency switching. I appreciate all the help
 
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