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[SOLVED] Issue with Contactor Driver Circuit – MOSFET Burns at Higher Voltage

Recker039

Recker039

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Hello everyone,
I have designed a contactor driver circuit that operates in two modes:
  1. It initially provides an inrush current (limited) to the contactor coil to engage it.
  2. Immediately after engagement, it supplies a constant holding current of approximately 50mA.
This topology worked well for 95A contactors. However, when I scaled up to high-power 265A contactors, I encountered issues.
  • For the 95A contactor, it engages with an initial current of 3.5A.
  • For the 265A contactor, it requires approximately 6.5A to engage.
Problem:
When operating at voltages between 110V and 180V, the MOSFET does not burn. However, above 180V, the MOSFET burns immediately when I switch on the circuit.
Some colleagues suggested:
  • Increasing the gate resistance
  • Adding an RCD snubber in parallel with the existing snubber
However, these modifications did not resolve the issue.
Components used:
  • MOSFET: STP30N65M5
  • IGBT (tested but same result): FGH60N60SMD
I would appreciate any insights on what might be causing this issue and possible solutions.
Thank you!

WIN_20250227_21_11_56_Pro.jpg
 
Solution
C
It sounds like the voltage across your shunt is going up and deleting from the VGS...so the FET is in linear region.

What is the required coil voltage and current of the contactor's control coil?......surely the contactor's control coil shouldnt be across the rectified mains?
Or is that a "low side buck converter" for driving the contactors coil?....beware your 250m inductor is saturating?
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I would suspect an overvoltage spike, but without actually measuring anything with an oscilloscope, it's just a guess. Maybe trying running it at the lower voltage and observe how much of a spike you have; that should give you some idea of what it will look like at higher voltage.
 
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It sounds like the voltage across your shunt is going up and deleting from the VGS...so the FET is in linear region.

What is the required coil voltage and current of the contactor's control coil?......surely the contactor's control coil shouldnt be across the rectified mains?
Or is that a "low side buck converter" for driving the contactors coil?....beware your 250m inductor is saturating?
 
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Solution
Have you put a 'scope to the drain to see whether
it's exceeding breakdown rating? Looked at
sensing coil current to see if the high current
FET is staying on (like snapback, after the flyback
event)? That 1.3 ohm resistor looks handy for that.
Debug wants waveforms.

The 50mA "hold" current does not appear in the
schematic .jpg, as far as I can tell. If that is made
by weak gate bias, that could get ugly at any time.
What's the deal there?
 
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dick_freebird said:
Have you put a 'scope to the drain to see whether
it's exceeding breakdown rating? Looked at
sensing coil current to see if the high current
FET is staying on (like snapback, after the flyback
event)? That 1.3 ohm resistor looks handy for that.
Debug wants waveforms.

The 50mA "hold" current does not appear in the
schematic .jpg, as far as I can tell. If that is made
by weak gate bias, that could get ugly at any time.
What's the deal there?
Click to expand...
Sorry, the holding current is 250 mA. I used a constant current controller IC for this. It controls the current by changing the duty cycle, which is now at 2%.
What do you mean by weak bias? Does it mean that the VGS voltage is low?
 
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I use switching mode...
Click to expand...
Thanks, so this is a low side buck to control the current in the contactor control coil.
Can you show the connection from the 1.3 ohm shunt back to the PWM controller?
Or are you doing this open loop?....(the control of the buck).
Also, i suspect the contactor control coil you show is saturating.....and you are using it as your buck converters inductor.

I think you may well have to use a separate inductor rated for the current...as the 250mH coil is likely saturating......they arent meant to carry several amps.
 
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As a rule I have used a conventional relay in the output stage when dealing with Contactors.
You could consider using it at least for the inrush step.
 
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What is gate drive Tr and Tf times ? Have you got a screen shot at lower contactor supply V of gate drive, to see if its oscillating ?

Is there a gate short as measured when out of circuit, eg. the destroyed MOSFET ?

Is it turn on or off that seems to be the problem ?

Show actual PCB/wiring well focused picture of prototype. Especially the clamp diode/contactor coil connections/wiring.
 
Last edited:
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I don't like this way...The relay is a slow and short-lived element.
Click to expand...

I can't see any scenario on which it is required to switch a Contactor in the range of less than few dozens miliseconds, but I may be outdated.
 
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Your original MOSFET has 8.8 nF input C, not counting miller.

The driver chip ratings shown in specs are for 1.8 nF, and I dont see any typical
Cload chart versus Tr, Tf performance. Then Tr, Tf further degraded with 100 ohm
input R to gate. Just a thought....you do need some R to prevent oscillation.



Loading…

toshiba-semicon-storage.com

The requested picture of board design crucial as stray L, especially to/from
clamp diode very important.
 
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ucc27524

Loading…

www.ti.com

What is your gate drive voltage?
What is the approx VGS rise time to get up above say 8V?
What is the saturation current of the 250mH inductor?
What are you using to "drive" the UCC27524A?
 
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Do you have a fast scope you can capture Vgate transient ? Do this at low V so non
destructive, to get an idea how your layout is performing.
 
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Can you show any more of the schem..?...i sympathise with you here as i know people will be on you for "giving away secrets"...but this is just a plain old Low-side buck tell them.....just like a spanner to a mechanical engineer...no secret.
 
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cupoftea said:
Can you show any more of the schem..?...i sympathise with you here as i know people will be on you for "giving away secrets"...but this is just a plain old Low-side buck tell them.....just like a spanner to a mechanical engineer...no secret.
Click to expand...
photo_2025-02-28_17-23-06.jpg

It's really hard for me to do this... I know it's pointless. But I hope we find the problem.
 
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