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

[Recker039]

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!

 

[D.A.(Tony)Stewart]

TL;DR I'm not sure why the cause of heat is a misunderstood. It shows high power spikes on the edges which are due to all node capacitance from Coss of FET , and clamp diode.

By some limited choices in LTspice I reduced the FET average power to 170 mW (1.2kWpk) using a low Q FET and a low Q clamp diode.

 

[Recker039]

TL;DR I'm not sure why the cause of heat is a misunderstood. It shows high power spikes on the edges which are due to all node capacitance from Coss of FET , and clamp diode.

By some limited choices in LTspice I reduced the FET average power to 170 mW (1.2kWpk) using a low Q FET and a low Q clamp diode.
View attachment 197738

Do you guess about the power dissipation of the Mosfet?

 

[D.A.(Tony)Stewart]

No LTspice measures power Alt+ hover of part (click thermometer) and if you select trace label then again Alt + click, it will parse to a cycle and display avg P in the period in a small box on screen.

 

[Easy peasy]

Ah yes - we can't see the layout - turn off spikes will easily kill the mosfet when you get above 180VDC supply - a good scope would see this straight away

explains sort of OK operation at lower Vcc

--- Updated Mar 2, 2025 ---

Also - at the higher Vcc - RF can easily get into the controller and upset things.

 

[D.A.(Tony)Stewart]

Ah yes - we can't see the layout - turn off spikes will easily kill the mosfet when you get above 180VDC supply - a good scope would see this straight away

explains sort of OK operation at lower Vcc

--- Updated Mar 2, 2025 ---

Also - at the higher Vcc - RF can easily get into the controller and upset things.

Leading edge power spikes (green) were observed in the initial simulation provided and reduced with my low Q=CV choice of semi's. Rated > 600V.

But this wasn't actual parts used nor had any layout parasitics been included.
The TS (thread-starter) ought to do this to simulate the failure again and confirm root causes.

 

[Easy peasy]

It is almost certainly layout related.

 

[danadakk]

Forum has asked for pictures of design/prototype, and scope shots of
it working at lower V, eg gate/drain transients.

We can hopefully stop the guessing and get to the root problem.

 

[Recker039]

 

[Easy peasy]

The snubbers are too small for HV operation - as said just above - if we can see G-s and D-S at turn off at 150V - then the issue will reveal itself

( waveforms at turn on too will be useful )

 

[D.A.(Tony)Stewart]

Too small ? 1nF 220 ohm? or size ? or severe lack of creepage gap that needs milled slots

 

[Recker039]

Drain's Mosfet shorts to source'Mosfet....

 

[Easy peasy]

that is typically over volts on the drain, perhaps coupled with too high switching loss.

 

[Recker039]

The snubbers are too small for HV operation - as said just above - if we can see G-s and D-S at turn off at 150V - then the issue will reveal itself

( waveforms at turn on too will be useful )

supply voltage is 150Vac.
Vgs: (Volt/Div divide by 2) => example: Min Volt is 57/2 = 28.5V

Vds: (Volt/Div *5)=> example: MAX Volt is 38*5 = 190V

I am changing shunt resistor value to 0.54 ohm.
Vs:

 

[danadakk]

Last 3 of your pics unlabeled.....? What test point are they ? What are we looking at ?

You are violating Vgsmax specs -

What is the sampling rate scope screen shots acquired ? Scope probes set to 10X right ?


Regards, Dana.

 

[Recker039]

supply voltage is 150Vac.
Vgs: (Volt/Div divide by 2) => example: Min Volt is 57/2 = 28.5V
View attachment 197771
View attachment 197772
Vds: (Volt/Div *5)=> example: MAX Volt is 38*5 = 190V

View attachment 197773View attachment 197774
I am changing shunt resistor value to 0.54 ohm.
Vs:
View attachment 197775

I dont have any idea about Vs peaks...

--- Updated Mar 4, 2025 ---

Last 3 of your pics unlabeled.....? What test point are they ? What are we looking at ?

You are violating Vgsmax specs -

View attachment 197777

What is the sampling rate scope screen shots acquired ? Scope probes set to 10X right ?


Regards, Dana.

I calculated the probe attenuation for you... and wrote it in front of each image as a multiplication or division. Vsource-gnd doesn't have probe attenuation.

 

[D.A.(Tony)Stewart]

Vds looks like too noisy with probe capture errors. Is it calibrated and probed well?

Gnd path < 2 cm , < 5 ns Tr , flat. , show calibration with probe, 10 MHz pulse

1:1 probes with a R divider is NG

 

[danadakk]

Tony made a great observation, you have 100V of noise on Vdd ? Thats crazy.

What are you using for caps on Vdd, combo of ceramic and tants, polymer tants
best in bulk class.

OS-CON is polymer tant.

 

[mtwieg]

supply voltage is 150Vac.
Vgs: (Volt/Div divide by 2) => example: Min Volt is 57/2 = 28.5V
View attachment 197771

This one especially does not make any sense. Suggests some major issue with how you're probing it, or some huge parasitics somewhere in the circuit itself.

From your schematics, it seems that the circuit is supplied from a bridge rectifier, so unless you use an isolation transformer on the AC supply, you can't connect your scope GND anywhere on your circuit. Are you using an isolation transformer, or a differential probe....?

 

[Recker039]

This one especially does not make any sense. Suggests some major issue with how you're probing it, or some huge parasitics somewhere in the circuit itself.

From your schematics, it seems that the circuit is supplied from a bridge rectifier, so unless you use an isolation transformer on the AC supply, you can't connect your scope GND anywhere on your circuit. Are you using an isolation transformer, or a differential probe....?

I used an insolated probe.

 

[D.A.(Tony)Stewart]

I suspect the hold current is DSO noise with PWM in a slow sweep.

I indicated your leading edge current caused dynamic power loss and demonstrated a solution in a simulation.

The Vds snubbed pulse appears overdamped but ok.

Yet you closed the question as solved without listing solutions. It would be helpful to readers and courteous to do so.