[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!

 

[Easy peasy]

The wave-forms are not especially helpful without detailed notes on how the probes are connected ( and to what exactly ) it appears there are no caps across the rectified mains ( AC = 150V ) this is likely the cause of over-volts on the mosfet at turn off ( no power rail decoupling ) => bang !

also the max Vgs applied should be <= +18v and -5V - exceeding the data sheet Vgs ratings can have bad effects

I hope you are learning something

--- Updated Mar 5, 2025 ---

Also - Vds is not sharp - I suspect quite high switching losses at higher Vin.

--- Updated Mar 5, 2025 ---

Ah - I see the issue - the shunt was subtracting lots of volts from the gate drive - causing excessive turn on losses !

 

[D.A.(Tony)Stewart]

Huh?

If the coil is driven with rectified AC . Even at 3300 Hz it operates in CCM with low ripple. But the little 220R snubber RC will see almost 1A pk. If Vcc is 15V then Vgs cannot exceed specs.

I have no idea what are the optimum coil specs but 6.5A for the actuator and 250 mA holding sounds rather low for vibration immunity. So I suggest this. The Isense cap degrades the performance. The transistor switch with CR filter from Vref gives the surge current and then decays to 75 W coil power from 1.2 kW pk decaying slowing after 2 cycles using 3kHz PWM.

 

[Easy peasy]

7A x 0.5 ohm source resistor = 3.5v subtracted from the G-S voltage. ( it was worse before, 1.33 ohms )

 

[std_match]

I used an insolated probe.

You mean a differential probe? It is good to know that most differential probes are not isolated. They normally have a few MegaOhms from the inputs to ground on the oscilloscope side.

 

[Easy peasy]

yes > 100Meg-ohms from the probes to the scope side ....

 

[std_match]

yes > 100Meg-ohms from the probes to the scope side ....

Have you measured that? I have 2 different brands of diff-probes which can handle about 1000V. If I remember correctly, one has 2 MOhm and the other has 4 MOhm from each input to scope ground.

 

[Easy peasy]

actually, I looked into it, some are indeed as you say, as low as 2 meg ohm, <=400 V usually, we are designing one for 1500V buses, it will be much much higher...