Power MOSFET blew up when increased duty cycle from 5% to 10% [With Load condition]

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
First, these are the componentes I'm using on my PCB, click on them to see the datasheets:

Gate driver
Power MOSFET
Freewheeling diode
Current Sensor

The schematic is attached below. Sorry if I'm not using some right symbologies. I took the schematic from the PCB software.
Vbus voltage comes from rectified and filtered mains (110VAC).
PWM frequency is 10 kHz
The gate driver is isolated, so GND1 is ground from rectifier, an GND2 is the digital ground.

I tested the PCB with two different motors.

Motor 1:
130VDC
25.5A

Motor 2:
130VDC
27A

These are the steps that I followed before the transistor blew:

1- I tested the circuit with a light bulb first. Everything ok.
2- Then, I connected motor 1 with no load. Everything ok.
3- I connected motor 2 with a slight load. It started ok at 5% of duty cycle. Then, I increased duty cycle to 10% and the transistor blew up, so the motor went to max speed uncontrollably.

I haven't programmed the fast overcurrent detection from the current sensor. That way I would have probably saved the mosfet.

The question is... what could be the cause of this? high dv/dt and di/dt? need an snubber? I have a lot of mysteries in mind right now.

 
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RichardO

Joined May 4, 2013
2,273
I don't see anything that is obviously wrong.

The MOS-FET driver is transformer coupled. I wonder if there is a minimum PWM frequency needed for proper operation.
 

Uilnaydar

Joined Jan 30, 2008
118
I'd love to see some waveforms on this. Rectified mains into a 250V part might be seeing spikes that creep into the avalanche area. Also, is the FET heatsinked (heatsunk... heatsanked...)? A 125A with no heatsink, in a box, could get bad fairly quickly.

For now, let us know if you have a heatsink on her.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
I'd love to see some waveforms on this. Rectified mains into a 250V part might be seeing spikes that creep into the avalanche area. Also, is the FET heatsinked (heatsunk... heatsanked...)? A 125A with no heatsink, in a box, could get bad fairly quickly.

For now, let us know if you have a heatsink on her.
Yes, it has a heatsink. In fact, I found it pretty well heatsunk.

I would also like to see some waveforms. Just that the environment where the board is located, is not suitable for me to take those measurements. I'll look for the way to do it anyways.
 

Uilnaydar

Joined Jan 30, 2008
118
Might be I'm not understanding the circuit but are you controlling two motors with one heatsink or are you saying one motor runs, 2nd motor blows up the circuit, or am I completely off on this.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Might be I'm not understanding the circuit but are you controlling two motors with one heatsink or are you saying one motor runs, 2nd motor blows up the circuit, or am I completely off on this.
The circuit can control only one motor at time. With motor 1 and no load, everything was ok. When I tried motor 2 with a slight load, it started ok, but after increasing duty cycle, the transistor blew up.
 

Uilnaydar

Joined Jan 30, 2008
118
Is your circuit fully turning on the mosfet?
How about ripple, maybe the mosfet is being partially turned off or on while it is switching states.

http://www.onsemi.com/pub/Collateral/AND9093-D.PDF
I was concerned about running in linear mode but he's got a beefy gate driver and is applying 12V to it. TS, can your 12V supply provide enough current to the gate driver? You could look a the waveform on the gate drive to see this also. You want the gate to not camp out between On & Off too long.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
I was concerned about running in linear mode but he's got a beefy gate driver and is applying 12V to it. TS, can your 12V supply provide enough current to the gate driver? You could look a the waveform on the gate drive to see this also. You want the gate to not camp out between On & Off too long.
Maybe the problem could be the 12V supply. My 12V comes from an isolated 5 to 12V DC/DC converter, which is rated at 84mA. I thought that as I'm switching at 10 kHz, the mean current from the 12V supply is low, i.e, below 10 mA. So, for the peak currents, I placed the 1 uF decoupling capacitor. I can attach my DC/DC converter datasheet if needed.
 

Ylee5763

Joined Sep 23, 2015
27
I think the issue might be due to gnd2 issue to cause the wrong timing. also check the Vds of the fet.
 

ebp

Joined Feb 8, 2018
2,332
Is the layout very tight so that the driver, FET and current transducer are all very close together with short connections? The total loop area with those components needs to be as small as possible.

The parts look to be very well suited for the job. The driver has low voltage detection, so that worry is eliminated. I can't imagine that the DC-DC converter is not adequate for the job, though I didn't look carefully at the driver datasheet for its own power requirement.

Do you know what to expect for stall current for the motors? I presume the values you stated for current are for full-load.
Is there a difference in how the two motors are connected, such as cable length?
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Is the layout very tight so that the driver, FET and current transducer are all very close together with short connections? The total loop area with those components needs to be as small as possible.

The parts look to be very well suited for the job. The driver has low voltage detection, so that worry is eliminated. I can't imagine that the DC-DC converter is not adequate for the job, though I didn't look carefully at the driver datasheet for its own power requirement.

Do you know what to expect for stall current for the motors? I presume the values you stated for current are for full-load.
Is there a difference in how the two motors are connected, such as cable length?
I can upload the PCB layout if needed. It's my first PCB involving a little of power electronics. So, it's hard to get everything perfect from the first attempt.
The gate driver specifies minimun peak current of 6A. My DC-DC converter is 84mA, but the decoupling caps for the peaks are there. As I said above, the mean current provided by the converter should be pretty small.

Those values of current that I specified for the motors are at full load, and I never reach full load. The cable length of both motors are very similar. The difference was that I didn't put load on motor 1, but in motor 2 I put a slight load like shown below. It was in a treadmill, but no people on it, just the belt.

 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Look at this fragment of the PCB. This is my first PCB. Of course, I will have to make a better design, but if I have to add something, I want to use this PCB as a test board.

 
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Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Could it be that I connected source of the mosfet to the current sensor instead of directly to GND1 which is the reference ground, as suggested in the driver datasheet?
 

ebp

Joined Feb 8, 2018
2,332
There will be substantial voltage developed between the source of the FET and the current sensor terminals during the switching transition due to the inductance of the PCB track. It is good and wide, which helps somewhat, but there won't be much magnetic field cancellation from the drain connection. This is one of the the perpetual problems with this sort of circuity - trying to minimize loop areas and get mag field cancellation to minimize inductance.

You may be getting some ringing or unwanted transients on the gate that are causing grief. A scope would reveal that, but you'd need a good isolation transformer to power your board, and even then it can be difficult to really see what is happening.

It probably would be better if the ground of the driver were connected as directly as possible to the source. Your driver is a moderate distance away, so there is going to be some inductance there, too. I would be inclined to try making a twisted pair of any convenient small wire (I typically would use 28 or 30 AWG Kynar-insulated wire wrap wire) and running it on the back of the board from the via that connects to the two resistors to the gate and the via for GND1 near the driver to the gate and the source, respectively. Connecting to the source on the solder side of the board right at the FET's pin is almost a Kelvin connection, leaving only the pin's inductance - and the pin looks very short so the inductance is about as low as it is possible to achieve. Another advantage of this arrangement is that it keeps the gate current from flowing through the current sensor. For your purposes, this isn't important, but in closed-loop systems where the current forms part of the feedback, the gate current spike can be a nuisance.

Your layout of the filter cap, HF cap, FET and diode looks about as good as it is possible to make it without putting "out" and "return" power paths on opposite sides of the PCB, which often is nearly impossible anyway and often blocks something else - like access to the gate. Well done.

Is that big cap really marked 1500 point zero microfarads and 250 point zero volts?! Very odd.
 

dendad

Joined Feb 20, 2016
2,977
Can you try driving a resistive load, like a bar heater to see if you can get to full power?
If so, it is then maybe a problem with the inductive load producing ringing or transient voltage spikes.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Can you try driving a resistive load, like a bar heater to see if you can get to full power?
If so, it is then maybe a problem with the inductive load producing ringing or transient voltage spikes.
I tried with a light bulb until 60% duty cycle. I actually don't want to reach 100% for my application. 80% would be the max. I think that it could be voltage spikes, where RC, RCD snubbers come to play. But I'm not sure.
 

bertus

Joined Apr 5, 2008
20,040
Hello,

Is the part with se 0512/H2 on it, a DC/DC converter?
What is the power of the part?
It might be that it has not enough power for the gate driver.

Bertus
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Hello,

Is the part with se 0512/H2 on it, a DC/DC converter?
What is the power of the part?
It might be that it has not enough power for the gate driver.

Bertus
Yes, it's an unregulated/isolated DC/DC converter 5 to 12V, 84mA, 1 W. I thought it's suitable as its only task is to activate the gate. And as said before, at 10 kHz, the mean current is pretty low. Am I right?
 

bertus

Joined Apr 5, 2008
20,040
Hello,

The gate of a mosfet behaves like a capacitor.
You can see the gate of the mosfet as a dead short when the gate pulse is initiated.
The driver you are using has stated that a gate drive current of minimum 6 A is used.
Try to use a stronger external 12 Volts supply and see the difference.

Bertus
 
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