Why do I keep burning my mosfet?

Question

Following this question, I ended up with the following diagram to drive a custom made dc motor using Arduino (omitting the 5V and 12V power supply stages which work properly): I keep burning instantly the irfp054n mosfet. An by instantly, I mean it. I tested with a voltmeter the voltage output by the Arduino PWM pin 5 and it's between 0 and 5V. The voltage coming out from the TC4422 (pins 6 and 7) is between 0 and 12V. Right after giving power to the whole system, I see the motor running at full speed, so I turn off the power and measure a resistance of about 0.2 Ohm between drain and source on the mosfet, which means it's gone. What am I doing wrong? Any hints?

UPDATE: Ok, ok, I'll get rid of that diode... Anyway, I installed it right after burning the first mosfet so I wouln't think it's the cause. Just out of curiosity: could it be that the problem comes because the Schottky and the mosfet are screwed on the very same aluminium heatsink?

UPDATE: It works, I replaced the diode between driver and mosfet with a 120 Ohm resistor and the mosfet is now a much stronger IRFB4115Pbf. I still have some glitches to work out: when I shut down the 5 V and 12V circuit, the motor runs up to full speed. Any ideas?

Answer 1

Lose the diode. I can't even guess what you think it does, but it will prevent the FET from being turned off quickly.

The FET driver will drive the gate high quickly, which turns on the motor. However, when it tries to drive the gate low, the diode prevents it from doing so. That means the gate just floats. It probably slowly drifts low, running the FET in the intermediate region where it dissipates significant power.

You need to switch the FET quickly to prevent significant dissipation. The power the FET dissipates is the voltage across it times the current thru it. When it is off, there is no current, so the power is 0. When it is on, the voltage drop is very small, so the power is also small. In the middle of its operation, it would have 18 V across it and 5.7 A thru it, for a dissipation of over 100 W. Poof!

The job of the FET driver is to slew the gate voltage quickly to have the FET only spend a few ns at a time in the high dissipation region. The diode is preventing that from happening.

Added:

You now mention that the reverse diode across the motor and the FET are bolted to the same aluminum heat sink. That could be a problem, depending on what pin of each part is connected to the mounting tab. This is, of course, all clearly spelled out in the datasheets. If it's not the FET drain and the diode anode, then that is very bad. At least one of the two then needs to be insulated. Or, use two separate heat sinks.

Another problem you may have is that the FET or diode aren't connected properly. Again, you have to actually read the datasheets, then double check that you have things wired right. This could explain why the FET driver blew out.

Also, do what Tom Carpenter suggested, which is to replace the motor and diode with a resistor for debugging. However, I'd use different values. Use a 1 kΩ resistor between the drain and the 12 V supply. Until the drain is switching crisply and opposite of the gate drive signal there is no point going further. With 12 V and 1 kΩ nothing else can get hurt, including the FET driver even if you flipped some pins on the FET.

Don't forget the bypass cap across the FET driver power and ground pins, and a 10 Ω or so resistor between the FET driver output and the FET gate.

Answer 2

As others have said, that diode is bad news- take it out and never put it back under any circumstances. Throw it away.

The start-up surge of a brushed DC motor from a low-impedance source such as lead-acid batteries can be very substantial- in the hundreds of amperes, and can destroy a MOSFET fairly readily. The current ratings for most TO220 and TO247 parts come with a lot of caveats. For a similar application we had to use two paralleled MOSFET modules as below, even with ramped-up current.

I suggest measuring the motor resistance with a milliohmeter to get an idea of the surge current you're exposing the MOSFET to. If you have an active high current Hall current transducer (you can buy them for a reasonable price- tens of dollars- as components) you can capture the startup transient surge with a digital oscilloscope (you don't need the MOSFET, a switch will do). Once you know what you're dealing with, it becomes a lot easier to do a design rather than guessing about what is doing on.

Also, make sure the grounds are connected solidly between the different circuits. A voltage transient caused by even a small amount of inductance in the power circuit can cause a spike that will punch through the gate oxide and destroy the MOSFET. v = L * di/dt. A series resistor from the drive circuit of 20 ohms or so and a fast TVS from gate to source would be cheap insurance, but try to make the layout good from the beginning.

Answer 3

has anyone checked BEMF?

Reminder: motor peak current is I = (V - BEMF) / R, while BEMF is 0 when the motor starts. So if you don't have current control (do you?) you may easily receive tens or in some cases hundreds of amps, depending only on ohmic resistance and bus voltage.

Add: it's another topic about motion control. And another time that best advice is: take servo. Google Galil servo or Adthech, they are the cheap high end devices that i know. But even buying random servo on ebay is better than trying random motor with random transistor. It's not even safe! Be careful.