I'm working on a BLDC control system for a micromotor, and have been using the Vishay Si4804 MOSFETs in output stage.
Due to availability issues, I ordered Diodes Inc DMG4800LSD-13 for a second board, since they were a bit cheaper, and the specifications match up well.
The problem: 4804 runs cool to touch, regardless of current draw. The 4800 runs WAY hotter, and one even blew during testing. Any idea what could be going on?
I'm running at a PWM frequency of 40kHz. The current draw during normal operation is only ~200mA.
EDIT:
The driver for this output stage is the Allegro A4964. I'm currently using a dead time of 1us.
Experimenting with gate resistor value, I can get the 4800 MOSFET to function without heating. Probably the slow turn-on and turn-off, and perhaps the small overlap, caused the heating.
Gate drive waveforms with 100 ohm gate resistor:
Gate drive waveforms with 33 ohm gate resistor:
Update:
I have switched to the 6016LSD MOSFETs and removed the GS resistors. They draw less current than the 4804's. These are the gate drive waveforms measured individually, since my cheapo scope doesn't to differential measurements. Both measured from Source to Gate.
Here I can start seeing quite some ringing on the high side gate. Does this warrant a higher value gate resistor again?
Best Answer
Both transistors are rated up to Vds=30V, you have 36V as supply. You would need at least 50V or even 75V rated devices.
Then, 100 ohm gate resistor is a way too high value if you want to switch at 40kHz, do calculate rising and falling switch time and you will see that your circuit doesn't even turn completely on, so you have lot of switching loses.
The resistor connected between gate and source, tells me that you don't have a gate driver IC, but rather some discrete components to drive the MOSFETs, which is undocumented in your question, which also could be a source of problems.