This question is related with the one I asked sometime ago about the heat generated when driving a BLDC motor BLDC drive inverter circuit problem. The problem is, the MOSFET chip got heated up obviously even the RMS current is about 200mA. The motor to be driven is 24V 90W BLDC motor with a phase resistance of 0.15 ohms and phase-to-phase inductance of 200uH. The MOSFET and its gate driver I chose was SI4564DY (P+N) and FAN3278.
@Bruce Abbott gave some excellent explanation why the chip got heated up in my last question, but after some digging, I still have some more questions regarding how to select the proper MOSFET.
Correct me if I am wrong, but according to datasheet, SI4564DY is supposed to handle up to 9A at 24V VDS, which I though is more than enough for the bldc motor used, and the "Continuous Source-Drain Diode Current" is 2.3A, which I assumed to be RMS value rather than instant value. So I didn't expect it to become so hot when the RMS drive current is about 200mA, although as @Bruce Abbott pointed out and verified by experiment that current spikes at 15% PWM duty cycle can as high as 2.3A. Even TI has a design using similar MOSFET chip to handle 24V/5A bldc motor without using heat sink. If I drive the motor at full load, I guess the MOSFET chip will burn out. So my question is
- Did I select the suitable MOSFET to drive the 24V/90W motor (phase resistance 0.15 ohms and phase-to-phase inductance of 200uH)
- Normally, when is heat sink needed for the MOSFET chip? I though SI4564DY is more than enough and heat sink should not be needed, as TI's example shows.
- If SI4564DY is actually not enough, what kind of current capacity should I look at when selecting another chip?
- To drive a 24V bldc motor, normally at what wattage should I consider using heat sink?
I am still new at handling this kind of bldc motor control and hopefully these questions don't appear to be silly to you gurus.
Here are some related info. The size of the PCB is 15mm*5mm with mainly the MCU, gate driver, MOSFET and some sensing circuit on it. The PWM freq used is 20Khz, and I did try 10KHz, but no obvious temperature drop. Currently I am using PWM on lower arm N-channel MOSFET.