Do you understand MOS driver in motor controller?

Introduction

The I/O voltage of a general purpose MCU or DSP is typically 5V to 3V, and I/O output current is below 20mA, which is not enough to directly drive a power MOSFET. Therefore, when using a general purpose MCU or DSP to develop a motor driver, it usually needs to be equipped with an external MOSFET driver, which we call "pre-driver". When developing motor drive controllers for automotive fans, water pumps, oil pumps, etc., the use of automotive MCU + automotive pre-driver + automotive N-channel power MOSFET can adapt to different power, different communication methods and different control methods.

Power MOS driver in controller

As shown above, power stage drive circuit of a three-phase brushless DC motor (including brushless DC motor and PMSM) uses six N-channel MOSFETs to form a three-phase full bridge, which is divided into three high-level sides connected to positive power supply pole (VBus). The MOS and three low-side MOSs are connected to negative pole of power supply. The controller completes commutation by controlling on/off of six MOSFETs so that motor rotates as expected. The motor may stall during operation and cause overcurrent, so MOSFET drive circuit must have a protection function to prevent controller or motor from burning out.

For a single CMOS, when turned on, an instantaneous high current must be provided to charge parasitic capacitance in MOS. After gate-to-source voltage (VGS) reaches a certain threshold, MOS can be fully turned on. Once MOS is turned on, a suitable gate-to-source voltage (VGS) must be maintained before it can remain on.

For a low-side MOSFET, source (S) is connected to negative pole of power supply, gate-source voltage is easily observed, and control is relatively simple.

For a high level MOSFET, its source (S) is connected to motor phase line and its voltage is unknown. If it needs to be enabled, gate voltage must be provided through bootstrap circuitry. , and drive is more complicated.

In general, on-state MOS resistance is shown in figure above, it decreases as VGS increases, but after VGS exceeds 10V, decrease curve becomes smooth. To achieve Minimum On-State Resistance (RDSON), VGS is typically 10-15V.