I have been looking around trying to design a simple but working H-Bridge for an RC car motor (12V and 2~3A).
This bridge will be driven from a microcontroller and need to be fast to support PWM. So based on my readings, Power MOSFET are the best choice when it comes to fast switching and low resistance. So I am going to buy P and N channel power MOSFETs that are rated at 24V+ and 6A+, Logic Level, have low RDSon, and fast switching. Is there anything else that I should consider?
Ok so on to the H-bridge design: Since my MCU will be running at 5V, there will be a problem with turning the P-channel MOSFET off, since Vgs needs to be at 12V+ to totally turn off. I see that many websites are solving this problem by using an NPN transistor to drive the P-channel FET. I know this should work, however, the slow switching speed of the BJT will dominate my fast switching FET!
So why not use an N-channel FET to drive the P-channel FET like what I have in this design?
Is this bad or wrong design? Is there any problem that I am not seeing?
Also, will the reversed diode built in these FET be enough to handle the noise which is caused by stopping (or maybe reversing) the inductive load of my motor? Or do I still need to have a real flyback diodes to protect the circuit?
To explain the schematic:
- Q3 & Q6 are the low side N-channel transistors
- Q1 & Q4 are the high side P-channel transistors, and Q2 & Q5 are the N-channel transistors that drive those P-channel (pull the voltage down to GND).
- R2 & R4 are pull up resistors to keep the P-channel turned off.
- R1 & R3 are current limiters to protect the MCU, (not sure if they are needed with MOSFETs, since they don't draw much current!)
- PWM 1 & 2 are coming from a 5V MCU.
- Vcc is 12V