Freescale Motor Driver

This is a schematic diagram of a motor drive

Before understanding this picture, let us first understand the conduction principle of the H-bridge drive circuit.

Assuming that when the motor rotates forward, Q9 and Q12 will be turned on, and Q10 and Q11 will be turned off.

During the reverse phase, Q10 and Q11 are turned on, while Q9 and Q12 are turned off.

Then going back to the schematic diagram, in order for the motor to rotate forward, Q2 and Q8 must be turned on.

Understand the N-channel MOS tube IR7843. In order to make it conduct, a 4.5V voltage (Vgs=4.5V) needs to be applied to the G pole of Q8. For Q2, a voltage of at least 10V is required at the G pole. However, the power supply voltage of our microcontroller is only 3.3V.

In order to solve this problem, let us first popularize the concept of bootstrap boost.

What is bootstrap boost?

It can be seen that when the transistor is turned on and enters the charging state, when the transistor is closed, the current of the inductor is discharged, plus the original voltage of the capacitor, the output voltage will be greater than Vin.

So what components can achieve this? Here I choose IR2104.

Using IR2104 chip, its advantages are:

1. Increase the level.

2. And it can output a team of complementary PWM waves

3. This pair of complementary PWM waves can simultaneously provide power to the front half bridge and the back half bridge MOS of the H-bridge.

4. There is dead time inside, so software design is not required.

This is the connection diagram and timing diagram of IR2104. IN port is the input terminal of PWM signal. SD is the enable terminal. It can be seen from the timing diagram that when SD port is low level, HO and LO are not output normally. Therefore, a high level is added to it during design.

I believe you have also discovered that the key to our boost is the diode D2 and the capacitor C4. The size of C4 needs to consider the frequency of the signal and the inter-electrode capacitance of the MOS. Referring to the parameters given by IR2104, I have selected 1.5uF here based on past experience.

From this picture we can see that the dead time is 520ns, which is great.

After the drive circuit is completed, the motor can be driven directly. However, an important point is missing. The motor has a coil winding inside, which is quite inductive. A large induced voltage will be generated at the moment of power failure. If there is no isolation, this induced voltage may break down the MCU. Therefore, it is necessary to add an isolation circuit.

The 74HC244PW chip is selected here. While isolating, the PWM signal is also increased to 5V.

But the disadvantage is that the system delay is increased.

Finally, for the servo, since the servo has only one signal, it would be too wasteful to use an isolation chip, so we directly use power-off coupling isolation.