Driving a DC Motor without diodes

Question

As you know, H-bridge circuits are used in order to control DC motors.

Switches S1-S4 are closed to turn the motor in one direction, S2-S3 are closed for another. But at the time of changing motor direction, when switches (transistors) are opened, Back EMF kicks in and makes the switches blow up. That's the reason of using diodes parallel to switches (See figure below).

On the other hand, these diodes consume significant amount of power. I am currently trying to solve this problem, and make a "diodeless" H-bridge circuit work.

I am using an mCU to control the motor with PWM signals. Is it possible to make a flawless timing for switches to not blow up?

For example, on the schematic above, assume that the situation is as this: S1 and S4 are closed and motor turns in that direction. When I want to change the direction, I open S1-S4; and close S2-S3. By this, I have burnt the switches several times. Now, my idea here is to open S1, and close S3 for a while. By this, I think I'll be able to let the remaining current be absorbed (on the circuit S3-Motor-S4). Then, I open S4 and close S2, which reverses the direction of the motor. As I said above, I am not sure about that if it is possible with an mCU. Also, I would need to analyze the characteristics of the motor and switches in order to find the perfect timing for the algortihm.

What do you think about this idea? Is there any method that you know or can suggest to overcome this power consumption issue? I have searched online but couldn't find anything.

Answer 1

Well, what you're asking for has already been done for many years in a completely different field. We call "Diodeless H-bridges" class AB push pull amplifiers. You'll likely want some pretty fast transistors that have a low saturation voltage. Bonus, the less switching you do the more efficient this method is!

Schematic below:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

What are you talking about with diodes in parallel with the switches? That is never done.

Anyway, what you are doing is called "Plug Reversing", and yes, you need heavy duty contactors to handle that. The typical and inexpensive solution is to put NC contacts with a low ohm resistor around the armature itself. When all of your 'switches' are open, the armature is shorted through the resistor, which acts to dynamically brake the motor to a stop. Then use a time delay between enabling the forward/reverse switching.

For the record, inexpensive (probably less then the cost of your switches) DC Drives can handle this easily. You would need a regenerative speed control.

Answer 3

When I want to change the direction, I open S1-S4; and close S2-S3. By this, I have burnt the switches several times. Now, my idea here is to open S1, and close S3 for a while. By this, I think I'll be able to let the remaining current be absorbed (on the circuit S3-Motor-S4). Then, I open S4 and close S2, which reverses the direction of the motor.

1. Probably the transistors burn because they can't turn off/on so quickly. When you switch from S1-S4 to S2-S3, in a certain period of time you have S1,S2,S3,S4 that are half conductiong each, the current passes trough pair S1/S2, S3/S4 directly from Vcc to GND and burns them, this is normal if you don't have a short dead time.
2. So first close S1/S4 then wait certain ammount of time then open S2/S3. While in this dead time the diodes are conductiong, without diodes the back EMF will rise until the transitors or motor winding breaks down and game over.
3. With your proposal to have S3/S4 turned on is nothing else than creating a short circuit between Vcc and GND, so byebye transitors.

Answer 4

The diodes are required to protect the switching devices. The power "waste" is low and unavoidable.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1 and 2. Half-H-bridge (for clarity) with catch diodes.

In the Figure 1 we can see the switches are closed and current flows through the motor.

In Figure 2 we see that the switches have opened but due to the inductance of the motor current is still flowing. The current will flow no matter what else happens.

With the catch diodes in circuit current can be provided from the negative rail and fed out back onto the positive rail to be reabsorbed by the power supply. There is no waste other than the unavoidable VI loss caused by the diode forward voltage drop.

^{simulate this circuit}

Figure 3. Circulating current through Q3 and Q4.

Your idea to circulate current through Q3 and Q4 won't work. One of them will be reverse biased - Q3 in this example.

Answer 5

The diodes are required.

Even wimpy old 1N4148s can handle more current (200mA) than those transistors (100mA).

Maybe you need a more capable H-bridge- that one is suitable only for the tiniest of motors- remember that motor current under startup conditions will be much higher than run current, and if you 'plug' the motor (reverse while it is spinning) double that again.

If your diodes are getting hot then your motor is too big for that design of H-bridge.