If you are a robot builder, there is a good chance you used an H-Bridge to drive your motors. This would have been in the form of an IC such as the SN754410 or the L293D H-Bridge IC. The problem with these H-Bridges are that they don’t usually go above 1A and when looking for a motor driver with higher capacity, it can get expensive very quickly. To overcome this problem we can build our own H-Bridge using MOSFETs to do the switching. The H-Bridge we are building can handle 12-24v motors at up to 10A of stall current giving 240 W total power handling ability.

The H-Bridge is a configuration of devices that is used to control the speed and direction of rotation of DC motors. The common design of an H-bridge is as shown above. It consists of 4 semiconductor switches, usually transistors, MOSFETs of IGBTs arranged in a way they resemble a capital H. The semiconductor switches in an H-Bridge are driven by a PWM signal usually at around 20 kHz (this frequency is just at the limits of the human range of hearing and is beyond the dynamic range of most motors you will use). A lower switching frequency is desirable as if we use a frequency that is too high, it will result in heating as the MOSFET is neither fully on of fully off.

A H-bridge can change speed using the PWM duty cycle. According to the duty cycle percentage of the PWM signal you are applying, you can control the the speed of the motor. A faster PWM signal means more energy is supplied to the motor, thus it will spin faster. A slower PWM signal means less energy is given to the motor and it will turn slower.

To change the direction of the motor. The H-Bridge would change the polarity of the voltage applied to the motor. Note that care must be taken when changing the direction of a motor, a change in polarity of the motor that is too sudden will result in damage to the motor. For example in the image above if we turn Q1 and Q4 on while Q2 and Q3 are off, the motor will have forward rotation. However if we turn Q2 and Q3 on while Q1 and Q4 are off, we would get reverse rotation. This is very important to maintain as switching on the wrong pairs of MOSFETs at the same time will result in damage to your motor.

To manage switching of an H-Bridge usually some sort of microcomputer such as a microcontroller is used to control the switching of the MOSFETs as well as provide controlled polarity switching and is usually used for driving H-bridges.


240W H-Bridge Design

For our H-bridge design MOSFETs are used to provide the switching. Though they cannot handle as much power as IGBTs, they are fine for this application. Usually when designing an H-bridge, one would use a half-bridge driver to drive a pair of the MOSFETs. However we can build our own half bridge driver using a transistor and a MOSFET driver. We use the BC547 NPN transistor and the TC4420 MOSFET driver to build our own half bridge circuit.

To use PWM signals to control the H-Bridge, we use the MOSFET driver. For testing the MOSFET H-Bridge I used a XR2206 IC and the H-Bridge worked as designed.

Vdd can is expected to be 12-24 volts.

*NB If you are using the driver close to the maximum specifications make sure to use heat sinks on the MOSFETs. If you plan to draw more than 2A of current from the driver heat sinks are MANDATORY.

 


Conclusion

If you want to control a robot, an H-bridge of some kind is required. Most H-bridges usually supply up to 1A of current. If you are building a mid-sized robot, you will require more power than that. We can build a H-Bridge that can supply up to 10A of current and supply up to 240 W of power using commonly available MOSFETs as well as bridge drivers for them using MOSFET drivers and low cost NPN transistors.

You can view this H-Bridge in operation here:

 

 

 

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