The H bridge

In today's post we will learn about what an H bridge is and how does it functions.

An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards.

High current DC motor speed controls, controlled with low power electronics can be quite difficult at first, but it's not really that hard to make an efficient controller at all.

Let us first show you what a H Bridge is, and then you'll see what a useful topology it is.

Switches are connected on either side, above and below the motor. When you switch on A1 and A2, the motor spins clockwise, when you switch on B1 and B2, can you guess? The motor spins counter-clockwise. When using transistors with diodes across the Source and Drain, when both switches are closed and we attempt to spin the motor, back emf generated by the motor flows through the diodes and back round to the opposite terminal of the motor, thus causing it to "short" itself and hence making the motor more difficult to turn. This is known as "Regeneritive braking" and is proportional to the back emf, eg the harder you attempt to spin the motor, the more it resists. Some Hybrid cars are now using this system to generate power when driving down hills.

Now, if we put MOSFET transistors in place of the switches, we can control a motors direction quite simply by applying voltages to the Gates of each pair of transistors. We can also vary the speed in either direction, by using a PWM signal with variable duty cycle. It only needs to be few to either one of the 2 lower mosfets, while the upper 2 remain high.

The accepted way to use Mosfets in a H bridge, is to use 4 N channel Mosfets with high current/voltage ratings and Low Rds values (on resistance, the lower it is, the more current can flow with less heat dissipations).

There lies the problem. N channel Mosfets are supposed to conduct to a ground reference. ie, the Gate voltage must be at least 10V higher than the Source (normally 0V). If we raise the S-D voltage, we have to apply 10V higher than the Drain voltage to turn the mosfet on.

The answer, is to let a H Bridge controller IC do all the work for you. A H Bridge controller IC has an inbuilt buck-boost convertor (a method of transforming a voltage to higher potential than the input) to generate the 10V (normally 15V to turn the Mosfet fully on) higher than the D-S voltage you intend to switch.

The most useful, and most known H Bridge controller IC used in motor control and robotics is the HIP4081 Dual Controller made by Intersil. we have used this chip and it seems well behaved and easy to integrate into your designs. The only down side is the chip is reasonably expensive at £7 each, but it does what it says.

 Operation of a H bridge :-

The H-bridge arrangement is generally used to reverse the polarity of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit. The following table summarises operation, with S1-S4 corresponding to the diagram above.

H Bridge structure

Driving N-Channel gates is essentially like driving a capacitance to a sufficient voltage in order to get the channel fully on. Dissipation is kept down to a minimum level when driving the gates to +15 volts (with respect to the sources), assuring that the transistors are on. Most of the same applies to driving both N-Channel gates and P-Channel gates, but the only difference is that the P-Channel gate to source voltage needs to be negative.

S1	S2	S3	S4	Result
1	0	0	1	Motor moves right
0	1	1	0	Motor moves left
0	0	0	0	Motor free runs
0	1	0	1	Motor brakes
1	0	1	0	Motor brakes
1	1	0	0	Shoot-through
0	0	1	1	Shoot-through
1	1	1	1	Shoot-through

Using the same logic in turning various transistors ON and OFF will allow sufficient "dead time" between a high side transistor and its low side transistor. This will then ensure that they are both ON at any given time.