An h-bridge is a bipolar driver circuit that is often used to control a load such as a brush type DC motor. This article covers the most basic concepts of a simplified h-bridge circuit. Later articles dig deeper into the details of practical h-bridge operation and design.
The h-bridge consists of four switches – two half-bridges with the load connected between them. The switches are usually solid-state devices such as bipolar transistors or power MOSFET’s. For now let’s depict them as mechanical switches.
When S1 and S4 are on, current tends to flow from the positive rail of the power supply through the motor left to right and then to ground. If instead we turn on S3 and S2, the current is driven the opposite direction through the motor. Obviously, these modes can be used to run a DC motor in a forward and reverse direction.
There also a third mode where the motor leads are effectively shorted together. This can be done by turning on both high side or both low side switches. This allows current to recirculate through the motor freely. When done briefly as the “off” portion of a PWM cycle, it allows current to decay slowly. When held in this state while a DC motor is spinning, the back-EMF of the motor will case a recirculating current to flow that will rapidly brake the motor to a stop. This is known as “dynamic braking”.
Note that your control logic must never turn on both the high side and the low side switches in the same half-bridge at the same time. This condition is called “shoot-through” and it creates a dead-short circuit straight from power to ground. Most bridge drive IC’s are designed to lock-out any command to do this. However, you must also be sure that one switch it fully off before the other switch even starts to turn on
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. We’ll cover the details of this in a later article.
There are some important details we haven’t covered yet and there’s a lot more to learn about h-bridge motor control circuits. In the next article, we’ll take a step back look at pulse width modulation (PWM) of a half-bridge unidirectional drive which will help us understand some important aspects of a real-life h-bridge.