So an H-bridge is just a way to control current direction with switches, but why is this useful? If you drive current and close two switches in series, for example, switch 3 and 4 in the image below, you will cause a short and burn out the H-bridge. Good thing I learned that lesson for all of you! I may have burned out my very first H-bridge at Digilent. If you create a short circuit, that’s a sure way to burn out your H-bridge. One thing you have to be very cautious about when working with H-bridges is to not create a short circuit.
This will cause the current to flow from the source, through switch 3, and then through the load, then through switch 2, then back to the load. In the image above, the circuit has Switch 2 and switch 3 closed. If you close switch 1 and switch 4, the current will flow from the source, through switch 1, and then through the load, then through switch 4, and then back to the load.Īn H-bridge circuit with S2 and S3 closed. The H-bridge configured to have switch 1 and switch 4 closed. If Switch 1 and 4 are closed, then the current will flow from the left to right on this image: Using one current source, you can drive current in two directions by closing two switches. In the image above, the load is the M connecting the two sets of switches. But what exactly is an H-bridge?Īs it turns out, an H-bridge is a super simple circuit in theory: The circuitry behind a simple H-Bridge.Īn H-bridge is built of four switches that control the flow of current to a load. If you’ve done much with motor control, you’ve probably heard of H-bridges. What’s the most common way to make things move? Motors. These are used to create a pathway for the inductive voltage spike to go.One of the most exciting things you can do with electronics besides blinking LEDs is make things move. The most common way to handle this voltage spike is to use “flyback diodes.” These aptly named diodes are typically just standard diodes, rated at many times the supplied voltage. The induced voltage is always the opposite polarity of the applied voltage. If there is no safe way to dissipate this within the circuit, this voltage can destroy the circuit. This voltage can be many times the supplied voltage used in the circuit. The only way it can do this is by increasing the applied voltage across the terminals of the motor. This field then tries to keep the amount of current flowing constant. When power is removed, the magnetic field collapses. Once the motor is up to speed, the stored energy is at a maximum. Once power is supplied to the motor, it stores energy in a magnetic field. The most important item to understand when driving a DC motor is that it acts as an inductive load. Inductive flyback protection Flyback diode basics Other than accounting for flyback spikes due to the inductive nature of DC motors, there are few external parts. One of the main advantages of using an integrated H-Bridge is the simplistic nature of the external circuit. The main topic of this video is to discuss the implementation of an H-Bridge IC in a schematic and printed circuit board (PCB) layout. Some ICs will contain circuitry to help protect from inductive loads, but the TC78 does not. At quantities of 2000 parts, they are available for around $0.60 each. For example, the TC78H620 contains over-current detection, thermal monitoring/shutdown, and undervoltage lockout.
The number of features that a modern IC contains, along with the reduced design time and part count make the usage of ICs quite attractive. This is especially the case when designing an H-Bridge circuit. In my opinion, for the majority of use cases, it is better to use an IC when one is available. The circuit I will be showing today is based on an integrated circuit H-bridge, specifically a Toshiba TC78H620. Integrate Chip (IC) Discrete componentsįigure 2: Basic layout of an H-Bridge circuit using discrete MOSFETs. Many H-Bridge circuits have protection to prevent a “shoot-through” condition from occurring, but it is still best to ensure that it will not arise programmatically. An immediate short circuit results, and can destroy the rest of the circuit, causing the magic white smoke to appear. This condition occurs when S1 and S2 are closed, or S3 and S4 are closed. If all switches are open, or only a single switch is closed, the motor will have no power, and will “coast.” If both S1 and S3 are closed, or S2 and S4 are closed, the motor is powered to resist motion and will brake if currently spinning.Care must always be taken to ensure a “shoot-through” condition never occurs. If S3 and S2 are closed, the motor changes direction and now rotates in the opposite direction. When S1 and S4 are closed, the motor is powered and spins in one direction. Image: Cyril BUTTAY, from Wikimedia CommonsFigure 1 shows a basic high-level view of an H-bridge circuit controlling a DC motor. Figure 1: Simplified diagram of an H-Bridge circuit (in red) driving a DC motor.