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An H-bridge is an electronic circuit which enables DC electric motors to be run forwards or backwards. These circuits are often used in robotics. H-bridges are available as integrated circuits, or can be built from separate components.

Structure of a H-bridge (highlighted in red)

The two basic states of a H-bridge.

The term "H-bridge" is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 (according to the first figure) are closed (and S2 and S3 are open) a positive voltage will be applied across the motor. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor.

Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and S4. This condition is known as shoot-through.

A solid-state H-bridge is typically constructed using reverse polarity devices (i.e., PNP BJTs or P-channel MOSFETs connected to the high voltage bus and NPN BJTs or N-channel MOSFETs connected to the low voltage bus).

The most efficient MOSFET designs use N-channel MOSFETs on both the high side and low side because they typically have a third of the ON resistance of P-channel MOSFETs. This requires a more complex design since charge pump circuits must be used to drive the gates of the high side MOSFETs. However, integrated circuit MOSFET drivers like the Harris Semiconductor HIP4081A make this easy.

Another method for driving MOSFET-bridges is the use of a special transformer known as a GDT (Gate Drive Transformer), that gives the isolated outputs for driving the upper FETs gates. The transformer core is usually a ferrite toroid, with 1:1 or 1:2 winding ratio. However, this method can only be used with high frequency signals. The design of the transformer is also very critical, as the leakage inductance should be minimized, or cross conduction may occur. The outputs of the transformer also need to be usually clamped by zener diodes, because high voltage spikes could destroy the MOSFET gates.

• Motor controller
• Switching amplifier
• Push-pull converter

• Using the HIP4081A for H-bridge control
• Brief H-Bridge Theory of Operation
• Tutorial: Build a 5A H-Bridge motor controller
• Building an h-bridge controlled motor with photocells to track light

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