![]() ![]() During this brief “dead time,” when neither FET is enabled, internal body diodes on the FETs carry the current.Current decay: Fast or slowThe two methods of current decay using the FETs are termed “fast decay” and “slow decay.” It’s important to note that, in this context, the terms “fast” and “slow” refer to the speed at which current decays, and are inversely related to how quickly the motor stops.With fast decay, negative voltage is applied to the winding by switching on the opposite FETs and reversing the direction of current flow through the H-bridge. Therefore, when the FETs on one side of the H-bridge need to be turned on sequentially, a method referred to as “break before make” is used.This method disables the first FET for a set amount of time (typically a few hundred nanoseconds) before the second one is enabled, preventing shoot-through. Alternating the sides of the “on” switches changes the motor’s direction of rotation.Image credit: Texas InstrumentsTurning on two FETs on the same side of the H-bridge causes very high current (referred to as “shoot through”) that could damage the FETs. ![]()
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