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Half bridge

Our controllers use either a full bridge or a half bridge PWM circuit to to control the speed of an electric motor.

In the half bridge configuration two sets of MOSFETS are connected across the battery, as shown in the diagram below.

pwm circuit

The drive MOSFET switches rapidly on and off in a series of pulses  20kHz. When it is on, current flows from the battery, through the motor, through the drive MOSFET and back to the battery (path ‘A’). The Drive MOSFET switches off – but the motor has inductance (inductance is to electricity what mass is in mechanics) so the motor current cannot stop suddenly. Some controllers use a power diode (where the diagram shows the ‘flywheel’ MOSFET) connected across the motor so the motor current can circulate when the drive MOSFET is off – a sort of ‘flywheel’ to smooth out what would otherwise be electrical surges caused by the sudden changes in electrical flow. However – power diodes get hot and waste a lot of heat. For this reason we use a MOSFET which is a much better device since they don’t get so hot. Moreover, depending on the method used to switch the MOSFET, the controller can give regenerative braking. To get regen braking, the top MOSFET is switched on when the bottom one is off.

There is a much more detailed description in our circuits archive.

 

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