bridge


Posted on Feb 7, 2014

Before trying to understand a full bridge circuit, you may care to read up on PWM controllers, which page also describes how a half-bridge circuit works. Most full bridge motor controllers also use pwm! Designing a full-bridge PWM circuit that is reliable and `user-proof` is actually quite difficult! 4QD released our first such controller in 1992:


bridge
Click here to download the full size of the above Circuit.

it was probably not the first but was certainly early in the race. The circuit shows a full bridge of four MOSFETs. In forward drive, current must flow in the direction of the pale green arrow A from battery positive, through Hi1, through the motor and so through Lo2 to battery negative. In reverse drive, current must flow in the direction of the red arrow C. With the current state of ICs and N channel MOSFETs, only a fool or a masochist would use the hi-side as the main active switching element, so it is normal to apply PWM to the loside MOSFETs, and to let the Hi-side ones take a more passive role. So, for forward drive, Hi 1 would be permanently on, Hi2 off, Lo1 off and Lo2 on, maybe chopped in PWM mode. If PWM is used, then, when Lo2 is off, motor current will still need to keep flowing in the same direction, forced to do so by the motor`s inductance, and it will use Hi2 as a `flywheel` diode as shown by the dark green arrow B. It matters not a lot whether the hiside MOSFET is turned on during the flywheel period: if it is on, then resistive losses apply rather than the diode drop that would cause more rather more loss. Note that you will need to use a static, pumped supply for the hi-side gate drives. A Bootstrap type charge pump won`t work unless the output is switching and, at full motor speed, nothing in the bridge is switching, so the bootstrap will fail. The rest of the design rather depends on how `safe` you...




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