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crutschow

Efficient MOSFET Bridge Rectifier for DC Motor or Other Resistive Loads

Simple MOSFET bridge provides efficient AC rectification for resistive loads

  1. crutschow
    A MOSFET bridge rectifier can provide much lower forward drop than a standard or Schottky diode bridge for resistive loads, such as powering DC motors or plating baths from an AC source, where steady DC is not required.
    This makes for higher efficiency and often no bridge heatsinking requirement, even for relatively high bridge currents.

    The bridge consists simply of two N-MOSFETs and two P-MOSFETs cross coupled so that their gates turn on at the proper time in the waveform.

    During the low voltage part of the waveform, below their Vgs threshold voltages, the current is carried by their substrate diodes.
    Thus for best efficiency, low Vgs(th) (logic-level) MOSFETs should be used.

    Note that the bridge current direction is opposite the normal MOSFET drain-source direction, but that works since MOSFETs conduct equally well in either direction when ON.

    Below is the LTspice simulation of the bridge.

    upload_2017-3-7_23-18-9.png

    The voltage drop through the bridge is only about 100mV at a 7.5A peak current for the selected MOSFETs, with an average dissipation of about 100mW for each MOSFET.

    The selected MOSFETs must be properly rated for the peak AC voltage you will use, with an ON resistance low enough so that the MOSFETs don't require a heat sink at the maximum load current, if feasible.
    And as previously noted, low Vgs(th) devices will be slightly more efficient.

    If the AC peak voltage is near or above the Vgs maximum rating of the MOSFETs, then an additional resistor must be added from each gate to ground with a value such that the resulting voltage divider will keep the peak Vgs below its rating.

    Note that this simple circuit will not work with a capacitive load, such as for a DC supply, since the capacitor charge will leak back through the MOSFET when the AC voltage drops below the capacitor voltage.
    For that you can use a circuit that includes comparators to turn off the MOSFET and prevent the capacitor discharge, such as discussed here.

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