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I've been looking into MOSFET bridge rectifiers, also known as "zero IQ" rectifiers, as a replacement for traditional diode-based bridge rectifiers.

Here's a simple example:

MOSFET bridge circuit diagram

The benefit, of course, is that the MOSFETs don't exhibit a forward voltage drop like a regular diode does, allowing for significantly higher efficiency in cases where the current is high or the voltage is low.

The major limiting factor for higher power designs seems to be that you can't get MOSFETs with a Vgs(max) high enough to rectify voltages exceeding 80V or so. There are some control ICs that are designed to help with higher voltage rectification (e.g. LT4320, LM74670) but they too are limited to around 75V continuous.

My first thought to solve this issue was to clamp the voltage with zeners, but I ran into the issue that you can clamp the voltage on two of the transistors just fine, but when you try to do all four you just get shoot-through because the diodes provide a path directly across the AC input.

I also thought about using IGBTs instead, but they appear to also have fairly limited gate voltages.

Is there a way to solve this issue, either by designing around the gate voltage limit somehow, or by changing the topology?

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    \$\begingroup\$ You need isolated gate drivers and I don't think you can control them off the line. You need a controller. This all makes it significantly more complicated than what you seem to have in mind. Complicated enough that you would just go with all NMOS because PMOS isn't saving any circuitry. \$\endgroup\$
    – DKNguyen
    Jan 3, 2020 at 2:26
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    \$\begingroup\$ The transistors basically get controlled individually as if they were the high side transisors in an inverter or motor driver. Look that up. \$\endgroup\$
    – DKNguyen
    Jan 3, 2020 at 2:35
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    \$\begingroup\$ is this aosmd.com/res/data_sheets/AOZ7270DI.pdf of interest? \$\endgroup\$
    – user132893
    Jan 29 at 20:41
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    \$\begingroup\$ @onepound Looks good. $US2 in 100's from LCSC - and $3.35 from Digikey in 1000's. \$\endgroup\$
    – Russell McMahon
    Jan 30 at 0:10
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    \$\begingroup\$ @onepound Nice! Looks like they have a whole brand new line of HV AC/DC bridge rectifiers based on this tech. \$\endgroup\$
    – Polynomial
    Jan 30 at 21:32

2 Answers 2

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This rectifier may not work as you expect or need.

The MOSFETs will turn on and conduct for the appropriate leg of the circuit, but unlike a conventional rectifier, they will remain conducting on the 'trailing' edge of the AC input. Thus if you want to peak-rectify the signal, it won't work -- the FETs will discharge the capacitor also.

You can add a diode (or even a Schottky) at the output; or you can replace 2 of the MOSFETs with simple diodes. That way, the whole bridge rectifier will only have a drop of one diode + FET resistance, not 2 diodes as a conventional rectifier.

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Note: Dave Tweed noted that this circuit has issues.
I'll leave it here as it helps inform the problem. See comments.

This is based on Dave Tweed's now deleted circuit with 4 added diodes.

The diodes in series with the 10k resistors drive the FETs on only when Vmains is above V_DC_out for a given FET by Vf_diode + Vth_FET.

enter image description here

Diagram is a hacked about cut and paste version of Dave Tweed's diagram.

Dave Tweed said:

If you use, say, 10 V 400 mW zeners, they can handle an average current of 40 mA. 10k resistors let you handle around 400 V.

But pay attention to the power dissipation in the resistors, too. Larger values of resistance will dissipate less power, but they'll also slow down the switching somewhat.

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    \$\begingroup\$ This doesn't work. The additional diodes prevent the MOSFETs from ever turning on. \$\endgroup\$
    – Dave Tweed
    Apr 19, 2020 at 4:00
  • \$\begingroup\$ @DaveTweed Whoops - true. Rotate the diodes 180 degrees. Silly me. You get the point though. With diodes of correct polarity I think it works. Yes? \$\endgroup\$
    – Russell McMahon
    Apr 19, 2020 at 4:05
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    \$\begingroup\$ No. Then you still have Tony's problem of shoot-through whenever the voltage is high enough to turn all of the transistors on at once. This "trick" only works for low-voltage applications, where it has the most value. High-voltage applications need a much more complex control circuit. That's why I gave up and deleted my answer. \$\endgroup\$
    – Dave Tweed
    Apr 19, 2020 at 4:08

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