MOSFET BRIDGE RECTIFIER

Looks like junk to me. If you cant feed a capacitor from the output, what sort of power supply are you going to build ?

I am sure you could build a bridge with mosfets, but you probably need at least 2 back-to-back in each arm, with some complex gate switching to make sure the right ones are on and off at the right times. Is it worth it ?

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Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

Both of your FETs are duals; i.e., both transistors in one package and presumably both built on the same substrate. In an integrated circuit, which is what these likely are, with two or more parts on single piece of silicon, all of the elements of the various parts are electrically isolated from the chip by building them on reversed biased diodes. The gates are oxide isolated but not the drains or sources which are diode isolated. These parasitic diodes are not shown on schematics but require that the substrate never becomes more positive than the most negative voltage in the device for an "N" device and the reverse for a "P" device.

When you use FETs built on the same chip on an AC source, the voltage on one will go below the substrate by conduction through one or more of the parasitic diodes to the opposite polarity, violating the rule. The result will be some sort of half wave output like you describe.

The solution is to use individual FETs, one per package not duals in this kind of a circuit. This circuit is a synchronous rectifier usually built full wave with two FETs and is quite common. It's big advantage is that the forward voltage drop is due only to the on resistance of the FETs and not PN junction voltage. It can also operate at high frequencies.

First of all, circuits depending upon the source voltage for gate enhancement require source amplitudes sufficient to enhance the gates.

2V peak is unlikely to do this, for many mosfet types. In this circuit you will need Vgs plus a parasitic diode drop to start, though the extra diode drop reduces to Vf as enhancement is achieved.

Secondly, as drawn, this schematic does not attempt to peak rectify the source, which is the usual aim of a power circuit. If you stick a storage capacitor on the output of the bridge, it will be discharged by the source as the source returns to or passes through zero volts on each half cycle.

If vyou just want to enhance the fet for reverse current flow, you'll need smarter circuitry, though it can be a simple linear circuit function.

RL

Hi,

I changed the input signal source to 12 volts peak to peak sine wave and still got the same result. I was measuring the input signal and the output of the bridge rectifier using two oscilloscope probes. While touble shooting I removed the input signal measuring probe and the output looks good.

I think that usually the bridge rectifiers need their output grounds to be separtaed by the input grounds. I removed the ground or neutral pin from the power cable of the oscilloscope and the output looks better. Did I do the right thing? Plus even with this new setting if I try to meausre the input signal with the other probe, the output does not look good. Any advice!

Thanks John

You need isolated inputs on the scope if you're going to do such a thing.

I have a cheap hand held that works great for that.

I'd like to correct your grammar.

"This seriously effects it's suitability" should read "This seriously affects its suitability"

Fire away.

...and I'd like to correct the link, recently (yesterday) updated to include ZXGD devices released in 2009:

RL

Shorting nodes through the scope ground is a good explanation. Better to find that out now before dealing with higher-powered circuits or mains supply voltages.

This error is also possible through the use of a grounded signal generator as a source in this particular circuit.

Removing the ground pin from the scope line cord, or using a 'cheater' adapter to the same effect is one method of opening this inadvertent inter-ground connection.

For general use in an electronic power or an electrical workbench, setting up a suitably isolated 120vac supply for low-power test instrumentation is recommended.

Please review comments about this circuit's use in peak rectifying applications.

RL