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  #1  
Old 05-24-2017, 04:32 AM
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Mosfet as Diode

There are many ways to use mosfets as diodes to have a lower voltage drop than a dedicated diode. Synchronous rectification and other terms describe this concept.

Does anyone here have experience with this?

Is it similar to something like this - tying gate to source?



I thought it was more than just using the intrinsic diode in the mosfet, which from my experience are slow and clunky.
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Old 05-24-2017, 03:23 PM
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the versions I have seen are quite complex.
they watch the input voltage and run a signal to the mosfet to turn it on and off at the correct point, and because there is no inductive load, the internal diode they have is not used at all. usually set up in sets

found this for you,
MOSFETs: Increased Efficiency In Bridge Rectifiers | EDN
not quite how I remember it being done, but I am sure there are many ways to set up the control circuity
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Old 05-25-2017, 11:08 AM
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A lot switching controller manufacturers use that technique for any voltage change controller. Mostly to improve efficiency through a reduction in heat. Since mosfets can sink more current.
But using the fet by tying the gate to the source is not really practical. Since the fet is going to come on at about 10 volt and stay on through 18-20 v. But anything higher than that will cook the gate. Anything lower than 10 volt will not turn it on.
A good shotkey diode can do a lot. The voltage drop is based on the current. At max current you have the highest drop which the case here is only 1 volt.
Based on what you writing about lately you might be looking for something better and in that case a mosfet still may not be the best choice as the internal diode will always allow current to come backwards in the circuit.
I would look into logic level IGBT. Most have no diode and act like transistor BASE instead of gate.
To make one work in automated fashion is pretty easy since they turn off internally in either high or low position. And no diode to deal with.
Now how to control the mosfet to turn on when its time for current to flow is another story. That would depend on the topology of what your trying to do. If you just need a rectifier you can use a current sensing resistor, an op amp and a small 2 channel SSR (NO/NC),2 voltage regulators and 4 drivers for each gate. I can give more detail if thats the case.
If your just building a simple switching regulator to jump voltage up with an inductor again the 2 channel SSR works great. Still requires 2 drivers and a voltage regulator.

Matt
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Old 05-25-2017, 11:05 PM
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when he posted, seemed like he wanted a bridge rectifier like I posted, but also thought maybe he had been talking of this kind of setup
Reverse voltage protection with a P-FET | Hackaday

or is he working on a DC switching power supply and wants to replace just the main diode ?
I have a neat circuit that someone already optimized efficiency, but can't seem to locate it, but will keep looking

also some IGBTs have reverse diodes (I know because I own some)
and some mosfets do not have them (I own some of them as well)
but no idea as to how popular they are lately
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Old 05-26-2017, 09:30 PM
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found it
original website is gone
but here is an archived copy of it
https://web.archive.org/web/20150315...roller.org:80/

you can see how they replaced the diode with a mosfet in the switching supply if you look at the old versions VS the new versions of the schematic
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Old 06-07-2017, 02:57 AM
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Exclamation Mosfet as Diode

I would not use a mosfet gate tied to the source without a resistor . and in that config would not an PNP be more satisfactory being that the gate is on the low side .? an NPN would have to be tied to the high side before it would switch on.



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Last edited by marathonman; 06-07-2017 at 03:05 AM.
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Old 06-07-2017, 10:39 PM
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mosfet as diode

Space, Matt, Marathon,

Thanks for all your feedback on this, I really appreciate it.

What I have works for now to prove the point, but I will be looking to optimize it with a mosfet as diode in the not too far off future. It is for a project I'm working on.
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