I recently saw this video that did inspire a little bit of interest on my part.
https://www.youtube.com/watch?v=IrB-FPcv1Dc
It appears that when the drain of a N channel mosfet is connect to the gate it can act as a diode. Also according to this video it can do that very efficiently. So I messed around a little bit on a very primitive inverter that I built a while back. I tried to rectify the output current of the inverter which was stepped down to 12 volts ac.
So I tried to use the transistors as diodes and my results where the following.
I tried an NPN and I got 8 volts of DC on my multi meter.
I tried the MOSFET and got less than a volt even without the gate being connected. Simply put the Mosfet acted a lot like a wire and had no effect whatsoever.
So I am curious how would you achieve rectification using transistors and is it even possible.
Electronic – Using MOSFET’s For AC Rectification
mosfettransistors
Related Solutions
Consider the equivalent BJT circuit, which may be more familiar:
simulate this circuit – Schematic created using CircuitLab
This holds provided the input voltage is >0.65V, with of course some variation based on temperature, output current, manufacturing variation, etc. However, as a first approximation this circuit outputs a constant 0.65V.
The MOSFET circuit is no different, but instead of the 0.65V from a forward-biased silicon PN junction, we get the threshold voltage of the MOSFET. This parameter varies between models of MOSFETs, but is usually some volts. If the output voltage, which is also the gate voltage, is above the threshold voltage, the MOSFET turns on more, shunting more current to ground, increasing the current through the resistor, lowering the ouput/gate voltage such that an equilibrium is reached:
$$ V_{GS} = V_{out} \approx V_{GS(th)} $$
This sort of circuit would be useful as a reference voltage, for example, to implement a voltage regulator, because the output voltage is relatively unaffected by the input voltage. A single transistor such as this isn't necessarily a good voltage regulator on its own, but it could be the basis for something better. A good regulator starts with a reference such as this which might vary based on other parameters (output current, supply voltage, temperature), then isolating or compensating for those parameters from the reference.
The drain appears to be ringing at about 600KHz.. Probably due to inductance in the drain circuit. The simple circuit you are using to drive the MOSFET is very asymmetrical.. the turn-off will be much faster than turn-on because your pull-up is much higher value than the ~50 ohm resistance of the MOSFET in the Si8710AC, so you are seeing the ringing.
I suggest a complimentary BJT emitter follower after the Si8710AC with a series resistor to the gate selected to trade off ringing vs efficiency. You can also minimize inductance in the drain circuit by shortening or twisting the wires arc., add a snubber or even a flyback diode to the supply.
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Best Answer
It is possible to create a very efficient rectifier with a MOSFET - but not by just connecting the Gate to the Drain!
All discrete MOSFETs have an inherent reverse 'body' diode between Source and Drain. This diode is as good as normal silicon rectifier, so the FET will work quite well as a rectifier provided that it is not turned on at the wrong time (ie. when the Drain voltage is positive relative to the Source, since that would nullify the blocking effect of the body diode).
If the Gate is left floating then it might pick up a static charge causing the FET to turn on continuously, which is obviously bad news. However if the FET is only turned on when Source-Drain voltage is negative then it will bypass the body diode and cut losses to almost zero. The trick is to synchronize the Gate drive to the incoming AC voltage.