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Why use 2 mosfets? Answered


In the circuit, they use 2 mosfets namely q1 and q2.
I was thinking what does q1 do.
I tried simulating the circuit with and without q1 and the result were identical.

Almost all synchronous rectifiers are following this pattern, what could be the purpose of q1?

input and output conditions...

Input: sine wave similar to AC mains (100Vrms 60Hz)
Output: half wave rectified sinewave
Load: 1kOhm resistor

The red curve in the graph represents the voltage in the output
The blue curve in the graph represents the voltage of a 1 ohm resistor in the input (for current reading purposes)


Damned good link. Won't fixed Codestroy7's broken simulation though.

Constructed right the output of that circuit should look like this.


Sir may I ask, without the filter capacitor, how will the output waveform look like?

Post your model data, in LTspice format.

What have you simulated this thing in ? LTspice is a very good free, pro-grade simulator.

Simulators are really for expert users. Its VERY easy to not get what you should, and only an expert can often see why.

Some circuit simulator programs are picky about what you do with grounds.

Some programs don’t like the grounds being connected together like in the pic you provided.

Try this.

output c.bmp

With one mosfet you only use half the sine wave with two mosfets you use both halfs of the sine wave.

Sir, thank you for answering. That was my earlier assumption but after simulation i found out that with or without q1 the output will be half wave rectified sine wave.

Then you did it wrong. Garbage in, Garbage out.
Use a proper SPICE simulator....

The concept is so simple, build one and try it.

Sir I was trying to understand how q1 works so I removed the inductor and capacitor since they are only for filtering purposes. Without this filters i will be able to see the true waveform that the transistor combination creates. Am I right? or there is a correct way of analyzing this circuit?

Watch out for simulator programs they are only as good as the programmer, I design circuits in my head and build them on a bread board and they work as planned, yet the same circuit doesn’t work in a simulator program.

Show your sim outputs in your question would be a big help.

I assume you've connected a load to the circuit ?

It probably has something to do with the waveform generated. Its probably a smoother wave with that mosfet there. Which is helpful if you want a stable output.

Or something like that.

I think you're taking it out of context

Don't dump a snippet of a complete circuit and ask questions about it ! What voltages is this designed for ? Current levels ? Its a synchronous rectifier. What waveforms would you expect to see where ?