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Why use 2 mosfets? Answered

http://electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/content/14978/60488_fig_01.gif

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)

Comments

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steveastrouk
steveastrouk

Answer 7 years ago

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

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Josehf Murchison
Josehf Murchison

7 years ago

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

output.png
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codestroy7
codestroy7

Answer 7 years ago

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

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Josehf Murchison
Josehf Murchison

Answer 7 years ago

Without the capacitor it should look like this.

output b.png
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steveastrouk
steveastrouk

Answer 7 years ago

+1 to Josehf. You have more patience than I do mate !

Steve

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steveastrouk
steveastrouk

7 years ago

Post your model data, in LTspice format.

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codestroy7
codestroy7

Answer 7 years ago

My apologies Sir, but how do LTspice format looks like?

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steveastrouk
steveastrouk

Answer 7 years ago

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

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codestroy7
codestroy7

Answer 7 years ago

I used multisim by National Instruments

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steveastrouk
steveastrouk

Answer 7 years ago

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

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Josehf Murchison
Josehf Murchison

7 years ago

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
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Josehf Murchison
Josehf Murchison

7 years ago

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

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codestroy7
codestroy7

Answer 7 years ago

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.

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steveastrouk
steveastrouk

Answer 7 years ago

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

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

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codestroy7
codestroy7

Answer 7 years ago

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?

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Josehf Murchison
Josehf Murchison

Answer 7 years ago

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.

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codestroy7
codestroy7

Answer 7 years ago

Thank you Sir!

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steveastrouk
steveastrouk

7 years ago

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

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

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transistorguy
transistorguy

7 years ago

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.

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steveastrouk
steveastrouk

7 years ago

I think you're taking it out of context

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codestroy7
codestroy7

Answer 7 years ago

what do you mean?

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steveastrouk
steveastrouk

Answer 7 years ago

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 ?