AC to DC conversion without diodes

[rahulponna]

-------------------------------------


Hi,

Does anybody have any idea to design a AC to DC conversion circuit
without using diodes? We can use MOSFETs, but we would need a control
circuitory to switch ON and OFF the MOSFETs.
Is to possible to somehow use the secondary voltage of the transformer
to turn ON and OFF the MOSFETs?

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[George Jefferson]

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Hi,

Does anybody have any idea to design a AC to DC conversion circuit
without using diodes? We can use MOSFETs, but we would need a control
circuitory to switch ON and OFF the MOSFETs.
Is to possible to somehow use the secondary voltage of the transformer
to turn ON and OFF the MOSFETs?

Not directly in any efficient manner. You want to turn on the mosfets when
the voltage is 0. Thich mosfet depends on the direction(sign of the
derivative of the voltage). If the voltage is 0 for any long period of time
you actually wan the mosfets to be off. This also causes a problem for near
0 voltage.

It's better and easier just to use a controller. This way you can turn the
mosfets on much more effectively by having them alternatate once per
1/120s(to prevent oscillations) and monitor the transformer when it is off
to turn the mosfets off.

The good thing about using a controller is that you can easily create a
variable power supply PWM the mosfets among other things.

 

[JosephKK]

On Sat, 10 Jul 2010 09:13:36 +0000,

-------------------------------------


Hi,

Does anybody have any idea to design a AC to DC conversion circuit
without using diodes? We can use MOSFETs, but we would need a control
circuitory to switch ON and OFF the MOSFETs.
Is to possible to somehow use the secondary voltage of the transformer
to turn ON and OFF the MOSFETs?

There is always motor generator sets.

 

[Jamie]

On Sat, 10 Jul 2010 09:13:36 +0000,



There is always motor generator sets.

Hmm
I've taken a PNP and NPN set to be biased via an AC signal to produce a
steady DC with no diodes..
The PNP simply derived the (+) directly with an R bypass over the
NPN.. the NPN circuit was decoupled via a 90 degree shift circuit which
also then, biased the PNP.. both were common E to the (+) and (-) rails..
I basically used that switch for a sensitive AC to DC switch which
produced a nice DC saturated switch with fast response..

It works find If you're working with a small range of freq..

Just a thought..

 

[Tim Williams]

Hmm
I've taken a PNP and NPN set to be biased via an AC signal to produce a
steady DC with no diodes..
The PNP simply derived the (+) directly with an R bypass over the
NPN.. the NPN circuit was decoupled via a 90 degree shift circuit which
also then, biased the PNP.. both were common E to the (+) and (-) rails..
I basically used that switch for a sensitive AC to DC switch which
produced a nice DC saturated switch with fast response..

It works find If you're working with a small range of freq..

Just yesterday, I invented a chopper circuit which performs synchronous rectification with 2N3904s. DC in, DC out, doesn't matter which direction. It should work a few tenths of a volt negative as well (i.e., a true ideal transformer for small signals).

Tim

 

[Jamie]

snip





Could you please draw a diagram?

I would have to do one in Spice and spit it out here.

The above circuit description is a rough draft of what I did..

The original circuit used 2 PNP's from the (+) rail and one NPN
circuit. The input signal was cap decoupled because of the signal
source would've otherwise biased the first PNP to saturation...

A basic R from the base to the (+) rail and R in series to the cap
of the input signal.

The 90 degree shift was a basic T network with cap and R which then
input to a buffer amp, this amp then switched on the second PNP.

Both collectors of the PNPs are joined and both Emitters are joined
to the (+) rail.. The second PNP base(B) is part of the 90 degree shift
buffer stage. The (B) r for that transistor simply was the collectors R
for the buffer stage which was inverted.

The shift was calculated for the lowest frequency of interest to
insure the time domain would cross over at 90 degrees. Higher
frequencies didn't need the shift to be exact due to the tome constants
put in place
for the final switch signal.

So when all said and done, I was able to get a wider pulse at the
final output with a short gap in the signal. A final cap on the end into
a comparator gave me the near perfect DC on/off I was looking for..

Basically I created a speed switch from a Eddy current motor that
needed to have a very fast on response when the rotor started to turn
with a very narrow gap in the pulse stream and voltages below .5 volts
into a compressed safety circuit..

This was done years ago and I later on did basically the same thing
using a 555 timer which worked out just fine...


This circuit isn't any good for a linear DC representation of AC levels.


I hope this was helpful enough, its a lot to do in Spice. I could
look up the old print in the files at work when I return monday
and photo scan that area and then post it. I will need to look in
dirty place to find that one, yuk...