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single ended output stage

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 Author Topic: single ended output stage  (Read 1843 times)
ec
Newbie

Posts: 1

 « on: May 30, 2011, 04:27:43 PM »

Hi

I am building a single ended MOSFET output stage and the circuit is attached. I am using Wideband transformer WB4-6.Its datasheet is attached. I need to drive a a transducer which can be assumed to be a 20 ohm load. The problem I am facing is the max. input current rating of the transformer is 250mA. My current is exceeding this limit. How can I make sure that current is lowered without changing the power supply. I have fix power supply of 5V, 15V and 8V.

Thanks
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Hero999
Global Moderator
Electronics God

Posts: 2472

 « Reply #1 on: May 31, 2011, 05:33:06 AM »

How are you using the transformer?

That topology is only any good for a flyback which produces high voltage negative spikes, not a sine wave or even a very good square wave.

The only solution is to drop the single ended approach, use a push-pull driver and reduce the turns ratio of the transformer.

This is confirmed by my calculations below:

Turns ratio:
Np:Ns = sqrt(Lp/Ls) = sqrt(50/200) = sqrt(0.25) = 0.5:1 = 1:2

Reflected impedance of the 20R transducer:
Zp = Zs *(Np/Ns)2 = 20*(1/2)2 = 20*0.52 = 20*0.25 = 5R

So at 5V, the current will be 1A, Ohm's law.
I = V/R = 5/5 = 1A.

Why are you using the transformer? Is it for isolation? Do you need to boost the voltage? If it's the latter, you need a transformer with a higher power rating. If it's just isolation the same size transformer will do but it needs to be 1:1.
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ec
Newbie

Posts: 1

 « Reply #2 on: June 01, 2011, 03:36:20 PM »

Thanks

I need the transformer to boost the voltage. Can you suggest me any transformer with small size and with higher power rating? I tried but was not able to fine one
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Hero999
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Electronics God

Posts: 2472

 « Reply #3 on: June 01, 2011, 05:38:30 PM »

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I also post at:
http://www.silicontronics.com

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question: if I know the answer, I'll be happy to help.

ec
Newbie

Posts: 1

 « Reply #4 on: June 01, 2011, 05:51:27 PM »

whole output stage is going to be put on a PCB. So transformer similar in size to the one I used before will do. No specific size constraint is there
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