Author Topic: Switching voltage regulator  (Read 1221 times)

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Offline newInRoboticsTopic starter

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Switching voltage regulator
« on: May 27, 2011, 09:56:34 AM »
Would a circuit attached below work?

I got myself thinking about switching voltage regulators and came up with a simple circuit.

Short explanation:
  • 5V linear regulator is used as a Vref for non-inverting pin of Op-Amp; as Op-Amp does not draw much current, 5V regulator would not waste much power in heat
  • 2N3940 NPN transistor is operated as a switch which is ON when output voltage drops below Vref and OFF when output voltage raises above Vref
  • As load changes, voltage drop across it (Load resistor) is instantly compensated
  • Max current draw is only limited by NPN transistor itself as Max current through the Load should not exceed Max current handling of transistor
  • As Op-Amp has very high gain, output voltage with very low voltage ripple can be achieved

Would such circuit work?
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Offline waltr

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Re: Switching voltage regulator
« Reply #1 on: May 27, 2011, 10:48:23 AM »
That is not a switcher as the Op-amp and transistor are linear devices. What will happen is the transistor's base current will be driven to the point where the transistor's output voltage is equal to the op-amps Vref.
Except that the wrong transistor is being used. Remember that an NPN transistor passes current from collector to emitter when current flows from base to emitter.
The transistor should be an PNP with its emitter connected to the 12V. Then the op-amp sinks current out of the base to turn on the transistor.

This circuit is really just a linear voltage regulator using a pass transistor not a switcher. You could use a biased zener diode on the transistors base instead of a Vregulator and op-amp.

Or you can just put a PNP transistor across the linear regulator to increase the regulator's current. See the application section of linear regulator's data sheet that typically shows how to use a pass transistor. Try looking at the data sheet for an LM7805 regulator.

The other part that is missing is an output filter cap.

« Last Edit: May 27, 2011, 11:01:40 AM by waltr »

Offline newInRoboticsTopic starter

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Re: Switching voltage regulator
« Reply #2 on: May 27, 2011, 10:55:08 AM »
That is not a switcher as the Op-amp and transistor are linear devices. What will happen is the transistor's base current will be driven to the point where the transistor's output voltage is equal to the op-amps Vref.

Hmmm, I do understand that transistor is linear, but how come that Op-Amp is? I have always thought that Op-Amp used as comparator gives either LOW or HIGH output and nothing in between.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

Offline waltr

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Re: Switching voltage regulator
« Reply #3 on: May 27, 2011, 11:07:48 AM »
It is connected with negative feed-back. An Op-amp changes its output so that the two inputs are the same.
This works as a comparator if the output is not feed-back to the inverting input. And many times one adds a small amount of positive feed-back when using an op-amp as a comparator.

Get out your proto-board and the parts then wire it up and play until you understand how it works.

 


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