LM317 Adjustable 20V, 1.5A Supply with Simple SMPS Tracking Preregulator 2015-04-14

crutschow submitted a new article:

LM317 Adjustable 20V, 1.5A Supply with Simple Tracking Preregulator - LM317 Power supply with switching tracking prereg for good efficiency and minimal heat sink reqmts.

The LM317 is a commonly used, rugged linear regulator with an adjustable output but suffers from the normal inefficiency of linear regulators which can require a large heatsink for the higher output currents, especially if input to output voltage drop is high.
To minimize this, a tracking, switching preregulator can be used to keep the voltage drop across the LM317 essentially constant, at a voltage slightly above the minimum required (3V) for proper regulator operation. This increases...

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Excellent. Now make an PCB for it an build it.

Wow, Nice trick! I am using the 1st part of the circuit designed by you
here, to charge my cell phone. So it's interesting to me. My cellphone takes maximum 750mA current through it and MOSFET needs medium size heatsink though.)

I guessed heat dissipation is divided into two part (minimised too around switching area)- around MOSFET little and around LM317 chip. To save component (I think LM317 is expensive than TL431) we could totally remove the LM317, and could add a TL431 based adjustable regulator circuit as a reference voltage for LM393, couldn't we?

After that our output part would be the the P-MOSFET. After that I cannot assume its heat dissipation though. (But I know that it would be a noisy power supply with huge ripples!)

Hi cruts,

You do some stuff.

The ciruit is fine as is, of course. I was just thinking: At the moment you have a buck pre reg. Have you thought about another version with s SEPIC pre reg. That way the supply volts could be anything, within reason. I only ask because it would be ideal for an application I have in mind.