LM317 Variable Switch Mode Power Supply (SMPS)

So far in this website we have studied LM317 based linear power supply circuits, here we will learn how an LM317 can be executed as a variable switch mode power or SMPS with zero loss.

LM317 as Linear Regulator

We all know that an LM317 IC is internally designed to work as a linear voltage regulator IC, which has a serious drawback of power dissipation through heating. Moreover such topology also requires the input to be minimum 3V higher than the desired output, adding further restrictions to the given regulator configuration.

Here we discuss how the same IC could be simply implemented as a 0-40V variable power supply using SMPS topology and therefore eliminating the losses mentioned in the above paragraph.

Modifying LM317 Circuit into a PWM Switching Regulator Circuit

The LM317 variable SMPS circuit explained here effortlessly converts an ordinary LM317 IC into an inductor based switching regulator power supply counterpart, as exhibited in the following diagram:

Circuit Diagram

Referring to the above shown diagram we can see that the LM317 is configured in its usual variable regulator mode but with some additional parts in the form of  R6, C3, and D1.

We can also see an inductor attached with D1 and an associated power BJT Q1.

How it Works

Here the LM317 IC performs two tasks together. It varies the output voltage through the indicated pot R4, and in turn causes a PWM to initiate for the base of Q1.

Basically, the introduction of R6/C3 transforms the LM317 regulator circuit into a high frequency oscillator circuit, forcing the output of the LM317 to switch ON/OFF rapidly with a varying PWM, which is dependent on the setting of R4.

The BJT Q1 along with the inductor L1 and D1 forms a standard buck converter circuit which is controlled by the above explained PWM generated by the LM317 circuit.

This implies that while the pot R4 is varied, the voltage pulse width developed across R1 also varies proportionately causing Q1 to switch L1 in accordance with the varying PWMs.

Higher pulse widths enable the inductor to produce higher voltages and vice versa.

Capacitor C4 makes sure that the fluctuating output from L1 at the output is adequately smoothed and eliminated, this consequently enhances the ripple current into a stable DC.

In the proposed LM317 switch mode power supply circuit since the IC LM317 is not directly involved with the handling of the load current, it's restricted from dissipating current, and thus ensures an efficient regulation of the high input voltage into the desired low output voltage levels.

The design also allows the user to upgrade the circuit into a high current SMPS circuit simply by changing the Q1, L1, D1 rating as per the required output current specifications.

L1 can be built by winding bifilar enameled copper wire over any suitable ferrite core.

Although this LM317 SMPS circuit promises a near zero loss output, Q1 must be mounted on a heatsink and some degree of dissipation may be expected from it.

Interesting Feedback from one of the Avid Readers:

Mr. Swagatam:

I am a retired EE, but continue to have an interest in various areas.  Happened to come across your website when I was researching power  supplies using LM317. 

Saw the interesting switch mode power supply schematic using the LM317. 

As it turns out, the exact circuit shows up in the 1978 National Semiconductor Voltage Regulator Handbook, with additional verbiage to explain its behavior.

However, I found it even more helpful to simulate the circuit using LTSpiceVII (which is free to download and use) to get a better idea of how the circuit operates with component value changes. 

Anyway, I decided to scan the two pages from the 1978 Handbook and email to you in the event you care to post them with the schematic for others who might be interested in a little more detail.

Regards,

Denton Conrad

Raleigh, NC