LM317 Variable Switch Mode Power Supply (SMPS) Circuit

So far 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.

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.

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:

LM317 Variable Switch Mode Power Supply (SMPS) Circuit

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.

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. 

16 thoughts on “LM317 Variable Switch Mode Power Supply (SMPS) Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Sir can I replace bjt/pnp transistor with MOSFET P-channel? sure i will try this after work ^_^
    by the way sir is this circuit applicable also on LM7812 or 7805 regulator??
    Thanks again Sir. Have a nice day.

    • Joseph, if you are using a mosfet then the 22 ohm resistor will need to be increased proportionately such that it develops the required 9V for the mosfet gate to trigger it satisfactorily.

      No, it is applicable only with LM317 or LM338.

    • hello chucks, you can use it as a power supply circuit for getting an efficient power output and also as a solar charger. It can be also modified to work as an MPPT

  3. Sir i have make an an inverter instant to rate at 220v but it rating at 320. what shall i do to decrease it to 220 ?? And later that i just observe that may be because i am using 6-0-6 transformer 12v battery but last time u say if going to use 6-0-6 transfer the battery also must be 6v so please how shall i reduce the battery from 12v to 6v ??

    • Hi Rabie, current is supplied by Q1 not by the LM317, LM317 is used just for initialization of the buck converter and for the PWM control.
      the output current is dependent on the inductor specs and the input supply current range.

  4. hello sir,please i have a project i would love you to help me with.will love to e-mail the details to you.i wouldnt mind if you give me your email.thanks alot



Leave a Comment