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. It varies the output voltage through the indicated pot R4, and in turn causes a PWM to initiate for the base of Q1.īasically, the introduction of R6C3 transforms the LM317 regulator circuit into a high frequency oscillator circuit, forcing the output of the LM317 to switch ONOFF rapidly with a varying PWM, which is dependent on the setting of R4. 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. 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. Moreover such topology also requires the input to be minimum 3V higher than the desired output, adding further restrictions to the given regulator configuration.
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