Understanding this self-power power supply design

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
See below a self-power supply design I took from a App Note from Texas Instruments.

My question is about the voltages at the inputs of the comparator U6A. At the inverting input, the reference is set to 0.7 V because of the diode D12, right? At the non-inverting input, the voltage is defined by the resistor R41//R42. If the voltage across C27 increases, the voltage drop across R41//R42 will increase so triggering the comparator. Am I right? In addition, what's the purpose of C34 in parallel with R43 at the comparator's output?




Joined Jun 22, 2012
The Voltage across the supply will have to be over 39.7V to trigger the Zener on pin 3 of the U6a, the capacitor C34 is to assist fast turn off for the fet.