In some applications, users expect protection for boost converters against reverse polarity, short circuits and other anomalous conditions. However, these step-up converters are susceptible to output shorts due to a DC path from input to output. Usually a fuse, resettable or non-resettable, protects the system, but it is not always accurate or fast enough.

 

Figure 1: The basic PWM boost-converter topology  

This design idea shows a way to safeguard a boost converter from short circuits and reverse polarity.

 

Figure 2: Simple boost converter with additional protection (inside the dashed line)  

The added circuitry comprises of opto-coupler IC1, N-channel MOSFETs T1 and T2, and a few passive components. When power is switched on, current flows through F1, L1, D1, IC1’s LED, C1, and the intrinsic diode of T1. C1’s momentary displacement current allows IC1 to turn on T1 and T2.

As soon as the boosted voltage appears at the LED of IC1, its current path through C1 is taken over by ZD1. T1 and T2 remain on as long as there is enough output voltage to breakdown ZD1. The Zener diode is chosen, so during a short-circuit–or perhaps even over-current–condition, it will not break down. R3 discharges C1 when there is no source power.

In this design, the MOSFETs provide reverse-polarity protection for free and with much less voltage drop than a simple diode.

First published by EDN.

“T.A. Babu is a design consultant and author with a strong interest in lighting who has worked in Dubai and India.”