The Mighty Zener

We are by now sadly conditioned to expecting that nothing good is ever going to come our way, at least not easily and cheaply, writes Sanjaya Maniktala in the latest installment of his power supply design column. Surprise, surprise The gate-source zener on your power FET (a couple of cents) can really save your bacon on safety issues. 

Yet even they couldn t ultimately avoid this rather stubborn gate-source zener. Note that this is actually a gate to ground zener, because there usually is a current sense resistor between source and ground. 

The trouble actually started after their first manufacturing sample had been built with no such zener in place. It was submitted for the mandatory UL1950 safety approval. At the test house, various abnormal tests were carried out. In some of these tests the switching FET exploded. And in all such destructive tests, the controller driving the FET failed too, sometimes quietly, and sometimes quite spectacularly. But either way, this was perfectly OK as per UL, since safety was the only concern here, and this should never have been compromised. But in fact, in one case safety did get affected  

This is the rapid chain of events that had apparently occurred  

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