Looking at any typical off-line power converter, we will usually find a controller IC driving a high voltage FET. At the Gate, besides the usual pull-down resistor to (Primary) Ground, we may also see a paralleled 500mW/18V zener diode. Yes, this diode does cost a few cents, but omitting it can be even more costly! This is just one of those examples of minor 'details' that will ultimately distinguish a bad power supply design from a really good one.
On the face of it, this zener does look as if it is there simply to 'protect' the Gate oxide layer under various transients and noise spikes that may be encountered in the field. But while the effects of this one single component can be subtle, they can be dramatically helpful. This is admittedly a rare situation, especially in Power Conversion, where we are by now sadly conditioned to expecting that nothing good is ever going to come our way, at least not easily and cheaply enough, without some sort of indirect or unforeseen price to pay somewhere along the way. In fact, here too, we do have our share of some rather reflex-action pessimism abounding. People still seek to question the basic wisdom and validity of this rather crucial zener.
But let us start off by first describing how I personally encountered this issue. The Singapore based design cum manufacturing outfit I worked in at that time had a policy of never (knowingly) putting in even one superfluous 1-cent component. They considered that equivalent to 'shipping free parts with every power supply'. They understood that at their current manufacturing volumes, they could probably hire another power supply design engineer for every zener diode they could eliminate from the schematic.
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!
In this case, the optocoupler, which was as usual connected to the controller for regulation purposes, cracked open. Now that was something unacceptable to UL, since it meant that the 'sacred' Primary to Secondary insulation barrier (inside the opto) had somehow gotten breached. That could conceivably lead to a hazardous voltage level from the Mains line input (Primary side) reaching a user who may have been in physical contact with the system (Secondary side) at that very moment. Of course, thereafter, the input fuse would also blow up, disabling the entire system. But a fuse can never be relied upon to blow up fast enough to prevent electrocution its main use is only in preventing a fire.
This is the rapid chain of events that had apparently occurred:
- a) The FET blew up and its Drain and Source shorted together
- b) the resulting high current ripped through the current sense resistor in the Source, causing this MOF (Metal Oxide Film) resistor to fail open
- c) the inductor current coming in through the Drain, still needing a path to freewheel through, diverted into the Gate, raising its voltage and then entered the controller IC
- d) The controller IC then failed and the high voltage/current damaged several components connected to the pins of the IC . including the optocoupler!
- e) The optocoupler cracked, and its Safety barrier was breached
- f) Finally, the fuse blew (but too late)
- g) About half an hour later, the 'prime culprit' (one sleepy and hapless power supply design engineer somewhere out there) receives a midnight call from his fulminating Boss. Surely the topic this time is not any upcoming promotion.
Now, had we pored over some older power supply designs we may have seen that a Transient Voltage Suppressor ('TVS') was often 'mysteriously' placed across the current sense resistor. In fact, its purpose was to circumvent this very chain of events. It takes us from the end of Step b) above, straight to step f). A TVS is basically just a rugged zener diode, and one that is designed to always fail in a shorted condition. So when the MOF resistor fails in step b) above, the resulting rising voltage would cause the TVS to almost simultaneously fail too, thus maintaining galvanic continuity for the current to keep flowing from Drain to Source to Ground, till the fuse interrupts. So in this case, the current wouldn't need to divert into the Gate (and the IC) as happened in step c) above.
But we note that a TVS is a fairly expensive solution. So in Singapore, we decided to try a zener between Gate and Ground. In this location, the zener also always fails in a shorted condition. It thus protects the controller IC and all its associated components (including the opto), till the fuse interrupts.
During debugging stages, or in initial proto-typing, a Gate-Source zener comes in real handy too. It not only saves a lot of soldering/desoldering, but it dramatically extends the life of the constantly reworked board. Because though the FET fails, the controller IC and all its associated components always survive. So even after an otherwise 'impressive' blow-up, we usually just need to replace the FET, the current sense resistor, the zener, the pull-down Gate resistor, and input fuse, to be up and running again in half an hour.
And the skeptics. some are still convinced that the small anode-cathode zener capacitance, can combine with the input capacitance of the FET and the lead and trace inductances to form a high-Q pi type of tank circuit (C-L-C). So they recommend a small resistor of about 10 ohms placed between the zener and the Gate lead, to damp out any oscillations. Yes, inarguably, the zener must be very close to the FET, but about the oscillations.?!! Well, one prominent FET manufacturer earlier was quite sure that this doomsday scenario could really happen, and had even stated as much in a certain Application Note (though this section was later removed). On further inquiries (from me in particular) they backed off, and in fact provided fresh data to actually disprove their own earlier assertion. So ultimately, privately they blamed it on one lone engineer of theirs, who didn't quite 'follow the book' when he reported he saw 'oscillations'. Probably a bad scope probe. We will never know.
As you can see, an engineer's job is not getting any easier. If you have time, do drop me a line, at firstname.lastname@example.org… just don't blame me if your converter circuit isn't working.