Sunday, 6 July 2014

Protect your MOSFET final stage

by G3YCC

Protect your MOSFET final stage

This idea was described in Sprat 52 in 1982 and basically shows how, by the addition of two cheap components, a resistor and zener diode, mosfet PA's can be protected from destruction by over driving. This is described in the article by Alan G3UZU who mentions seeing this recommended in Radio Spares data sheet 5342 on power mosfets. The resistor limits the dissipation in the zener diode in series with it from gate to earth and can be 10 ohms, 1/4 watt. The zener voltage is found from data sheets. For example the commonly used VN46AF needs a zener of less than the maximum drive volts of 15, say 13v, 400mW. The ubiquitous VN10KM would need a zener of 4.7v, 400mw. For other devices, look up the data in many books and catalogues and note the maximum gate voltage quoted and choose an appropriate zener. This simple modification will be found very useful in homebrew transmitters.

1 comment:

  1. Thank you for sharing this wisdom. Recently building mini-boots for my Mongrel (g0kjk's design), I have counted on that zener more than a few times during the build process. I have read that the capacitance of the zener may need to be taken into account. With values from 30-300pF being cited as common, requiring one to consider this when calculating the values for that stage, such as the LPF.This seems like it would be an issue if the diode was connected directly to the output stage (say, the collector of a BJT) but I am curious if it may be an issue at all when used in the manner described in your post.

    Nice to see more QRP discussions and more of that Sprat wisdom being shared. Keep up the great work.