Sounds like you have you have some high frequency feedback. The Miller capacitance and the capacitor to ground probably formed a Coulpits Oscillator. One trick is to use a high value resistor in series with the gate. The resistor in combination with the distributive capacitance forms a low pass filter. Since there is no gate current the resistor doesn't cause a DC gain error.
Fred Townsend Peter Vogel wrote: > I have a problem that has me stumped. I'm using an n-channel power > mosfet as a source-follower pre-regulator to increase the input voltage > range of a 5 volt regulator. That is, 50 volt supply to drain of FET, > zener from gate to ground, resistor from gate to drain, source goes to > input of 3-terminal 5 volt regulator, capacitors to ground from both > source and drain of FET. > > This works okay however my original design had a capacitor across the > zener as well, and the FET oscillated at high frequency and got hot. > Taking t he capacitor of the zener fixed that. > > But I stil lhave occasional blowups of the regulator, which I assume is > to do with instability of this circuit. > > Have I done something stupid? > > Peter Vogel > ZapTV Pty Ltd > 30 Adeline St, Faulconbridge 2776 > Australia > Tel: 02 4751 8735 > Fax: 02 4751 2601 > email: [EMAIL PROTECTED] > > -- > Author: Peter Vogel > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). -- Author: Fred Townsend INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
