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Re: How can I make my simulation oscillate like my poorly designed PC board does?


 

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That sounds like you have a trace on your circuit that is acting as what used to be known as a Lecher line, but these days would be classed as a transmission line stub.

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At the critical point on the wave form this has a higher Q than the rest of the circuit and so the oscillation occurs.

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I can¡¯t help but get the feeling that increasing the miller capacitance is only helping it oscillate. Put about 10 ohms in series with the drain, close to the FET, and see what happens. A series resistor right at the gate might be an alternative stopper to oscillation.

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From: carlvanwormer@... [LTspice] [mailto:LTspice@...]
Sent: 29 May 2014 16:25
To: LTspice@...
Subject: [LTspice] How can I make my simulation oscillate like my poorly designed PC board does?

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I have a PC board with a FET that has a high-frequency oscillation because of my sloppy layout.? I'm trying to simulate the oscillation effect, but I'm unable to make my simulated circuit oscillate!? I know that most requests to the group are to stop the time-wasting oscillations in our simulations, ?but I need help making my circuit oscillate.? I've uploaded my circuit (Oscillating Miller FETs.asc) for your viewing pleasure.

The goal of my final circuit is to slow down the rise and fall times with a large cap between the Drain and Gate, increasing the Miller Effect that we often try to minimize.? I usually tweak the value of the Miller Effect cap in the final circuit to balance the reduced noise switching harmonics (30MHz, for this design) with the increased heating losses of the slower switching speeds.? Unfortunately, I get a burst of oscillation (150-220MHz) about half way up the switching waveform on my circuit board.? I've traced the problem to a long trace to a poorly placed Miller Effect cap.? Any help in making my simulation oscillate would be appreciated.

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Thanks,
Carl




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