by **Tom WA3KLR** » Tue Mar 15, 2005 3:33 pm

Hi Dave,

I did a lot of simulations of the Class E PA operation recently (using LTC SwitcherCad III) and after adding (by guess) realistic series inductances to the FET (voltage controlled switch with defined resistance) and shunt capacitors I saw the ringing you are now referring to. As you say, a recent photo showed the ringing on the drain line about the same as I got in my simulations.

What I came up with after some analysis of the situation is this:

The ring frequency during the period the FET is ON was the result of the FET package inductance divided by the number of FETs in parallel + shunt cap. series inductance, and the shunt capacitor value. In my simulations this came out to 5.333 nH and 1178 pf = 63.5 MHz.

The ring frequency during the period when the FET is off is again the FET package inductance divided by the number of FETs in parallel + shunt cap. series inductance, and the shunt capacitor in series with the net total capacitance of the FETs in parallel. This came out to 5.333 nH and 500.4 pf = 97.4 MHz.

The results were approximate to the ring frequency observed in the simulations. This is the closest I could come to defining the ring frequency observed.

Also, the ringing seen on the drain could become cancelled out when the product of one FET's capacitance x one FET package inductance = shunt capacitor x shunt capacitor's series inductance. This is not to say there is no ringing at the FET die.

In the simulations, the shunt capacitor and the FET capacitance is a fixed capacitance. In real life, the FET's capacitance is voltage dependent.

This change in the FET capacitance while the FET is off (ring frequency changing over that half cycle) could be seen in one of the recent photos. It's unescapable as there will be series inductance on the FET package leads and the shunt capacitor.

It is best to minimize the inductances to lower the peak ring voltages at the FET. Looking across the bare FET switch in a simulation allows you to see the higher voltage that is present across the die, as opposed to only being able to see the voltage out on the FET leads in real life; one good use for a simulation.

I was able to measure the series resonant frequency of a capacitor fairly easily by having a pigtail coax from an r.f. signal generator across the capacitor and also putting a scope probe across the capacitor and watching for a minimum in the r.f. voltage. Note the frequency and solve for series L. (You know the capacitor value.)

I hope this is a help.

Tom WA3KLR AMI #77