Re: Re: [Lowfer] LF: Re: FET change, hmmm

[Andy Talbot <andy.g4jnt@gmail.com>]

Cross conduction spikes can also be dealt with by inserting a bit of inductance between top and bottom FETs of a totem pole, or in each leg of a push-pull pair.  As done in the old Decca transmitters, and I cribbed for my   700 Watt 137 design.

The inductor absorbs the short spike and returns the energy via clamp diodes to the supply 

See http://www.g4jnt.com/137tx.pdf

A lot simpler than worrying about generating a special drive waveform with gaps

Andy

www.g4jnt.com

On Fri, 30 Nov 2018 at 20:19, Hans-Albrecht Haffa <dk8nd@web.de> wrote:

Dear OMs, dear Paul,

To prevend current spikes by cross conduction of both fets it is necesary to use a gate drive circuit with dead time, as used in power supplies. But in this case there is a no current flow time in both transistors, what may cause spikes in the drain circuit. Damping of oscillations in the drain circuit may also reached by quiescent current, not used for class D PAs. To load the spikes to the load is not possible in an LW-PA, like in an flyback-transformer of an switch mode power supply, as the load is an LP-Filter that filters by reflections of the applied voltage.