The common base configuration has been well documented for the BJT .Small signal analysis shows better high frequency capability then common emmitter for the same transistor.I have seen common gate used on JFETs at VHF where the low input impedence would be a better match to 50 ohm. Would a common gate circuit arrangement which would be more complex on an orthodox SMPS give lower switching losses ? Is there a definative statement about how much lower they would be ? I did this on a small ZVs royer where the circuit complexity penalty for common gate was minimal but the actual switching losses of ZVs are so low anyway when input volts are high and input currents are low .Is it worthwhile to look at common gate ?

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    \$\begingroup\$ Just to give you one idea: I have seen MOSFET cascodes used as a switch in SMPS. \$\endgroup\$ – Nick Alexeev Nov 12 '15 at 21:07
  • \$\begingroup\$ Would you need to actively turn off the main HV switch by turning off the bottom LV switch and actively discharging the gs capacitance of the main switch by turning on say a small LV p channel device ? \$\endgroup\$ – Autistic Nov 12 '15 at 21:12

If the gate is set at maybe 10V (fixed) and the source is hit with a 20Vp-p square wave (0V to 20V) then I can see good reasons to trial it - the MOSFET would turn off very quickly because the gate would be taken negative with respect to the source and to turn the device on the source would be at 0V.

However, the source signal is having to conduct the same current as the load so this is its chief disadvantage I reckon. If the drain current is 10A the signal at the source has to cope with 10A when it is low falling to virtually nothing when the source is high.

I'm not saying that there is a big power loss in the drive signal, just that the drive signal has to cope with a rapidly changing load.

Of course on the use of a common gate, any CMOS op-amp using MOSFETs as the differential pair uses a common gate by virtue that the sources are connected.

It's an interesting idea.

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  • \$\begingroup\$ Is it reasonable that if Vin is very high compared to your 10V like say rectified mains then maybe the extra drive losses may be more than offset by reduced drain circuit switching losses . Could you show that you can win here?Remember that the bottom mosfet sees about 20V so it will be cheap and small and have minimal impact on total conduction losses . \$\endgroup\$ – Autistic Nov 13 '15 at 7:05

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