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[SOLVED] Capacitor Switchover (NOT switching capacitor), simple implementation?

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righteous

righteous

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Gents,

I want to implement the following circuit the simplest way possible. The simple idea is to switch over C1 and C2 so that R3 does not (directly) draw current from V2. But I'm having trouble selecting the right components, especially as I believe FET's are out the window due to their body diode. Any creative and ingenious suggestions are highly appreciated.

Screenshot from 2018-04-02 19-25-17.png
 

I found this LTC4412 It's simple but high in price, then the solution can be implemented as a high side power switch (Fig.7 Page 12)
 
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Here's the circuit using P-MOSFETs.
I used two MOSFETs back-to-back to prevent reverse current through the body diode.
That works because MOSFETs conduct equally well in either direction when on.

Capture.PNG
 
Thank you very much! It's exactly what I needed, much better than the stupid IC's (which probably don't work as advertised)

But please tell me if my memory is failing me; in the god olden days, didn't FET come without body diodes? Or is the body diode really intrinsic to the FET design?
 

The bottom line is that yes, the body diode is intrinsic to the FETs.

The long answer is that there are always two PN junctions which form body diodes, one from bulk to source, one from bulk to drain. In discrete devices, the bulk is always shorted internally to the source, so the bulk-source diode is shorted out. This leaves the bulk-drain diode, which is now effective a source drain diode. That's the "body diode" in power MOSFETs. In principle, you could tie the bulk to some other potential, but it only makes sense to do so in very specific cases (analog CMOS circuits, never power MOSFETs).
 

righteous said:
..............
But please tell me if my memory is failing me; in the god olden days, didn't FET come without body diodes? Or is the body diode really intrinsic to the FET design?
Click to expand...
JFETs don't have a body diode.
All MOSFETs have an intrinsic body diode.
 

crutschow said:
JFETs don't have a body diode.
All MOSFETs have an intrinsic body diode.
Click to expand...

I knew there was something with FET's without body diode, my memory still works occasionally! So your solution could have been implemented with four JFET's? Or what's the reason now we don't use JFET's for power switching?
 

righteous said:
So your solution could have been implemented with four JFET's? Or what's the reason now we don't use JFET's for power switching?
Click to expand...
Yes that could be a solution, except they don't make power JFETs. :-(
You could use a power MOSFET that brings the substrate diode out on a separate terminal which could be connected to the most positive voltage, but they are not commonly available.
 

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