High Side PFET bidirectional switches not working??

[ridgerunnersjw]

Hello.
I have the following circuit that I built up on the bench and tested in ltspice.  It seems that the powering of the input power to the circuit is enough to turn the two PFETs on independent of the NFET.  If I disconnect the 1Meg resistor from the gates the circuit will power up and supply load power without at the moment I connect the 60V power switch.  Can someone please tell me how to solve this?  Is it a leakage current issue?



Thanks

[moffy]

Try putting a 1M resistor in parallel with D1/C1 while having R1 disconnected and see what happens.

[ridgerunnersjw]

Although I don't see the logic with this I did try it.  It made no difference.  Removed R1 and placing a 1M in parallel Source to Gate the powering of the 60V triggers the devices to turn on.

[shapirus]

.doc, really? Not even a pdf, not even a docx or odt, but .doc?

Come on, attach a picture. Chances of getting help and hints are higher when people don't have to run external programs (that they may not even have installed) to open your files.

[Peabody]

But it does turn back off doesn't it?  Or does any power getting to the load automatically power up V2?

You might try switching the positions of the two mosfets so that the drains are connected together, then move the pullup resistor, the zener and the capacitor over to the 60V side.  Also try a much smaller (ceramic) capacitor.

Assuming V2 is not coming up too, I think the basic problem is that when 60V is initially applied, the source rises faster than the gate, so for a while the mosfet is turned on.  But it's not clear why that is happening.  In theory, the 60V should pass through the left mosfet's body diode, through the capacitor to the gate, so the gate should also rise.  But maybe switching things around as described above will make it work.

A separate question is why you are using two mosfets back-to-back.  In what sense does the circuit need to operate in a "bidirectional" manner?  Does current ever flow from the load back through to the 60V side?