Normally closed p-mosfet

Hi, I am trying to implement a normally closed switch. In the shown circuit:

Vcc = 16..60V (Vcc>>Vz)

The characteristics of the P-mos are: Vth = -1V Rdson = 0,055ohm

The circuit works correctly: in normal condition Vz=Vgs=-10V so the mos is fully opened. When the zener is shorted (using an optocoupler not shown in the circuit) the Vgs goes to 0 and the load (Vout) is de-energized. When a load is connected I have an 1V unexpected voltage dropout. When supplied with 15V and the load is 4Amps, I expected the following voltage dropout:

V = Rdson x Iload = 0,055 x 4 = 220mV

With a 1V dropout the equivalent Rds is:

R = Vds / Iload = 1/4 = 0,25ohm.. NOT 0,055

I suspect that the mos is not fully opened but I cant understand the reason

Thanks a lot

  • \$\begingroup\$ Have you measured the actual Vgs? It may be less than you think, since you're only putting about 6.6V/20k = 0.33 mA through the zener. \$\endgroup\$ – Dave Tweed Mar 16 at 21:42

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