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P mosfet turns on/voltage at drain during powerup

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tiwari.sachin

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Attached is the waveforms for P MOS and sch

SCH.jpg

1cap.jpg

Yellow is Vs
Blue is Vg
Pink is Vd
Orange is Vsg

I have a Switch (used tactile for reference but is a rocker switch) at the source as shown

The moment I press the switch I see a rise at Vd and it comes back to zero after about 20ms (Why does this happen)

Note: Q8 is off initially

Ideally Mosfet should turn ON only when Q8 is ON.

What could be causing Vd to rise and fall the moment switch is turned ON.

- - - Updated - - -

Also Note:
D45 and C136 not loaded
After the switch I have a 470uF x 2 electrolyte caps
 

Hi,

* defective scope probe
* bad solder joint
* wrong scope setup AC/DC

Klaus
 

Remove C139. You should not have a capacitor between gate and drain.
When you press the button, C136 and C139 form a capacitive voltage divider that will switch on the MOSFET for a while.
If you want a capacitor on the output, the negative side should be connected to ground.
 

Hi,

You should not have a capacitor between gate and drain.
I often have a capacitor between gate and drain. Especially for a power distribution switch it prevents from fast switch ON of the load.
In combiniation with the drive resistance it generates an about constant dV/dt...which causes a calculable inrush current on load capacitance.

Klaus
 

Hi,

I have a cap on drain and gate to control the turn on time of mosfet (as explained by KlausST)

Meanwhile I have not loaded C136.
 

I have a cap on drain and gate to control the turn on time of mosfet (as explained by KlausST)
The cap also causes slow turn off. As the FET has initially zero Vds, the cap further causes the observed output pulse after switch closure because Vds can only ramp up slowly.
 

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