CD4013 cannot drive Mosfet

[Sultanpepper123]

General Circuit information :

1. Function/Purpose: After the user press a tactile and hold for 6 seconds the circuit latches and turns on the mosfet to in turn turn on the power for a system

2. Power source : 4 X AA batteries 6.8V - 5V

I've tested this circuit first in a simulation application which verified that the Circuit will turn on after 6 seconds, then after building the exact circuit it worked as expected with no problem.



Now after replacing some components to a smaller alternative, the circuit now latches after 3 seconds and the output from the CD4013 which goes into the gate of the N-channel Mosfet which drives the power of circuit immediately shuts down , by playing around with the resistor 10R-470R (gate resistor) and pull-down and the pull-down resistors I am able to prevent the shutdown and get a good 4.5V-5V at the gate of N-channel, but the P-channel at best at its gate would get a 1.8V-3V at gate which seems to be not enough to close the P-channel. So now it's like a pendulum if I try to ease out the pull up at the P-channel gate the CD4013 shuts down and otherwise the p-channel fails to close.



Any help would be greatly appreciated

[Peabody]

I would just be sure the 10x resistors are in the right place  - R5, R6 and R9 - and that you haven't switched a 10K with the 10R.  Also double check the pinouts of the mosfets to make sure you don't have them backwards.  Otherwise I don't see any reason why it wouldn't work.  Technically, after coming out of Q, the pulldown to ground through R9 should come before R6 so as to avoid setting up a divider, but with these resistor values that shouldn't really matter.

[stoyanoff]

The supply voltage is quite low! There is a possiblity you don`t fully open the Mosfet. Fifure 4 in the datasheet suggest around 1.1ohms resistance, but with the voltage drop inside the CD4013 and adding the fact your load resistance it`s not so high(10ohms on the schematic), the voltage drop on the mosfet can be significant. Try to use little bit bigger supply voltage!

[Doctorandus_P]

6V is quite low, but should be enough for most FET's.
You can verify if this has an effect by increasing the supply to 8V or 10V.
To fully utilize AA batteries, your circuit should be able to work down to 1V per battery, and with 4V you're getting into the zone where most MOS fets start having trouble. So check the gate voltages of your FET's.

Loose MOSfets are quite delicate devices and can easily be damaged by just touching them.
Partial damage from ESD is also a common occurrence with these devices. Switch your FET's for new ones to check this.

If you plan to use MOSfets more often on breadboards and such, then also get yourself some bipolar TVS diodes (12 to 16V) and solder them directly to the MOSfet Gate and Source, close to the body of the FET.

[David Hess]

4000 series CMOS can drive power MOSFETs just fine at low speeds.  The output resistance is high enough that a gate dampening resistor is not required unless the line length is long.

[jfiresto]

I was wondering when someone would write that. A BSS138's input capacitance is also low enough to be considered a typical load for the CD4013's datasheet.

EDIT: You can also delete the 10K gate pull down resistor: 4000-series outputs sink current just as well as they source it. The "C" in CMOS is one of its original charms.