MOSFET delay circuit query

[yanc]

Hi, I have a query regarding a dual MOSFET delay circuit I'm trying to implement.

I'm need to switch a 16V 5A load using a 16V control signal and once swiched there needs to be a 3sec delay before the MOSFET actually turns off.
I thought I would be able to do this using a N & P channel MOSFET configuration in a dual device package with a R/C delay circuit on the gate. The circuit does work but the time delay changes considerably depending on how long the MOSFET has been conducting. I have tried different capcitor types (Tant and ceramic) and values with the same effect. So initially the time delay is approx 10sec but if the the circuit is left on this gradiually reduces to less than 3sec.

Assuming this due to the gate resistance changing in Q1A as it warms up. I didnt think this would be critical as Q1B is the device doing the main switching, but maybe as they are in the same package there is enough thermal transfer between the two devices to have this effect?

Any help would be most appreciated..
Thanks

 

[FvM]

I'm quite sure that Q1B safe operation area will be far exceeded if it is turned on or off in the given circuit with load current. You want a kind of positive feedback circuit (Schmitt-trigger) to avoid linear transistor operation with large power dissipation. I would also prefer a precision timer.

 

[BradtheRad]

Your capacitor appears to lack a definite discharge path.
This alternate RC arrangement is more predictable.

The potentiometer allows you to customize for unseen/ unknown resistances.

Values are selected so voltage takes a few seconds to drop.

 

[Easy peasy]

How about a TLC555 astable one shot timer ... ?

 

[stenzer]

Hi,

the HEF4541B from Nexperia [1] (or CD4541B from TI [2]) is a versatile programmable timer IC which may siuts your application. Have a look on the single cycle mode.

[1] https://assets.nexperia.com/documents/data-sheet/HEF4541B.pdf
[2] https://www.ti.com/lit/ds/symlink/c...=https%3A%2F%2Fwww.ti.com%2Fproduct%2FCD4541B

greets

 

[yanc]

Many thanks for the input on this problem.
The Si4564 dual N & P package should work fine with correct timing control.
I was just trying to keep things simple and compact as I dont have much board space.

I take a look and see what best works.
Many thanks..