N-Channel Vs. P-Channel (driving a 24V load from CMOS)

[Morcego]

Hi all,

I recently build a circuit using Logic Level P-Channel Mostefs to drive a 24V load from a microcontroller 0-5V level.

Gate to microcontroller, Drain to Load and source to 24V.
That dind't work. Works only at 5V. From 5V up the transistor is allways on. I think I need to use some kind of mosfet driver. (I was thinking that using a Logic level transistor was enough). My mistake.

Aparently such a simple circuit is possible using a N-Channel Mosfet. Gate to microcontroller, drain to Load and source to ground. Since we are "giving ground" to the load everthing works.

Can you guys confirm this?. I think I still have a lot to learn concerning circuit design..

I dont want to make more mistakes.

Thanks in advance

M.

 

[IRstuff]

P-channel needs inverted logic. You can use open collector TTL or NMOS RTL inverter to to the level conversion.

TTFN

 

[benta]

Morcego, you do not give an indication of the current involved.
I prefer to use a bit of extra money and buy an integrated switch for this kind of stuff, they are available from a lot of suppliers.
Do an search on "High side switch", and example can be seen here:

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC33981&nodeId=01435979968459

Regards,

Benta.

 

[geekEE]

Morcego, if your load does not have to have a common ground with the rest of the circuit, then the scheme that you describe with a single N-channel MOSFET should work.

If your load can only be switched on and off from the 24V side, then you can use your P-channel MOSFET in combination with one of the schemes that IRstuff mentioned. I typically implement the latter by adding an N-channel MOSFET with a resistor.

And in regards to your not wanting to make any more mistakes, good luck with that. I generally make several mistakes before I even start work each day.

 

[BobbyNewmark]

"Logic Level P-Channel Mostefs to drive a 24V load"

For those MOSFETs, the maximum gate-source voltage is generally 20 V. Use a resistor divider to keep it under the max.

 

[LionelHutz]

Yes, use the N-channel instead. Switching the ground is better in that application. The N-channel will likely have a lower on resistance and less losses for the same size of Mosfet.