suggestion on High power rating MOSFET needed

[Mathan]

NDS352AN and NDS352AP were used for switching as shown in the circuit. It works well for a bus-powered device. But for self-powered devices, it doesn't switch ON/OFF the device.
For ON condition, 5v is the output and for OFF condition also it gives 4.5v , thus the device is always ON.

Any suggestions?

Thanks,

Mathan

[kittmaster]

switch the source and drain, looks like its flipped vertical

[lacy]

I agree with Chris, but there is more here. He is using Depletion Mode MOSFETs and these things conduct without a voltage present on the gate and require a reverse bias on the gate to turn them off. Since he is using a digital signal that will never occur. Therefore they will never give what he is looking for. I would suggest using Enhancement Mode MOSFETS as these act more like a normally open switch and probably would be better suited to this application.

I hope this helps.

[kittmaster]

agreed

Message Edited by kittmaster on 08-28-2008 10:43 PM

[Mathan]

/////lacy wrote: /////I would suggest using Enhancement Mode MOSFETS as these act more like a normally open switch and probably would be better suited to this application. Dear lacy, Thanks for the ideas. Also could you please suggest what Enhancement mode MOSFET shall i use? Thanks for your time, Mathan

[kittmaster · Accepted solution]

Only you can answer that, you need to know how much current will pass through it, how much power loss there will be based on Rds_on.  If your doing just simulation, anything will be "ok", but only you know what your app is and should research what your needs are......I like to run my max at 50% of the device capability........because I can......but typically avoid anything over 85%  of the max values..........good luck