Load switch MOSFET

[dark]

I have following supplies going to my module:

+15V,-15V,GND . I want to switch it ON/OFF , which is the best way to do this . Is it enough to switch the +15V line , or a switch at -15V is required too ? if yes how can -15V be switched N-Channel Mosfet?

-Adi

 

[Boncuk]

The best way is using a DPDT-relay rated properly.

Boncuk

 

[dark]

The best way is using a DPDT-relay rated properly.

Boncuk

I am running out of PCB space , I forgot to mention the load is quite low 200mA max . Is disconnection of -15V altogether mandatory ?

-Adi

 

[user_88]

If you can access the Jan. 2009 issue of Circuit Cellar Magazine, there is a relevant article on the application of NFETs. ... That would be issue #222. ... not the current one displayed on their website ..... ought to be out in print in a week of two... The article diagrams show the schematic icon for depletion mode NFETs, but the author specifically states that he is talking about enhanced mode NFETs .... if I understand correctly.

From the text content, it looks like you could handle the GND to -15 V NFET case by placing the load above the NFET, with the NFET source terminal at -15 V ... Then turn the NFET on by raising the gate to GND potential, and off by lowering the gate to -15 V .... I have not actually tried this myself...

 

[Roff]

I am running out of PCB space , I forgot to mention the load is quite low 200mA max . Is disconnection of -15V altogether mandatory ?

-Adi

Yes, you need to switch both of them. How are you turning off the positive supply?

 

[dark]

Yes, you need to switch both of them. How are you turning off the positive supply?

Actually I want to switch off an IC when not in use for the measurement . Can you please tell me what damage could result if only -15V and GND keeps connected and +15V is raised. I think to use PMOS and NMOS to do this .

-Adi

 

[duffy]

Shoot, if it's under 200 ma you could just use a 2n3904 and a 2n3906.

Dead simple - emitter of the 2n3906 on +15v, emitter of the 2n3904 on the -15v, collectors are outputs. Now, make a series circuit running the base of the 2n3906 through a 220 ohm resistor to the collector of a third 2n3904, the emitter of that to the base of the 2n3904. Switch that transistor on and it turns on both supplies. This second 2n3904 will bias on at -13.6v. If that isn't convenient, there are other ways to do it - what is delivering the switch voltage? A port pin or something?

 

[Mikebits]

Many devices that use +/- supplies have power sequencing requirements. Often if this sequencing is not follwed the device can go into a latchup mode where tons of current is drawn and can blow the suplly fuse or the part. I once encountered this problem with a Nat Semi CODEC device. The boards blew fuses for months before I determined the cause.

 

[duffy]

dark - something like this -
I replaced the 3904 with a 2n2222 and the 3906 with a 2n2907 to increase the current capability from 200 to 600ma.

 

[Roff]

dark - something like this -
I replaced the 3904 with a 2n2222 and the 3906 with a 2n2907 to increase the current capability from 200 to 600ma.

Q2 has to be driven by a signal referenced to -15V.

 

[duffy]

Yes, like I mentioned in an earlier post, it will turn on at -13.6v. The could be an advantage or disadvantage depending on the rest of the circuit. You could change Q2 to a 2n3906 and flip that part of the circuit upside-down to get it to work with a ground reference, but then it's inverted and sitting near +15v, so you would add a fourth transistor...

Better to wait and see what dark is driving it with.

 

[dark]

Yes, like I mentioned in an earlier post, it will turn on at -13.6v. The could be an advantage or disadvantage depending on the rest of the circuit. You could change Q2 to a 2n3906 and flip that part of the circuit upside-down to get it to work with a ground reference, but then it's inverted and sitting near +15v, so you would add a fourth transistor...

Better to wait and see what dark is driving it with.

Duffy , I do have a reference GND for the IC . I have the circuit for running upto 85C and I think 2N3906 cant be sinked for worst temperature conditions or am'I worrying too much.
Is it mentioned in the datasheet regarding the power sequensing threat . Mikebits it would be appretiated if you please help me as to this regards , the chip is (AD 6 9 8).

Thanks
-Adi

 

[bassnut]

Sheesh, its not hard, a minature DPDT relay would solve the problem simply. No leakage and as small as a circuit involving transistors.

 

[ericgibbs]

Sheesh, its not hard, a minature DPDT relay would solve the problem simply. No leakage and as small as a circuit involving transistors.

I'll go with that, simple, small and cheap.

 

[duffy]

You still haven't told us what you are driving the switch with.

 

[dark]

You still haven't told us what you are driving the switch with.

Please see my previous thread I did mentioned there.


-Adi

 

[ericgibbs]

Please see my previous thread I did mentioned there.


-Adi

I have looked at your threads, where does it say how you are going to control this additional power switching device.

Is it a switch input or a logic input, etc.?

 

[Mikebits]

Is it mentioned in the datasheet regarding the power sequensing threat . Mikebits it would be appretiated if you please help me as to this regards , the chip is (AD 6 9 8).

Thanks
-Adi

From the data sheet, power sequencing does not appear to be an issue with your device. At any rate I would not remove +v while maintaining -v. So you should go with some of the proceeding suggestions.

 

[dark]

I have looked at your threads, where does it say how you are going to control this additional power switching device.

Is it a switch input or a logic input, etc.?

Actually what I understand is Duffy wants to know if the GND pin is of any need in switching the negative rail.I gave hime the chip number for that . It is using GND as a reference .

Anyways Thanks everybody for your valuable inputs...

-Adi

 

[duffy]

That's not telling us what you are driving it with. Makes a big difference.