Hello All,

I would like to switch between two voltage sources, namely +28v and +20v. I have been trying to do with MOSFET. IRFR5305TRPBFand BJTMBT3904DW1T1G, I will be doing the switching using MICROCONTROLLER.Please advise on this. I also attached our circuit design please confirm it works or not


Or, Is there any other better way to do this ?



Thanks
Ashok

 

Why the back to back mosfets for the 20V supply ? Given timing is
controlled by UP ?

BJTs should have a R to ground right at base, to absorb leakage and
possible turn on when UP outputs are in tristate, like during UP power
up. I would make UP to base R 1K, and BJ base to ground ~ 10K. In
fact Rbasetognd <= Vbe / Ileak

Regards, Dana.

 

Why the back to back mosfets for the 20V supply ?

To block the 28V from backdriving the 20V supply through the substrate diode.

That circuit should work okay but you should add a resistor in series with the bottom BJT as you did for the top one to limit Vgs when ON, since the maximum Vgs rating of the MOSFET is 20V.

 

Also, I think you need back-to-back MOSFETs for both switches.

ak

 

Also, I think you need back-to-back MOSFETs for both switches....

Why?
The top MOSFET will never be reverse biased.

 

Why?
The top MOSFET will never be reverse biased.

Depending on the condition of the 28 V source...

ak

 

Depending on the condition of the 28 V source...

Yes, my statement assumes it stays at least 20V (or that it's not a problem if the 28V is back driven by the 20V if it's off).

 

Hello All,

I would like to switch between two voltage sources, namely +28v and +20v. I have been trying to do with MOSFET. IRFR5305TRPBFand BJTMBT3904DW1T1G, I will be doing the switching using MICROCONTROLLER.Please advise on this. I also attached our circuit design please confirm it works or not


Or, Is there any other better way to do this ?
View attachment 152589
us a LM5050 + Pmosfet + Mosfet to control Lm5050 (see datasheet)

Picbuster




Thanks
Ashok

 

Do you want to be able to switch between or just use which ever is the greatest voltage ?

You could also look for active diodes or loss less diodes,

( they are really fets, but have a hunt )

 

To block the 28V from backdriving the 20V supply through the substrate diode.

That circuit should work okay but you should add a resistor in series with the bottom BJT as you did for the top one to limit Vgs when ON, since the maximum Vgs rating of the MOSFET is 20V.

HI danadak,
Thanks for the details.where should i add the resistor in series with the bottom BJT.could please show me or redraw my attachement where should i add the resistor.


Thanks,
Ashok

 

For something completely different: I had to solve this issue for a test calibration jig.
I used a LM317 (or equ adjustable regulator) with one voltage set by a resistor, then modified by a relay to another resistor.
Just be sure to use a relay good at low current. I suppose a FET could replace the relay, but the relay is simpler.
An upside is no glitch in transitions.

 

Or you could put a SPDT relay which will switch between the 2 sources. If you do not want it to be automatically switched, you can put a slide switch.