Switching between two voltage sources.

Thread Starter

ashokchandra

Joined May 13, 2018
37
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 ?
upload_2018-5-16_14-41-56.png


Thanks
Ashok
 

Attachments

danadak

Joined Mar 10, 2018
3,793
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.
 

crutschow

Joined Mar 14, 2008
24,130
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.
 

Picbuster

Joined Dec 2, 2013
990
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
 

andrewmm

Joined Feb 25, 2011
157
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 )
 

Thread Starter

ashokchandra

Joined May 13, 2018
37
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
 

olphart

Joined Sep 22, 2012
78
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.
upload_2018-5-20_8-4-26.png
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
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.
 
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