Hi everyone,

I am looking to switch between two 12V DC power supplies without having an actual “switch”. I’d like for 1st power supply to switch to the 2nd one when the 2nd one is turned on. I believe the current rating for the system is 12A. I’ve read about using ORing controllers, diodes, MOSFET, etc. Is there a sort of all in one chip or component that can do this instead of creating my own circuit?

Thanks!
S

 

Hi everyone,

I am looking to switch between two 12V DC power supplies without having an actual “switch”. I’d like for 1st power supply to switch to the 2nd one when the 2nd one is turned on. I believe the current rating for the system is 12A. I’ve read about using ORing controllers, diodes, MOSFET, etc. Is there a sort of all in one chip or component that can do this instead of creating my own circuit?

Thanks!
S

I haven't heard of one but that doesn't really mean anything since I've been retired for a decade. Have you done any looking at all or were you just hoping for some manna from above.

 

Reading the TS post, " I’ve read about using ORing controllers, diodes, MOSFET, etc. Is there a sort of all in one chip or component that can do this instead of creating my own circuit?"

A completely reasonable question, based on the work he has apparently put into this.

 

Reading the TS post, " I’ve read about using ORing controllers, diodes, MOSFET, etc. Is there a sort of all in one chip or component that can do this instead of creating my own circuit?"

A completely reasonable question, based on the work he has apparently put into this.

The question is quite reasonable, but the implication was that among all the things he has looked at none of them do what he wants. I was asking if he had looked for exactly what he wants. That is what was not clear – at least to me. BTW have you ever heard of or seen such a thing?

 

Now that you mention it, aside from a dual diode, no.

 

Welcome to AAC.

A module like this one might do the job for you. Unfortunately, it is rated for only 8A—but you might be able to swap out the MOSFETs on the board and add some heatsinks to increase that.

​

 

Welcome to AAC.

A module like this one might do the job for you. Unfortunately, it is rated for only 8A—but you might be able to swap out the MOSFETs on the board and add some heatsinks to increase that.

View attachment 313612​

That is close, but it does not perform the exact function requested. Maybe that is OK, the TS will need to tell us if it is. What was requested was the first one goes off when the 2nd one is "switched on".

 

That is close, but it does not perform the exact function requested. Maybe that is OK, the TS will need to tell us if it is. What was requested was the first one goes off when the 2nd one is "switched on".

If the second one is wired as the primary, the behavior should be what is specified—unless you mean “powered down”, in which case a bit more will be needed but it shouldn’t be too complex since the primary supply can be used to signal that state an operate a relay or similar.

 

I’d like for 1st power supply to switch to the 2nd one when the 2nd one is turned on.

If that order is required, independent of the actual voltages of the two supplies, then no, I don't think there is an off-the-shelf solution, and you will have to build your own.
Two MOSFETs connected source-to-source in series can have a low ON voltage drop, and block current in both directions when OFF.
i can show a circuit for that, if interested.

 

Hi everyone,

Thanks for all of the input so far! I am new to designing a circuit for such application and so this is a bit beyond my scope (hence the lack of detail/knowledge). If there is not a component off the market that can do this already, would a circuit like this one in the image work if I use the correct resistors, diodes, MOSFETs, etc that can handle the necessary voltage and current rating?

Thank you

 

If you can make the 2nd power supply to output a voltage slightly higher, let’s say 12.2 volts, then the problem is trivial: a pair of O’ring Schottky diodes.
Very likely, because of output voltage tolerances, this condition is already met. Bring out your DMM and measure the voltages.

 

I am new to designing a circuit for such application and so this is a bit beyond my scope (hence the lack of detail/knowledge).

I'd do something like this:

If the voltage of the 2 12V supplies aren't close enough in value, the -2 supply needs to be the higher of the two.

 

Y’akov;
I find it amazing at the immense amount of electronics modules available on Ebay and other E-commerce web sites.
There is seldom the need to design and build anything oneself, a little search and one can already readily find what is required, or something that requires simple modifications.
And, most importantly, for a price lower than the total cost of the individual components.

 

Y’akov;
I find it amazing at the immense amount of electronics modules available on Ebay and other E-commerce web sites.
There is seldom the need to design and build anything oneself, a little search and one can already readily find what is required, or something that requires simple modifications.
And, most importantly, for a price lower than the total cost of the individual components.

Depending on the application, the QC on any given item may not be sufficient, but is non-stop operation is not needed, and it isn’t health and safety critical, the modules are often so cheap that buying several spares against the possibility of failure is quite practical.

I always buy more than one in case are DOA, or if they suffer infant death, or—if they are so useful I want more of them on hand.

 

If you can't find a module to do what you want, below is the LTspice sim of a circuit that connects V1 to the load if V2 is off, but always connects V2 to the load when its on, independent of the V1 voltage relative to V2 (which is my interpretation of what you want).
In other words V2 always has priority.
I set V1 (green trace) and V2 (yellow trace) to different voltages so you could see which one is connected to the output (red trace):

It uses two MOSFETs in source-to-source series for V1 so there can never be back current through the off MOSFETs due to conduction through their substrate diodes, regardless of the relative voltage difference between the two supplies.

The MOSFETs should have an on-resistance of less than 5mΩ to limit their dissipation to <1W so they don't need to be on a heatsink.

Edit: Simplified circuit, since realized that only one MOSFET is needed for V2.

 

If you can't find a module to do what you want, below is the LTspice sim of a circuit that connects V1 to the load if V2 is off, but always connects V2 to the load when its on, independent of the V1 voltage relative to V2 (which is my interpretation of what you want).
In other words V2 always has priority.
I set V1 (green trace) and V2 (yellow trace) to different voltages so you could see which one is connected to the output (red trace):

It uses two MOSFETs in source-to-source series for V1 so there can never be back current through the off MOSFETs due to conduction through their substrate diodes, regardless of the relative voltage difference between the two supplies.

The MOSFETs should have an on-resistance of less than 5mΩ to limit their dissipation to <1W so they don't need to be on a heatsink.

Edit: Simplified circuit, since realized that only one MOSFET is needed for V2.

View attachment 313658

Hello,

Thank you for sharing this schematic. I am trying to make sense of it. So when V1 is on and V2 is off, Q1 is turned on which provides the necessary gate voltage for M1 to turn on. But M2 is off or acting as an open switch so I don't quite understand how the current flows to the output in that scenario (however I am probably overlooking something, it has been a while since I have analyzed circuits). When V2 is turned on, Q2 is turned on and this turns on M2 to allow current to flow from drain to source towards the output. V2 gets the priority because Q3 is turned on and this pulls the gate of M1 lower turning it off from my understanding. Further explanation/clarification would be much appreciated since I probably got some of this wrong

Thanks!

 

So when V1 is on and V2 is off, Q1 is turned on which provides the necessary gate voltage for M1 to turn on. But M2 is off or acting as an open switch

No.
A MOSFET conducts equally well in both directions when turned on, so both M1 and M2 are on at the same time when Q1 is on.

Q3 is turned on and this pulls the gate of M1 lower turning it off

No.
When Q3 is on, it turns Q1 off, which allows R1 to pull the gate-source voltage of M1 and M2 to zero, turning them off.

All make sense?

 

All you need us a 12V relay. Power the coil from supply 2. NC goes to the supply 1, NO goes to the supply 2, COM to the device being powered. If you need isolation a DPDT can be used.

 

Further explanation/clarification would be much appreciated since I probably got some of this wrong

I suggest using my 2 component circuit. It can't be much simpler. As long as your two power supplies are close in voltage, you don't need to worry about one being high enough to disconnect the other, yet not so much higher that it forward biases the parasitic diode.

 

I suggest using my 2 component circuit.

For that circuit the MOSFET will always be on, with no isolation.