# Series/parallel buck converters...but different

#### LikeTheSandwich

Joined Feb 22, 2021
42
OK, so I know that normally you can't connect buck converters in series because it's creates a a short, and even in parallel it's somewhat risky because whichever one has the higher voltage will usually get all or almost all of the load. But here's what I'm wondering: buck converters, but the inputs are in series? So batt neg to buck1 neg, buck1 pos to buck2 neg, buck2 pos to batter pos, then the outputs of these in parallel? Again, I know you're not normally supposed to put them in parallel, but if they're adjustable then I can dial in the voltage outputs until they're basically identical and it should work OK...I hope.

#### AlbertHall

Joined Jun 4, 2014
11,498
If the inputs are in series then you force them to draw the same current from the supply and so the voltage will not split between them equally. If one of them draws slightly more current its input voltage will fall resulting in it trying to draw more current to maintain its output. This will further reduce the input voltage - and so on.

#### LikeTheSandwich

Joined Feb 22, 2021
42
If the inputs are in series then you force them to draw the same current from the supply and so the voltage will not split between them equally. If one of them draws slightly more current its input voltage will fall resulting in it trying to draw more current to maintain its output. This will further reduce the input voltage - and so on.
Yes, but it will achieve a settling point.

#### AlbertHall

Joined Jun 4, 2014
11,498
Yes, but it will achieve a settling point.
I think the 'settled' point will be with one converter having very loww input voltage and the other having a high input voltage. I have no idea whether this would be a stable state but it sure isn't going to work properly.

#### BobTPH

Joined Jun 5, 2013
3,652
A buck converter does not draw a continuous stable current, it is switching on on and off at a rate dependent on the load. Such circuits cannot work in series, because their input voltage and current will change each time the other one switches.

And, even if it did, how would that help with combining the outputs?

Bob

#### LikeTheSandwich

Joined Feb 22, 2021
42
I think the 'settled' point will be with one converter having very loww input voltage and the other having a high input voltage. I have no idea whether this would be a stable state but it sure isn't going to work properly.
Makes sense. But as long as each one has a sufficient input voltage, it should be fine, shouldn't it?

#### AlbertHall

Joined Jun 4, 2014
11,498
Makes sense. But as long as each one has a sufficient input voltage, it should be fine, shouldn't it?
No, it is not going to work. Remember they both must have the same input current as they are in series but hey will have different input voltages and so different output powers.