Can 2 boost converters outputs be put in parallel at the same voltage?

Question

I have a slight doubt here. Can the outputs of 2 boost converters set to output the "same" voltage (isolated, they would never be exactly the same obviously) be safely tied together, as long as there is a capacitive load? If so, is there a minimum to that load?

Context:

^{simulate this circuit – Schematic created using CircuitLab}

I need at least 4Ah of battery capacity, and I was only able to find single cells charger+boost modules, so I have to put them in parallel. It's for long term logging, the current is very low: 30mA peak, and around 5mA 99% of the time.

Answer 1

Given the diagram you have drawn in your question I would be tempted to look for two buck converters that each produce 2.5 volts on their outputs and then wire the two converters in series to give you 5 volts. You say they can be ran "isolated" and therefore this should work. When it comes to charging them, two charging power sources will be needed and these also need to be isolated.

Alternatively, find a charge-control circuit that is suitable for 2 x LiPo in series and then use a buck regulator to down size the raw output to 5 V.

Answer 2

I know this is an older post, but I wanted to mention that parallel sources like this do not inherently split the load current very well. Supplies are often paralleled in commercial power systems but they have circuitry built in to evenly split the current demand between all supplies. There is often an extra connection between each supply that allows the supplies to match voltages very accurately and thus supply currently evenly. It's true you can do OR-ing diodes, as they are called (they work as a logical OR function) but as you already said, this causes power loss, and it still does not result in real sharing. Sometimes this is done for simplicity in redundant systems where either supply can handle the load independently, in the case that one fails.

At minimum, OR-ing diodes are a good idea, but if you consider the topology of a boost converter, it has a diode inherently in the design. I don't think you would get reverse currents when one goes dead. If you aren't bothered by the fact that one battery would likely go dead before the other, then maybe this scheme could work. There could be efficiency tradeoffs in some ways also. I think you will get overall worse efficiency at low load conditions, because you will have two converters switching even if only one is needed. There are losses in switching converters any time they are switching the inductor.

One last note for the future, 3.3V is really suited well to running on Li-Po and a lot of devices are coming with Li-Po charging circuits built right in :).

Answer 3

I can not tell if you can put two of these modules in parallel without seeing a circuit diagram.

However if one module can deliver enough current for your application, you can do it with only one module.

LiPo cells are a bit picky when it comes to voltages above 4.2V. When you put two cells in serie the maximum charching voltage would be 8.4V. Now consider one of these cells is defective and ceases to charge at 3V, the other cell will now get 5.4V - too much. Thats the reason why in a serial configuration each cell needs it's own charging or balancing circuit.

The key here is that you like to put the two cells in parallel, they will be charged up to 4.2V, no problem. They will not catch fire or explode because of over voltage.

As usual there are drawbacks, in this case two.

• The charging current will be split, the charging time doubles.
• If one cell is defective it can discharge or even demage the second one, they are not isolated from each other.

As a sidemark, before putting them in parallel I would bring them to the same state, e.g. fully charge them separately.

Answer 4

Made and test this yesterday with two of these very excellent boards: https://nl.aliexpress.com/item/wholesale-2-pcs-lot-DC-DC-2A-Adjustable-Step-Up-Boost-Power-Supply-Converter-Module-2/32266587114.html

To power an Amp (2x50W, class D, very efficient) with 3.7V 8000mah (4x2000mah) battery to 24V (4A) and it's working great! Really.

On each board there is a 3A schottky diode present that protects the output so it is safe to connect these boards together (diode is not shown on the aliexpress product picture, dunno why).

For best results you NEED to be SURE both modules are connected to the same source (and switched on at the same time) and output EXACTLY the SAME voltage. In this case it is 23.8V and I use a large 25V capacitor 4700uF connected to both output 'lines'. Before you connect these modules to eachother, you have to setup first the voltage! I made a jumper on the back to be able to disconnect or connect the boards to eachother to be able to setup or to troubleshoot.

Tested the boards in this configuration and measure the voltage at high load and it seems to be pretty stable (almost no voltage drops until you reach INSANE because the Amp is able to output 100W and this config can't reach this). Very reliable, very nice. I will try to connect another board to get at least 5A so it can handle the full potentional of the Amp.

So yes, it is possible.