Hello Boost converters in parallel? Can it be done well? Some of the listings for these boost Converters on AliExpress and eBay say that you can put these things in parallel to increase their power ratings. The problem with these things is that I mean this particular one is called a 600 watt boost converter. but it's only 600 watts if you put 60 volts in. and that's because there's a 10 amp fuse there.

So 60 volts in times 10 amps, 600 watts. But what if you only want to put 12 volts in and then boost it up to say 24 volts? That's what I want to do? well. 12 volts in 10 amp limit. That's only 120 watts.

So I thought if I put two of these in parallel I could at least get 240. Watts Now of course you can get higher power boost converters. Uh, this one I've Got a Feeling is 1200 watts. Although there are some strange things here, it says on the board here: T30 amps And yet you can see that they've put two 20 amp fuses in here.

uh to limit the input current to 40 amps. so which is it? 30 or 40. And I Have seen these things with ratings as high as 2400 watts and they have a fan attached at the bottom. but again, you have the same problem this these ratings.

These specs are only relevant if you put 60 volts in and it's because of these fuse ratings. You multiply the voltage in by the current rating of the fuse and that limits the input power. So if you again, if you want to boost 12 volts up to 24 volts which is what I want to do, you have to derate these things quite substantially. Now, there's another problem with some of these boost converters.

see this current sense resistor here, which is quite clearly between negative of the output and the general circuit Negative, which you'll find will be negative of the input. So if you start paralleling these things up, when you parallel inputs and outputs with neighboring boost converters, are you going to disrupt the operation of this current sense resistor? I'm not sure because I haven't actually tried it. Interestingly, this particular type of boost converter doesn't have a current sense resistor um between input negative and output negative. In fact, if you look at the trace on the board underneath, you can see I think it goes all up around the top there, but you can see that input negative is in fact connected directly to Output negative and this boost converter measures current in order for it to do its constant current functionality.

It measures current with this current sense resistor here up near the inductor, and if we look at the Primitive schematic of a boost converter, you've got input with capacitance and there's capacitance on the output. Input goes through an inductor and a diode to the output, and then here there's a switch typically a mosfet, and when you turn this switch on, you pull this inductor down briefly, and then when you let go, the voltage on this side of the inductor Springs up flies through the diode and charges the output capacitors. And that's how you get your higher output voltage. And this particular boost converter has that current sense resistor down here in the arm of the mosfet.

So it's not measuring current in the negative of the output, it's measuring it in the switching device. And that means that I can parallel up all the negatives. So the negative input here. the negative input.

here. they're paralleled on this terminal block and I haven't even bothered to connect the output negatives because they are essentially the same as the input negatives. So I've simply taken a wire from that terminal block which is connected to the input negatives. and I'm using that as my output negative.

So here's my setup: Input comes in: Via this cable. Here, it's a cigarette lighter plug that will come from a 12 volt power bank. It's paralleled into both inputs of these well. 120 Watt boost.

Converters On both of the Boost converters, both pots are set to the absolute minimum, so initially, they won't be boosting and they also will be current limiting to the maximum extent. But I Got a feeling if I remember these things rightly that the minimum current limit was something like 25 watts. I Know that's not a current, but it's probably around 2 am. something like that.

And then the outputs of these two boost converters go separately through two ammeters. I Can't see that one very well. Perhaps if I lift it up a bit? Yeah, I'll do that separately through two ammeters and then they're connected together so they are parallel. But I Wanted to be able to see the output current separately from these two to see how they do current sharing.

And then that's uh. Passed on to this light bulb 24 volt brake light bulb. It's 21 watt, 24 volt I think so it's about one amp and as I say negative from the terminal block I've simply passed through here also to the light bulb. so we'll switch that on and see what happens.

And here's what's happening: One of the Boost converters is doing all the work 625 milliamps and the other one is doing almost nothing. now. the rear ammeter is for the rear boost converter and vice versa frontameter front boost converter. Now, this voltage uh, Vault meter I Put on this boost converter years ago and it's measuring the voltage on the output capacitors.

but interestingly, because these are paralleled, it's also measuring the voltage on these output capacitors. In fact, they have to be the same because they're connected through these ammeters and linked together there. And you'll see that if I increase the Boost voltage of the near boost converter, you can see the voltage is going up on the far boost converters voltmeter. So yes, they are linked together.

So let's put that back down until it clicks. One of them's very quiet and one of them is quite noisy. So let's try and balance the currents. Let's raise the Boost function the Boost voltage of the rear boost converter and as I said, it doesn't matter which boost converter I raise the voltage of you will see the voltage go up on that little voltmeter.

So raising the output of that one 12.8 Let's see when it starts to rise up there and they flipped over, the two ammeters have totally flipped. So now the rear boost converter is doing all the work and not the front one. So I'll wind that back a little bit and we'll see if I can balance them and the answer is no, I can't It just flips from one to the other And if you think about the way these things work, that kind of makes sense because the Boost converter that's putting out the higher voltage the other one is saying oh, the voltage on my output is higher than I actually want I'll back right off and go to Pwm 0 and so then it stops doing any work at all. So the situation here is that one boost converter does all the work, produces all the current and the other one produces almost nothing.

Let's raise up now the boosting voltage of the near boost converter and see if I can balance the currents. There's probably a bit of dead zone, right? Okay, it's flipped so zero on the front one. a slight tweak and it's gone up to 700 milliamps and nothing on the rear one can I find that midpoint? No. So I'm afraid it's one boost converter, all the other that does all the work.

However, if we keep going and I wind up the voltage on the rear boost converter, It's Tricky getting this thing to sit in the slot and get that one to do all the work. But I'm going to push it a bit further now and we'll go up to 18 volts. wind up the voltage on the near boost converter pushing that 18 volts a little bit higher. Now the front one does all the work.

Let's go up to 20 volts. Should better see the bulb is getting brighter. back to the rear one to see if I can get that one to share current and again I'll try and do current sharing, but it's very. Twitchy I mean I've kind of got it there, but it's extremely.

Twitchy you only have to turn this pot a few degrees and it literally jumps from one boost converter to the other. but I'll keep on going. And now I'm at 24 volts, which is the maximum. Well, these voltage.

These bulbs can go higher of course, because when the engine's running, it's more than 24 volts on a truck isn't it? And I've reached a point where I kind of can balance them it's a bit Twitchy but they are sort of able to be balanced and I believe. What's happened here is that these two boost converters have started to go into current regulation because they have a constant current part and it's set to the minimum the minimum current a couple of amps, maybe less than that, but they are starting to go into current limiting. And of course, when these devices switch from being voltage sources to current sources, then yes, they do share current. Okay, I'm going to wind these both right back down.

again. that one's clicking. You could probably even hear that one. it's pretty loud.

This one's much quieter because I'm wearing headphones I may not be able to. Yes, I can hear it clicking. And now I'm going to switch from the brake light bulb to the car headlight bulb with both its filaments in series so that it can take 24 volts. Okay, let's plug that in here.

and actually what's happened now is the Boost converters have sort of collapsed and you can see the little red lights have gone off because they can't cope with that load. Oh, the power bank's gone into short circuit mode as well. I Think what I have to do is raise this up to about to a high voltage. So let's go for about 18 volts and then I'll also raise this one up to 18 volts and I'll know where that is because it'll move the voltmeter up and then if I back off a bit I know that these are set roughly the same now.

Can I put this bulb in? Yes, and the power bank is allowing me to do that. And now, even at 17 volts, you can see that their current sharing already. and that's because they're both in current limit and after doing some tests yesterday, they're slightly different. These two I don't quite know why, but if I raise the current limit on this near one and you've got very fine control now, so I can get them very accurately balanced.

2.1 on the rear, 2.0 1.9 on the front. that's my meter's about to turn off. Well, let's tell it not to do that and I just want to come down slightly on this front one to 19 something. But there you can see that they're pretty much exactly balanced and so I can balance them.

There they are. They're absolutely bang on. So I can balance them using the current limiting pots. but when they're in voltage limiting mode, they simply flip from one to the other.

So once you put enough load on these boost converters such that you can control them with current limiting, then yes, you can get them to current share. Now what happens if I raise the Boost voltage? It's 17 volts at the moment. Uh, up to I Don't know. Of course it won't go up because it's current limiting.

So I can ask for whatever voltage I want on this part and it won't give me that voltage because it's now acting as a current source. This one has also now hit voltage limit. Uh, so if I slightly raise the current limit on the near one, I can rebalance them again. and so I think if you set the current limit 19 on the front, 21 on the rear, 20.5 About that, 20.6 Yeah, so that's got them balanced Again, if you set the current limits here such that you know you're not going to blow the input fuse.

If you've got 12 volts going in, 24 volts coming out and you set say a 5 amp current limit on the output, that gives you a 10 amp current limit on the input which these fuses should hold, then yes, you could, um, parallel these boost converters and get them to current share. Now if I wind down the load I can't do it because it's a light bulb. but if I put the electronic DC load on here which I plan to do later and wind this up and down when the load is very light, these may go into voltage limiting. In which case, one of these will do all the work.

but it doesn't really matter because it's not doing much work. It's not putting out much current, but once you start to draw more current on the output, then these will go into current limit and they will current share. And so yes, you can parallel up these boost converters well I Think that's it for this video. It kind of showed it quite well.

Cheerio.