Hello Can I charge a battery using this Buck converter? And if I try to do that, what are the implications? What do I need to look out for? Well, let's do a very quick test. This isn't the battery I'm going to charge. It's a much bigger battery. Um, but I Just wondered what would happen if I put a battery on the output of this Buck converter and there we go.

Battery on the output. Oh my. Connection's not very good. It seems to be feeding power back through the buck converter to the input where it's taken off and through a couple of regulators it Powers up this digital control module.

So the first thing I have to be aware of is that these Buck converters appear to back feed. Now the question is, why is that? So let's look at a simplified diagram of this Buck converter and it is a switch and an inductor. Like so and out and that's in. Now There's one other component which this needs to work and that is a diode whose anode goes down to ground.

And that's a simplified diagram of a buck converter. So on. here. The inductor is quite obvious.

It's a big chunky winding of enamel copper wire on a ferrite toroid. The switch is a five pin chip. We'll take a look at that in a moment. There's the diode.

It's got two connections there I think the cathode is on the tab now. The other large components are capacitors, so I'll add them into the drawing. There's one on the input here like so and there's one on the output like so there. but there's also a second one.

And there's this resistor here, which is um, 0.01 ohms. So it's a very low value resistor and that is in the negative line. So if I draw that resistor in, it's actually here. output negative is at this point here and I'll draw an output positive up there.

And then there's another capacitor which is on this side of the current sense resistor uh to provide the sort of best possible smoothing either side of that current sensor resistor because the voltage across there is measured by the microcontroller to provide current limiting. Now, there are also some smaller capacitors. Oh, it's down in there and they are simply across the larger capacitors. so there's one there.

There's one on the output here. These are 104. So 100 Nanofarads goes to there. and there's also a resistor.

and I Believe that's across this Um capacitor because this is such a tiny resistance Here, it's effectively across this capacitor. That's the resistor up there. It's a sort of half watt resistor and that just makes sure that these output capacitors discharge within a reasonable space of time so that you don't have a high voltage on there for a long period. So this resistor is a 2K7 This as I say is a 0.01 Ohms.

Uh, the big capacitors are 470 Microfarads. These are 100 n Now, the diode and the switching element I can get numbers off these devices. The inductor I can't tell you the value of that inductor I don't really know it. It's going to be micro Henry's I would imagine or possibly Millie Henry's Can we see the markings on the diode there? Yes, I believe it's an MBR 1060 and the marking on this device.

Well, it's an Excel Semi or it's not very easy to see. Is it? Oh okay, it's an XL 4012. So those are all the major components or the large components that we can see up in this part. Now if I take this controller board off, we can actually see a lot of components down here as well.

lots of resistors and capacitors. This little chip here eight pin is another Buck converter and the idea of that is it just provides a fixed voltage mainly for the microcontroller. the electronics part here but that is also rooted up here and it says oh, can we see that it says Dc5 volts and my belief is that that is for normally a fan that you might Mount If this would put in a case, you might put a fan on the side of the case that cools the major components that need cooling. The heatsinks here are a giveaway that these might get warm.

the inductor often gets warm. Um, the other giveaway as to the voltage of this little Buck regulator is this capacitor. You've got the bug regulator, an inductor, and a capacitor with a small capacitor as well. and this one says 6.3 volts.

That's a fair guess that this is 5 volts. We've got five volts as I say up here. So my guess is that this is, uh, fixed at 5 volts to generate power for this thing which itself has a 3.3 volt regulator on it. So it takes the 5 volts down to 3.3 for the microcontroller which you see a little St microcontroller and for these two CMOS Latch Chips hc595 Now I'm not going to go into any detail on the microcontroller circuitry or indeed this circuitry down here.

I Believe these two are dual Op amps and they're going to be for measuring the output current, measuring the output voltage, and driving the feedback pin of this chip. but I do think we need to look at this XL 4012 in order to see what's going on with what. I've just marked here as a switch. Well, this is far more than a switch.

This is a buck converter integrated circuit. Now just while I print out the data sheet for the XL 4012. I'm just going to attach this uh, small 12 volt bulb. this came out of a slot machine to the input of this Buck converter so that we can see this business of the power from this battery back feeding to the input.

I'm also going to remove the microcontroller thing so that that isn't part of the issue. Let's once again see if power goes backwards through this Buck converter. And it does. Because you can see this small lamp, it's a one watt lamp.

I Think light up. So yes, power is getting back through this circuit even without the microcontroller connected. so it's not involved in doing any switching. So how is power getting from this battery back to the input? Let's get this data sheet.

So here are parts of the data sheet for the XL Semi XL 4012 It's 300 Kilohertz fixed frequency Pwm Buck Step down DC to DC converter capable of driving a 12 amp load. Um, Okay, let's take a look at page three, which I've printed out and here's the internal functional block diagram of that chip. Here is an application circuit and it's very similar to what I've drawn here. We've got the switch element here: a diode whose anode is connected to ground cathode to this Central Point an inductor same as I've got an inductor output, capacitor, input capacitors.

Now there's a feedback circuit here, which is just a potential divider. Going back to the feedback pin of course. I Haven't drawn that in on my circuit because the feedback pin is unique to this type of buck converter. But let's take a look at why power is getting from my battery back through the buck converter to the input.

So here's the switched output and here's the input and power is getting from the switched output back through here to the input. Now they've shown this as a couple of. well it just says here switch But it looks to me like these are mosfets. but it doesn't say much about them.

So here's another printout page. three of a slightly different device. This is the XL 4016. It is very similar.

This one is rated for eight amps and has a 180 kilohertz uh oscillator. But what it does say in here is power. Pmos. So these are Metal Oxide semiconductors.

Uh, Mosfets Field Effect Transistors. Um, Pmos. So they're P-channel mosfets and that gives a clue as to why we're able to pass power from the output of these mosfets back to the input. So with a P-channel mosfet and I'll draw it in the same orientation as this.

What we have is a device like this. This is the source and this is the drain. This, of course is the gate on the input here and the diagram has an arrow here connected to there. This is down to the drain.

That's the source. But there's one additional part to a mosfet which sometimes is not drawn, sometimes is. And in the case of the p-channel mosfet, it is a diode like so and so we can see that there is a path from drain which would be the output of this chip back through this. What's called the body diode.

It's uh, not an intentional diode. It's a diode that simply exists because of the makeup of a mosfet. But yes, there is a path back from the output to the input through this body diode. So that's what I believe is creating a path from the output of this Buck converter back to the input so that I can light that bulb now I do believe Actually that um, when this microcontroller unit is not on here, the mosfet might be turned on.

But if I put this back on here, then the mosfet definitely is not turned on because the microcontroller has it switched off I can turn it on by pressing that switch. All that we see then is the voltage of the battery. But if I turn it off, then the mosfet is Switched Off But the body Diode part. This little diode here cannot ever be removed.

It's an intrinsic part of a Uh, P Channel monster and indeed an end. Channel Moss It also has a body diode and that I believe is why we're getting power from the output back to the input to power up this microcontroller and light to this small bulb. So back to me: wanting to charge a battery from a battery. Um, yes, I believe it's possible to do it I can put a battery on the output of the buck converter.

but I just need to be aware that with that battery on there and always charged, you'd never have a battery go down to zero volts. There's always going to be a potential across the input here and if that accidentally gets shorted then all the current is going to go through the body diode of this of the mosfets. The Pmos switches in here and they're likely to go bang so it. I'm sure it would be possible, but you just have to be careful not to short the input of the buck converter.

So if you liked this video about this, Oddity of this Buck converter. Oh well, obviously. uh, press the like button. but I can do another video going into more depth into the circuitry of this thing including the the other rig regulator and here, the other butt converter, step down regulator, the five volt one and how the Op amps work and also look at some other anomalies of this thing.

But for this video, that's it for now. So cheerio.