Boost converter for 100w led - Page 1
Forum section: electronics / beginners

author: RyanG
author_rank: Regular Contributor
timestamp: October 07, 2017, 07:47:38 pm
content: I'm trying to build a boost converter for some 100w less that I have. Obviously, using an smps controller ic is probably the best route, but I wanted to try to understand the general workings of switch mode power before I build something. Does the schematic I attached look like it would work? I know it probably isn't the best way to do things, and it lake OVP and stone other fairly important things. I'm just wondering if this basic idea, the way I set it up look alright. The 10 ohm resistor takes the place of the led in the schematic.

author: Benta
author_rank: Super Contributor
timestamp: October 07, 2017, 07:57:52 pm
content: Well, it's based on a 555, so I'm almost certain it will work perfectly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 07, 2017, 08:14:46 pm
content: Basic idea, more or less, but what happens if the load is shorted?Sure, it's an LED, pretty reliable, but who knows, eh?More importantly though, what about during startup, when that big honkin' 3mF looks like a short circuit?Or if the supply browns out, same sort of idea. Well, Vref to the error amp (well, it's drawn a bit odd, but that seems to be its function -- let me know if it's something else) varies with supply, but it might be worthwhile to put a stable reference in there so the light isn't dimming with low supply voltage; and then you'd have that concern, so it's something to think about if you want that as a feature.The fundamental problem is that the inductor inducts, but there's no way to sense that it's inducting. It stores current, but there's no current sense resistor or whatever to tell how much!But you don't want to sense current, do you? You want constant voltage output!Well, you need to make sure the switch will be okay. Exceed its current rating, and in a few hundred microseconds, it's toast. A few hundredmilliseconds later and the fuse blows. That's the direct consequence of missing inductor current.So say you regulate inductor current first. Great, now you get a constant current supply. But not actually, because that's input current, not load current.For an LED, that might be just fine, actually. But on the other hand, you get the inverse problem: what if the LED goes open circuit? You'll blow up the capacitor and transistor from excess voltage!Clearly, a solution is needed, that tracks the inductor current, and the output voltage and current.For lighting up LEDs, one of the two (voltage or current) doesn't need to be particularly accurate, but we need to control the other. (This is especially important for general purpose power supplies, where voltage regulation is usually priority. Or if you're making a bench supply, you'll want to regulate both!)Suppose we control inductor current, then we add another error amp that controls the variable we want (output voltage or current). Huh, well, that'd work, wouldn't it?And we can add a simple tweak to that error amp, to deal with the other variable. Say we're doing a constant current output, so the error amplifier is reading the output current through a shunt resistor. A zener diode, from output voltage to current sense node, will do a real cheezy voltage clamp, with just one or two parts. Nice and simple!To do that, you'd end up with a circuit much like this:https://www.eevblog.com/forum/projects/building-a-simple-switching-circuit/msg1252706/#msg1252706(Which, ironically enough, I created for powering LEDs.)Inverted the image for readability:Tim

author: RyanG
author_rank: Regular Contributor
timestamp: October 07, 2017, 10:13:36 pm
content: Thank you for that explanation. As far as it being based on a 555, that was just to get the idea of how it works. I don't like to build something up with an IC that has all the functionality built in and not understand how it works. If/When I build this circuit for real I will be using a SMPS controller IC, which I will have no idea how to use, but that's besides the point. I think it's better to understand how things work even inside of an IC than to just throw things together with a cookie cutter schematic from the internet and learn nothing. My power supply for the circuit would be an atx power supply