Inverting boost converter getting very hot - Page 1
Forum section: electronics / projects

author: fabiodl
author_rank: Frequent Contributor
timestamp: December 30, 2021, 09:36:57 am
content: I am trying to obtain -25V from 5V. I am using an MC34063. It works, but it gets very hot. Here is my circuit part choice.Am I doing something wrong?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 30, 2021, 09:52:33 am
content: This topology is called buck-boost, also inverting buck-boost, but usually not "inverting boost".Show the layout.Part number of the inductor?This part is truly ancient with extremely low f_sw, bipolar power switches (!). How hot is "very hot"? Any approximation? If you add a drop of water on it, how quickly it boils off?

author: fabiodl
author_rank: Frequent Contributor
timestamp: December 30, 2021, 09:58:45 am
content: Thanks for the comments. Here is the layout. The ground plane is not visible in the first image, it is in the second.Very hot as in 60C to the touch. U3 is an lm358, currently still unsoldered.This is the inductorWould a CS5172 be better? do you have any suggestion?

author: ajb
author_rank: Super Contributor
timestamp: December 30, 2021, 07:56:18 pm
content: In this topology, the input voltage the device sees is the difference between the positive supply and the negative output, so 30V in this case. Figure 8 of the datasheet shows that at 30V, the device consumes 3.3mAin standby, so that's 100mW right there, which with an RθJAof 160°C/W accounts for 16°C of rise above ambient while the chip is standing still. If you're seeing 60C on the chip with a reasonable ambient temperature (and presumably the immediate environment around the board is a little higher anyway), that accounts for almost half of your temperature rise right there.In the configuration you're using, the output switch saturates at something like 1V, so that's going to be a hit to your efficiency as well. In fact if you look at Figure 13 which shows this specific configuration, the efficiency is given as only 62.2% at a load of 100mA! That's 730mW of heat with 1.2W supplied to the load. If all of that were dissipated by the IC (some of it will be dissipated in the diode, the inductor, and Rsc) you'd expect a temperature rise of something like 116°C.That IC is just going to get some level of toasty no matter what you do.

author: nvmR
author_rank: Regular Contributor
timestamp: December 30, 2021, 08:42:45 pm
content: A very small heatsink may be able to fit over the device to take away some of the heat. You can buy them in bulk (say a 8.8*8.8*5mm ) and saw them in half with a hacksaw due to the space constraints with those capacitors. This is easier than a respin, depending on how many of them you need.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2021, 09:51:44 pm
content: Yes, a CS5171 or family will do fine. You need Cuk topology to use the inverting type (5172), which is perfectly reasonable with the range of small dual inductors available these days. Flyback is also a good idea, with a transformer of some other ratio, so the switching duty cycle can be closer to 50%.The one thing I don't like about the family is the terrific nonlinearity in the error amp; when it's stable, it's fine, but if it's unstable, it's very quickly chaotic, and no sense can be made of its compensation, you just have to keep trying values until it behaves. Start with the recommended/calculated values, and go from there. Varying values (while searching for best stability) say in steps of +100/-50% is fine, compensation is fairly inexact.Tim

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 30, 2021, 09:51:54 pm
content: Even being old technlogy, shouldn't happen, 3mA at 25V is nothing.I've used this ic in the past, got warm with high currents, but this circuit is almost floating.

author: fabiodl
author_rank: Frequent Contributor
timestamp: January 01, 2022, 07:23:02 am
content: Quote from: T3sl4co1l on December 30, 2021, 09:51:44 pmThis is something I worry about! I am using this circuit to power some magic eye tubes, and I do not want to damage them for any reason. Do you have any suggestion on more stable ICs?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2022, 08:49:10 am
content: I wouldn't worry about that; how much voltage would you think it would run away to? The semiconductors are far more vulnerable; just use 400V diodes, they'll blow up long before anything happens to the tube!Typical unstable behavior BTW for any control loop, is hunting, oscillating, or generally chaotically orbiting the quiescent point. It just can't find it and settle down, so to speak. How far it's jumping away from that point, does depend on the severity, but it will also be pretty obvious (on the scope, say -- you'll want a scope to debug these things) how far. Even just within a few cycles; you don't need it powered very long to figure that out (can avoid failures due to overheating, by just not letting it run long enough to overheat). And, again, it's more likely to result in destruction for the supply than the load. And you're testing with a dumb load resistor at that, right? So who cares anyway?And if you're having issues with a full scale design (scaled in terms of voltage, current, power, whatever), it pays to gain experience doing a smaller, simpler build, then scaling everything accordingly. For example, instead of targetting 250V output with a 1:10 transformer, how about 25V output with a 1:1? Dial that in first; use a current-limited supply so it doesn't choke itself to death, use a controller plus a somewhat overrated transistor so it doesn't burn up, etc. Then multiply the secondary turns by 10, and divide the secondary stuff by 100 (impedance ratio is turns ratio squared!). So, output filter cap --> 1/100th, load resistance and feedback divider --> 100x. The frequency response will be identical, the controller won't know the difference! And then make small adjustments from there, like accounting for the higher ESR of small HV electrolytics, or conversely the lower ESR of film caps, whatever the case may be; etc.Power scaling is a bit different to work with, as parasitic effects are exacerbated at high power: the circuit generally needs to be larger, while higher currents are switched, leading to higher transient voltages on switching edges; you may need to improve the layout, apply snubbers, reduce switching speed, etc. It helps to test at full current but reduced input voltage (so the peak voltages still land within ratings), full voltage but reduced current (make sure it handles the voltage, and stray capacitance isn't mucking things up), then finally testing at full power. (Not that power scaling should be important here, this is just a couple watts if that -- should be very easy. Reasonable even on a solderless breadboard, but do mind to use short length jumpers to avoid stray inductances.)Tim