LM5117 Overheating - Page 1
Forum section: electronics / projects

author: mattko
author_rank: Contributor
timestamp: August 04, 2018, 01:43:17 pm
content: Hello!I'm creating a Buck DC-DC converter with LM5117. 48 V in, 14 V out.At highest output current 7 A @ 14 V it starts overheating (after 10-20s) and overheat protection steps in, so it starts buzzing as it turns off for a period of time.I attached a quite big heatsink to it with thermal plaster, and it was better, but after a few minutes the chip fried. (heatsink is on the picture)I made a mistake of not adding a lot of vias under the chip. But adding a heatsink should fix the problem.Maybe there is a mistake in the schematic that I'm missing or do you have other suggestions?I attached a few photos. Some components have been changed - like snubber resistor and capacitor value.Thanks!

author: iMo
author_rank: Super Contributor
timestamp: August 04, 2018, 02:02:38 pm
content: When all is ok with your schematics you still have to dissipate aprox 3W of heat (97% eff), right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2018, 07:00:30 am
content: 3W? Geez, that's going to be marginal even with a large thermal pour on the pad, no?Plastic is a poor conductor. Afraid you've screwed the pooch here.If you can patch in a VDD supply (48 to 12V, 10s of mA) so the poor chip isn't dropping its supply through all that voltage, you'll probably be fine. Good idea anyway for efficiency's sake (or power it from the actual output, using diode logic?).Tim

author: iMo
author_rank: Super Contributor
timestamp: August 05, 2018, 08:49:06 am
content: The 3W heat is with an assumption the efficiency is 97%. That is almost an ideal situation with that design. In case the OP is using some lower quality components he can easily achieve 90%, that would be 10W heat (based on my rough estimate, the OP has to provide some measurements/calculations when applicable).

author: Siwastaja
author_rank: Super Contributor
timestamp: August 05, 2018, 09:05:28 am
content: Layout with the MOSFETs? Most of the heat is coming from:- The mosfets- The inductor- Possibly input capacitorsWhat's the dissipation in LM5117, or is the problem external heat, with the small amount of internal dissipation driving it over the edge?In this case, heatsink the components causing the heat, i.e., the MOSFETs. Not through the plastic; through thermal vias in PCB.

author: prasimix
author_rank: Unknown
timestamp: August 05, 2018, 09:09:48 am
content: Someone could easily assume that mentioned dissipation (3 W, 10 W or whatever it is in reality) belongs to LM5117 alone, not switching MOSFETs and power inductor. It seems that OP added heatsink only on LM5117 that is probably fine (but strange practice, since manufacturer didn't randomly choose certain package) if its going too hot. But if that is a case the first thing that come to my mind as possible cause of controller overheating is inappropriate selection of MOSFETs i.e. one that has e.g. enormous input capacitance for controller output driver(s). Selecting suitable MOSFET is not a simple task, therefore e.g. FOM (Figure of Merit) are introduced to simplify selection.Anyway whatever you manage to do with controller and MOSFETs after some level you should start to thinking about additional cooling with small fan and/or thermal pad to beneath metal chassis, heatsink, etc.

author: mattko
author_rank: Contributor
timestamp: August 05, 2018, 10:09:59 am
content: Thanks for your answers.Power loss of the circuit is about 10W, however the most power is dissipated on the MOSFETs (both are screwed in a big heatsink under the circuit). Any ideas how to lower power loss? I tried diferent MOSFETs with different Rds(on) (lower conduction losses) and Coss (lower switching losses), but the power loss stays about the same.High side MOSFET: CSD19505KCS - low Rds(on)Low side MOSFET: IRLI520NPbF - low CossHigh side MOSFET has high input capacitance, that could be the source of the problem, however I doubt it.I'm thinking to try another High side MOSFET and I could order another PCB with additional vias under lm5117 and big copper area to dissipate heat.

author: prasimix
author_rank: Unknown
timestamp: August 05, 2018, 10:18:49 am
content: Good, so you are aware of the issue with MOSFETs. Few more (or couple of tens) of miliohms for Rds, onon the MOSFET side that is already on the heatsink doesn't make a big difference for its heating, but could be a real relief for controller's driver.

author: Circlotron
author_rank: Super Contributor
timestamp: August 05, 2018, 10:23:05 am
content: Is the switching frequency needlessly high?Can you dial it down a bit and if necessary use a higher value inductor?

author: mattko
author_rank: Contributor
timestamp: August 05, 2018, 10:55:05 am
content: Frequency is 400 kHz. My calculations were based on this assumption. But it is a good idea, I will keep it in mind.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2018, 12:13:07 pm
content: The high side has ~25% duty cycle, why would you put the low-Rds(on) there...Maybe that was a typo.My experience with LM25117 (the dual version, AFAIK) is the efficiency is simply poor no matter what you do, at higher frequencies. It's capable of up to, whatever it is, just under a MHz, but the timing is awful: dead time is whopping large, and not guaranteed well enough to make up for it with gate resistors.The dev kit runs at ~100kHz IIRC, claiming okay efficiency. But you need huge caps and inductors for that, obviously.Tim

author: prasimix
author_rank: Unknown
timestamp: August 05, 2018, 12:35:18 pm
content: Quote from: T3sl4co1l on August 05, 2018, 12:13:07 pmNo it is not dual, version with prefix 2 is low Vin, maxversion (45 instead of 65 V).

author: mattko
author_rank: Contributor
timestamp: August 05, 2018, 12:51:26 pm
content: Quote from: T3sl4co1l on August 05, 2018, 12:13:07 pmI started with CSD19505KCS for high and low side. But I encountered oscilations in SW node. Based on this:http://www.ti.com/lit/an/slyt682/slyt682.pdfSearch: Equivalent RLC circuit after Qturns ON. For this reason I had to lower Coss of low side MOSFET. And as far as I understand from my experience, if you choose a MOSFET with lower Coss, then Rds(on) goes up. (correct me if I'm wrong please).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2018, 01:29:12 pm
content: Oh, 25119* I used.Yes, Coss and Rds(on) are complementary, simply put: more or less die area at work.Ringing is a sign of trying to go too fast for the layout. As Lloop can still be significant, even down at 3nH or so (which you are unlikely to improve, even with DFNs and multilayer PCB), your only other option is to reduce switching speed, or to actually deal with it (damping it, or reducing dV/dt or dI/dt with snubbers).In my circuit, Lloop was approximately 5nH, at somewhat lower current (higher total output current, but less switch current because the two phases are wired in parallel). For the parts I chose, I got less than 10% overshoot and no significant ringing.You can reduce switching transients by managing the gate waveforms just right. Again, this is dubious given the poor timing tolerances on the controller. It may not be something you want to do for production, but it is fine for short runs at least. In particular, since the buck output current is positive, you want the low side to turn off slowly (to prevent forward-biasing its body diode and incurring recovery). Low side turn on, and high side turn on or off, don't really need to be managed, but do evaluate these for treating EMC, if needed.Consider improving your layout. That TO-220 alone has twice the lead inductance of my layout, plus whatever else is around it. That's a big red flag for running at higher frequencies.Your overall power dissipation is already within the reach of DPAK, and also PDSO-8 and PSON-8 packages, if well heatsinked. The further savings from lower Lloop, and somewhat lower Fsw, should cover what's left.Tim

author: chris_leyson
author_rank: Super Contributor
timestamp: August 05, 2018, 02:24:58 pm
content: QuoteI had to re-design someones power supply, TPS40061 48V to 15V followed by TPS54550 15V to 5V. 15V output didn't need to supply much current but the 5V output was maybe 1.5A, not a lot of power really. It turned out to 65% efficient overall because the designer had used a 50A mosfet on the TPS40061 highside and the driver stage had to supply nearly 3W of drive power. On a warm day the TPS40061 would cook itself and fail. I replaced the design with a 48V to 5V "flybuck" and few extra windings to generate 15V, plus a foldback current limit on the auxiliary 15V output because flybucks are not short circuit proof. Got 80% efficiency, 10C cooler case temperature and no more customer returns. The moral of the story is low Rdson doesn't always mean better efficiency.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 05, 2018, 02:45:35 pm
content: Also, remember that the MOSFETs in the synch buck don't need to be symmetrical, and often are not. When going from high to low voltage so that the conversion ratio is far below 50%, you want to optimize low switching losses (Qg and switching speed) for the top MOSFET, and low Rds(on) for the lower MOSFET. Sometimes you see, for BOM reduction, a single MOSFET on top and two paralled on the bottom.Attached is how I heatsink SMD power parts (part of a 500W 2-phase synchronous buck). I add a bunch of 0.3mm or 0.4mm thermal vias just next to the part, as close as possible, while keeping a thin solder mask sliver. I add paste mask openings to these thermal vias as well (when not under a component), to get some solder flow into them (not expecting much improvement, but maybe 10-20%, and it comes for almost free). Note that here, the solder for the component does not flow into the vias. I have used vias directly below the component in the past with no ill effects, but it risks solder voids below the component. If you do that, the via size should be 0.25mm or 0.3mm maximum.Then, I add fills on the bottom side to spread the heat from the vias a bit - no need to go too far, the sideways heat conduction is not that great, especially with 1oz copper. Then mount the board with thermal pad material to heatsink. That's why the mounting holes.Note the same style for the inductor, as well. As you can see, the switch node is heatsinked this way, as well; this poses a risk of increasing EMI if the heatsink fills are unnecessarily large.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 05, 2018, 02:55:42 pm
content: Thanks Siwastaja, that's a nice layout, I like the idea of pasting over the thermal vias, it's free and it gets more heat through the board.

author: mattko
author_rank: Contributor
timestamp: August 07, 2018, 10:43:53 am
content: Update!I figured out that I did not solder LM5117 properlyThe bottom exposed pad was not soldered properly, so heat did not travel into the PCB. I changed that and now it is not overheating any more.Now I have to focus on other improvements such as:-Spread spectrum (dithering) - adding another 10x bigger resistor to RT pin and have a triangle 1,2 V signal, that the chip sees different resistances over different times.-Shottky diode over low side MOSFET is very hot.-Output and input voltage has a lot of swithing noise. I have to have good EMCIf anybody has any recommedations (maybe good application notes) for these topics I would appretiate a lot.Thanks!