LM2576-5 buck converter overheating advice. - Page 1
Forum section: electronics / projects

author: DanEdwards
author_rank: Contributor
timestamp: February 06, 2021, 12:14:57 pm
content: HiI've currently got a PCB that has a buck converter circuit taking 11-18VIN down to 5VOUT using the LM2576-5.I've followed the datasheet schematic on driving 3A.Running low loads, under 500mA the IC stays slightly warm, but when I start demanding some more power, max 1.5A the inductor starts to whine and the chip gets so hot so quick that it thermally cuts out.I have attached pictures of the PCB and my schematic.My parts are genuine from Mouser.Am I or have I done something wrong? or is there anything else I can do?

author: jmelson
author_rank: Super Contributor
timestamp: February 06, 2021, 03:44:47 pm
content: Does your inductor saturate at 3+ amps? I'll bet that is the issue. I'm using an LM2575 up to 500 mA or so, and my inductor is probably 3X the size of yours.Jon

author: langwadt
author_rank: Super Contributor
timestamp: February 06, 2021, 04:02:41 pm
content: is that the actual schematic? the diodes is in the wrong place

author: temperance
author_rank: Super Contributor
timestamp: February 06, 2021, 04:05:14 pm
content: The inductor in question can handle 1.96Arms. Temperature rise Dt will be 40°C.The LM2576 uses an emitter follower with a saturation voltage between 1.4 and 2V. With 12V at the input, the duty cycle for a 5V output will be around 41%.With a 1.5A load, neglecting switching losses:P-LM2576= Vsat x I x DP-LM2576= 2V x 1.5 x 0.41P-LM2576= 1.23WP-diode= Vf x I x (1-D)P-diode= 0.4V x 1.5A x 0.59P-diode= 0.34WP-inductor= I² x RP-inductor= 1.5A x 0.163P-inductor= 0.35WP-total= 1.92WWith the PCB you have and taking into account the heat dissipated in the inductor and the catch diode, your PCB might become 80...110°C with Ta being 25°C.There are much better regulators available with MOSFET's. Even synchronous ones for which you don't need a catch diode able can handle 2.5A on a few square cm of board space without getting too warm.And as someone else noted: the diode is in the wrong place. Also, you don't have proper HF decoupling. You will need some 100nF capacitors on the in- and output.

author: KT88
author_rank: Frequent Contributor
timestamp: February 06, 2021, 04:27:43 pm
content: The wrong placement of the diode is the culprit here. You are using the internal ESD diodes instead of the external shottky. It has very likely more Vf than an external schottky and it is not designed for that purpose. It might work kinda...but the part will fail very early.In general I won't use a non-synchronous buck for any RF circuits at all: their potential for interference is much higher than with a synchronous version. Reason is the larger loop area of loop between high-side FET and diode. At a synchronous buck the transient currents stay mostly within the chip. Only exception is the input capacitor, which has to be a ceramic type that has to be placed as close as possible to the part (shortest possible traces)CheersAndreas

author: DanEdwards
author_rank: Contributor
timestamp: February 06, 2021, 05:23:17 pm
content: Welp. Thanks, everyone.Certainly was the diode. Just swapped it over to the correct place and we're looking much better now.Clearly, the lesson here is don't make schematics in a hurry and double-check everything...Quote from: temperance on February 06, 2021, 04:05:14 pmCould you recommend any such regulators?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2021, 05:36:41 pm
content: Quote from: Daned33 on February 06, 2021, 05:23:17 pmOpen Digikey or Mouser parametric search,Look for dc/dc regulators (not just controllers),Look atsynchronousconverters, this means diode Vf loss is removed by using a mosfet instead (at 5Vout, this is already quite significant). This also means, in case of an integrated regulator, that you get rid of the external diode, but OTOH usually need to add a small external bootstrap capacitor, but it can be a cheap and tiny 100nF 0603 part for example.High frequency, low cost, and high availability numbers are good indicators of a suitable, modern, well tested design. Also the example circuit showing only MLCCs being used.High frequency converter can work with MLCC only. I recommend, though, an additional parallel electrolytic capacitor on the input for damping any hotplug ringing. Assuming you have enough MLCC to manage the bulk of the ripple current, the exact ratings of that elcap become non-critical.LM2576 is indeed ANCIENT.

author: temperance
author_rank: Super Contributor
timestamp: February 06, 2021, 10:21:55 pm
content: AP63200, AP63300, TPS562207, BD9D321EFJ,...Inductor example: NRS6028T100MMGKVAll available on mouser.