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I have designed a board that uses the MIC2251 (SOT-23 package) boost converter to boost 5v from a USB charger to 10v, to power some 2 series LiPo chargers.

http://docs-europe.electrocomponents.com/webdocs/1065/0900766b81065118.pdf


(source: think-engineer.com)

I am experiencing a serious overheating issue, when a single battery is connected the boost converter should be supplying 10v @ 40mA (These are button cell LiPo's). The boost converter and the diode get very hot.

When hooked up to a bench supply the boost converter was drawing an expected current of 10's of mA's, but it got very hot, and after ~5 seconds, the current draw jumped to ~250mA and the boost converter cut out for a second, then this cycle repeated... until the boost converter died. By died, I mean that there now seems to be a short between VIN and GND.

I am using a 10uH inductor, rated at 2.2A, 58mOhm: http://docs-europe.electrocomponents.com/webdocs/0f90/0900766b80f90d1a.pdf

Both capacitors are 10uF 0805 16v.

R1 is 100K, and R2 is 14K

Diode is S1JB-13-F Switching Diode, 1A 600V: http://docs-europe.electrocomponents.com/webdocs/104b/0900766b8104bc04.pdf

I did not put much in the way of thermal relief on the PCB, as I assumed that drawing only ~120mA would not be to strenuous for a boost converter rated at 2A. I have just tried adding a small heat sink to the boost converter and the diode, and I am experiencing the same problem.

Have I done something wrong regarding component selection?

Please can someone explain why the components are getting so hot, and what design considerations would effect the temperature, and how I can get this module to run stably and at a happy temperature.

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  • \$\begingroup\$ Can you add a diagram that shows what is on the load side, ie., downstream from the 10V output. And what it's load is? How many mA DC, and are there transients? \$\endgroup\$
    – dbrwn
    Aug 7 '15 at 14:40
  • \$\begingroup\$ I have tried multiple loads, and I get the same result. The intended load is three LiPo chargers based on MCP73844-840I/MS. When I power these with 10v from the bench supply, they work fine and draw the expected current. I have tried powering an arduino with this converter circuit and I get the problem \$\endgroup\$
    – Steve
    Aug 7 '15 at 14:43
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The S1JB-13-F diode has a pretty slow reverse recovery time (trr is 1.8us typ, 3us max). The boost switcher appears to be variable frequency, but at some loads it's switching frequency may be uncomfortably close (or right on top of) this reverse recovery time.

The MIC2251 data sheet recommends a Schottky diode but points out a fast switching diode can be used (their recommendation is the LS4148, whose trr is less than 8ns).

See this question for more on diode reverse recovery time.

A boost converter works by grounding the SW node during the 'on' time, which charges up the inductor. When the converter switches the internal FET off, the current flowing through the inductor needs to go somewhere, and the diode is preferred. So the voltage rises until the SW node is a diode drop above the desired output voltage. When the inductor has dumped its energy into the output cap/load, the diode needs to turn completely off. While it's turning off, you will get current flowing in the reverse direction, and depending on how long that recovery time is. The longer it is, the more time there is for current to flow backward and cause heating. In the worst case, if the switching frequency lines up badly, you can have the FET turning on again before the diode has recovered. I suspect that is what was happening in your case, since the converter was also exhibiting some temp rise and eventual failure.

You can see a couple examples of the operating frequency and switching waveforms in the graphs on Page 5 of the MIC2251 data sheet. And there is a graph of operating frequency vs load current on Page 4.

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  • \$\begingroup\$ Thanks Dave, as I've got the PCB's now I'll try to find a suitable diode that is the same size. Can you explain what happens in the circuit if the reverse recovery time is too slow? I could not see anything in the MIC2551 datasheet about what effects the switching frequency, do you have any idea of typical switching frequency range?? Will ES2G-13-F do the job, Trr of 35ns rated for 2A and the same package size? How confident are you that this is the cause of the problem, as I really want to get these working for a demo next week, so I'd like to have an idea of all possible problems Cheers \$\endgroup\$
    – Steve
    Aug 7 '15 at 16:17
  • \$\begingroup\$ I've just dug out an old COTS boost converter module and reflowed the schottky diode from that to my custom board, and the problem is solved. Thanks very much for the solution, I've got some replacement diodes on there way, and I should be good for my demo next week. Thanks again! \$\endgroup\$
    – Steve
    Aug 7 '15 at 17:35
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    \$\begingroup\$ Glad to hear it's fixed, nice job scavenging the needed components. This is why we keep piles of dead/old electronics around...Or at least that's what I tell myself! \$\endgroup\$
    – dbrwn
    Aug 7 '15 at 21:51
  • \$\begingroup\$ Expanded my answer to include an explanation of why, and pointer to the operating frequency info (which Micrel doesn't specify in a parameter table, just characteristic graphs). \$\endgroup\$
    – dbrwn
    Aug 8 '15 at 15:30

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