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I am trying to assemble a boost converter using the LM27313 (datasheet link), following the reference design and calculations given in the data sheet. The load is 6 white LEDs (Vf=2.95V) in series.

Each assembly will drive the load once or twice, but after that, it will not work again. After replacing the chip, I noticed a faint 'click' from the inductor when I applied power, but Vout would never rise above Vin. Upon replacing the chip for a third attempt, the assembly delivers a 10-20 ms pulse of light immediately after power is applied, after which Vout decays back to Vin minus the diode's forward voltage drop. At no point was the chip ever too hot to touch.

What am I doing that's killing these ICs this fast?

I have checked the power supply: it is capable of supplying the required current, it does not overshoot when turned on, and delivers (to my inexperienced eye) ripple-free 12V DC. The load draws ~150mA when connected to a bench power supply.

schematic

(Schematic download also on mega.nz)

Rb and D2-D7 are not on the same board as the other components, but are connected with a few inches of hookup wire. Ground is connected from J2 to J3 on the bottom layer.

PCB layout:

PCB layout

(PCB layout download also on mega.nz)

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    \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Commented Feb 7 at 3:42
  • \$\begingroup\$ What inductor are you using? Are you sure the Vin is not higher than 14V and Sw does not go above 30V even for a little time (peak)? \$\endgroup\$ Commented Feb 7 at 4:15
  • \$\begingroup\$ Inductor is a Pulse Electronics PA4335.103NLT. I'm certain that Vin is not more than 12V. I am not certain of the voltage at SW: I did not see any requirement in the data sheet to control it other than by selection of suitable feedback resistors. \$\endgroup\$ Commented Feb 7 at 4:50
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    \$\begingroup\$ Are you hot-plugging power to the device? Standard problem, inrush overshoot. Put a >10uF electrolytic in parallel with C1. Let us know if that fixes it. \$\endgroup\$ Commented Feb 7 at 6:04
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    \$\begingroup\$ It seems that hot-plugging power to the converter was a problem. I measured spikes up to 24V (from the 12V supply). Using the switching power supply, I tried adding electrolytic capacitors in parallel with the converter input, ranging from 1uF to 1000uF, but none mitigated the spike. A series resistor of 4.3Ω helped, but is hardly an efficient solution. Given that available power supplies are likely to be of poor quality, what's the best way of absorbing this transient? Now I'm powering the converter with a regulated-output linear supply which I connect to the converter before turning on. \$\endgroup\$ Commented Feb 8 at 4:22

3 Answers 3

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There is nothing obviously wrong with the schematic or the layout.

Failure without overheating points towards excessive voltage or current on some semiconductor or capacitor. Because replacing the chip didn't help, I would measure input and output capacitors, the inductor and the diode when the chip is removed.

It's also possible that your power supply is delivering voltage spikes when being plugged in. Some cheap DC supplies have bad regulation that spikes up the voltage when hot-plugged into ceramic capacitors.

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  • \$\begingroup\$ That appears to have been the problem. I measured the voltage at the converter's input pins as I connected the power supply (which was itself powered on) and observed spikes up to 24V. \$\endgroup\$ Commented Feb 8 at 4:11
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Power Inductor at 1.6MHz is challenge. D1 has to conduct after switch off as soon as possible, otherwise a huge voltage spike at SW pin is developed. The inductor leakage makes it worst.

Check if D1 is a high speed diode.

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  • \$\begingroup\$ Diode is a On Semiconductor SS26FL (datasheet: mouser.com/datasheet/2/308/1/SS26FL_D-2320248.pdf) I had these laying around, and assumed they would work. \$\endgroup\$ Commented Feb 7 at 6:45
  • \$\begingroup\$ Hmm, seems “super-fast” to me. Do you have a scope to measure SW pin voltage? If there is an issue, would try to place series RC snubber accross inductor (like 100 ohm, 220p). \$\endgroup\$ Commented Feb 7 at 7:24
  • \$\begingroup\$ I couldn't find much switching speed information in the datasheets of either SS26FL or the recommended MBR0520. What should I be looking for? \$\endgroup\$ Commented Feb 8 at 4:24
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Have you set the output voltage correctly? You need to measure V_FB first in order to properly adjust the V_OUT. Disconnect the load and check whether your circuit could work in open load (before and after the failure). From what you've described, an irreversible failure is very likely due to over current. LM27313 has current protection, so my bet is on the over current of the inductor, somehow.

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