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I have a project which uses a TI step down converter (LMZ14201) to buck an input voltage (up to 41V) down to a regulated 5V. Until now, I had successfully used an adjustable (regulated) DC power supply for testing. In that setup, the step down converter worked like a charm.

Today I proceeded to assembling the complete system (i.e. using a lithium-ion battery pack, 36V, as the input for the buck converter). For testing purposes, I further connected a switch between the positive rails of the buck input and the battery.

With nothing else changed, the buck converter failed (produced sparks coming from under the package; thermal/GND pad?) on the second time connecting the battery. What could be the issue here?

  • I diligently followed the recommended layout depicted in the datasheet of the DCDC converter
  • I tested voltages up to 41.6V with the power supply. The aforementioned battery pack had 36V (approx. nominal voltage), the power supply is rated up to 42V (recommended) and 43.5V (abs. max) respectively
  • The input capacitance connected to the buck converter is 3x4.7uF. Apart from that, there were no input filter elements separating the battery pack and the converter

Could possible inrush current (caused by switching/input capacitors), paired with stray/parasitic inductance be the culprit here? If yes, a viable solution would be a series NTC current limiter. What other causes could this have, apart from random part failure?

UPDATE: This problem is reproducible. Changing the soft start delay to 10ms did not help either. I managed to capture the voltage transient at the battery terminal. The battery voltage is about 37V, but it seems to swing up to 50V as soon as the DCDC is connected to the battery. The absolute maximum rating of 43.5V is therefore not satisfied anymore.

Battery Voltage Scope Capture

This does not seem to happen with just the input filter caps connected, but only when the input filter caps and the DC/DC converter IC are connected.

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  • \$\begingroup\$ Sparks from under the package sounds suspicious. Could be a soldering issue. You shouldn't need an NTC with this part, just use the external soft-start capacitor. \$\endgroup\$ – John D Jun 3 '16 at 16:06
  • \$\begingroup\$ The external soft start cap was connected. I soldered it via reflow and a laser cut stencil for the solder paste - in any way the connections seemed (looked at least) perfect. \$\endgroup\$ – fscheidl Jun 3 '16 at 16:08
  • \$\begingroup\$ What is the load on the buck converter's output? What is the expected continuous current draw on that 5V rail? As John suggested in his answer, I believe it was just massive current supplied by the Lithium Ion battery that normally isn't/can't be sourced from bench supplies. \$\endgroup\$ – KyranF Jun 3 '16 at 16:12
  • \$\begingroup\$ Some of the load is capacitive, but the (maximum) continuous load on the 5V rail is about 100mA. \$\endgroup\$ – fscheidl Jun 3 '16 at 16:26
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    \$\begingroup\$ The primary difference between a bench PSU and a battery (especially a Lithium-Ion one) is that there's no protection to stop you from delivering 100s of A if you short out / cause that condition. Commercially sold cells should have a built in circuit to protect OVP/OCP/UVP, but your raw cells might now. \$\endgroup\$ – Krunal Desai Jun 3 '16 at 17:35
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I understand why you see the initial fast droop, because the electrodes in the battery are quite complex electrically:

schematic

simulate this circuit – Schematic created using CircuitLab

At the application of the load, an EDLC forms at the electrodes and the capacitance can be enormous - 60μF to 100μF is not unusual (the effective plates are literally Angstroms apart) with a sub milliohm ESR and that gives you a very effective differentiator at initial transient loading of the battery into a heavy load.

Once the EDLC has formed, it stops conducting and the only resistance is the ESR of the battery electrodes themselves. As the terminals recover to being limited only by load and internal ESR, there can be an inductive effect (after all, self inductance exists anywhere current flows, to a greater or lesser degree).

Your waveform looks like a damped resonance, so an appropriate snubber may work.

You can see how EDLCs form at battery electrodes in this paper.

I have seen this effect a few times and it was quite confusing the first time it appeared (in a seawater activated circuit).

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  • \$\begingroup\$ This model of the electrodes combined with parasitic inductance seems to indeed perfectly describe the phenomenon I am seeing. \$\endgroup\$ – fscheidl Jun 5 '16 at 8:21
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The batteries can provide currents an order of magnitude higher than your bench supply. Possibly your bench supply was giving you a "soft start" by current limiting during startup. Did you implement the soft start? This is especially important when the power supply is under load at start-up.

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  • \$\begingroup\$ That is what I thought in the first moment too. I did not implement a custom soft start mechanism or current limitation for my buck converter whatsoever, thus the question if that is the case. The power supply on the other hand is able to do CV and CC, so I take it has that feature. \$\endgroup\$ – fscheidl Jun 3 '16 at 16:07
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    \$\begingroup\$ I would note from section 8.2.2.1.3 in the datasheet: Use of a 0.022-μF results in 2.2 ms soft-start interval which is recommended as a minimum value. I always implement soft-start, even when I think it may not be necessary. \$\endgroup\$ – Peter Smith Jun 3 '16 at 16:14
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    \$\begingroup\$ If your load is capacitive, your converter tries to deliver high current during start-up. If you check your load capacitance, you can determine the required charging current. The output voltage will rise more slowly on your capacitors if you soft start, so set it so that the inrush current plus load current is within your inductor's range while the caps are charging. Once they are charged, you will be OK. \$\endgroup\$ – John Birckhead Jun 3 '16 at 16:20
  • \$\begingroup\$ Oh I'm sorry. I was thinking about a custom soft start mechanism in series with the buck converter. I surely connected the SS cap for the internal soft start. \$\endgroup\$ – fscheidl Jun 3 '16 at 16:25
  • \$\begingroup\$ @PeterSmith Thank you for pointing out the recommended minimum value. I just checked my setup and realized I had used a 4.7nF capacitor (this was based on a design from the TI website though, might send them a hint). I can't believe I missed that. I will try to increase the SS capacitance. \$\endgroup\$ – fscheidl Jun 3 '16 at 16:35

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