Help me not blow up my buck converter

[SmoggyTurnip]

I have an LED headlamp that is powered by 3 AAA batteries that I use when I go running. I wanted to power it using my battery from my Dewalt cordless drill.  The battery is a 5 cell rechargeable lithium ion battery.

I bought a buck converter on ebay:

http://www.ebay.com/itm/1pcs-DC-DC-Buck-Converter-Step-Down-Module-Power-Supply-4-75V-24V-to-0-92V-15V-/400349988919?pt=LH_DefaultDomain_0&hash=item5d36b80837

I connected the input of the buck converter to my bench power supply and the output of the buck converter to the headlamp and adjusted the voltage to 4.5 volts to simulate 3 AAA's in series.  Everything worked fine.

Next I replaced the power supply with the battery.  As soon as I turned it on there was a big poping sound and the buck converted burnt out and my head lamp also burnt out.  I checked the wires going from the battery to the converter to see if I had them reversed but the were OK.

I went and bought another LED head lamp and went through the whole process again and had the same result.  There goes another headlamp and another buck converter.  I have 12 more buck converters left so no problem there but I only want to buy one more headlamp.

I don't understand why this setup would work with a bench power supply perfectly but have such instant failure with a battery.

Any suggestions?

[SmoggyTurnip]

Some facts I left out in the first post.  - I had the bench power supply set to 20 volts.  The battery measured 19 volts.  The LED headlamp uses 150 mA when supplied with 4.5 volts.

[mariush]

What headlamps do you buy? What's the voltage and the wattage (or the current) the lamp uses ?

The headlamp may pull a lot of current when it starts which your bench power supply limited (maybe you have a 3A max power supply or something like that?)

The batteries are probably able to provide a lot of instant current and the switching element in that IC probably blew up. The inductor may also not be rated for your peak currents.

Also be careful with those buck regulators - just because it says 4.75v-24v it may not be wise to use 19-20v as that's close to 24v. Also notice they use ceramic capacitors, and if those are only rated for 50v when you push 20v on them the capacitance drops significantly and that may make the circuit unstable.

Here's the datasheet for the regulator chip : http://www.richtek.com/download.jsp?t=UyUyRmFzc2V0cyUyRnByb2R1Y3RfZmlsZSUyRlJUODI3MiUyRkRTODI3Mi0wMi5wZGYlM0QlM0Ql%0AM0REUzgyNzItMDJD

Study it, compare with what you have on that pcb and see what broke.

I also suspect the inductor may not be up to par like i said above.

[minime72706]

I've gotta say --- those buck converters on ebay look pretty awful. The inrush current from the Lithium Ion battery pack is probably destroying the regulator IC and causing a higher-than-expected voltage to reach the headlamp.