Debugging a failing DC-DC power module

Manufacturing
#1

I am trying to debug a failing DC-DC module in a LED lighting project I have just built.

The design can be found here https://upverter.com/StephenHamer/73d5e5450f4a34ee/12V-PWM-Controller/

I have populated the board, hooked up a 12V power supply and LED lighting strand. I am able to control the brightness using a PWM signal driven from a BeagleBone. The lighting works well for several hours. Then I turn the lighting off for the night by setting the PWM to 0 duty cycle at 0Hz (using the http://beagleboard.org/Support/BoneScript/analogWrite/ API).

Sometime during the night the 5V module (https://upverter.com/upn/738daca67feacadd/) fails. When I test with a multi-meter I only see 1.2V

I have tested for shorts, verified resistor values, and visually inspected the module and board. I have had this same failure happen two times. Each time with an entirely new board and parts.

How should I go about debugging this issue? I’m not sure what I should be looking for.

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#2

Are there any problems with the 12V rail?

Does the issue reproduce if the beaglebone is completely removed from the system?

Does the issue reproduce if you disconnect the beaglebone from the node “control”?

Note 5 of the “Functional Specification” of the datasheet for the converter says “The On/Off Control Input should use either a switch or an open collector/open drain transistor referenced to -Input Common. A logic gate may also be used by applying appropriate external voltages which not exceed +Vin.”. Figure 1 also seems to suggest that the designers of the module had something in mind other than just shorting the On/Off control pin to one of the rails. It might be worth going through the datasheet with a fine-toothed comb for advice on how to connect this pin.

#3

The datasheet says this part can handle a maximum of 7.5 watts. Given that its efficiency is around 90% at 12vin/5vout, your output current should then be kept under ~1.35 amps. Are you sure you’re staying under that?

They recommend a 3A fast blow fuse on the output; that might save some modules while you troubleshoot.

Additionally, they strongly recommend input and output caps (page 14):

For best performance, we recommend installing a low-ESR capacitor
immediately adjacent to the converter’s input terminals. The capacitor should
be a ceramic type such as the Murata GRM32 series or a polymer type. Initial
suggested capacitor values are 10 to 22 ÎĽF, rated at twice the expected maxi-
mum input voltage.
…
The minimum external output capacitance required for proper operation
is 50uF ceramic type. The maximum external output capacitance is 100uF
ceramic and 470uF POSCAP. Operating outside of these minimum and maxi-
mum limits may affect the performance of the unit.

Do you have a scope that you can use to look at the input and output waveforms? I suspect they may be pretty bad without input and output capacitance.

They also spec a pretty fancy trim resistor, but this should be less important if you can tolerate a bit of drift.

The Rtrim resistor must be a 1/10 Watt precision metal film type, ±0.5% accuracy or better with low temperature coefficient, ±100 ppm/oC. or better.

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