Does heat have a big effect on DC-DC step-down converters (LM2596 board)?

[Jay112]

I have an xbee module hanging outside in the shade in a protected enclosure. It's been working well there for over a year now.

A few months ago I implemented a feature where I can ask the xbee what temperature and voltage it's running at, and get the information on my computer. I have a 5v power supply getting reduced to 3.500 volts by an ebay LM2596 board like the following:
http://www.ebay.com/itm/5pcs-Re-DC-DC-3A-Buck-Converter-Adjustable-Step-Down-Power-Supply-Module-LM2596S-/221991448900

For months the voltage has remained constant at 3.500, where I originally set it at. But yesterday and today were the first hot days of the year, with the xbee reporting that it's running at 90 degrees F. I noticed from these last 2 days that whenever the xbee is above around 85 degrees, the voltage drops to about 2.8. Then at night when the temperature lowers the voltage goes back up to 3.500. It's not a linear drop, and it seems to switch immediately to 2.8 when it gets hot, and then when it cools down at night it jumps immediately back to 3.500.

I looked at the datasheet, and it looks like it's only supposed to fluctuate 1 or 2% over those temperature ranges. Do you know why I'm seeing such large voltage drops whenever the temperature is high? Is there a way to prevent this?

Just as a sidenote: the xbee is still functioning as normal when the voltage is low like that, but I'm guessing maybe the range isn't as good (I'd need more time to test out the range to be sure).

Thanks!

[sleemanj]

Step 1: swap LM2596 module with another one, they are cheap, no sense chasing geese if you can just buy another

Step 2: if problem still exists, how much power is being drawn, what is the temperature of the DC-DC converter, (measured at the tab of the IC)

The LM2596 does have thermal protection built in, but it's a bit of a long draw to assume that the datasheet applies to an IC on one of these modules from China, which may or may not be legitimate LM2596 onboard.

[Jay112]

Step 1: swap LM2596 module with another one, they are cheap, no sense chasing geese if you can just buy another

Step 2: if problem still exists, how much power is being drawn, what is the temperature of the DC-DC converter, (measured at the tab of the IC)

The LM2596 does have thermal protection built in, but it's a bit of a long draw to assume that the datasheet applies to an IC on one of these modules from China, which may or may not be legitimate LM2596 onboard.

Thanks for writing! Your advice sounds great! I'll try swapping the board first, and also I'll try replacing it with a different step down that isn't LM2596 based. And I'll check the temp of the boards with an IR thermometer when they're reporting that they're >85°.

Yeah, that makes sense that the cheap boards from eBay won't necessarily follow the datasheet specs.

Part of my reason for posting was just to see if anyone else has experienced something similar, but my use case might be too specific.

I think the XBee Pro specs say it draws 250mA at top transmit power, but is more likely to be consuming 100-150mA whenever not transmitting. I was surprised how much power that little thing can draw. I don't remember if I ever tested it myself though. Another neat experiment would be to test the draw at different temperatures.

[matkar]

I have tested one of my chinese LM2596 buck converters under same conditions. I increased the temperature of the module with a hair dryer. The module became so hot I couldn't hold my finger on it, so the temperature had to be above 55°C. The voltage dropped from 3,528V (room temperature) to 3,439V when heated.
Try switching your trimmer with a new one or with a resistor pair to see if this has any effect.