You can’t shrink heat

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Regular readers may recall that I volunteer at a nonprofit computer recycling facility. Recently, the incoming e-waste stream yielded a batch of approximately 100 castoff HP Model 1530 15-in. LCD monitors. While many still worked, approximately one third showed only black screens.

Shining a flashlight on a typical screen showed faint black characters, indicating that the display’s LCD worked but not the backlight. Investigating further, I found a blown fuse that supplied power to the inverter that drives the backlight’s fluorescent tubes. Replacing the fuse restored operation, but then the fuse failed again.

An hour later, the screen went black, and an inverter transistor (a TO-126-packaged Sanyo 2SC5706) released a wisp of smoke, obligingly unsoldered itself from the circuit board, and dropped onto the workbench. I incautiously poked it and received a small burn on my fingertip. Can you say “thermal runaway”? I could and did, along with a few unpublishable words.

I examined several failed displays and noted that the inverter circuit boards, manufactured by BenQ, had discolored areas around the inverter transistors. Mounted close to the board, the 2SC5706s’ 7.5-mm² collector tabs hardly qualify as convection heat exchangers. Heat produced by the transistors dissipates primarily via the board’s copper traces.

The single-sided, low-density printed-circuit board layout crams the transistors next to the inverter’s step-up transformers. Finding suitable substitutes for the fast-switching and high-current 2SC5706 proved surprisingly difficult. I wanted to use TO-220-packaged switching transistors in place of the 2SC5706s, but there’s barely room for TO-18 transistors and small heat sinks.

Driving fluorescent tubes more closely resembles an art than a science (see sidebar), but in our industry’s crazed pursuit of “smaller, cheaper, and faster” electronics, we’ve evidently forgotten “hotter”—and heat doesn’t take kindly to miniaturization.