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HB-415M Stepper Driver Heatsinking: Lack Thereof

Just to see what’s inside, I took those HB-415M drivers apart. They’re not all identical inside:

HB-415M Driver - interior top

HB-415M Driver – interior top

The other side shouldn’t come as much of a surprise:

HB-415M Driver heatsinking

HB-415M Driver heatsinking

Now, admittedly, I’ve applied a heatsink to the top of an epoxy package, but that DIP package has thermal tabs that should connect to the heatsink through a low-thermal-resistance path. A dab (!) of heatsink grease and what might be a thermally conductive plastic sheet atop the package seem, well, insufficient.

The driver chip sports an Allegro A3992 marking that might be genuine. The datasheet goes into some detail as to how you should lay out the PCB; none of its recommendations made it into the finished product. In particular, the hulking current sense resistors surely have more inductance than you’d like.

The resistor color code seems odd: black red red silver brown.

HB-415M current sense resisors

HB-415M current sense resisors

Using black as the first band is unexpected, but it’s probably the only way to indicate a low-value resistance without printing the numbers: 0.22 Ω ±1%.

Ah, well, the peak current isn’t as high as they claim, so it all probably works out in the end.

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  1. #1 by Red County Pete on 2013-02-27 - 12:03

    I’ve seen DIPs with heat spreaders to give a bit of thermal connection to the outside world. They look like mutated butterflies, with the “wings” out at the ends of the package. The single dab is in almost the right position to catch one of the wings, but a matching one (well, I’d run compound across the top) would work better. Used to have to do this for bipolar memories–plastic packages suck in comparison to hermetics for thermal resistance, but with the heat spreader, the suckage isn’t quite too terrible.

    Looks like the center pins are also trying to get heat to the PCB, too. I’d make sure the plastic gasket has some compound to link to the metal lid. They did get thermal contact from package to the lid, didn’t they?

    • #2 by Ed on 2013-02-27 - 12:20

      There’s actually a decent compression fit between the DIP IC, the gasket, and the aluminum cover, so a few blobs of thermal compound should improve the situation. Now that I’ve given the supplier a negative rating, I can proceed to do that.

      The datasheet highly recommends heatsinking the PCB to the case, with extensive copper pours to conduct heat from those pins in the middle, but … I’ll work with what it has. Given the current levels, I think it’ll be OK without too much improvement.