RAMBo Platform Heater MOSFET: VGS Specs

The improved platform for the M2 runs at 30 V, but the RAMBo board specs limit the max HBP voltage to 24 V, presumably because the 15 A ATO fuse won’t clear a high-voltage, high amperage DC short. While setting up the SSR that drives the new platform, I looked up the specs for the PSMN7R0-60YS MOSFET controlling the bed heater and … it doesn’t have a logic level gate.

The rDS spec is an impressive 6.4 mΩ max, but that’s at VGS = 10 V. The 1 mA threshold voltage VGS(th) = 4 V max, which means there’s only 1 V of headroom to turn the transistor on enough to pass upwards of 10 A.

The typical ID vs. VGS curve (Fig 6) shows 20 A at maybe 4.2 V, but the typical RDSon curve (Fig 8) shows the resistance skyrocketing for VGS under maybe 4.8 V; sliding that curve a wee bit to the right would cause a Very Bad Thing to take place.

A 20 mΩ resistance dissipates 4.5 W at 15 A, which seems rather aggressive for the small PCB copper-pour heatsink on the RAMBo board. It’s a somewhat more bearable 2 W at 10 A, but I think that’s still too high. Of course, the typical dissipation will should be much lower…

A good engineering rule of thumb is to ignore the datasheet’s “Typical” column and design using the “Minimum” or “Maximum” columns, as appropriate. When you depend on typical specs, getting “the same part” from a different supplier can provide a real education in supply-chain management.

I suspect tolerance stacking works well enough that nearly all the MOSFETs on nearly all the RAMBo boards run cool enough to survive, but I’d rather see logic-level MOSFETs in Arduino circuits where the maximum gate voltage won’t ever get above 5 V.

5 thoughts on “RAMBo Platform Heater MOSFET: VGS Specs

  1. But if people didn’t do dumb sh.. er stuff, then you would have so much less to write about. :-)

    1. In truth, the RAMBo board has a lot going for it. I don’t like the overall dedication to All Things Arduino and prefer themocouples, but it’s a nice solution within its limitations.

      And I’ve misread the occasional datasheet, too… [wince]

  2. I’ve run 300W through the RAMBo bed MOSFET without issues. Close to 3k hours of runtime across 5 machines. This is at 24V too, with no fans directed at the PCB.

    1. Agreed, most of those MOSFETs will work perfectly well with 5 V gate drive, because the tolerances stack up on the side of having higher ID for a given VGS. Any given MOSFET will continue to work fine, too, so if you have boards that work, they’ll keep on keepin’ on.

      But it’s entirely possible to buy a batch of in-spec MOSFETs that require, say, VGS = 6 V to pass 12 A without blistering the silkscreen, because the spec limits tell you guarantee nothing about the MOSFET’s behavior below VGS = 10 V.

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