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I have a power mosfet (STP55NF06FP) used as a low-side switch to cut-off a current of 40 A. Using pcb calculator, I estimated that the trace width is too large (50mm) and as a result I can't connect the trace directly to the pin because there is not so much space between the pins of the mosfet. Please advice what can I do. The pcb is 1oz. Thank you

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The mosfet cuts-off the Motor- to C-.

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  • \$\begingroup\$ Depending on your pcb calculator (that you didn't share) there is often a max temperature rise that you can change. Regarding this specific problem seems like you are using through-hole so you can use a footprint with a wider space between pins and just pull the legs of the transistor more \$\endgroup\$
    – Mat
    Feb 28, 2022 at 12:20

3 Answers 3

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Please advice what can I do

The data sheet of the STP55NF06FP says it is rated for 35 amps continuous current at a warmed-up temperature of 100°C. You are saying you are switching 40 amps. This can only mean that: -

  • You have misread the data sheet
  • You are switching 40 amps for a short period of time

If the latter then it's unlikely that there will be an issue with the 1 oz copper fill shown in your question. However, I'd be tempted to spread the pins and put a little more copper around the device legs (just to be safe). This applies to both drain and source pins. If you have more than 1 copper layer, then double up on each side and connect the two with a few vias.

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There are pcb manufacturers offering special high current pcbs. It is possible to reinforce only the traces used for high currents. The pcb has normal traces with 35 µm copper and the usual trace widths of 0.2 mm to 0.5 mm or more for low currents as well as high current traces with 200 µm or more copper and widths of 5 mm or more.

About 10 years ago I made such pcbs working for an university. The pcbs were plated two times, the first time to build the 35 µm traces, after etching a second time to reinforce only the high current to 200 µm.

The current for galvanic reinforcement of the traces was connect to the edges of the pcb. Only those traces connected to an edge were reinforced, the other traces without a connection to an edge got no additional copper.

Here is an example with 350 ; 700 and 1050 µm traces.

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I don't know what trace-width calculator you are using but my calculator says you should have 127mm trace for single-sided 1oz PCB.

If you duplicate your pattern on the second side, you effectively transfer 20A on each side of the board. can get down to 48mm.

Allowing a temp rise of 10°C is typical but you can make sure all is good with wire and housing material and to avoid heat build-up in the housing and allow 20°C temp rise and 32mm trace (when using both sides of the board at 20A each side.

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