Power supply circuit sense check

[jamesportman]

Hi,
Hope it's OK to post and ask like this, pictures attached.
I couldn't work out how to easily share the whole project from easyeda, I can post the gerber file/whatever else if it helps?

I am trying to plan a PCB to run some mosfets (high side power switch - BTS7002-1EPP) to act like relays, but also to allow them to run with PWM input rather than just on/off.
They are 21A continuous each, and planning to run at car voltage so about 9v-14v.
Potential to run this with an arduino or similar to make use of the sensing and diagnostic functions work, but as a minimum just as on/off or pwm inputs.

Ignoring the protections for microcontroller etc for now, I was just wondering about heat and power issues, using vias to transfer heat like they have done on the datasheet example where the temperatures were tested.

Is it OK to run the vias to a large pad on the underside of a 2 layer board as opposed to an internal layer of a 4-layer board?
I think I've done it so the copper is NOT exposed on that bottom side pad.
I've tried to do the vias as exposed copper so guess the vias would be exposed on the bottom - is kapton tape enough to cover that? Is there a better way to only expose the top of the vias?
The bottom of the IC is the main VS/power supply so it seems risky.

Is it a waste to run the main VS lines on the top layer then also run the heat pad on the opposite side? Is it easier to combine them into one huge power supply/heatsink "trace"? I was thinking having them separately will keep power supply separate from the heatsink and ideally allow more heat transfer since the power supply lines might already be hot at max rating?

Is it OK to run huge traces under the output pads like I have done? They are all output pins and meant to be eventually joined together.

Is it OK to do the huge hole for a bolt type mounting of incoming power?
I would guess more of the copper around the hole on that needs exposing really.


Any advice welcome,
and a note that I'm not about to immediately have this made and run it somewhere that could set on fire/be dangerous.

Thanks

 

[KlausST]

Hi,

doesn´t look ban on a quick view.
You may wireuse the blue layer to wire the supply to the bolt hole. Yes add copper around the hole.
This enables to route all the microcontroller wires on the red layer avoiding a lot of vias.
For sure any copper on the red layer helps to spread the heat.

You may glue some heatsinks on the Mosfet packages. then less heat needs to travel through the PCB. At least it doesn´t hurt.

Those PROFETs suffer from high switching time. They are suitable for low PWM frequency only.

The PROFETs are quite self protecting. I don´t expect fire ;-)

For those high currents you need a proper GND plane to avoid ground bounce between microcontroller and MOSFET.
Especially take care the the load current does not flow between microcontrolelr and MOSFET. --> set the power_input and load_ouput(including GND) in close proximity to the MOSFETs.

Klaus

 

[jamesportman]

Thanks for your comments, I will have another attempt soon.
I actually found an arduino shield that Infineon themselves make, and they have a really good datasheet along with it which explains a lot and gives example code too!

Thanks for pointing out the pwm frequency.
They do quote something like 500hz max, I see the switch on and off times as some number of microseconds, I guess if you set pwm frequency too high it will always be in the transition states and never have any time actually on?

 

[jamesportman]

OK I had another go, screenshot attached.
I can have it made in 2oz copper (for a lot more money), but I think it will help with the current and heat.
I added all of the suggested caps/diodes/resistors now, also tried to make a sort of ground and power plane.

Remaining unknowns:
- Heat - heatsinks are a good idea like you suggested
- Is the low PWM frequency an issue? Is it too "spiky" for some devices? I can set the MCU to run 500hz no problem otherwise.
- What do you mean about "set the power_input and load_ouput" - is that an option somewhere in easyeda designer? Or did you just mean placement on the board?
- would ground bounce still be bad even with the filtering caps and sort of ground plane?
- I would probably add more chips to bring the total up to 6 or 9, the board is tiny at the moment so size is fine to double or triple it

 

[KlausST]

Hi

set the power_input and load_output(including GND) in close proximity to the MOSFETs.

You already did the placement correctly.
* power_input: hole/bolt
* Load_outputs: OUT1, OUT2, OUT3

I didn´t go deeply through your design. The R, C, D package sizes seem all to be the same. They are small, especially the pads.
Don´t think it makes sense. But maybe ...

Klaus

 

[jamesportman]

Sure about the rcd size. I just picked 0805 smd size which the caps and resistors do come in, I’ll check all the other parts and resize them to whatever I can order.



I’ll check the pad size too, I might have blanked on that!

 

[KlausST]

Hi,

in your PCB layout there surely is no (even almost) standard 0805 package.
Even without any scaling information...

Klaus

 

[jamesportman]

I think the yellow outlines in the silk layer are approx 0805 outline, so the pads need to match and will that size basically,
I think so anyway..
0.080" x 0.050"