Mounting a surface mount FET/Controller

[cupoftea]

Hi,
We are using Innoswitch eSOP package for an offline flyback.
The PCB sits 5mm above a big metal heatsink.......the innoswitch will be on the PCB underside.....so as you can guess, it is 3.4mm shy of the heatsink.

..so we wish to put the innoswitch on a 1mm thick piece of (little) PCB, with thermal vias through...then we put legs on the little PCB....then we mount a square piece of aluminium or copper to the wee PCB's back......then there will still be a gap of about 0.2mm or so to the big heatsink ....so a bit of gap pad will do for that.

But does anyone sell such little "platform PCBs " off the shelf?

InnoSwitch-EP Data Sheet

Off-Line CV/CC Flyback Switcher IC with Integra

www.power.com

 

[dick_freebird]

If they're thermal vias then they're electrical vias and a sheet of
aluminum (or aluminium as you might say) would do as well.
You can get that in any thickness you like and leave room for
some Kapton tape if you need isolation at high temp.

But why not just source the correct gap pad of the lowest
thermal impedance you can find and call it done?

--- Updated Aug 13, 2023 ---

If they're thermal vias then they're electrical vias and a sheet of
aluminum (or aluminium as you might say) would do as well.
You can get that in any thickness you like and leave room for
some Kapton tape if you need isolation at high temp.

But why not just source the correct gap pad of the lowest
thermal impedance you can find and call it done?

 

[cupoftea]

Thanks, we may do that...indeed...or a "liquorice allsort" approach involving an aluminium square between two gap pads.......

So...

We wish to mount INN3675C (inSOP-24D) across the isolation barrier as ever.

Its a 20W, 90-265VAC flyback...but its in CCM at 105VAC, so we need to heatsink it well.

We have it on a daughter board, which is resting on two kind of PCB "pieces" which have castellated pads so that we can bring the signals/power up to the innoswitch on the little PCB.......so we can put a gap pad to the bottom of this little PCB , and then gap pad it to the underlying earthed heatsink. However, its obviously across the isolation barrier, so do you know any gap pads that we can use for this.?

So we have the INN3675C mounted on a small PCB (just a bit bigger then the innoswitch)....and this little PCB has thermal vias to bottom copper, and then this is mounted on the main PCB via the PCB "Pieces"....so the innoswitch is kind of the other-way-round (upside down) to the usual way, and so this means we can gap pad the thermal via'd bottom copper of the little PCB to the earthed heatsink which is 4mm below the main PCB.

So do you know of gap pad which is good for this isolated purpose...where the gap pad will span the isolation barrier under the innoswitch's little PCB?

We are also thinking of a "liquorice allsort" heatsink approach, where we have a little (2mm thick) square of aluminium in between 2 gap pad pieces....and this "allsort" will be in the gap between the little PCB and the below earthed heatsink. Do you know of gap pads that can do this without needing a "harder" electrical insulator piece?

Innoswitch3 EP

InnoSwitch3-EP

The InnoSwitch3-EP family of ICs dramatically simplifies the design and manufacture of flyback power converters, particularly those requiring high efficiency and/or compact size. The InnoSwitch3-EP family incorporates primary and secondary controllers and safety-rated feedback into a single IC.

www.power.com