LM5116 - 15A @ 5V. Choice of FETS.

Andrew Frazer

Other Parts Discussed in Thread: LM5116

I'm wanting to use a LM5116 to get 5V@15V. I'd like to use this driver because its in our inventory already.

I've done the numbers and it looks feasible,

I picked a fsw of 150khz as a bit of a compromise, I have enough space to use a reasonably large indicator with low DCresistance. I think i'll use Polycaps on the output, i need about 200uF, so ceramics just become unworkable, as they will drop to about 30% of rating with the DC bias. 4 in parallel brings the ESR down to point that is workable.

My big question really is about the choice of FETS. The FETS i'm currently using are rated at 22A, and should do the job ok, but i was wondering if its possible to drive two FETS in parallel, which would help with the thermals for this. Again using these FETS over other ones helps us keep inventory down.

the smaller the number of line items in our inventory, the larger the per item Qntys get, which really helps on pricing.

I'd really appreiciate any feedback on this.

over 10 years ago

0 EricLee over 10 years ago

TI__Guru 58371 points

Parallel MOSFET connection is okay with LM5116

By using parallel MOSFET connection, switching loss increases and Rdson loss decreases.

Thermally, it is always good since two MOSFETs are sharing the loss.

Regards,

0 Andrew Frazer over 10 years ago in reply to EricLee

Expert 2305 points

what is the limiting factor for how many fets can be driven in parallel.

0 EricLee over 10 years ago in reply to Andrew Frazer

TI__Guru 58371 points

I can think of two factors.

'VCC current limit' limits the maximum driving current which is calculated as 'Qg times Frequency'.

Switching loss : This cannot be estimated accurately. The best way is to test on the bench.

Regards,

0 Andrew Frazer over 10 years ago in reply to EricLee

Expert 2305 points

I've updated my design,

(i) I've reduced the Fsw to 70Khz, to reduce the effects of the Switching losses. In this application, i'm not tightly space restricted so i have gone for a pretty large ( 30A ) Inductor, with low Rdc

(ii) After looking at the power loss calcs, changing Fets made sense. They cost a bit more, but the power efficiency is improved.

(ii) output capacitors, i've drawn eight, in, but this might be overkill. I think i'll just have to measure this experimentally when i build it. If I don't need so many, i can leave some off.

My only concerns is about how the LM5116 is driving the Fets. Does my schematic look ok?

0 EricLee over 10 years ago in reply to Andrew Frazer

TI__Guru 58371 points

I think it is okay. I could not find an error.

Regards,

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LM5116 - 15A @ 5V. Choice of FETS.