TPS40195 / calculate Ron of rectifier MOSFET

S.Satoshi

Other Parts Discussed in Thread: TPS40195

Hi,

Now I'm trying to calculate the Ron of rectifier MOSFET by referring formula (23), but I’m not sure where 0.338W of Pcon came from. Please let me know.

Best Regards,

Sonoki / Japan Disty

over 11 years ago

0 S.Satoshi over 11 years ago

Guru 21685 points

Hi,

I'm waiting for your response. Your response will be appreciated.

Best Regards,

Sonoki

0 JohnTucker over 11 years ago in reply to S.Satoshi

TI__Guru**** 191445 points

In that example, target efficiency is 90% and to achieve the figure requires the total losses to be 1.8 W. The example assumes these losses are shared equally among the high side FET, low side FET and inductor. So the losses in the low side FET should total 0.6W. The body diode losses are estimated at .162 W and the gate losses are estimated at 0.1 W. So the remaining conduction losses should not be more than 0.6 - 0.162 - 0.1 = 0.388 W. You can then solve EQ 23 for RDSon.

0 S.Satoshi over 11 years ago in reply to JohnTucker

Guru 21685 points

Hi John-san,

Thank you for your response.

You said the total losses to be 1.8W, but I believe it should be 2.0W at this condition.

---

Pout = 1.8V x 10A = 18W

Target efficiency = 90%

Then Pin = Pout / Eff = 18W / 90% = 20W

So Ploss = Pin - Pout = 2.0W

---

Is my understanding correct?

Best Regards,

Sonoki

0 JohnTucker over 11 years ago in reply to S.Satoshi

TI__Guru**** 191445 points

Yes you are correct. I suppose these calculations make a lot of estimates and assumptions. So instead of calculating the actual losses for Eff = 90%, the author used a rougher approximation of losses = Pout * .1 = 1.8 W. I did not write that datasheet though...

0 S.Satoshi over 11 years ago in reply to JohnTucker

Guru 21685 points

Hi John-san,

OK, I understand !!

Best Regards,

Sonoki

0 S.Satoshi over 11 years ago in reply to S.Satoshi

Guru 21685 points

Hi John-san,

I'm sorry but please let me ask again about Pqsr calculation.

Now our customer is trying to calculate Pcon, Pbd and Pgate by using formula (23), (24) and (25). And then they are using value from Design Example 1 to target following results. However we can not get the same or similar result.

Can you modify attached spread sheet and teach which value we should use for these formula?

Best Regards,

Sonoki

TPS40195 Pqsr calc.xlsx

0 JohnTucker over 11 years ago in reply to S.Satoshi

TI__Guru**** 191445 points

I'll take a look at it later today.

0 S.Satoshi over 11 years ago in reply to JohnTucker

Guru 21685 points

Hi John-san,

I'm sorry to be pressinｇ but I'm waiting for your update.

Best Regards,

Sonoki

0 JohnTucker over 11 years ago in reply to S.Satoshi

TI__Guru**** 191445 points

I had to finish a datasheet yesterday. I have a few items ahead of your request today as well (they keep me pretty busy). Normally I can answer these types of questions quickly, but I do not normally support TPS40195. I have to research everything from the beginning as I have never used TPS40195 before, so it takes longer.

0 JohnTucker over 11 years ago in reply to JohnTucker

TI__Guru**** 191445 points

I looked at your spreadsheet just now (it is before working hours here). You are correct. I cannot make the numbers work out either. I will try to locate the original author to see if he can explain it. Otherwise you will just have to ignore the example. We have already found one bit of sloppy math for the efficiency to power dissipation so i will not be surprised if there are others.

0 S.Satoshi over 11 years ago in reply to JohnTucker

Guru 21685 points

Hi John-san,

Thank you for your effort when you are busy. I recommended our customer to calculate each value by using formula by themselves.

Best Regards,

Sonoki

0 JohnTucker over 11 years ago in reply to S.Satoshi

TI__Guru**** 191445 points

I have located the original technical owner of the datasheet. he is looking into this issue. I'll let you know when he reports back with his findings.

0 S.Satoshi over 11 years ago in reply to JohnTucker

Guru 21685 points

Hi John-san,

Thank you for your effort. I'm waiting for the response from the technical owner.

Best Regards,

Sonoki

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TPS40195 / calculate Ron of rectifier MOSFET