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# UCC28063A: UCC28063A: Maximum input current equation in datasheet incorrect?

Part Number: UCC28063A
Other Parts Discussed in Thread: UCC28063, UCC28061, UCC28060

The peak inductor current for one boost converter is correctly given by equation 20 in the datasheet.

Equation 37 however, which is the sum of the two inductor currents, is incorrect. Since the two inductor currents are working out of phase, the sum will never be twice the peak inductor current of a single boost converter as given in equation 20. I agree that the sum will be more than the current of a single boost converter when the on-times overlap, but not twice, since they are still out of phase.

Equation 37 is therefore an over design of the current sense resistor.

• Hello Stephan

Thanks for spotting this, I don’t think the resistor is quite so overdesigned as you think for the following reason. Eq 20 deals with a single phase and each phase carries half the output power. The peak inductor current will be twice the average inductor current. Therefore EQ 20 has a factor of 2/2 which  cancels out and of course it’s not actually written down. The ‘bottom’ 2 divides the output power by a factor of two to get the average inductor current in each phase. The ‘top’ 2 multiplies the average inductor current by two to get the peak.

Eq 37 does the same thing except that the current being measured is now the sum of the currents in the two phases. There is no cancellation of the low frequency (line frequency) current – half of which flows in each phase. There will of course be some cancellation of the high frequency component of this current but this is a complex function of the line voltage and the peak current (which is what is used to trip the over current protection comparator) may not be reduced very much.

I agree, Eq 37 is conservative and probably somewhat of an over-design but not by the factor of two.

The comment in the DS that the most critical parameter in selecting the current-sense resistor is the surge rating is correct. This part has to withstand inrush current surges and line surges (IEC61000-4-5).

I hope that this clarifies things

Regards
Colin

• Hello Colin,

Everything you say is correct.
I am inclined to be too-mathematically precise.
I am designing with the UCC28063A and am looking forward to see it working on my first PCB layout.

Regards,
Stephan.
• Hello Stephan

Glad to be of help. It's good to see that these equations do get read and used in practice - even if they can be a bit confusing at times.

Do re-post if you need any further help.

Regards

Colin

• Hello Stephan

A colleague of mine pointed out to me that the reason that Eqn 37 uses 2X the peak current is that the 2 legs of the PFC always start up in-phase, where the currents’ peaks sum up directly.  Eventually, they move to be 180-deg out of phase.
There can be various circumstances which may result in a (temporary) cessation of switching, after which the 200us restart timer expires and it starts up again, in-phase.

My earlier comment about the average line currents summing is valid but this new insight makes it clear that the HF ripple cancellation is not always present.

Regards

Colin

• Hello Colin,

Nice to hear from you again!
This makes sense.
It will be wonderful if Texas Instruments could mention this in an update of the datasheet some time.
I am busy with the board-layout and am waiting for my ics.

Regards,

Stephan.
• Hi Stephan

I've added this issue in our 'submit documentation feedback' system for the UCC28063A and for the related UCC28063, UCC28061 and UCC28060 devices. The correction should be included in the next update cycle for these parts but I don't know when this may happen.

Thanks again for spotting this.

Regards
Colin