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UCC28070A ramp circuit is wrong?

Flyback

Well-Known Member
Hi,
Would you agree, the App Note sluu312b (below) contains a sub-circuit that is wrong?....it concerns the bit on page 3 which imposes a compensatory ramp onto the current sense signal. This sub circuit would clearly not provide a ramp, since the burden resistor acts as such a low value “bottom divider” resistor that the ramp would be too insignificant. The attached LTspice sim proves this (see circuit number 2, which demo’s the incorrectness)

(BTW, the ramp is needed because the Current transformer magnetising current subtracts too much from the “ideal signal” at low load and at the mains zero crossings)

(There is also a DC level added, and this is needed to overcome other effects at light load/zero_x.)

Also, why on page 3 do they try to use the UCC27324 Gate drive chip output to produce the ramp?…..surely they should use the gate drive output on the UCC28070A chip itself?….it would allow far less noisy layout.

Do you agree that in the PDF and LTspice sim attached, the method of circuit number 4 is the best way to achieve what the App Note is trying to actually do?

(Unfortunately, we cannot go to the ti.com forum, since you need a company email address, and we are merely hobbyists)

App Note sluu312b
https://www.ti.com/lit/ug/sluu312b/sluu312b.pdf?ts=1653521265498&ref_url=https%3A%2F%2Fwww.google.com%2F

UCC28070A PFC controller (see pages 35 , 36 for relevant pages to this post)
https://www.ti.com/lit/ds/symlink/ucc28070a.pdf?ts=1653493319157&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FUCC28070A
 

Attachments

  • LT1248 Boost PFC 1000W 100vac_CST.asc
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  • UCC28070A with ramp signal added.pdf
    238.5 KB · Views: 14
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Flyback

Well-Known Member
...it seems that they have sized the upper R of the RC , low enough to get some ramp out of it......but then they end up needing a big C, and then the gate driver is left with the task of discharging this very very quickly.
Its a bit puzzling why they didnt use a BJT buffer like circuit 4, but maybe it is something to do with the max duty cycle setting, since the BJT adds a delay, but this shouldnt be an issue, as the max duty feature of UCC28070A doesnt actually measure duty and use it in a control loop?

They have an upper 2.49k, into a burden R of 16.5R (total)....this means a maximum ramp peak of 85.6mV.....thats not very much...the datasheet suggests 10% of max...ie 300mV....i dont see how you can practically get 300mV ramp without a buffer BJT like in circuit 4?
 
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Flyback

Well-Known Member
...sorry to harp on.....the question looks like becoming, do you know of a NPN type that can do the job of circuit 4 in the top post, but turn off more quickly?
Presumably a NPN with high fT and low-ish hfe and low max Ic , eg 100mA? (so not get swamped with minority carriers).

....Also, could we agree that the bottom waveform on page 36, Fig28, of the UCC28070A datasheet is nonsense?….the pulses don’t even line up with the inductor current shown there in the middle trace. That bottom trace is nonsense…you simply don’t see a waveform like that at the output of the current sense transformer when near the zero crossings.

UCC28070A datasheet
 
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