Other Parts Discussed in Thread: UCC28070A, UCC27324
Hi Team,
We would like to ask your help regarding our customer's inquiry below.
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)
The attached LTspice sim proves this (see circuit number 2, which demo’s the incorrectness)
LT1248 Boost PFC 1000W 100vac_CST.zipUCC28070A with ramp signal added.pdf
(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 light load/zero_X effects.)
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?
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
(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 light load/zero_X effects.)
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?
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
..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 of UCC28070A, since the BJT adds a delay, but this shouldnt be an issue, as the max duty feature of UCC28070A doesn't 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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...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)
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 of UCC28070A, since the BJT adds a delay, but this shouldnt be an issue, as the max duty feature of UCC28070A doesn't 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?
.............................................
...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)
Regards,
Danilo
