Does Boost PFC ramp circuit need diode to be added? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 21, 2022, 05:23:55 pm
content: Hi,Is the diode D13 necessary to reverse recover the Base-emitter diode in the NPN Q1?(the sim works without it, but I suspect real life may be different?)It’s a ramp producing circuit to inject ramp into the current sense signal during light load and during mains voltage zero crossings. This is needed for UCC28070A Boost PFC controller.UCC28070A datasheet:https://www.ti.com/lit/ds/symlink/ucc28070a.pdf?ts=1658423975378&ref_url=https%253A%252F%252Fwww.google.com%252FLTspice sim and JPEG schem attached(Note to Mods if wished: I am only logged in due to this being a different browser, which autologs me in)

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2022, 06:00:14 pm
content: There is something missing from the schematic. D10 in series with D11 clamp a negative going voltage at 52 volts which without D13 would be applied to C16 through the emitter upsetting the slope generator, but there is no indication where that voltage would come from because D9 would block it.

author: Faringdon
author_rank: Super Contributor
timestamp: July 21, 2022, 07:26:56 pm
content: Thanks, my mistake, ive put the rectifier diode on CST sec the wrong side of the reset cct.Sorry about that.....corrected as attached.Please may i still ask the original question?

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2022, 08:45:27 pm
content: I am thinking out loud. Maybe it will prompt someone else to comment.When the switch turns on, the current measured from L9 starts rising at the ramp injection point. Emitter follower Q1 starts rising from RC network R20 and C16, adding another current to the ramp injection point.When the switch turns off, current in L9 reverses and is clamped at a high level by D10 and D11. D12 quickly resets RC network R20 and C16.When L9 resets, there is a high voltage applied across D10 and D11, which capacitively couples through D9, is attenuated by C17, capacitively couples through D14, and acts through R22 to pull the emitter of Q1 down, but the emitter of Q1 should handle the now tiny pulse easily.When the gate voltage of M1 is driven low, Q1 operates in reverse with low beta to discharge the capacitance at its emitter, but D13 does not do anything useful for that either, and D14 blocks most of C17, so there is an explanation for D14, although I am not sure it is needed either because R22 limits the current. Q1 might suffer a bit of beta degradation, but so what?Could D13 be left over form an earlier design?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2022, 09:51:51 pm
content: B16 is a supernode. This is a bunch of noise.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: July 21, 2022, 10:52:51 pm
content: Thanks,The circuit is a form of page 18 of the following...https://www.ti.com/lit/an/slua479b/slua479b.pdf?ts=1658443855014&ref_url=https%253A%252F%252Fwww.google.co.uk%252F...you can see the RC(s) and discharge diode(s) there at the output of the FET gate driver(s)...i just added a NPN to buffer the ramp.The use of the NPN buffer is like the diagram on bottom pf page 12 of this...(AYK in that case for adding a slope compensation ramp)https://www.st.com/resource/en/application_note/an246-uc3842-provides-lowcost-current-mode-control-stmicroelectronics.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2022, 11:06:27 pm
content: But that connects to CSA/CSB.Did... you mean to connect to the controller ISENSE or whatever pin, from the current transformer thingy?Tim

author: Faringdon
author_rank: Super Contributor
timestamp: July 22, 2022, 06:09:16 am
content: QuoteThanks, sorry i should have said, this was just to demo the ramp only , using a quick simulation, i didnt mean it to connect to the LT1248......in the actual cct, a UCC28070A is used.In the (corrected) sim of post #2, the ramp is seen working well, giving a good ramp signal at the zero crossings, but i wondered if without D13 it would work so well in real life?I also wondered why ti.com hadnt used a BJT to buffer their ramp in their above link, page 18 (300W PFC....)?