Why does PFC controller need large protection diodes? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: August 05, 2022, 02:21:40 pm
content: Hi,Page 3 of the UC3854 Boost PFC controller App Note (DN-39E, below) says that three Schottky diodes are needed to protect the UC3854……They recommend a DO201 (1N5821) Schottky at the gate drive output to prevent oscillations due to “stray inductance” in the gate drive circuit. But surely at most all that’s needed here is a SMA Schottky, or even a SOT23 such as BAT54?....after all, a big axial diode like IN5821 is going to introduce lead inductance anyway. And surely if it has to be thru hole, then why not a little DO35 Schottky, after all, gate drive ringing is going to be very short transients.Also, why are DO41 (1N5817) schottkys needed at the inputs to the current error amplifier?…again, surely only SOT23 schottkys are needed here?…or DO35’s if insistent on thru hole?Also, if you are only using the gate drive output to drive a fet driver IC through a 5k1/5k1 divider, then is it right to say that the schottky at the gate drive output can be omitted?DN-39E App note on UC3854https://www.ti.com/lit/an/slua172/slua172.pdf?HQS=ti-null-null-productcentre_appnt-manupromo-mc-ElectronicSpecifier-eu&ts=1659708754055UC3854 datasheethttps://www.ti.com/lit/ds/symlink/uc3854.pdf?ts=1659624624801&ref_url=https%253A%252F%252Fwww.google.com%252F1N5821 DO201https://www.onsemi.com/pdf/datasheet/1n5820-d.pdf1N5817 DO41https://www.vishay.com/docs/88525/1n5817.pdfAlso, on page 2 of DN-44, it says that the UC3854 needs a diode on its "voltage amplifier output". Do you know what this means? Neither datasheet or App Notes show any diode at the output of the "voltage error amplifier output"...which is what i assume they mean by "voltage amplifier output"?DN-44https://www.ti.com/lit/an/slua177a/slua177a.pdf?ts=1659711251494&ref_url=https%253A%252F%252Fwww.google.com%252FDoes anyone have experience of using this chip?.....from the above it sounds hyper sensitive and difficult to use?There doesnt seem to be a demo board for sale for it.

author: mzzj
author_rank: Super Contributor
timestamp: August 13, 2022, 02:29:19 pm
content: It almost sounds like you would need real power electronics Engineer? Do you agree?Previously you or "treez" were wondering why there is so little SMPS production&development in Europe/UK.. competent engineers are hard to come by.Chinese company can get incompetent engineers lot cheaper than in UK. Do you agree? (competent SMPS engineers are not cheap in China either from what I have heard)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2022, 03:49:49 pm
content: Would think it more likely with transformer coupling, though you shouldn't need that with PFC for fairly obvious reasons. Have done that before, TL598 etc. need clamp diodes. But those outputs are pretty weak (~200mA) so a high inductance GDT is needed (many mH) and mere BAT54S, B0540, etc. suffice.Keep in mind the age of the part. Most everything UCxxxx is ancient Unitrode stuff. 1N58xx also very ancient (still in common supply, mind, as there hasn't been much improvement in schottkys over the decades). Back in the days when single layer boards were standard, and BJTs were just starting to be displaced by MOSFETs. There could easily be a long inductive path between switch and controller. Not that that should still matter, because the gate resistor should always be chosen to, at worst, slightly underdampen the gate drive loop.Tim