Where are high PF rectifiers used? - Page 1
Forum section: electronics / beginners

author: bonzer
author_rank: Regular Contributor
timestamp: December 02, 2019, 11:50:14 am
content: Hello everyone, sorry if I sound stupid with such questions but I studied time ago rectifiers with high power factors that emulate variable load after graetz bridge to reduce current harmonics number. They use buck boost or fly-back converters for that.Where are they used? Do they already use them in new generation battery chargers for mobile phones? Or in some particular applications? I think such a type of system is efficient but has a higher cost than classical simple graetz rectifiers with ICs like 7805 because there's also a control part and duty cycle generation etc ...

author: ferdieCX
author_rank: Regular Contributor
timestamp: December 05, 2019, 05:51:20 pm
content: I think, that the PC power supplies with 80 Plus certification use this kind of rectifiers.Also the quality on-line UPS systems are usually of this type

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 06:34:30 pm
content: FYI, we usually call that "active PFC", but rectifiers can also be used passively to give high power factor -- what might be terms "high PF rectifier".In three-phase systems, we can use transformers to construct any phase angle we like; by interleaving many phases, we get a "6N-pulse" rectifier (N = 1, 2, ...). The normal 3-leg FWB is a 6-pulse rectifier, which has ~0.9 PF. Using a delta-wye transformer, we get a 30 degree phase shift almost for free, and adding another 3-leg FWB, we get a 12-pulse rectifier with even better PF. And so on. These are relevant to very high power (MW+) industrial systems, where simplicity and reliability, and economy of scale to some extent, overcome the size/weight/cost concerns that are more important to smaller equipment.This isn't applicable to single-phase systems.Tim

author: bonzer
author_rank: Regular Contributor
timestamp: December 05, 2019, 06:43:25 pm
content: Thanks a lot for your answers!I'm glad you all mentioned the different categories by power. I also know about 6-phase modulation (space vector approach etc), thanks for mentioning.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 09:28:38 pm
content: Well, that would be the other side of it -- synthesis; an inverter turning DC into AC, rather than the other way around.Tim

author: sibeen
author_rank: Frequent Contributor
timestamp: December 05, 2019, 11:01:55 pm
content: The higher pulse numbers, 12, 18 , 24 etc used on larger three phase systems was normally used to lower the harmonic content, so a 6 pulse rectifier would be giving around 30% harmonic current a 12 pulse system would be down about 15%. That is without any filtering being implemented. With systems with a controlled DC voltage, and therefore using thryistors as the switching device, the input power factor was closely related to the firing angle of the thyristor.Modern rectifiers in large UPS and VFD systems often use an active style front end utillising IGBT switches and space vector modulation. Using these techniques the input harmonic current is kept below 3% and the power factor at better than 0.98.

author: Yansi
author_rank: Super Contributor
timestamp: December 05, 2019, 11:05:59 pm
content: Quote from: T3sl4co1l on December 05, 2019, 06:34:30 pmYou don't need to use multi-phase transformers, that kind of thing is used when you go the megawatt scale.Small industrial drives (up to tens kW) use mostly just a passive full bridge rectifiers followed by a choke. That alone can provide good enough PF of more than 0.9

author: langwadt
author_rank: Super Contributor
timestamp: December 05, 2019, 11:16:27 pm
content: Quote from: Yansi on December 05, 2019, 11:05:59 pmsure, three-phase alone helps alot

author: sibeen
author_rank: Frequent Contributor
timestamp: December 05, 2019, 11:21:46 pm
content: Multiphase transformers for 12 pulse rectifiers were very common for UPS systems above approximately 50 kVA. These have now been replaced by transformerless active front end systems up into the few MW range which give far superior results of 0.98 PF and harmonics of <3%.