What's this DC buss circuit add on doing?

[AlfBaz]

Recently I had to fix our AC unit and managed to find a schematic for it.
It's an "inverter" style unit where the compressor is driven by one

Whilst I had it apart I decided to have a bit of a poke around when I came across this part of the circuit.

I've had some ideas but I'm not entirely convinced any of them are right
The one I'm leaning to the most is it creates a variable frequency ripple current in an attempt to increase the "reactor's" inductive reactance, somehow limiting current into the DC buss capacitors but that just seems like a crappy way to do things

Any ideas?
Attached is a pdf page of the whole schematic plus this pic here of the relevant circuit

[Circlotron]

Looks like active power factor correction. Bit of an odd cct though.
http://www.powerfactor.us/active.html

[Berni]

I think it's a cheep attempt at power factor correction too. When ATX computer PSUs claim they have power factor correction this is the kind of thing you find inside.

We have a heat pump that runs a 3 phase compressor on a variable frequency drive, but it does proper active PFC using IGBTs.

[AlfBaz]

Thanks guys. PFC had crossed my mind but unfortunately I know precious little about active PFC and the topology of this cct is unfamiliar to me.
Thanks for the link Circlotron. The link in the references led me to what appears to be an in depth article note on active PFC by OnSemi... a good opportunity to brush up

Seeing that 71.5k resistor in parallel with 0.025ohms makes me think something's not right here

[Phaedrus]

I think it's a cheep attempt at power factor correction too. When ATX computer PSUs claim they have power factor correction this is the kind of thing you find inside.

Not since like 2004. Turns out copper for inductors costs more than mosfets and a PFC controller. No one uses passive PFC unless they're modifying an older design to pass EU/AU/NZ regulations; and nowadays there are efficiency requirements too, so those older designs are getting more sparse by the quarter.

Most (not all) PSUs on the market with so-called "Passive PFC" really have fake inductors, usually cement or rocks wrapped in yellow tape.

[DanielS]

Not since like 2004. Turns out copper for inductors costs more than mosfets and a PFC controller. No one uses passive PFC unless they're modifying an older design to pass EU/AU/NZ regulations; and nowadays there are efficiency requirements too, so those older designs are getting more sparse by the quarter.

The disappearance of passive PFC has more to do with 80+ certification requiring a power factor of at least 0.9 to qualify and passive PFC cannot achieve much better than 0.8. PSU manufacturers who want to put an 80+ label on their product have to include active PFC on top of achieving at least 80% efficiency. The 80+ certification makes passive PFC effectively worthless marketing-wise, so there is no point in bothering with rocks aside from defrauding people who believe heavier is better.

The cheapest PSUs do not bother with the added costs, weight and complexity of faking passive PFC; they simply omit it altogether. Low-end PSUs from at least somewhat reputable vendors also omitted PFC until 80+ certification started becoming popular.

[Phaedrus]

The other aspect driving fake passive PFC is the EU regulations requiring all SMPS devices over 75W to feature power factor correction. So Chinese factories that sell non-80PLUS garbage PSUs for €10-€20 add the rocks in hopes of fooling lazy/unknowledgeable customs inspectors, mollify distributors, etc. That's why you almost never see passive PFC in the US and Canada, as these countries have no legal requirement for PFC, so it's cheaper to omit the rocks.

There's a lot of regional factors like that at play. For instance, fake 80PLUS badges are way more common in Latin America than they are in the US or Europe, because the "heavy = better" meme never took root there. Instead they have the "80PLUS = good" notion, donde si el fuente es certificado, es muy bien! Doesn't matter if the unit is a 0.5kg gray box with a 115V/230V switch and unsleeved, 20cm cables, if it's certified it must be good!

[DanielS]

For instance, fake 80PLUS badges are way more common in Latin America than they are in the US or Europe, because the "heavy = better" meme never took root there.

I think stronger consumer protection, fraud, marketing and other laws or regulations may have more to do with it: once customers' (or local competing manufacturers, vendors, distributors, etc.) complaints about your goods reach the customs agency, your shipments are more likely to get inspected, seized and possibly destroyed depending on the severity of whatever laws and regulations your products might be breaking.

[Phaedrus]

To an extent, yes. But US consumer protection laws are actually pretty toothless unless your product is endangering life and limb. So you can claim all the false efficiency or power ratings you like and no one cares; but if you fake a UL badge you're getting a visit from the feds. Overrated power supplies are very common, and companies like Raidmax have gotten away with sending completely different PSUs to Ecos Consulting (firm that does 80PLUS cert) than they actually sell to consumers, using the same model number: Raidmax AE series. The unit sold to the end user is Andyson E series, 80-86% efficient. Unit sent to Ecos for certification was Andyson K series, 87-90% efficient. It's been pointed out many times, including by professional reviewers, but the AE series continues to be one of the most popular (and cheapest) "Gold" rated PSUs on Newegg and Amazon. Ecos doesn't care, because they got their $5,000 check per SKU, and the feds don't care, because consumer fraud protection?