How do you do 3kw PFC with no significant output capacitance?

[treez]

Hello,
We are using a 3.3kw Brusa NLG514 electric car battery charger (Air-cooled, mains powered). We dropped it many times and thought something was loose inside, so opened it up.
We saw absolutely no electrolytic capacitors at all!

Brusa NLG514 charger.
**broken link removed**

There were two separate film capacitor banks of about 6uF each, that was it!!
The power transformer was potted into the corner so we could not see it. (~65mmby65mmby45mm)
We saw two torroids in one of the side panels which we assumed must be boost PFC inductors but dont know because they are sandwiched inside two panels.

Surely electrolytic capacitors (or at least several 100 uF's of capacitance) are needed following a 3kw boost PFC stage?

How have they avoided having a proper post PFC capacitor bank?

We wondered if perhaps it was not PFC'd? ..but then surely PFC is essential for 3300W single phase mains chargers, or else the reactive current is going to be too high and they wouldn't be able to draw enough current to get 3300W output from the single phase mains...since it is illegal to draw more than 16A from a single phase mains socket.

What topology do you think they have used?...and how have they avoided a significant post PFC cap bank?

(We cannot remove the bottom PCB as it is glued down, and has the FETs layed flat underneath it. Theres no room there though for electrolytic caps)

 

[FvM]

If you have the device, why don't you measure it's characteristics in a tester instead of guessing?

By the way, according to the datasheet the output ripple is quite high, they are apparently using the battery as PFC energy storage.

 

[treez]

thanks, unfortunately when taking it apart , we smashed one of the inductors, (it was right near a stiff screw from which the spanner slipped) so are waiting for the replacement to come in from farnell before we can power it up.
As you know, a boost PFC output is not usually isolated from the mains...wouldn't that be a problem?
Unless it was some kind of single stage pfc/isolation thing.

 

[FvM]

a boost PFC output is not usually isolated from the mains

I don't understand the relation to the original question.

According to datasheet, the device provides PFC (pf = 0.99) and galvanic isolation. So the question isn't if but how the features are implemented.

At PFC battery charger must not necessarily use a large filter capacitor if it forwards the pulsating input current and power to the battery. There are two simple disadavantages:
- the isolating DC/DC converter must be designed for double peak power
- the battery has to absorb the ripple current

 

[mtwieg]

If the intermediate DC/DC converter is designed properly, it can be controlled such that it is a transparent DC voltage transformer (constant duty cycle), and the battery can be used to smooth the voltage at the PFC output. Though as FvM states the DC-DC converter will need to handle the high 100/120Hz ripple between the PFC and battery.

The bigger question is how the battery feels about the large ripple current...

 

[treez]

Thanks, ive always been of the impression that batteries dont mind ripple current , no matter how big, as long as its at frequency greater than 1Hz or so.
I think this because years ago we had a doctor from Saft come and visit us at a different place. I remember him being asked about ripple current into batteries and he said it was ok as long as it wasn’t below about 1Hz.

Theres a big heatsinked "DSP like" chip in there..so I guess this is doing some exotic control algorithm to deliver the 0.99 power factor.

 

[D.A.(Tony)Stewart]

I dont understand why you think there must be electrolytic caps to control the PF. I would only use sub-class X1 rated plastic film that can handle much higher ripple current with a Dissipation factor better than 1e-3, but then filtered with suitable choke to pass on conducted emissions.

If the PFM or PWM regulates the current to match the Vac voltage waveform you get unity PF.

The current can be regulated to not exceed 16Arms AC and not exceed battery current limit nor Vdc battery voltage .

 

[Warpspeed]

About forty five years ago we were taught that high mains ripple current into lead acid batteries was very bad because it eroded and pitted the plates. No actual mechanism for this was ever suggested, but our most learned and venerable lecturer assured us young innocents, that this was so.
And there were scary pictures of badly eroded and pitted plates in a book to "prove" it.

Today many of us are experimenting with battery desulphators, that pulse huge currents at Khz rates into batteries, with seemingly good results as far as lowering internal resistance and restoring capacity of lethargic batteries.

For other battery chemistries I have no idea, but the incredibly low source impedance of some modern battery technologies suggest that high ripple may not be a problem.

 

[D.A.(Tony)Stewart]

Cars use 3 phase alt. so ripple is low compared to peak

Whereas single phase , the current is only limited by the diode resistance and can be 100x the average at 1% duty cycle due to the million Farads in a car battery unless regulated somehow. Thus Single phase it far more stressful on rectifier circuits and reactive loads.

Heat is deadly to batteries.

 

[treez]

thanks, interesting that the following 3.3kw charger does contain 810uF of electrolytics, and is about 6 times cheaper. ..

https://www.google.co.uk/url?sa=t&r...-mu3UYfLLlcKKhARQ&sig2=aZpGCg7UpKf35euC8Rkimg

 

[FvM]

Some posts are only loosely related to the original topic.

Particularly we are talking about 100/120 Hz ripple current with sine waveform, peak-to-peak current up to double the DC charging current.

I agree that the Li-ion battery of an electric can basically handle the ripple current. If you go for fast charging, the extra resistive losses are probably unwanted. But fast charging would use a three-phase supplied charger with zero low-frequent ripple.

 

[Easy peasy]

There is no PF stage on this charger, 6 diodes off the 3 ph mains, this should be obvious by inspection (also data sheet) depending on the power stage used the o/p is unfiltered 100kHz which uses the L of the cables to smooth out, batteries can easily handle 300Hz (if any) and 200kHz ripple when charging (only charging), a fast feedback loop would remove almost all of the 300Hz ripple.

 

[FvM]

There is no PF stage on this charger

Which charger are you talking about? Apparently neither NLG514 nor the Eltek product, both are clearly single phase + PFC.

 

[Easy peasy]

the one the post is about, it shows 3 breakers, hence 3 phase in....

 

[FvM]

The wall box, charger cable and car connector are optionally three phase, NLG514 seems to be designed for 230V/16A supply. May be with a three-phase option for 100 or 120V grid connection. In any case it's utilizing PFC (PF = 0.99).

- - - Updated - - -

The connector is IEC 62196-2 type 2, supporting up to 400V 32A three phase. But it can be used with 230V 16A as well, if the power supply or charger don't supply/need more power.

 

[Mr.Cool]

i agree this charger is using the battery for energy storage to the battery will see large low frequency ripple. also this means it is a single-stage PFC topology which means it is not using the entire power cycle to make correction but rather just the diode conduction period. its normal for single stage PFC topology to not have an output cap because that cap would have moments where it is floating which is not good.

i would say that brusa makes a charger that is economical and meets minimum requirements but it is not a good example of a superior product (despite what their product marketing says)

 

[D.A.(Tony)Stewart]

Maybe they have a sale on their green technology.

https://www.eltek.com/photoalbum/view2/P3NpemU9b3JnJmlkPTQ4MzgzNw

But then again they say 95% efficiency at 100% load of 3000W then specify ....

Cooling External liquid cooled cold plate or convection cooled heat sink. Worst case heat dissipation approximately 200W

... which is 6.7% loss or 93.3% efficiency.

.. which may be for 10% variation of nominal line voltage where conduction losses usually increase with lower input voltage for constant output power, even though they are not clear about if this is nominal or best case... probably best case at nominal voltage, even though they spec 85-275 VAC (Nominal 230VAC) input range while the Brusa is spec'd for 100VAC to 264VAC

meanwhile the Brusa NLG514 is 93% efficient reported in specs in both air and water cooled options.

Methinks , this is marketting playing spec games.

 

[treez]

thanks, alsom I wouldn't mind knowing what topology Brusa are using with their single stage PFC/isolation.
Surely not LLC....wouldn't have the gain at mains dips
Surely not full bridge....can't see an output inductor, unless they potted it in with the transformer...
Surely not flyback(s)..too much reflected voltage and leakage inductance related loss.

 

[Easy peasy]

possibly one of our single stage implementations using resonant technology... ) EP.