# Active PFC and square wave supply

P

#### Pimpom

Jan 1, 1970
0
I started using a Tagan 700BZ PSU in my computer a couple of months
ago - my first PSU with active PFC. The UPS is a basic one that has
served me well for 10 years, and outputs a square wave on battery.
While setting up this particular combination, I briefly wondered how
the PFC circuit would react to working with non-sinusoidal power but
then forgot about it - until it died a couple of days ago. It blew the
fuse on the UPS and shows a short at the AC input. One of the two
paralleled 20N60C3 MOSFETs was a dead short.

The UPS battery - a 70Ah car battery - is of the same age as the UPS
(10 yrs) and has little backup power left. So I always shut my
computer down quickly in the event of a power outage, of which there
were dozens during the 2 months I've been using this PSU.

The PSU died while I was in another room. I didn't notice a power
outage or fluctuation during that time but can't be 100% certain. Is
it likely that there was an outage and the PFC transistor was finally
killed by the square-wave supply?

To put it another way, if I repair the PSU, is it likely to go poof
again unless I change my UPS to a more sophisticated one? I'd never
done a detailed study of practical active PFCs before and could well
miss something with a quick analysis.

supplies after my PSU died but they're mostly end-user discussions and
are all inconclusive).

C

Jan 1, 1970
0
Jon Elson said:
Pimpom wrote:

Hmmm, I can't imagine a PFC can work properly when fed a non-sinusoidal
wave. But, the typical PFC is a boost regulator, so the transistors
are ACROSS the DC bus (with an inductor in series). I can imagine
that maybe the boost regulator is asking for high duty cycle = maximum
boost at the zero voltage point, then the UPS switches on and the
PFC creates excessive boost voltage before the PFC regulator can recover.

But, that is just a guess, and I'm assuming the UPS is not really true
square wave but "modified sine wave" which means there is a zero Volt
perion between each output pulse. Anyway, I cannot imagine a PFC
supply can properly handle a UPS with non-sine output. As these
PFC supplies become more common in larger appliances like computers,
the UPS makers surely are going to have to address these problems.
And, it sure isn't clear how to do that very cheaply. Mostly, they'd
need PWM synthesis of the sine wave, and then enough filtering to
smooth out the PWM carrier.

Maybe smarter PFC's could detect square-wave input and modify their
PWM logic to compensate.

Jon

An interesting test would be to see what a PFC does when you feed it plain
old DC.

Has anybody exploded any switching power supplies of any sort by just
feeding them DC, filtered or with 120Hz ripple? They should int theory
love that, but I'm not sure about the autoranging device they use before
the filter caps that either rectifies or doubles the AC input.

M

#### miso

Jan 1, 1970
0
My advice is use a double conversion true sine UPS or none at all. With
a double conversion UPS, you will know soon enough if everything is
compatible when the fecal matter hits the fan. That is, line and battery
operation are similar since the inverter is always running.

If you go through
http://www.xbitlabs.com
website, they test power supplies run from a crappy UPS. just to see how
much power they can deliver before the UPS protection circuitry cuts in.
Some supplies really piss off the UPS!

The drawback to double conversion is the thing is running 24 and 7. And
it makes noise. Oh, and they aren't cheap.

The advantage to double conversion is you have no brownout issues, line
to battery switchover is fast since the inverter is already running, and
the surge suppression is as good as it gets since they are filtering DC
prior to the inversion.

P

#### Pimpom

Jan 1, 1970
0
Jon said:
Pimpom wrote:

Hmmm, I can't imagine a PFC can work properly when fed a
non-sinusoidal wave. But, the typical PFC is a boost
regulator, so
the transistors
are ACROSS the DC bus (with an inductor in series). I can
imagine
that maybe the boost regulator is asking for high duty cycle =
maximum
boost at the zero voltage point, then the UPS switches on and
the
PFC creates excessive boost voltage before the PFC regulator
can
recover.

But, that is just a guess, and I'm assuming the UPS is not
really true
square wave but "modified sine wave" which means there is a
zero Volt
perion between each output pulse. Anyway, I cannot imagine a
PFC
supply can properly handle a UPS with non-sine output. As
these
PFC supplies become more common in larger appliances like
computers,
the UPS makers surely are going to have to address these
problems.
And, it sure isn't clear how to do that very cheaply. Mostly,
they'd
need PWM synthesis of the sine wave, and then enough filtering
to
smooth out the PWM carrier.

Maybe smarter PFC's could detect square-wave input and modify
their
PWM logic to compensate.

Jon

You're right in that my UPS is not true square wave but has a
dead time in between transitions. My son has been using a PSU
with active PFC for more than a year and his 800VA UPS ($45 with a 9Ah internal battery) outputs the same type of waveform except that mine has noticeably steeper edges. Maybe that's what makes the difference. Some of my friends are also using PSUs with PFC and, judging from what they paid for their UPSes, I'm sure none of them is a sinusoidal type. The focus here is on how the PFC components bear up to a non-sinusoidal input rather than on the actual PF correction. Similarly, although my computer needs less than half of the PSU's 700W rating, I used it because it returned good performance figures in reviews and I got a used unit cheap. C #### Cydrome Leader Jan 1, 1970 0 Jon Elson said: Cydrome Leader wrote: Years ago I worked on an outboard PFC that supplied PCs with pure DC at 340 V, worked fine as LONG as the PC had a classic voltage doubler/switch PWM supply. But, it wouldn't fly, as the PC's power switch and fuse were not rated for DC. And, of course, woe be to anyone that plugged in, say, a monitor with a 60 Hz transformer or AC degaussing coil! I think many PFC front ends might work fine with DC input within the right range, they'd just become a nice boost converter. But, of course, it depends on the control loop of the PFC chip. Jon I am pretty curious about this, but not enough to test it on my computer. Modern computers do lack AC blowers, real power switches and LCDs lack degaussing coils, so I decided to see what I switching power supplies I can explode with DC. Then things got "interesting". The laziest place to get 160or 340VDC is from a switching power supply. I grabbed some genericy open fram 110 watt unit from Artesyn and noticed it only had one, not two filter caps on the HV side- a single 400V fed by just a 4 pin bridge rectifier and the usualy noise filtering stuff. There was no autoranging chip, that that would do anything too useful with a single filter cap. The label on the power supply clearly states 100-240VAC 50/60Hz. I plugged it in and sure enough, there was only 164V on the filter cap. I'm going to have to guess you 330-ish if you feed it 240. I've never seen that before. So at least for this power supply, it would seem feeding it any type of DC would probably be OK. The real test is to run this power supply off the filtered 164V from it's twin. Then it's off to see what happens to stuff like power adapter bricks and maybe a CFL bulb. Anybody want to vote on if a low end CFL bulb with light up (flames don't count in this case) on DC? C #### Cydrome Leader Jan 1, 1970 0 Pimpom said: You're right in that my UPS is not true square wave but has a dead time in between transitions. My son has been using a PSU with active PFC for more than a year and his 800VA UPS ($45 with
a 9Ah internal battery) outputs the same type of waveform except
that mine has noticeably steeper edges. Maybe that's what makes
the difference. Some of my friends are also using PSUs with PFC
and, judging from what they paid for their UPSes, I'm sure none
of them is a sinusoidal type.

The focus here is on how the PFC components bear up to a
non-sinusoidal input rather than on the actual PF correction.
Similarly, although my computer needs less than half of the PSU's
700W rating, I used it because it returned good performance
figures in reviews and I got a used unit cheap.

PFC methods and how they work:

http://www.fairchildsemi.com/applications/motor-control/solutions/power-factor-correction/

My guess is whatever is cheapest to make and just passes interference
standards is what you're going to see in a PC power supply.

The boost one IS just a plain switching power supply anyways, so I'm not
sure why you'd string yet another switching power supply after that.

F

#### Fred Abse

Jan 1, 1970
0
Hmmm, I can't imagine a PFC can work properly when fed a non-sinusoidal
wave. But, the typical PFC is a boost regulator, so the transistors are
ACROSS the DC bus (with an inductor in series). I can imagine that maybe
the boost regulator is asking for high duty cycle = maximum boost at the
zero voltage point, then the UPS switches on and the PFC creates excessive
boost voltage before the PFC regulator can recover.

Most, if not all PFC circuits use feedforward from the incoming waveform.

M

#### miso

Jan 1, 1970
0
I ruined a notebook power supply with a modified sine inverter. Use that
shit at your own risk. Like I said, get a true sine double conversion
UPS or use nothing. You are far better off losing power than feeding a
PC with a modified sine.

They should call it a modified square wave! That is closer to the truth.

M

#### [email protected]

Jan 1, 1970
0
Has anybody exploded any switching power supplies of any sort by just
feeding them DC, filtered or with 120Hz ripple?

A few years back I fed a laptop "brick" power supply, 40 W nominal
output, with 216 V DC from a string of 12 V batteries. It worked fine
in a 5-minute test; the scope showed a lot less voltage ripple and
somewhat less current ripple. The supply didn't seem to heat up or
make noise any differently than it did when fed with AC. Load was a car
tail light lamp.
They should int theory love that, but I'm not sure about the
autoranging device they use before the filter caps that either
rectifies or doubles the AC input.

I think the one I used didn't have an auto-ranging function; I think
it's just a single wide-range input. The nameplate says "Input:
100-240V~ 1.0A-0.6A 50-60Hz" and "Output: 15V= 2.7A". It is for a
1997-ish laptop.

A lot of desktop computer power supplies have a manual 120 V / 240 V
switch on the back that controls "rectify or double". I have heard of
some that automatically control this decision. When used in a 230 V or
240 V country, and there is a momentary power dip, they sometimes pick
120 V, with predictable results.

Matt Roberds

J

#### josephkk

Jan 1, 1970
0
I started using a Tagan 700BZ PSU in my computer a couple of months
ago - my first PSU with active PFC. The UPS is a basic one that has
served me well for 10 years, and outputs a square wave on battery.
While setting up this particular combination, I briefly wondered how
the PFC circuit would react to working with non-sinusoidal power but
then forgot about it - until it died a couple of days ago. It blew the
fuse on the UPS and shows a short at the AC input. One of the two
paralleled 20N60C3 MOSFETs was a dead short.

The UPS battery - a 70Ah car battery - is of the same age as the UPS
(10 yrs) and has little backup power left. So I always shut my
computer down quickly in the event of a power outage, of which there
were dozens during the 2 months I've been using this PSU.

The PSU died while I was in another room. I didn't notice a power
outage or fluctuation during that time but can't be 100% certain. Is
it likely that there was an outage and the PFC transistor was finally
killed by the square-wave supply?

To put it another way, if I repair the PSU, is it likely to go poof
again unless I change my UPS to a more sophisticated one? I'd never
done a detailed study of practical active PFCs before and could well
miss something with a quick analysis.

supplies after my PSU died but they're mostly end-user discussions and
are all inconclusive).

PFC power supplies are designed with the ASSumption of sinusoidal sources.
The are likely fundamentally incompatable with non-sinusoidal sources.

Alternate solution: Huge LC filter, about the size of your computer due to
the large inductors and capacitors needed.

?-)

J

#### josephkk

Jan 1, 1970
0
An interesting test would be to see what a PFC does when you feed it plain
old DC.

Has anybody exploded any switching power supplies of any sort by just
feeding them DC, filtered or with 120Hz ripple? They should int theory
love that, but I'm not sure about the autoranging device they use before
the filter caps that either rectifies or doubles the AC input.
So far as i know, they would love straight DC and they do rectified ac as
part of their normal operation. Check some of the chips data sheets, it
is pretty clear there.

?-)

J

#### josephkk

Jan 1, 1970
0
Then things got "interesting". The laziest place to get 160or 340VDC is
from a switching power supply. I grabbed some genericy open fram 110 watt
unit from Artesyn and noticed it only had one, not two filter caps on the
HV side- a single 400V fed by just a 4 pin bridge rectifier and the usualy
noise filtering stuff. There was no autoranging chip, that that would do
anything too useful with a single filter cap. The label on the power
supply clearly states 100-240VAC 50/60Hz. I plugged it in and sure enough,
there was only 164V on the filter cap. I'm going to have to guess you
330-ish if you feed it 240. I've never seen that before. So at least for
this power supply, it would seem feeding it any type of DC would probably
be OK. The real test is to run this power supply off the filtered 164V
from it's twin.

Then it's off to see what happens to stuff like power adapter bricks and
maybe a CFL bulb. Anybody want to vote on if a low end CFL bulb with light
up (flames don't count in this case) on DC?
My bet is that CFLs will work fine on 90 to 250 V dc. Hell that is so
interesting i might try it myself.

?-)

C

Jan 1, 1970
0
josephkk said:
My bet is that CFLs will work fine on 90 to 250 V dc. Hell that is so
interesting i might try it myself.

?-)

It may take a few days to get to this project. I was also also startled to
learn that those bi-pin U shaped compact flourescent bulbs, at least the
pile of 13 watt ones I examined all had starters in the plastic base. The
best part is the leads of the cap and the starter glass thingy weren't
even soldered or crimped- just twisted together to the leads on the glass
envelope. The caps were as least plastic film and not wax impregnated
paper.

I seriously though the last place you'd see a flourescent starter module
was in the corner of an old, old hardware store.

C

Jan 1, 1970
0
josephkk said:
PFC power supplies are designed with the ASSumption of sinusoidal sources.
The are likely fundamentally incompatable with non-sinusoidal sources.

Alternate solution: Huge LC filter, about the size of your computer due to
the large inductors and capacitors needed.

?-)

a ferroresonant power conditioner would be fun too, but there's no way an
el-cheapo UPS could magnetize the core of one of those beasts and get it
started up.

F

#### Fred Abse

Jan 1, 1970
0
flourescent bulbs

They're only "flourescent" if you break one, and white powder comes out ;-)

F

Jan 1, 1970
0

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