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Why have Active PFC in Audio SMPS?

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grigson

grigson

Electrical
Aug 21, 2011
69
Hi,

I have just seen the datasheet for a 180W (peak) offline SMPS which comprises a single stage active power factor corrector...

here it is, its the SMPS180 by Hypex......


I cannot , for the life of me, understand why Hypex have made a Power Factor Corrected SMPS for Audio usage.

There is not a single country in the world that requests Power Factor Correction for Audio usage.

Since this (single stage) SMPS is power factor corrected , it will comprise a dreadfully slow feedback bandwidth, (~10Hz)which is totally unwanted in audio applications, where good transient response is required.

In the datasheet, Hypex extol the virtues of them having the bulk storage capacitance on the secondary side...........

.....i can't think why they believe that this is virtuous, the entire switch-mode industry knows only too well that capacitive storage banks are best placed where you have the highest voltage...........generally at the mains side.
Capacitive energy storage quadruples with doubling capacitor voltage because of the square law of capacitive energy storage.

The lack of capacitance on the primary side of the SMPS180 means that it is harder for the SMPS180 to filter the high frequency switching harmonics from the mains....i'm not saying it cant be done, but your hampered by the lack of primary side capacitance.

The SMPS180 will, however, reduce mains harmonic current levels, in comparison to a non-PFC design.........however, what on earth is the point of doing this when the regulatory bodies have no requirement of it.?

A PFC design will be more expensive and require more engineering effort than a non-PFC design, and since there are no advantages in the Audio world of using PFC designs, why have Hypex chosen to do a PFC design.?

Perhaps i am being too cynical there, the inrush current will be less with a single stage PFC design....however, inrush is easily circumvented with NTC's.

One point about Single stage PFC design is that the peak FET currents will be higher, and the transformer will need to be bigger.
The FET RMS current will also be higher, and a bigger FET heatsink, or more expensive low RDS(on) FET wil be required.

Can any reader think of a reason for using the SMPS180 in an Audio application?

Here are Audio SMPS's of several 100W's power level, which have no PFC stage..........................




The deleterious point about Hypex SMPS180 is that it is a *single stage* PFC design............if it had comprised a PFC Boost converter, followed by a downstream SMPS, then that would have made sense, since the high voltage bus provided by the Boost PFC stage, would mean a convenient high input voltage for the downstream SMPS stage, which would allow it a very good transient response.
 
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"There is not a single country in the world that requests Power Factor Correction for Audio usage"

EU has a 75 W lower limit for non-PFC devices. That includes computers and all other mains connected devices. Like audio amplifiers. Have a look at especially page 8, where equipment classes and their harmonics limits are listed.

This applies to EU - and that is a lot more than your "not a single country in the world".



Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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Filtering the HF stuff out shouldn't be too hard on the secondary side, and it is entirely possible to push the PFC switching frequency well above the audio spectrum. Any decent amplifier will have a good supply ripple rejection ratio even if the supply isn't perfect.

If you're one of the 'audiophile' golden-ear whackos then you won't like it here. Too many objective people who believe in measurements.


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If we learn from our mistakes I'm getting a great education!
 
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Another quote: "Capacitive energy storage quadruples with doubling capacitor voltage because of the square law of capacitive energy storage"

True. But the fact is that energy also is proportional to capacitance and that low voltage capacitors are smaller and cheaper than their 'high' voltage equivalents. So, that part of your reasoning is also not very valid.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Hi

Skogsgurra:
"But the fact is that energy also is proportional to capacitance and that low voltage capacitors are smaller and cheaper than their 'high' voltage equivalents"

OK but i'm certain, after working in many SMPS companies, that when a compnay is looking to add hold-up time to an offline SMPS, the place where they add the storage capacitance is on the 400V post-PFC bus...in all cases.

i can't believe they would be doing this if it was more expensive.

Adding PFC to sudio devices would add inefficiency...especially since most of the worlds audio equipment is operating at 5-10W average power (over time) or less....even if its peak power is several 100's of Watts.

I remember working at a flat screen TV place and the test setup for the audio amplifiers at maximum was a mere 10W test jig.....it was just some power resistors which were supplied at 10W to simulate the audio amps in the TV on max load....since obviously you didnt want to be doing max load testing with the audio amp on max as you couldnt hear each other talk....so just use resistive load instead.

So PFC like SMPS180 means higher rms input currents and greater inefficiency...greater rms fet current etc...
 
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As Gunnar rightly pointed out, the "EU has a 75 W lower limit for non-PFC devices". I'm sure this company could design their unit to be non PFC compliant. It would probably be cheaper to design and manufacture.

Sales may be a little slow though in places that use the standard. In my little part of the world the equivalent standard became mandatory about 3 weeks ago.
 
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EN61000-3-2 regulations refer to constant power......but smps for guitar amplifiers, even if rated to 400W peak, draw <<<<75W on average.

So EN61000-3-2 exempts guitar amplifiers due to the low avergae power.
 
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Really you need to re-read that table, because you are wrong.

From the flowchart, "Household equip., tools excluding portable, audio equipment?" to which I assume you would say 'yes' because the word "portable" is associated with tools, not audio equipment. That makes it a Class A device according to EN 61000-3-2 (2006). Table 1 on page 7 applies.


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sorry yes, i didnt see the comma.

Bu there is an exemption from PFC for guitar amps, because there must be since companies have 350W guitar amps with no PFC....out there in the market, now.

I think the exemption is that guitar amps are approval tersted on 1/8th nominal power?
 
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This thread has gotten rediculously pointless.

EN 61000-3-2 clearly dictates the requirement for such a power supply configuration. So, the question was answered.

The company's UdC modules are designed for other uses besides guitar amplifiers So, the guitar amplifier arguement is pointless as well. Besides, there has been no reference for this claim.


 
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Wriggling like a worm on a hook.

Why this obsessive desire to refute what is clearly in the standard? Have you designed a non-PFC amplifier and are worried you can't sell it? [poke]


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If we learn from our mistakes I'm getting a great education!
 
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My thinking is, that PFC is pointless for 350W guitar amplifiers that , on average , draw <<<<75W.

So if we market it without PFC, will the regulatory bodies show discretion and turn a blind eye?

-Or will they make us do a complete product recall and refund to the customer?

- and then close us down?

The extra Watts consumed in the PFC stage would be a waste of energy........and the power is just too low for us to be clogging the supply network with harmonics....
 
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"the power is just too low for us to be clogging the supply network with harmonics"

Right. But there is not only you. And, who shall be allowed to do the pollution - and who shall not be allowed to do so?

Do you have a reason to be treated extra nicely?

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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I like to see products evolve , an in the case of SMPS equipment ,
that includes , not Exploding a 440V capacitor from violent in-rush
currents , If PFC circuitry is developed with years of practical experience ,and some innovation .
Higher cost + Quality , hopefully brings reliability !
 
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