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Why use PFC for audio SMPS?

Discussion in 'Audio' started by eem2am, Aug 21, 2011.

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  1. eem2am


    Aug 3, 2009

    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.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It's polite, it's nice, and why do you insist people don't do it just because there are no regulations to force them to?

    Many power supplies for audio (power) use are pretty much unregulated anyway.
  3. davelectronic


    Dec 13, 2010

    I cant see what the problem is, as long as the SMPS meets the ratings for its use there should be no problem for audio and other apps, its not that critical, the unit looks well made, and an ok looking company, unless its critical, and for audio i doubt it, the psu looks ok to me. Dave.
  4. eem2am


    Aug 3, 2009
    sorry but it is not nice, and it is making the supply more inefficient.

    since audio equipment (most of it) generally operates at well under 75W average, adding PFC stage just means an inefficiency is added here.

    (the RMS FET current will be much higher with a single stage PFC, and i^2R losses will arise)

    So there are no green credentials for SMPS180, and i beg to differ in that it is not nice at all.

    Was the SMPS180 a "nice" one for some Engineer's CV?

    It is a total mystery why its been done.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Hey, maybe because "Regulatory agencies such as the EU have set harmonic limits as a method of improving power factor. Declining component cost has hastened implementation of two different methods. To comply with current EU standard EN61000-3-2, all switched-mode power supplies with output power more than 75 W must include passive PFC, at least. 80 PLUS power supply certification requires a power factor of 0.9 or more." (see here)

    Maybe they have designed for the EU and don't want different designs for different parts of the world?
  6. eem2am


    Aug 3, 2009

    I know that SMPS for guitar amplififers definetely dont need PFC.

    EN61000-3-2 says nothing at all about guitar amplifiers, and thus it is taken as wrote by the guitar amplifier industry that they dont need them.....since EN61000-3-2 does not mention guitar amps, and says nothing about SMPS's (such as guitar amplififer SMPS's) which draw very low average power (<<75W) over time, even if they have peaks of 100's of Watts.

    I have taken many guitar amplifiers apart and none have a PFC stage, even if they are for 100's of Watts...and these are on sale in the well as worldwide.

    the Coldamp and Abletec supplies above dont have PFC...and indeed , most of Hypex's SMPS's for audio usage dont have PFC even if for >75W.

    I am not slating Hypex as you will know, its just that i am wondering which supply to buy, and came across this one that has PFC
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Personally, if it was me, I'd buy the one with PFC.

    And of course EN61000-3-2 says nothing about guitar amplifiers. It almost certainly says nothing about a lot of things. I do expect it says a lot about power supplies, which this is one.

    I would suspect that this power supply would help in reducing the amount of audio frequency power line interference by reducing the amount of weird harmonics it puts on the mains.

    It would also contribute to lower I^2R losses in the power network and make life easier for your power generators.

    There's nothing magic about any particular power level. It's not like a single 100W SMPS places a weird load on the mains that two 50W units would not.

    I'd much prefer if ALL my SMPS had PFC. My home shows a base line load of 900W using a current clamp -- that's almost 4A, however my power meter shows that my actual load is much smaller. At a rough guess, I have about 30 small SMPS's operating. It would be convenient for both myself and the company generating my power if my load was closer to a unity power factor. -- And on that point, I'm planning to replace many of these small SMPS's with a couple of larger ones that do have PFC.
  8. eem2am


    Aug 3, 2009

    Regarding "audio frequency power line interference"

    yes the non-PFC smps will generate more mains harmonics, but these are too low in frequency to interfere with the circuitry.

    ....A PFC SMPS would generate more high frequency interference which definetely could interfere with the electronics....since PFC stages have limited input capacitance.....and so more high frequency noise can get into the mains, and also interfere with other things.

    It may be worth PFC'ing some SMPS....but definetely not guitar amplififer SMPS supplies.
    -This is because the average guitar amplifier, when used in the home by someone for the pleasure of generally going to be drawing much less than 10W.

    Seriously, pick up an electric guitar with an amplifier and start strumming away.....i gaurantee you that you'll quickly get won't be strumming for hours on end like a heavy metal guitarist...and the louder you play..the quicker you'll get bored and stop....

    -So the inefficiency of a PFC stage would be deleterious.

    I used to work in a flat-screen TV company, and we saw the inefficiency of PFC SMPS first hand.....................

    Modern flat-screen tv's have an "active standby" mode.....where they download updates from the web etc......the TV consumes 15W whilst in "active standby".

    We found, if we switched off the PFC stage for "active standby"...that the TV consumed just 11W.

    So there we have it......for light loads, or for loads with low average power, PFC is a complete and utter waste of time.
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