A short piece on PFC in SMPS... there are many but this one is in easy to understand form for all to benifit: www.ospmag.com/files/pdf/whitepaper/Power-Factor-and-Input.pdf
THx-RNMarsh
THx-RNMarsh
And about 50% efficiency. Actually, you can get all kinds of ground to ground issues. Been there, done that. ( Ironic, we were an OEM to Uniwack)
Something to read while I am sitting home at tax payers expense because we freak out around here with 4 inches of snow.
Well, they can't drive worth a lick in the sunny dry weather here either! If I was still in Boulder, yea, spring t-shirt skiing. (The ladies dressed a bit differently ) That's Boulder, Colorado for the international set.
Read the bit on active PFC. It is looking at the problem from the SMPS input issue as to not cause issues back on the mains. Fine, but no help if we are building an amplifier other than in understanding the cheap PS in my computer does not have such devices in it and is putting those harmonics back onto the same branch as one of my amps which is why my computer strip is fed from a big multi-stage pi filter. So this is causing a harmonic noise problem, rather than the phase problem motors cause.
Other than the need for a decent input filter, what does this mean to building a power amp? Big inefficient E-cores still rule? Does the active PFC effect the OUTPUT of the SMPS if we are using it for an amp? The paper does not suggest this.
) That's Boulder, Colorado for the international set.Read the bit on active PFC. It is looking at the problem from the SMPS input issue as to not cause issues back on the mains. Fine, but no help if we are building an amplifier other than in understanding the cheap PS in my computer does not have such devices in it and is putting those harmonics back onto the same branch as one of my amps which is why my computer strip is fed from a big multi-stage pi filter. So this is causing a harmonic noise problem, rather than the phase problem motors cause.
Other than the need for a decent input filter, what does this mean to building a power amp? Big inefficient E-cores still rule? Does the active PFC effect the OUTPUT of the SMPS if we are using it for an amp? The paper does not suggest this.
Question: How large a local on-board filter cap can I safely use if I am feeding it with an SMPS? Example, I am getting one of these cheap Chinese chip amps to drive my wife's keyboard speaker. I will feed it with an old 2A-12V laptop power supply. Can I add a small cap bank inside the amp or do I rely solely on the regulator to provide slean dynamic power? Clearly, not ultimate fidelity, but it need not be worse than necessary.
I have argued against the suitability of small SMPS for High Fidelity Amplifiers.
I simply ask:
where does the speaker get it's peak transient current from?
The very fastest transients come from the HF decoupling.
The slightly slower transients come from the MF decoupling as the HF decoupling becomes depleted.
The slowest transients come from the smoothing capacitance as the MF decoupling becomes depleted.
Most SMPS cannot feed conventional smoothing capacitance that is designed for 50/60Hz operation. The SMPS smoothing capacitance can be around 47uF to 150uF.
Where does the speaker get it's peak transient current from?
The peak current capability of the SMPS should be around the expected peak current demand of the speaker.
A 150VA transformer/rectifier/smoothing, fed 100W into 8ohms ClassAB amplifier can deliver a far high peak transient current than a 150W smps fed amplifier, simply because the smoothing is substantially absent.
I simply ask:
where does the speaker get it's peak transient current from?
The very fastest transients come from the HF decoupling.
The slightly slower transients come from the MF decoupling as the HF decoupling becomes depleted.
The slowest transients come from the smoothing capacitance as the MF decoupling becomes depleted.
Most SMPS cannot feed conventional smoothing capacitance that is designed for 50/60Hz operation. The SMPS smoothing capacitance can be around 47uF to 150uF.
Where does the speaker get it's peak transient current from?
The peak current capability of the SMPS should be around the expected peak current demand of the speaker.
A 150VA transformer/rectifier/smoothing, fed 100W into 8ohms ClassAB amplifier can deliver a far high peak transient current than a 150W smps fed amplifier, simply because the smoothing is substantially absent.
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Andrew, you also have to think about how quickly the storage caps are being replenished. For a 50/60 Hz system, it's one every 10/8 ms for FW rectification with a very short window (2-3 ms perhaps) in which to charge the caps. OTOH, an SMPS running at a typical 100 kHz is pumping energy into the caps once every 10 us. So, it's the SMPS that can deliver and sustain the higher peak currents.
Still wouldn't use 'me on any of my amps though!
Some SMPS's are able to handle multi-kilo uF reservoirs at the output. Specifically for audio purposes.
There was a Spanish outfit building/selling them, forgot the name.
Also think Hypex has some of those.
I am using a pair of eBay SMPSs with 4700uF on each rail, seems to run fine.
So, it's matter of selecting the right unit.
Jan
Hi Andrew,
This a good concern, and I have addressed it before in my book and in some discussions online. The simple answer is that designers of amplifiers using SMPS must not skimp on the reservoir capacitors. Ideally, the reservoir capacitors should be just as large as if the supply were conventional. As you understand, it is not just about ripple. There is a reason the call them "reservoir" capacitors.
There is no reason that a well-designed SMPS cannot feed large reservoir capacitors. However, there are some potential subtleties regarding the fact that the SMPS is often very tightly regulated, and what it may do when the even large reservoir capacitors sag a bit under a transient. It is important that the SMPS handle this situation gracefully.
Cheers,
Bob
Another possibility is to interpose a DIY soft-start / slow rampup circuit between the SMPS and the many-dozen-millifarad capacitor bank. Keep I = C x (dV/dt) under control by deliberate choice of looooong dt. Have the soft-start circuit communicate with the I/O relay controller (esp. muting) and all's well.
Hi great subject
so how to specify a reasonable sized reservoir cap for a class AB audio amp , or maybe the Q is how to specify a regulated SMPS properly for an audio power amp ?
I'm sure there are unregulated SMPS for audio but I think they have draw backs as well. Maybe users can adjust the rails to their line voltage and put up with the rail sag under load.
How many SMPS specify the transient current capability?
If a 150W +-45Vdc smps can output 1.7A per polarity, is that a continuous 1.7A or 50% duty cycle,
Can that same 150W smps supply 200% (3.4Apk) for 100ms?
Can it supply 300% (5Apk) for 50ms?
Can it supply 900% (15Apk) for 1, or 5, or 10ms?
All of these current pulses are available from a +-20mF smoothing (the size I recommend for good bass into an 8ohms speaker) cap bank to feed a reactive 8ohms speaker.
The smoothing caps and the inductors (that I have seen) on the output of the SMPS certainly can't supply 15Apk.
Many SMPS will refuse to start up with a decent cap bank attached.
That is why I ask that question. Where does the current come from, when there is no smoothing cap bank as found on conventional PSUs?
It matters not how often the capacitors are recharged.
If one were to specify a +-45Vdc, +-15Apk SMPS then the question is answered for us.
That is, use a 1350W SMPS for a 100W into 8ohms amplifier and one ensures that the SMPS can supply the 15A pk currents that can be demanded by a single 8ohms severe reactance speaker.
If one knows that the decoupling and small smoothing allowed on the output of the chosen SMPS is say 4Apk, then one can downgrade the SMPS specification to speaker peak demand minus the short term capacitor capability. i.e. 11Apk @ +-45Vdc. This 990W smps would indicate a secure 15Apk into the amplifier if the 8ohms speaker demands that much.
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Mr. Self,
Could you address the influence of loadshare of the output devices on performance. Is having more output devices than required by usage and dissipation guidelines beneficial?
The relation with dynamic related distortion due to dye heating and thermal stability due to bigger heat transfer to heatsink (thermal compensation benefit?)
As well as influence on component life due to lower temp fluctuations.
Are there penalties like higher distortion and required bias?
As well as influence of lowering the supply voltage and using differential output with both complementary output devices.
BTW. My compliments on the previous books!
Could you address the influence of loadshare of the output devices on performance. Is having more output devices than required by usage and dissipation guidelines beneficial?
The relation with dynamic related distortion due to dye heating and thermal stability due to bigger heat transfer to heatsink (thermal compensation benefit?)
As well as influence on component life due to lower temp fluctuations.
Are there penalties like higher distortion and required bias?
As well as influence of lowering the supply voltage and using differential output with both complementary output devices.
BTW. My compliments on the previous books!
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