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Author Topic: Switch Mode Power Supplies  (Read 4196 times)

John Roberts {JR}

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Re: Switch Mode Power Supplies
« Reply #10 on: August 05, 2014, 09:11:14 pm »

Ok and from another question the capacitor holds and releases a charge to keep things smooth.  That charge goes both directions on the circuit and back to the diodes and creates a reverse bias on the diodes pushing noise back at the mains.   ??   Would it not also push some DC onto the AC mains ??

No, by definition diodes only conduct current in one direction. A diode with a reverse bias is just a high impedance, like an open circuit, within normal operating voltages. Caps in power supplies after rectifier diodes are often called "reservoir" caps since they store up and release charge to the power supply between charging pulses.

Note: an asymmetrical load that pulls more current in one direction than the other does in fact presents an effective DC term back to the mains.  I recall old light bulb life extenders that were simple diodes in the base of the lamp socket so the lamp only drew current for one half of the wave form.  One lamp no problem, lots of lamps a possible DC problem, unless you carefully alternated the direction of each of them, so 1/2 blocked one polarity and half the other.

JR
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Stephen Swaffer

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Re: Switch Mode Power Supplies
« Reply #11 on: August 05, 2014, 10:45:22 pm »

IIRC, it was fairly common for early tube equipment to use half wave rectifiers in their power supplies due to the cost of tubes.  Full wave required 2 rectifier tubes plus a costly center tapped transformer.  Full wave bridge rectifiers did not become common until solid state diodes become common.

Probably the strangest tech support call I ever placed was in the early 2000's.  We had a magnetic clamp on a surface grinder that had quit working-and it had a vacuum tube rectifier.  I was fairly certain the rectifier tube was bad-but figured I would never get any help with it.  The first tech I spoke with knew the problem and the part number and I had a replacement on the way in under 5 minutes on the phone.  In a day when 5 years old is obsolete, the 40 yr old tube stuff was a simple fix, go figure.

Would active PFC be more economical to properly implement in one large supply vs a number of small ones?
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Steve Swaffer

Steve M Smith

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Re: Switch Mode Power Supplies
« Reply #12 on: August 06, 2014, 03:48:40 am »

I recall old light bulb life extenders that were simple diodes in the base of the lamp socket so the lamp only drew current for one half of the wave form.

Many US voltage photographic enlargers are wired like this.  I had one which I ran via a 240 - 120v transformer.  They are fitted with a bulb rated at 82 volts.

I have no idea why they didn't just fit a 120 volt bulb and do away with the diode.


Steve.
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John Roberts {JR}

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Re: Switch Mode Power Supplies
« Reply #13 on: August 06, 2014, 10:42:19 am »



Would active PFC be more economical to properly implement in one large supply vs a number of small ones?

Interesting question... Probably not, but PFC is not cheap so not widely used for audio amps. The Euro zone government was talking about making it mandatory so most major amp makers developed the technology, and even one or two amp models were introduced but AFAIK this is still not mandatory. 

I guess we could make an AC to AC PFC correction unit, that works kind of after the fact so it puts out a low impedance AC waveform, then spreads out that current draw proportionately over the input sine wave.  The output wave form doesn't even need to be a perfect sine wave, like a cheap inverter, but still not trivial or cheap. 

JR
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Mike Karseboom

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Re: Switch Mode Power Supplies
« Reply #14 on: August 07, 2014, 11:34:29 am »

This discussion is interesting to me because I have 8 QSC PLX (1) series amps which are, I believe, switching amps with no PFC.  I have run these off a variety of power sources including generators.  I have tried to reconcile some of the general statements I read/hear about these type amps with my personal experience.  The clarification of what happens to the current wave form seems to help explain some of the general statements.


Statement:  You need a "stiff" power supply to get the most out of the PLX amp. 


Well that is no doubt true for all amps but I have definitely noticed a subjective improvement in sound quality when I can dedicate a 20A circuit to a maximum of 2 amps.  Subjectively the sound is just more "present", "spacious", and "punchy" at the same time.  I also have some older RMX 2450's and some even older Crown Powerbase 2 amps and have not detected the same "improvement" when the power supply was very robust.


Statement:   PLX amps suck on subs,  old iron amps work better


I suppose this relates back to the all too typical situation where the power supply is less than optimum. 


Statement:  PLX amps "sip" power compared to old iron amps


The arguments put forth with the kino light vs. fresnel light seem to indicate that switch mode amps are less efficient and draw more peak amperage and waste a lot of power.  So how is it that switch mode amps get the reputation for not needing as much power?



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Steve M Smith

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Re: Switch Mode Power Supplies
« Reply #15 on: August 07, 2014, 11:47:26 am »

With enough capacitance in the output, it shouldn't really matter if a power supply is linear or switch mode.  I suspect the perceived differences are due to manufacturers using the smallest capacitors they can get away with when designing switch mode supplies.


Steve.
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John Roberts {JR}

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Re: Switch Mode Power Supplies
« Reply #16 on: August 07, 2014, 01:03:12 pm »


Statement:  PLX amps "sip" power compared to old iron amps
Kind of depend on how old that "old iron" is... CS1200 old, or RMX2450 old? The modern amps like 2450 are more efficient so deliver more power to speakers for the same line cord draw.
Quote

The arguments put forth with the kino light vs. fresnel light seem to indicate that switch mode amps are less efficient and draw more peak amperage and waste a lot of power.  So how is it that switch mode amps get the reputation for not needing as much power?
There seems to be some confusion regarding "peak current" and "efficiency"... Efficiency is about the average power output vs average power consumed. Power supply topologies like PFC that smooth out the current draw over the entire waveform cause less peak IxR voltage drop, in the mains wiring, so actually allow the amp to pull more voltage and power from a given mains distribution. This "lost" power difference is not literally wasted or dissipated in the mains wiring but is more of a missed opportunity. The power that an amplifier can put out is limited by the internal DC power supply rail voltages. The voltage of these internal rails is a simple function of the Peak mains voltage. With high peak current draw concentrated at the top of the mains power sine wave, IxR voltage drop at the line cord limits how much voltage the internal PS rails get charged up to.   

I can imagine a rack full of power amps putting a hurt on the top of sine waves, with or without switching supplies, while the raw diode cap front end of many switchers will try to draw the same current over a narrower window, so higher peak current for the same average current compared to big transformer amps.

JR

PS: One of my old amp ideas, that I never developed due to cost/complexity was to perform a variation on the classG/H multi-rail power amps where instead of using separate transformer windings for each rail voltage, each charging their respective PS caps only at the top of the mains sine wave, use a high power switch to connect and disconnect from the same transformer winding, before and/or after the peak voltage to charge the lower voltage amp rails. The high voltage rail still draws current from the top of the sine wave waveform, but the other rails draw their power away from the sine wave peak. This approach would be like a poor man's PFC and spread the current draw away from the top of the sine wave so effectively reduce the IxR voltage drop that limits max power.  I did not pursue this because in my estimation it would not be clean enough to qualify as true PFC to meet european harmonics regulations, so relatively expensive for the partial benefit it could deliver.  Note: This switch disconnect could even vary the actual voltage of the lower rails to optimize amp efficiency for real world audio signals.

PPS: I have other unexplored amp ideas that are even more bizarre but academic at this point with class D now cost effective. All we need now is cost effective PFC.
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Re: Switch Mode Power Supplies
« Reply #16 on: August 07, 2014, 01:03:12 pm »


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