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Old 18th December 2011, 06:12 PM   #1
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Default Why so many Bridge SMPS's here?

Hello,

I am confused that there are so many Bridge SMPS's being built on the diyaudio forum, for powering Class D amplifiers.

Since we are diy'ers, and since we are powering our audio equipment, and since our main goal is Audio Amplifier performance, why are so many people choosing to vastly over-engineer their SMPS?

Below 500W, for an audio application , there is absolutely no need to do anything more than a Flyback.

The average power of a Class D amplifier is Peak Power /8

So for a 500W amplifier, that means its basically 62W.....and that is flyback territory.......do we want a half-bridge or full bridge for 62W?

Also, a flyback is the simplest SMPS.....it gives you well-coupled split rail outputs without needing external coupled inductors.

A split output flyback has just one FET and two diodes.

The 500W Single switch flyback will give you about 4W or so of dissipation in the primary RCD clamp even when on no load......but we in the audio world are not required to conform to "Blue Angel" type efficiency requirements..........just like we are not required to conform to PFC requirements.....so its no problem........if it is a problem, then simply do a two switch flyback instead.

Doing a Bridge SMPS for a 500W amplifier which is going to be on 62W average power is massive overkill.

Anybody would think that this was a Switch Mode Power Supply website.........rather than an audio website.

Another thing is that Class D supplys need fast transient response, and so they need current mode control, with duty cycle of 0.4 maximum.

-That means that the flyback primary RMS current need be no bigger than any other topology.

-Admittedly a full bridge will have less rms current *per FET*.....but for 62W power, who cares?

The Half-Bridge is a real bogus smps......it may have two fets switching alternateley, but it scores a MASSIVE own goal in that only half the bus is switched...what a waste of time that is for audio!

So , does any reader know why the SMPS's being built on this site are miles over-engineered?

Last edited by eem2am; 18th December 2011 at 06:29 PM. Reason: MORE INFO REQUIRED
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Old 18th December 2011, 11:15 PM   #2
magnu is offline magnu  Sweden
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I see what you mean and somewhat agree.

But we need to be able to deliver 500W instantly. This can be solved with reservoir capacitors.

Otherwhise we need to design a power supply with 62W average and 500W peak. This means that we can use less cooling. We can use FETs with higher RDSon.

But the transfomer, here we have two criterias, it shall not overheat and not saturate. A transformer must never saturate, this gives a current rush, since the inductance disappears.

But the PSU must be able to handle a sine output from the amplifier at full power.
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Old 18th December 2011, 11:36 PM   #3
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A 62 watt flyback supply that has to provide 500 watts peak is a mess. Let me try to explain before you resopnd. Any supply has to be designed to provide the maximum "rated" power no matter long it lasts. Otherwise it will sag significantly when the high power load is applied. Therefore, you must design for 500 watts. The net result will be that at 62 watts the duty cycle will be very small and the currents will be relatively large at this power and duty cycle. The efficiency will suffer. In a "forward" type converter the duty cycle is a function of the output voltage, not out power. Therefore it will have a higher efficiency.
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Old 19th December 2011, 08:38 AM   #4
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Remember i'm speaking of Continuous conduction mode at 500W for flyback, forward or bridge.....

The primary current profile, whether flyback, forward or bridge, will be the same.

This is because we cannot allow the duty cycle to be more than 0.4......if we allow more than 0.4 then we have slower transient response and need slope compensation.

So we are looking at trapezoidal shaped primary current for flyback, forward or bridge.................

So the duty cycle will not vary with load for flyback, forward or bridge......that is, until the load gets really light and we go into discontinuous mode.....in which caase the duty cycle will then decrease in flyback, forward or bridge.

The shape of the primary current will be the same whether flyback forward or bridge.

Regarding bridges.....
The textbooks tell us that , for example , a full-bridge is superior to a two transistor forward because it more greatly utilises the transformer....in that it can have duty cycle greater then 0.5.
...but in supply for class D audio, we want to be in current mode, and we do not want a duty cycle >0.4 because it just means a slower transient response...so the bridges ability to have duty cycle > 0.5 is useless to us.




I will however, admit that the flyback would be less efficient than bridge or 2-transistor-forward..................but many amplifiers use mains transformers these days and are very inefficient...............................in audio, efficiency isnt really bothersome.......no government officials are going to crack open an amplifier to test it for blue angel conformity.

The flyback would be more inefficient than bridge or 2-tran-forward because of switching losses and the use of ~800V fets which have higher rdson.............but with 62W average power and for audio use, are we concerned about shaving the efficiency to hell and back.?

I would certainly rate the components for the 500W, and it still wouldnt be that large, and in any case for diyaudio, its absolutely fine, leaves more time to get on with designing the class D audio amplifier
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Old 26th December 2011, 03:35 PM   #5
dtproff is offline dtproff  United States
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Default topology choices are not quite so straight forward...

To each there own...

I have done multi-kilowatt flybacks and I have done 65W HB designs (in fact I have even designed a 9W full-bridge before). In each case the end application ripple requirements,EMI performance,and cost determined my topology.

Most of the time the driving consideration for me is output ripple at peak load. Usually a flyback has a duty cycle less than 75%. The secondary capacitance has to support the output the entire time that the primary side switch is conducting. For HB/FB applications this means that on average my output capacitance has to be larger for a flyback than a HB/FB topology. If my end application can tolerate this, then there really isn't an issue.

EMI is the other portion of this that seems to be overlooked a lot in the design considerations. In general I tend to use the resonant topologies for that reason.

The rule of thumb I have usually used is that for average power to about 65W I use standard flybacks. When I have a PFC front end I can stretch the Flyback to 130-180W. Above that I am using HB/FB topologies. When I have high peak to average power ratios such as you describe, I tend to end up with slightly higher complexity designs to make sure that I can handle the corner cases (low line and peak current) in order to ensure design margin.

One final thought... I design power supplies for a living so what is easy for me is not so easy for others. Thus... I leave the topology choice recommendation to what the designer is comfortable with as long as the cost and performance criteria are met.

Tony
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Old 26th December 2011, 04:22 PM   #6
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Regarding EMC, a 500W flyback for Audio Class D supply would be EMC tested at 1/8th power....ie 62W.....this does ease the situation with EMC a bit.

For Class D amplifier Power supplies, the Class D output has a huge LC filter and so switching frequency ripple on the power supply output isn't too much of an issue, as switching frequency is typically well above audio frequency and so gets filtered out by the LC filter of the Class D amplifier.

Also, An audio offline flyback solution at 500W isnt necessarily all that physically big.

....also, the flyback means you avoid coupled output inductors if its a split rail supply.
(pse see pg 3 part 5.............
http://www.ti.com/lit/an/slua119/slua119.pdf
)

Also, its pointless to use a high side drive if theres no need.

The single switch flyback does give primary clamp resistor losses, though the "RCDQ" clamp can mitigate this at light load.

I think Class D audio amplifier design, as well as all the tweaking of the pre-amplifier circuitry so that the best sound can be achieved, is hard enough, and more than a lifetime's work for anybody, if you really go into audiophile territory.......the same for other amplifiers too.........

...given this fact, its surprising that there's so much over-engineering of the SMPS being done...........Definetely for 500W and below, flyback is all you need in the audio world.

Some would say some type of resonant converter should be used as it can give small solution size at high switching frequency...........but small solution size, certainly for guitar amplifiers, is not wanted............the guitar amplifier and speaker cabinet is part of the stage "furniture"......no self-respecting guitar player is going to stand there with some tiny cube of a guitar amplifier head........on the contrary , its the bigger the better.
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Old 31st December 2011, 05:00 AM   #7
dtproff is offline dtproff  United States
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My reasoning for resonant is several fold. First I am more efficient thus heat sinking, airflow, etc... are minimized. It has very low harmonic content on the secondary side so that the noise is lower (less chance of the noise coupling into the amplifiers). I can run higher power levels with better core utilization, EMI is better almost everywhere, the Sync Fet drive is easy unlike CCM Flyback and finally, there is no need for a couple inductor since I use the leakage of the transformer as the resonant element.

But again, to each their own. There are a number of ways to arrive at the same endpoint. I mention resonant because I am fond of them for the upper power ranges. Just as I am for a phase shifted full bridge or interleaved resonant converters. They all have their place and I won't denigrate anyones choice on the issue.

It all comes down to what you are comfortable with and what design goals you are trying to achieve. That doesn't mean that any one topology is the only way or even the "right" way.

Tony
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Old 31st December 2011, 09:36 AM   #8
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One limitation of LLC resonant converters is the dynamic range of the gain......this means that universal operation (even with a changeover link) is not really possible.

.....ie please see under "frequency and gain" section of the following......

http://www.eetimes.com/design/power-...ant-converters

Also, with LLC resonant converters, they are voltage mode controlled.....and so the transient response is not as quick as a current mode controlled topology......transient response is very important for class D power supplys.

There is a way round this, ie you can run the LLC resonant converter at a very high switching frequency and end up enabling a high loop bandwidth like that, but high switching frequencys cause problems even for LLC converters..........the main problem is the gate drive transformer, because at high frequencies, it is particularaly sensitive to leakage inductance, so it has to be manufactured with a very strict tolerance.

Another problem for LLC converters is multiple outputs, including bias windings........The modus operandi of an LLC converter with an integrated transformer is that there is *not* good coupling between primary and secondary............this means problems for multiple windings, where you have to worry over the way in which each coil is coupled in to the core..............with an integrated transformer, the secondary is often 'scrunched' into a tight space, and this is not a good starting point to get an extra output winding to couple up to it.

The same problems arise for the bias winding, .....and where llc converters are supplied by PFC, a coil is often used on the pfc inductor to supply the LLC resonant converter.

Although it might be worth thinking about it with an external "leakage" inductor......although it may end up being rather large.

So LLC converters for Class D amplifier supply at least, is not necessarily the best way.

The thing about audio is that the equipment is going to be big and chunky anyway..........speaker cabinets are big pieces of furniture......you cant make them sleek and flat, so the name of the game in audio is basically having the amp and speaker as big blocks of furniture, and making them part of the stage set up.

This means that small 30W/inch^3 SMPS's are not really all that required in the audio world.



The one application that actually screamed for an LLC resonant converter is the LED streetlight........theres a PFC front end, so no line regulation problems, and the load is constant load......so the LLC converter is basically the only realistic way forward there due to the superb cost and efficiency for that particular use. (being streetlights, very numerous, the authorites insist on efficiency requirements, unlike the audio world, where the authorities turn a blind eye even to very inefficienct mains transformer based power supplies)
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Old 1st January 2012, 02:53 PM   #9
Cristi is offline Cristi  Europe
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What's the EMI content, radiated and conducted of a 500W flyback converter with moderate EMI filters compared with the same power level LLC converter, with the same filters. please give us numbers, not hypothesis.
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Old 1st January 2012, 03:09 PM   #10
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I dont have the figures to hand but the 500W flyback gets EMC tests done at 500/8 Watts = 62W, so the EMC filtering components do not have to be as big as would think.
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