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Old 9th December 2004, 12:40 PM   #1
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Question Switching Power supply

What about a switching power supply for a LM3886 amplifier?
My idea is to make a switching power supply for two mono amplifiers based on LM3886 chip but the power supply is made in a different box from the two amplifiers.
How many possibilites I have to do a great job if I think to use a good output filter for power supply's output voltage and big capacitors for amplifier's input supply voltage?
I want to connect the two amplifiers to power supply with a shilded cable to avoid trasmission of high frequency noises and a big filter in the power supply to avoid trasmission of noises below 100kHz.
I know that it's probably a well-known stupid idea but I want to know if someone have tried this solution and eventually results obtained.
Thanks

Giuliano
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Old 9th December 2004, 02:30 PM   #2
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it's been discussed on different and varying threads -- but DONT use big caps on the output of the SMPS -- a lot of heat is generated in these caps and usually 200uF or 330 uF are adequate. ripple is one thing, switching transients quite something different.

the output choke design is critical -- the value is inversely related to the current drawn -- your PC draws a relatively constant current from the SMPS, but in an amplifier the current will vary depending on the biasing. an smps has to be loaded at all times.
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Old 9th December 2004, 03:28 PM   #3
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Quote:
Originally posted by jackinnj
...an smps has to be loaded at all times.
Why did you say so?
In my opinion SMPS is able to work fine with light and heavy load.
Thanks for suggestions, infact probably I will use tantalum capacitor + MKT capacitors for SMPS output voltage to eliminate the ripple at high frequency in addiction to an big electrolitic capacitor (that has the function to be the bulk capacitor).
Thanks

Giuliano
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Old 9th December 2004, 03:55 PM   #4
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Quote:
Originally posted by allanon77


Why did you say so?
In my opinion SMPS is able to work fine with light and heavy load.
Thanks for suggestions, infact probably I will use tantalum capacitor + MKT capacitors for SMPS output voltage to eliminate the ripple at high frequency in addiction to an big electrolitic capacitor (that has the function to be the bulk capacitor).
Thanks

Giuliano
because most of the energy in an SMPS is stored in the inductor --

is there need for a reservoir capacitor -- a SMPS has poor transient response compared to a linear supply -- you might find it better, although less efficient, to use a high quality, discrete linear post-regulator.
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Old 9th December 2004, 04:02 PM   #5
Eva is offline Eva  Spain
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Your project is perfectly achievable but your appear to have very little knowledge and no SMPS experience

You will have to do some smaller and simpler SMPS projects in order to learn and gain the required experience. You will also require a >10Mhz dual oscilloscope and some home-made equipment to test transformer and inductor saturation characteristics

The output filter is usually just a second order low-pass filter. Its transfer characteristic depends on capacitance, capacitor ESR, inductance and inductor Rdc. Capacitance may be chosen to be small to reduce size and cost or to be huge to reduce output peak current requirements, but the value of the inductor and its Rdc have to be chosen to match the capacitor and get little overshoot [use a simulator to learn how each parameter affects the overall behavior of the filter]

Capacitors should be rated in excess of the ripple current requrements [you have to learn how to calculate it]. Also, inductor saturation current should be rated in excess of the peak current requirement [again you have to learn how to calculate it and how to design inductors]

Then, the entire system, including the 2nd order roll-off is enclosed into a feedback loop controlled by an op-amp, and you have to learn how to frequency-compensate this feedback loop for stability [the more peaky the filter, the more compensation required]

There are also more complex control techniques involving load current sensing and two nested control loops [op-amps] each with its own frequency compensation to make it stable [another subject to learn about]

Also you have to learn about transformer design and about optimizing switching device behavior to make it ringing-free. And proper PCB routing is like an art...

As you can see, there are several subjects involved in designing such an SMPS. Try simpler projects first
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Old 9th December 2004, 04:05 PM   #6
AGGEMAM is offline AGGEMAM  Denmark
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Doesn't that depend entirely on the construction of the SMPS? If it's built as a comparator type, ie you compare the regulated and filtered output to a reference voltage to control the switching instead of a fixed clock cycle, that should give excellent transient response as well as no fixed clock.
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Hmm .. no .. I really haven't got anythig cool to say ..
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Old 9th December 2004, 05:09 PM   #7
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Quote:
Originally posted by AGGEMAM
Doesn't that depend entirely on the construction of the SMPS? If it's built as a comparator type, ie you compare the regulated and filtered output to a reference voltage to control the switching instead of a fixed clock cycle, that should give excellent transient response as well as no fixed clock.
It takes several cycles for the loop to stabilize itself -- so the problem can long be in the rearview mirror before it is fixed (or the loop can go into a condition of never stabilizing -- as when you use a converter for a purpose for which it wasn't intended) -- the linear regulator burns the energy in about 1/10th the time (according to something I read from National Semi -- will dig up the source somewhere.)
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Old 9th December 2004, 05:10 PM   #8
Eva is offline Eva  Spain
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'Poor transient responde in SMPS' is just a myth created from a lack of knowledge and by people too lazy to learn to analyze the actual circuit behavior

SMPS just behave as 2nd order filters driven from a programmable current source whose output depends both on input voltage and on the driving signal from the error amplifier [the exact relationship is topology-dependent]

There is actually a crossover frequency, at wich the control loop action is reducing the actual output impedance of the 2nd filter to half the expected value. Above the crossover frequency the impedance of the capacitor dominates, below the crossover frequency the action of the control loop dominates. This crossover frequency is a design parameter free to play with, it depends on filter design and error-amp compensation

So all SMPS at high frequencies behave just like the output filter capacitors driven from a 'slow' current source and show the same HF transient response as these capacitors

Typical crossover frequencies may range from 50Hz to 5Khz [strongly depending on output filter size]

And to correct jackinnj statements :

Major energy storage happens *allways* on capacitors. SMPS filter inductors usually store 5 to 20 times less energy than its associated filter capacitors. Also, maximum inductor energy storage happens only at full load [E=.5*L*I^2], while capacitor energy storage is *constant* [E=.5*C*V^2]. This figures out to be pretty obvious when you design some output filters and inductors. Four 2200uF 25V capacitors charged to 15V use little space and store 1 Joule. How big is an inductor capable of storing 1 Joule or more energy?

Answer : Bigger than the entire SMPS, maybe bigger than two or three SMPSs each with 1 Joule energy storage on output capacitors. To better understand think that the usual 5cm diameter yellow-white iron-powder toroid core stores only 8 mili Joules before saturation
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Old 9th December 2004, 07:00 PM   #9
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Quote:
Originally posted by Eva
Your project is perfectly achievable but your appear to have very little knowledge and no SMPS experience
Sorry but why do you think so? because I don't talk about every SMPS's rules ... ... thank you for your suggestion I will talk with my boss about my lacks ...
I'm a electronic designer, young, but however I designed some SMPS in the last years.

Anyway ... I thank you for everything you said. You gave many useful information for new designers.

My dubts are about the results of the use of an well-designed SMPS with an audio chip to obtain, finally, a good audio result.
I haven't big problems with SMPS design but every audiophile I heard say the same thing: "the only supply for audio amplifiers is bridge+huge capacitors". So I thought: "Is it a problem of poor SMPS designers or it's really impossible to obtain great results with SMPS for audio amplifiers?
I'm interested with some personal experience of some audiophiles in SMPS supply for good audio amplifier.
Thanks to all.

P.S. I agree with you, I think that it's possible to design a SMPS with good transient response; my dubts are about the possibility to have a low noise (comparable to "bridge+huge capacitors").

Giuliano
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Old 10th December 2004, 05:32 AM   #10
mike.e is offline mike.e  New Zealand
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Recently I got this book out of my local Library.

"the power supply cookbook' by Marty brown

SMPS = not for the faint hearted.

SOmewhere locally on the net,there is a webring of SMPS builders that use a certain control chip with success. It might be related to the ESP website.
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