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Class D Switching Power Amplifiers and Power D/A conversion

Line-powered SMPS for Power Amp?
Line-powered SMPS for Power Amp?
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Old 15th February 2004, 03:49 PM   #21
Peter Daniel is offline Peter Daniel  Canada
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Originally posted by marcus66
I'm sorry if my post above was against the rules! I did check the forum rules and moderator's statement, and assumed the subject to be OK, but if discussing a SMPS is not allowed, I apologize!

Originally posted by Eva
Anybody could suggest me a cool forum where switching knowledge could be freely shared? I feel this board is not the right place for serious switching discussion and power conversion has evolved a *lot* since 50-60Hz 'passive' switching

Originally posted by joan2
I would be happy if there is a permanent thread regarding smps on this website....
Actually, the discussion of SMPS itself is not breaking the forum rules. The rules are mainly about direct "main power supplies". We could allow an SMPS discussion as long as the idea of direct main operation is not included.

It is still more dangerous than typical SS PS (at least for somebody who just starts with them) and any one of them can kill you. We just have to make sure that safety is addressed. But it is the feeling among the moderating crew, that such discussions, when evolving sensibly, should be a part of the forum.
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Old 16th February 2004, 08:47 AM   #22
li_gangyi is offline li_gangyi  Singapore
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hell even a 12V battery can shock if given the right corrections...I'm just browsing as I get along...don't really understand SMPS and stuff too well but I have played with the LM series of switchers and the results really look promising...well I guess I just have to learn more...
Kids in the back seat cause accidents...Accidents in the back seat cause kids...
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Old 2nd March 2004, 04:25 AM   #23
slowhands is offline slowhands  United States
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Switching power supplies have been used off and on in audio gear. Virtually all car audio amps use them to step up the 12 volt battery voltage to what the amp needs, say +-30 to 50v. Sony had a line of receivers in the mid 80s that used them, very light. Carver used SCR switch regulated power supplies in his lightweight amps. So they are nothing new to audio, and certainly fair game here.

They get rid of weight, particularly in high powered amps. Tranny's and filter caps for 60Hz just have to be bigger and heavier than those for 20-100kHz. That's the big attraction. But they may end up being more complex than the amp, they are very challenging to do well.

Of course they are ubiquitous in PCs. There, high volume drives cost down for components, and amortizes expensive engineering design over lots of units too. The supplies used in most PCs that I have seen were incredibly optimized to shave every cent of cost, and then made in vast quanties to get economy of scale. There are tons (literally) of old AT style supplies lying about, and you can salvage quite a few components from them.

The typical supply is a half bridge pulse width modulated supply, typically doubling the US 115v mains and using a full wave bridge for 220v, thus allowing worldwide operation with a simple switch of the diode array. What that means is you have 330VDC stored in a pair of caps - it can knock you on your *** or worse. Caution and experience are really advised if you want to play with these.

But you can conceivably lift the front end, rewind the transformer secondary for the voltages you need, and possibly use some of the regulation circuitry. (Many use a plain Jane TL494 PWM controller, pretty well documented). It may be easy to open the transformer, it's probably 2 E cores epoxied together but I have not done it. Count the number of turns on the 5V winding to get turns per volt, and re-wind accordingly. You can't used any of the rectification or linear regulators on the board, but no big deal. You don't need very good regulation, because your amp has high power supply ripple rejection.

Probably the Achilles heel will be high frequency switching noise. It should probably be in its own shielded box, with input and output filters and proper power wiring (how you ground the system will be critical). A second difficult issue will be stability. Amps are challenging loads especially if driven hard at a frequency at which the power supply regulator is not stable. I've seen guys tear their hair out designing these, it is very challenging but potentially very rewarding too. Good luck!
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Old 2nd March 2004, 02:37 PM   #24
mwh-eng is offline mwh-eng  United States
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Originally posted by slowhands
Probably the Achilles heel will be high frequency switching noise.
I agree! Even for the experienced SMPS engineer, dealing with the noise or not creating it in the first place, will take the most effort. Minimizing high dv/dt and di/dt, and eliminating sources of ringing helps. Zero voltage and/or zero current switching helps. PCB layout is very important. Shielding may be necessary.

The PC half-bridge topology would not be my first choice. For low to moderate power, I may consider a modified forward converter. For high power, I would consider full bridge resonant switching. Many ways to accomplish the basic conversion, but I believe the complication may lie with noise suppression and elimination. I would probably approach this power conversion with a topology that has the lowest source of noise generation, so it will require the least amount of noise suppression circuitry. In other words, minimize generation of the noise in the first place, so you have less to deal with. If your converter generates a lot of noise, then it takes a lot of extra filtering to reduce or eliminate it.

If you find a design that has been developed and proven already, use it!
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Old 6th March 2004, 01:19 AM   #25
mwh-eng is offline mwh-eng  United States
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Originally posted by Eva

Windings are designed to operate at 31Khz [pretty slow for today standards but allows to use cheap bipolar transistors for switching and still get low losses, 4x MJE18008] and to mantain 1KW output [14,4V 72A regulated] with little air cooling [less than 8 watts losses in the copper]

Insulation between primary and secondary in this prototype consists of 3 layers of common insulating tape [I can't find the usual mylar tape in my country] and wires are protected with PVC tubing when they enter and leave the bobbin

PD: Anybody could suggest me a cool forum where switching knowledge could be freely shared? I feel this board is not the right place for serious switching discussion and power conversion has evolved a *lot* since 50-60Hz 'passive' switching

I'm surprised you cannot get inexpensive Mosfets for your SMPS designs. I prefer using Mosfets over bipolars in SMPS designs for several reasons.

Not sure, but www.digikey.com may ship international at decent prices.

I don't use mylar tape either; however, the thin white teflon tape that plumbers use works for me.

Maybe another topic of discussion could be added for power electronics so technology such as SMPS, Class D and other high efficiency amps, switching converters for replacing tube amp output transformers, etc., could be discussed. It could be something like "Everything Switching" or similar.
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Old 6th March 2004, 01:31 AM   #26
sss is offline sss  Israel
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i got an old magazine with 1KW smps for an amp where u can change the output voltage up to +-70V , the schematic looks very complicated .

i wanna make smps for car amp and i got those pc smps's ,
can i take the core from them to use with my car smps?
or is it too small (the core)
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Old 6th March 2004, 01:55 AM   #27
mwh-eng is offline mwh-eng  United States
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Size of a core, winding area, and frequency determine the obtainable output power . This assumes the core material is made for the application. If you are referring to cores from a PC SMPS, they will probably work. Would be better if you could buy new cores so you wouldn't have to unwind the PS transformers. SMPS designand development is not trivial.

Depending on the shape of the cores, they might be stackable. For example, two toroidal cores can be stacked for increased power handling capability.
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Old 7th March 2004, 03:56 PM   #28
sstoiana is offline sstoiana  Romania
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Default QSC audio PLX series amplifier schematic

ok, i've visited the qsc website and they charge $15 for the service manual/schematic... does anyone have the schematic and would like to share it ?
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Old 7th March 2004, 08:34 PM   #29
kilowattski is offline kilowattski  United States
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SMPS are also used in some High End gear also. If I am not mistaken the Rowland 112 power amplifier uses a SMPS.
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Old 7th March 2004, 11:46 PM   #30
Eva is offline Eva  Spain
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mwh-eng :

These oscilograms show why I like modern switching bipolar transisotrs. MJE18008 were good but MJE13009 are outstanding

The circuit is a full bridge of Motorola MJE13009 driving a transformer whose secondary is syncronously rectified and feeds a LC filter [transformer-coupled buck converter] to get a regulated DC output [12-14.4V at 72A or more, for automotive purposes]

Transistors are driven by a proportional-drive circuit through a transformer having a base current supply winding connected in series with main transformer, similar to the one used in AT and ATX PC power supplies but extended to full bridge.

The converter is self-oscillating like AT PC supplies so it starts to oscillate by itself until the control circuit, powered by the secondary side of the main transformer, has enough voltage to take control of the converter [through the base drive transformer]. This means the control circuit is fully isolated and connected to the secondary side [no optocouplers needed] without requiring a separate auxiliary supply to power it

Blue trace is the current through the main transformer at 2.5A/div, measured using a current transformer in series with the main transformer [that introduces some hysteresis]. 0A corresponds to the center. Switching frequency is about 31.6Khz [63.2Khz clock]

Red trace is the voltage to ground on one of the switching sides of the bridge at 50V div. Zero value is a bit misadjusted, voltage drop in the transistors while saturated is about 0,6V for 6A [equivalent to a Rds_on of 0.1 ohm]

This shows overall switching waveforms at 5uS/div :
Click the image to open in full size.

This is a detail of the turn on and saturation process at 200nS/div :
Click the image to open in full size.

This is a detail of the turn-off and desaturation process at 200nS/div:
Click the image to open in full size.

These captures were obtained with the converter delivering 1KW continuously to the load. RF ringing typical of MOSFETS [>10Mhz] is 100% absent [bipolars tend to be 'smooth' and generate very little RFI]

What do you think about switching times?
Who said bipolar transistors were too slow for switching applications?

Compare the price of a MJE13009 to the price of a MOSFET having similar losses. A 20A 500V or more device in TO-247 bulky case will be required. Dedicated drive circuits will be also required in order to speed up turn-on and turn-off. Take into account that a simple IR2112 IC is more expensive than a pulse transformer and doesn't provide enough current to drive a 'big' mosfet fast enough, so it has to be buffered

In my opinion, MOSFETS are not the best solution for switching more than 250V
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