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Class D Switching Power Amplifiers and Power D/A conversion

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Old 23rd October 2003, 05:06 AM   #31
subwo1 is offline subwo1  United States
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I think a 200v mosfet should work. It depends on the transformer and other things.
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Old 29th October 2003, 05:05 AM   #32
Eva is offline Eva  Spain
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Looking carefully with the scope to what happens in a traditional 50-60Hz mains supplied transformer-rectifier-capacitor power supply reveals some ugly things

- 50-60Hz transformer windings have very highg self capacitances and self inductances making them ring when diodes start or stop conducting or when any pulse or AC source excites these resonances. Ringing frecuencies depend on transformer size but its Q is markedly higher in toroidal transformers due to its higher capacitances between turns and windings and this also allows resonances in one winding to cross-talk easily to the others

- Mains waveform isn't a pure sine wave, actually it looks more like a hardly clipped sine wave or a square wave with limited slew-rate. Of course, this clipping effect ocurrs due to all the DC- equipment powered by rectifying mains directly or through transformers, making aprox 300% of their average current consumption only during 33% time and no current consumption during the rest of the waveform

- The amplitude and the waveform of mains suffer random variations over time and this causes random operating point variations on most circuits with unregulated supplies and random clipping points in unregulated amplifiers

- Mains carries lots of common mode noise to ground and some diferential mode noise between phase and neutral, including 50-60Hz harmonics up to 20Khz and RF of all flavors [AM, FM...]

- 50-60Hz transformers create stray magnetic fields, of much more intensity in the classical E-I cores, that induce audio-frequency voltages on all small signal circuits near them

- Loose wiring making loops and conducting AC creates magnetic fields that also induce voltages on small signal circuits near them and this includes all the common mode and diferential mode RF carried by mains

For all these reasons, the output of the traditional transformer-rectifier-capacitor power supply contains aperiodic ripple, random DC variations, ringing from the transformer, lots of common mode noise and diferential mode noise in all frecuencies

Even worse, it creates magnetic fields dependent on the load and up to RF, think that faraday-cage shielding only works for electric fields, magnetic fields suffer little atenuation

In comparison, a high quality well designed and shielded switching power supply has input and output common mode and diferential mode filtering introducing high mains and circuit-generated RF and ripple attenuation, very low capacitance from mains to output opposed to classic toroidal transformers, pure DC regulated output independent of input wavevorm/amplitude or load, output current limiting, undervoltage and overvoltage protections, soft start, low weight, etc... and uses about the same space as a toroidal transformer of the same power rating

For all these reasons, I seriously think that the traditional 50-60Hz transformer-rectifier-capacitor power supply :

- Actually has much dirtier output
- Affects more small signal circuits
- Is bulky
- Has poor power/space ratio
- Has very high weight
- Is outdated
- Component costs are similar than for same power SMPS
- Is not adequate for audio circuits
- GENERATES LOTS MORE CONDUCTED AND RADIATED EMI THAN AN OPTIMIZED SMPS, because noise is also present but there are no efforts in things like damp ringing, shielding, filtering, cancelling out magnetic fields, etc...

Having some SMPS understanding, its relatively easy to modify for your needs an old AT or ATX computer supply, that costs almost nothing

A model with complete mains filtering should be selected as some models lack common mode or diferential mode filters because in theory can pass EMI tests without them

Originally these power supplies have a few mV of ripple and ringing on their outputs but this is due to cost cutting in output filters and poor PCB layout to be able to put all the components in so little space

Usable things from these supplies include input filtering, rectification, half bridge driving, switching transistors, driver transformer for them [made from a saturable reactor in AT units to make them self oscillate to be able to start as control circuit is powered from output side], control circuit usually based on a TL494 or equivalent, heat sinks, fan and little more

Letting the primary-driving intact you can get about 250W of output and wiht some modifications maybe 500W, but in the units I modified It was enough with 250W

ATX units are a bit trickier because you have to mantain the standby auxiliar supply to power the control circuit

Some links of interest :

Typical AT PS schematic :
Click the image to open in full size.

Typical ATX PS schematic :
Click the image to open in full size.

Same Proportional-base-drive bipolar transistor half bridge design with self-starting but at higher power and info :
Click the image to open in full size.


Datasheet TL494 : http://www-s.ti.com/sc/ds/tl494.pdf

Additional info TL494 : http://www-s.ti.com/sc/psheets/slva001a/slva001a.pdf


PD: I suggest discarding the all-discrete control circuit alternative as you would need about 25-50 active components to get decent performance and reliability
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Old 29th October 2003, 05:08 AM   #33
Eva is offline Eva  Spain
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Sorry for missing the following link and for my bad english

Same Proportional-base-drive bipolar transistor half bridge design with self-starting but at higher power and info :
Click the image to open in full size.

For more info, google shows lots of pages on these topics
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Old 29th October 2003, 05:11 AM   #34
Eva is offline Eva  Spain
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Err ...

http://www.qsl.net/xq2fod/Electron/PS40/PS40.html
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Old 29th October 2003, 12:52 PM   #35
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the design is in the Radio Amateur's Handbook -- at least it's in my 2001 Edition. In the States every public library has a copy.
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