OK, now that actual PS design is being talked about I'll jump into this thread. (I actually have a couple hours not at work today!!)
I've done a huge amount of PS testing, design and building in regards to the Touch and other devices, what I'm presenting here is not just conjecture, its been tested many times. My conclusions are that a large percentage of improvements with PS design for the Touch are related to high frequency noise sent BACK into the mains and picked up by other components. power sent into the Touch itself has little impact on what goes on inside the Touch (not zero, but quite small). The differences in mains injected noise is vastly greater than any changes in the power actually delivered to the components inside the Touch. Given this the focus of PS design should be on decreasing what gets sent back down the AC line, not making the absolute lowest noise, lowest impedance feed to the Touch. Many of the attempts at getting the "best" power to the Touch increase the noise sent down the line. For a linear supply there are primarily three things that contribute to noise sent back down the line: reverse recovery noise when the diodes switch, transformer ringing and current spikes when the diodes conduct. Using Schottky diodes can get rid of the reverse recovery noise, transformer ringing can be dealt with with an RC network across the secondary which damps the resonance. This damping of the transformer ringing is very effective but almost never done. If you go and look at 100 different linear PSs there will be a very high probability that NONE of them have damped the transformer resonance. Its intriguing that the higher the "quality" of the transformer the worse the resonance is, thus using expensive "high end" transformers is one of the worst things you can do. (unless you damp it) The current spikes are the hard part. These are caused by the traditional PS design of transformer, diodes and a big cap. The diodes only conduct when the voltage from the transformer is greater than the voltage on the cap. Thus the power coming from the transformer is in short high current spikes. For example a common design for the 5V 2A for the Touch will actually have 20A spikes coming from the transformer. These 20A spikes have all kinds of high frequency components which get sent right back through the transformer and into the mains. They ALSO excite the above mentioned transformer resonance causing the transformer to ring like a bell, even if you use Schottky diodes. The DC signal coming out of such a design is a sawtooth wave. This also has large amounts of high frequency components. Unfortunately most rectifier designs have very little input rejection at high frequencies, they do great at low frequencies, but at high frequencies not so good. This is one reason that discrete regulators have been used in audiophile designs, they can have much better high frequency input rejection, which is needed to correctly handle the sawtooth. There is a solution, but its hardly ever used in low voltage PS designs. Its called a choke. Not the little high frequency things designed to run at 50KHz and up, but big heavy things which have significant inductance at 120Hz. If properly designed they allow continuous conduction through the transformer and rectifier, eliminating the high current spikes. Another advantage is that the output waveform is a pure sine wave, no high frequency harmonics. Put the choke filter, transformer damping and Schottky diodes together and you have a supply that injects almost nothing back into the AC line and delivers a clean pure sine wave to the rectifier. With this you don't NEED a complicated expensive regulator. I've put together a design using these principles, the schematic is at: http://home.comcast.net/~johnswenson1/stereo/SB_5V.GIF All the parts should be available at Mouser or Digi-Key. I'm sure there are other distributors in other parts of the world where you can get these parts. You should be able to get all the parts for $75 or so. Give it a try I think you will like this design. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=82648 _______________________________________________ Touch mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/touch
