re switching ps questions anyways i ve built the basic up down converters with pretty good results efficiency but i m running into problems when i try to design build anything that puts out serious amps i know it can be done i have some v a guts on my bench but something puzzles me i m using a simple choke as the storage element basicly a toroid with a single winding but all commercial models use transformers with many windings i traced a few and they seem to use some of the winding for the usual error feedback but some of the others seem to loose me what are they for better than that anyone have a full schematic for one of these that i could get a copy of i d love to see how they manage to squeeze out so much from such low volume most commercial switchers do not use magnetics for their energy storage that s handled by the big caps on the primary side those which in a linearly regulated power supply would be considered the filter caps coming right after the bridge the transformer is just that a transformer it s there primarily to step the voltage up or down although the inductance seen by the switch transistor on the primary side is not negligible as anyone who s zapped said transistor from either failing to get one with a high enough breakdown voltage or neglecting to include a snubber diode across it would tell you actually many transistors intended for switcher use today have the diode built in the basic switched mode power supply operates something like this in a somewhat simplified manner the ac line is rectified to produce a high voltage more or less unregulated dc rail the energy storage or filter caps appear across this rail as does the switching transistor the transistor chops the current into the primary side of the transformer resulting in stepped up or stepped down pulses out the secondary which are then rectified and filtered at least one of the transformer s outputs is sampled and fed back to the control circuit for the switching transistor which acts in a pulse width modulation pwm fashion to control by varying the pulse width the amount of energy being dumped into the primary and therefore the voltage coming out at the secondary the remaining outputs may be allowed to simply run at whatever value they will more or less tracking the regulated output or they may have some additional linear regulation added you may also note that the feedback between the regulated output and the pwm control which is most typically an ic is not done via a direct electrical connection this is due to various safety standards which require primary and secondary circuits to be electrically isolated often the feedback path involves an optoisolator to meet this requirement while the transformer isn t the primary energy storage device in these designs this does not mean that the energy stored in the transformer can be ignored besides the inductive kick giving the switch transistor a bad time as noted above you also need to worry about getting all the energy that went in to the transformer back out again one way or another in some designs this happens more or less automatically but in others you need to take special care to ensure that the transformer core doesn t saturate which again would have disastrous results best left to the imagination i m not by any stretch of the imagination a power supply designer hopefully a real one will come along soon and clean up any gross errors in the above bob myers kc ew hewlett packard co opinions expressed here are not systems technology div those of my employer or any other myers fc hp com fort collins colorado sentient life form on this planet
