--0__=CC256AB60074FC838f9e8a93df938690918cCC256AB60074FC83 Content-type: text/plain; charset=us-ascii Russel Yes, that was exactly the reason the zener & additional resistor were positioned there. I did also get compensation by adding a resistor from the emitter of the first transistor to the incoming rail - this provided quite good compensation also but prooved to be somewhat "select on test". The actual optimal resistor value was different for different transistors - and different again from the spice simulations. Since I require an input voltage range of at least 18-70V, and good immunity from component variations, I elected to go for the zener option to limit the +ve hysterisis effect. I did try a standard diode here also - can't remember what I am using in the latest version but I think a BAV99 double diode worked - probably with modified resistor values. There is a certain amount of RF filtering added as well - but not shown on my diagram. It may be important to note that the circuit will always exhibit a reasonable amount of ripple on the output. This is required as it is this ripple that is used to determine the condusct/block operation of the main switch. Reducing the ripple will increase the operating frequency and also the losses. The addition of an LC filter on the output (or an LDO regulator!) may assist here. Richard P Russell McMahon To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: Design Challenge - low power step down switching regulator Sent by: pic microcontrolle r discussion list 29/08/01 09:15 Please respond to pic microcontrolle r discussion list > From: "Dave Dilatush" > I wrote... >See attachment. > What attachment? I don't see any attachment... > Oh, THIS attachment. Just a note about this circuit (derived by Dave from the original relay driver cct). It looks really excellent but I can see one potentially significant problem. R9 provides positive hysteresis (that word again) by raising the effective reference voltage on the base of Q1 when the pass transistor Q2 is on and lowering the reference when Q2 is off. In the original circuit the extent of this effect was limited by splitting R9 into two resistors and providing a clamp zener to ground in the middle. This limited the maximum amount that the reference voltage could be increased by when the pass transistor Q2 was turned on. It is traditional in step down converters to "feed forward" the effect of the input voltage to reduce the on time as input voltage rises. (This effect acts in addition to the voltage feedback loop). Unfortunately, this circuit does the opposite - as Vin rises the positive increase in reference when Q2 is turned on INCREASES - the faster you go the faster you go :-). Presumably the split resistor and zener in the original were placed there as a result of practical experience by the original designers. The problem (if there is a problem) would increase as the range of Vin decreased. For modest ranges of Vin (eg say 20 to 28 volts in an automotive application) the effect may be tolerable. What is ideally wanted to drive is a voltage which increases at a decreasing rate as Vin increases. In such a simple circuit this is probably a bit much to ask for. The divided resistor plus zener (add 1 + ?5 cost units) is a simple compromise. Whether the result is bad enough to be important in practice depends on specific circumstances. regards Russell McMahon (See attached file: RPmodified.gif) --0__=CC256AB60074FC838f9e8a93df938690918cCC256AB60074FC83 Content-type: image/gif; name="RPmodified.gif" Content-Disposition: attachment; filename="RPmodified.gif" Content-transfer-encoding: base64 R0lGODdhLAJUAfcAAAAAAP////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// //////////////////////////////////////////////////////////////////////////// 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