Dave King wrote: > > >i've done some work on the 3A circuit and it's > >looking do-able. > > Thats fantastic, at 2-3A it becomes very very useful. Great its working fine and fairly cool, 18W in and 15w out (5v @ 3A). I modified the "monostable" timer part of my regulator to give more stable delay period, more independant from inductor and load type. We can still get rid of 1.5w by swapping the darlington for a low sat device. Now with a GOOD monostable delay it runs much more like a chip, most of these stepper chips we've been discussing do basically the same thing, 1. detect voltage, 2. timed off period (hysteresis etc) but the result is the same; longer more powerful pulses with a smaller inductor. The regulator now oscillates slower for a given inductor with a more stable frequency response and lower freq at light loads than it had before. All good steps as Mike and Morgan have pointed out. With this improvement it can now use inductors of 50uH and 100uH etc. :o) > >I just built a big version of my 2-tran regulator > >using a cheap TO220 darlington, all do under 1.0v saturated at 3A. > >By comparison, the Maxim Max727ECK switcher chip you > >mentioned has 1.7 volts saturated at 2A! That's not real > >good if you want a 2A switcher. > > That really is an improvement. I thought the Max727 or the LM2576 > were a real answer to things but your switcher seems like a real > improvement. Funny how expensive some of these chips are. They are definitely selling a "solution" as people are afraid of building their own switchers and will pay big bucks for a chip that makes it easy. :o) Re the saturation voltages of these chips, pretty bad at 1.7v @ 2A, when you can get a 50c zetex transistor that does around 0.3v @ 3A and have a device beta of about 300!! (I've ordered some!) > >the > >ability to handle wide input voltages is not perfect > >with it. > > At what point ie high end or low end will there be a problem with > the input voltage? Would it be better to build a high and a low input range > or any problems really just going to show as a loss in efficiency? I've solved some of the problem in anticipation that you wanted that difficult WIDE input range. Some of the solution was the improved monostable design. The problem is simple, the timed off period is dependent in some part on the current through RZ, to charge C1 and C2 which give the timed period. When you have input from 16v to 32v it will be good enough. But going down to only 2 volts above your zener voltage requires that Zener is fed from a constant current source IF you also need the high range. Either range can be cheaply tuned, for 12v auto or 24v auto type inputs. But to do all ranges well means the cost is another zener + transistor + resistor. No big deal for 10 cents in parts if you are saving $5 on a chip. :o) > Well 80% of what I'm thinking of using this for would be > auto voltages ie 12-13.8vdc, then say 10% around 30vdc > and possibly the rest being run off of a battery pack which > might be 7.2-9v which is the absolute lower limit. A 7.2v battery run at 3A is going to soon be a 6v battery, which is cutting it very close. Fortunately the 2-tran regulator will generally go linear at very low headrooms which is ok as the current gain with those voltages is nil anyway. How critical is it that it runs below 10v? That extra voltage option causes compromises. > I''d even like to try and adapt this to re switching regulator > on a bike alternator. That put out 40. You have to be careful again with the schottky diode voltage, the 16A pack I have is 60v rated and costs about 40 to 50 cents. For 40v schottky you can get 3 x 1A diodes with BETTER sat than the big TO220 pack for total of 25c. So a 60v input is going to cost you in semi price. > It solves any foreseeable use I would have. What about > setup that is optimized for a high or low range? ie both > retain the 5Vdc 3A out but say the first would work 7.2v > to 15v and the second might be optimized for 20-35v. > Not to make any work but is it easier to define it over a > smaller range to maximize the conversion efficiency? What happens is the switcher is about 85%+ efficient in it's happy range of 12v in, then maybe 80%+ at the higher range of 30v in. At 10v or less the efficiency is going to drop due to the nature of buck converters. Since the design easily tolerates 30% power waste that means anything under 8v will be possibly more efficient in linear anyway, and it will still regulate well. Can you confirm the load requirements, ie is this 3A all the time or does it have some reduced current periods etc. -Roman -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.