> > From: "Byron A Jeff" > > > 1) You talked about replacing BUKFET with a PNP bipolar. What about an NPN > > bipolar? Remove QBUK2, RBUK3, ZBUK2, and RBUK4 and tie the bottom of RBUK2 > > and the collector of QBUK1 to the base of the NPN. When QBUK1 is off then > > RBUK2 will turn the power NPN on. When QBUK1 conducts, the NPN will turn > off. > > I'm sure that the base current from RBUK2 will need to be higher so that > > the power NPN will saturate. > > I've paper designed something similar. The main problem is that the NPN > needs positive drive and when it is turned on the base needs to be driven by > a voltage ABOVE the input voltage. This can be accomplished by using an > extra high side supply. This can be provided by an extra isolated winding > on the inductor and a single diode and capacitor. Confusion sets in on the computer guy ;-( . Everytime I think I actually understand this semiconductor stuff, someone comes along and throws a curve: The statement you make above makes sense for N-channel MOSFETs. But that's not my understanding of how NPN bipolars work. First off I thought that bipolars are current amplifiers, and that voltage wasn't really the determining factor. Secondly my understanding was that a bipolar can be driven to saturation with the base voltage well below the collector voltage. And that in fact that once the base voltage gets above the Vbe that it doesn't in fact continue to rise but that it sucks down more and more current through the base while channeling more and more current through the CE junction all the way to the point of saturation, where Ice/Ibe = hfe. Now if this were not the case then very ordinary things like relay drivers where a PIC pin drives the base of an NPN and the CE junction of the NPN switches the relay coil. For a 12V or 24V relay the base voltage never goes above 5V. I guess my only question out of this is whether or not it is possible to oversaturate the base. For now I abandoned the NPN bipolar for a much simpler reason, the 6W of heat that would have to be dissapated if I maxed out at the 10A I'm shooting for. That .6V drop is devastating when you're pumping 10A through... My interest in the question is for the junkbox variety supply. My box has in decreasing frequency: NPN bipolar, Nchannel MOSFET (usually logic level, PNP bipolar, Pchannel MOSFET). > > 4) Is the BUKFET and the inductor the only power elements? My goal is to > build > > a 5V 10A supply. > > Not quite. > The flyback diode DBUK2 MUST be rated for a peak current capability of > several times the mean output current and MUST be a high speed part (eg NOT > 1N400X). That's not a problem. The peak current was 30A and it's an ultra-fast recovery diode. > > And my new design challenge: > > > > How could this circuit be augmented so that it could output 12V from an > > input between 6V and 15V? 15V: > > This requires a fundamental change in the circuit. > At present it is a pure BUCK step down circuit and switching occurs when the > input voltage rises above the design voltage. There is therefore no need for > an oscillator - it is self switching. A boost or boost-buck design NEVER > reaches a target voltage when Vin is below Vout-design so would not self > switch as this does > A similar simple scheme using eg input current would be possible but it > would be different than this. > > Your suggested boost and boost buck designs are along the right track. > In a one off situation or where the power level means moderate expense is > necessary for the power parts , then using a commercial control IC may be a > good idea. I have two goals in this arena: understanding and simplicity. Both give me the ability to throw together a switching PS with readily available parts. As I stated in my first message, I got the theory, but failed to grasp the control mechanism. Now I get it. For the sake of the following discussion I'm going to abrrev BUK with B only. This prompts one last question: What exactly is the purpose of RB2 and QB2 again? I keep seeing QB1 and QB2 in a darlington configuration but I'm not understanding why that config is required. Consider if we removed RB2 and QB2 and attached the collector of QB1 to the bottom of RB3. When Vin is applied the gate of BF will rise causing it to conduct. QB1 is turned off by PB1 pulling the base of QB1 low. The coil charges until ZB1 conducts. This turns on QB1, grounding the gate of the FET turning it off. Eventually the coil will start to lose energy and ZB1 turns off, turning off QB1 and turning on the FET again. So it'll oscillate right? It seems to be the same exact operation. The only difference I could find is that the MPSA42 (QB2) had a much higher Vce. But even that's confusing because QB1 is going to subject to a higher Vce via RB2. So anyway just wondering why the darlington config and whether or not RB2 and QB2 are absolutely necessary? Great project. Keep up the excellent work! BAJ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body