This is a multi-part message in MIME format. ------=_NextPart_000_0096_01C131B6.DB6DCE60 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: 7bit Following on from the "buck converter challenge" I was thinking about how one would implement an ultra low cost boost-buck converter - output can be below or above input. After brief thought the BooBu converter evolved. (Please note - not a BooBoo converter although it may yet be :-).) It may even work (although I haven't tried it yet). I'm throwing this into the PICLIST melting pot early on so anyone else who wants to can contribute to its evolution. By all means add critique but before adding criticism please do note its "thought experiment" status (and it's 12:30am here and I'm going to bed and ..... This is based on "my" prior 3 transistor buck circuit. The circuit will definitely need work and the additions to the basic circuit are certainly especially inspired ( :-) ) but I feel it may be able to be made to work quite well. Components values where shown should be ignored! I have taken some liberties in simplification which will probably not be justified in practice eg Q2 and Q4 will quite possibly need separate base drive "spreading" resistors. Lines crossing don't join. Tee intersections are joins. I will refer to inductor as "L" CORE GENERAL OPERATION Most buck-boost converters works as follows. Q3 and Q4 are both turned on. Vin is applied across L1. Current in L1 increases. After a suitable time Q3 & Q4 are turned off. L1 now 'rings" with LH end going to ground and rh end rising to supply load via D1. Cycle repeats. Switching off may be controlled by time period or by current in L. Switching on may be by time delay or Vout drop. THIS CIRCUIT Initial turnon is by RBUK2 biasing Q2 and Q4 on. LH end of L will rise to Vin. C2 will charge via R1. When Vc2 exceeds Vzener_Z1 then Q1 will be turned on as before. Q1 on will turn Q2 & Q4 off and L will "ring" and deliver power to output. The RH end of R1 will be at ground during ringing cycle and C2 will discharge. I have shown D2 allowing this discharge but on reflection a resistor in series with D2 is probably needed or some other means to prevent immediate discharge of C2 - needs thought. Regardless, when Vc2 reduces Q1 will turn off initiating another turn on cycle. However, if the output voltage has risen to the desired level on this cycle then zener ZBUK1 will conduct and hold Q1 on and Q2/Q4 off until the output voltage falls enough to stop zener conduction. If this happens part way through a discharge cycle L will deliver energy after switch off contributing to Vout rising and off delay. More thought is needed about the short cycling which could occur before desired output voltage is reached. Possibly a capacitor C3 from the collector of Q2 to Q1 base or R! C2 junction or some similar cheap solution will become clear. Possibly a capacitor to ground from Z1/Zbuk1 anodes (as per previous design). An interesting thought: If you want the ultimate in low parts count (as opposed to practicality) it MAY be possible to eliminate Q4 and use Q2 to serve both as driver (as now) AND as low side inductor switch. This may take a diode or 2. We would then have a 3 transistor Boost-Buck circuit !!! At approximately $nothing per transistor I don't see this as a major goal but it could be fun. And I bet they would never suggest you should build anything like these circuits in EE school :-) regards, Russell McMahon ------=_NextPart_000_0096_01C131B6.DB6DCE60 Content-Type: image/gif; name="boobu1.gif" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="boobu1.gif" R0lGODlh2AF4AYAAAAAAAP///yH5BAAAAAAALAAAAADYAXgBAAL/jI+py+0Po5y0gXOr3rz7D4bi SJbmiabqkgWt+8JAvNb2jef6zvc+i0nEWrSf8YhMKpfM5IU2DDan1Kr1il0+FVFDMQsOi8fkMXSG IJbX7Lb7DY/L5/S6/Y7P6/f8vv8PGCg4SNg384V0WIiBtuj4uHHYyCSJaCgJmanpUNnp+QkaKjpK Wmp6ipqKucnaWjn1WhjbSpupSHW7mFvL62iZaNsr7Hv1+2c8nHyH/MPM56wcDQfNQ51nLZ1Nhp3D beetHV48Dgkuft5kXqMux47+buSeIv9GD3/fTf5oj9+/wl8C4BqB/gqSICgCoRmDDHHpI9YwohKF ICiGsSgx4wOM/x04YvGoMWSah7pEmsxnBWTKkyzXkZTVMiYKlRVoOpSJc4TNCTvT5fxZ8SWhnkDf EYVwdGLRpRqScgrGNGoEpxagSr3KgGpWq1i7SqmiNZ7XsV6EDgpLlhVaIVzTFgUFq63bn0XWru1x d+5ZID7L6V1aN67fv0AD991HuDBfSnITszTMeLBjmYZXedmlpqzmkRTy7vA8eU/lr5ldkN7MqGbj 0CFHby4N2zRbaqBRsn7M17KMF7ChoLbwqeRt3FzKdpGNXM0Q3VuLwxx+0jXv069/W1/MWQhmzNSR 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