This is a multi-part message in MIME format. ------=_NextPart_000_082B_01C24FAC.43F9AA80 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit > > I am considering using your switcher as a pre-regulator to go from the 12V+ > > to 3V+ and use a linear 3V reg to make it clean. > > I would suggest using an extra transistor, cost > of about $0.04, will improve the regulation a lot > and efficiency a bit. You will be spared the 3v > regulator unless you specifically need current > limit and thermal protect etc. I think Russell > McMahon had a good 3-tran circuit. In its 3 transistor form my circuit probably wasn't as efficient as Roman's AFAIR BUT it has extremely good regulation considering its simplicity - AFAIR about 0.1v variation in output across wide load and input voltage variations. Also, in the form I used it it had an "immense" input voltage variation capability by normal standards - say from about 11 volts to 200+ volts input for 10 volts out. In its 11 - 200 volt input production form I use an extra transistor in the high side driver and a FET as the main switch. Attached is a circuit which is an *** EXAMPLE ONLY *** as an experimental starter. Cap CBUK2 is usually much smaller than this. 0r3 in output is probably unneeded - was used as part of a current limit which I have omitted. Actual output voltage is close to zener ZBUCK1 value due to zener knee. CBUK1 adds formal static hysteresis to the feedback path but is actually not needed in practice. Diode DBUK1 replaces a low side drive transistor and produces about the same result. LBUK2 provides an extra "almost free" output but this only gets fed during flyback part of cycle so will tend to sag somewhat as Vin approaches Vout. Note that FET is (of course) a P Channel device! PNP transistor can be substituted with suitable component changes. V100 is nominally a <= 100 volt rail but the cct still operates OK at 200v but at lower efficiency. ZBUK2 protects FET gate. While the circuit and method of feedback caused much debate when first introduced starting is very reliable in practice. Inductor value is quite flexible. The circuit shown provided about 600 mA out nominal (over an amp on demand) but can be scaled down with suitable component changes (left as an exercise for the student :-) ). 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