I need to design a ±12V supply for my project. In the interest of size and efficiency I thought I'd go for a wall-wart AC-DC converter, then converted and regulated on the board with a switching regulator. I don't know much about switching regulators and I'm having a little trouble.
I need to supply about 350mA per supply (actually less for the negative supply), so I've been using 500mA in my calculations (for margin). Most of the switching regulators I've looked at that can do voltage inverting can't seem to supply that much current in inverting configurations, and it also seems very inefficient.
I found this regulator: LTC1372 which shows a method for dual-output flyback design (text page 7, schematic page 11).
Some questions:
1. It mentions line regulation for the reference voltage. Would this also correspond to line regulation of the output?
2. I had to look up what flyback converters are all about, and they all include the turns ratio of the transformer/inductor in the output voltage calculation. However, the transformer specified in the example (page 11) doesn't have a turns ratio specified in its datasheet. Should I then assume it's 1:1?
3. Also regarding the transformer, in the schematic the primary side has pins "2,3" on one end and "6,7" on the other. Does that mean "2 or 3" and "6 or 7", or does it mean "connect 2 & 3" and "connnect 6 & 7", or something else?
4. Output ripple is a concern for me. A formula on this application note (page 35) includes C, ESR, and switching frequency. The LT1372 runs at 500kHz and says to use 22μF-500μF solid tantalum capacitors for low ESR. However, it seems capacitors of this type seem to have resonance around 100kHz (including the very one they recommend), and become inductive above that. How does one get a capacitor of at least 22uF when the rated capacitance is at something like 120Hz, and goes way down up at 500kHz?
And lastly: is there a better way of going about this than the above? Tight regulation and low ripple are what I'm after. I thought about using linear regulators, but I'd still have to do a voltage inversion for the negative supply in that case, and it seemed like it would be quite inefficient for 12V output at 500mA.
I need to supply about 350mA per supply (actually less for the negative supply), so I've been using 500mA in my calculations (for margin). Most of the switching regulators I've looked at that can do voltage inverting can't seem to supply that much current in inverting configurations, and it also seems very inefficient.
I found this regulator: LTC1372 which shows a method for dual-output flyback design (text page 7, schematic page 11).
Some questions:
1. It mentions line regulation for the reference voltage. Would this also correspond to line regulation of the output?
2. I had to look up what flyback converters are all about, and they all include the turns ratio of the transformer/inductor in the output voltage calculation. However, the transformer specified in the example (page 11) doesn't have a turns ratio specified in its datasheet. Should I then assume it's 1:1?
3. Also regarding the transformer, in the schematic the primary side has pins "2,3" on one end and "6,7" on the other. Does that mean "2 or 3" and "6 or 7", or does it mean "connect 2 & 3" and "connnect 6 & 7", or something else?
4. Output ripple is a concern for me. A formula on this application note (page 35) includes C, ESR, and switching frequency. The LT1372 runs at 500kHz and says to use 22μF-500μF solid tantalum capacitors for low ESR. However, it seems capacitors of this type seem to have resonance around 100kHz (including the very one they recommend), and become inductive above that. How does one get a capacitor of at least 22uF when the rated capacitance is at something like 120Hz, and goes way down up at 500kHz?
And lastly: is there a better way of going about this than the above? Tight regulation and low ripple are what I'm after. I thought about using linear regulators, but I'd still have to do a voltage inversion for the negative supply in that case, and it seemed like it would be quite inefficient for 12V output at 500mA.