Power transformer selection madness

[photo_marc]

I am very confused about my choice of power transformer. I would like to get around 330V. 250V RMS will get me this with a bridge rectifier configuration and a regulator (using the Morgan Jones set up).
Here is where my confusion starts. If I look at the Hammond web site, all there transformers are center taped. I.E. 250-0-250. That’s fine, I know I just have to use the two diode configuration. However, when I set this up in my modeling software
(Linear Technology switchercad III
http://www.linear.com/designtools/index.jsp
)
I end up with 0.5V of ripple at the output that I did not have with the bridge rectifier.
My inclination is to order a 125-0-125 transformer and leave the center tap un-connected. Presumably, this will get me 250V RMS and a lower ripple

Is my thinking correct?

 

[PRR]

> I end up with 0.5V of ripple at the output that I did not have with the bridge rectifier.

There's always ripple. How much is the bridge showing?

For practical values of load and capacitance, the raw-supply's ripple will be 1% to 10%, probably 5 Volts to 10 Volts. 0.5V ripple suggests very small load or huge capacitors.

> switchercad

Getting power supply results out of a SPICE engine is hard. Use Duncan's power supply calculator.

> I would like to get around 330V. 250V RMS will get me this

250VAC into FWB (or 500VAC CT into 2 diodes), sand rectifiers, gives 353V peak. Diode, copper, and ripple losses usually yield 80%-90% of this, though if the transformer is oversized you may get more. Still, 300VDC is a good bet. Normal wall-power variations can be +/-10%, so realistically you could get 270VDC to 330VDC.

> and a regulator

Allowing 10V minimum drop across the (sand-state) regulator, 260VDCreg is about the most you can be sure of.

 

[amperex]

Hammond transformers

Remember, Hammond current ratings are for a full-wave center tap rectifier & a choke input filter. Using a bridge rectifier, one has to drop the current rating in half with choke input design.

Using a capacitor input, multiply that current rating by .65 for full-wave CT & approximately .35 for a bridge.