Power supply design query

[TunerPhish]

Hi,

I am trying to design a 230 V smoothed unregulated power supply based on the following specification:

Output voltage 12 V
Output current 2 A
ripple factor < 0.024

Would anybody be able to offer any advice on suitable ratings and values for the transformer, bridge rectifier and smoothing capacitor?

Thanks

 

[d123]

Output voltage would be 12VAC, if you want 12VDC.
For output current a transformer rated at 3 - 4A.
Diodes should be rated to at least twice (or even three times) the DC current.
Capacitance depends on load, and as I'm not sure if you mean 0.024%, the answer is so bad for a 50 Ohm load, I won't put it...

Took my calculations from "Linear Power Supply Basics"

 

[treez]

Tunerphish you can easily check your answer with lspice which is free simulator

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here is the sim for ltspice...ill leave you to tweak it up nice.
Just convert it to .asc and then open it up in ltspice....then hit the running man icon

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remember to play with the coil resistances

 

[TunerPhish]

Hi would you be able to clear something up for me. When selecting a transformer using the calculation 230÷12 to get the number of windings. I get a ratio of 19.1667:1. I know the transformer I would use for my design would have a ratio of 20:1. At a 20:1 ratio my output would be 11.5V not the 12V I am looking for how would this effect my overall design? Thanks

 

[treez]

did you account for the diode drop and the winding resistance?...this will affect your turns calculation....youll need to slightly tweak the ideal calculation which considers zero volts diode drop and zero ohms winding resistances.

Regarding turns ratios...remember that NP/NS = VP/VS

Also, NP/NS = SQRT(LP/LS)

..i am not sure what else there is to go into about that.

 

[TunerPhish]

I sussed it. My notes referred to the peak secondary voltage which gives enough overhead to allow for voltage loss in the bridge and transformer windings. In this case Im looking at a peak of 17v which gives enough overhead.

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I have a question about your output voltage calculation. Should it not be 24VA? as the required output is 12V 2A. Thanks. My notes show a simple formula of the values multiplied by each other 12*2

 

[treez]

The VA rating of the transormer will have to be more than 24VA, because 24VA is the apparent power, and the real power is less than that because with transformer/rectifier/smoothing caps type supplies, the power factor is poor and so the "real power" is less than the "apparaent power" quite significantly.

 

[TunerPhish]

With that in mind and my Peak overhead being 17V would a transformer with a rating of 40VA be suitable or would that be overkill?

 

[d123]

Hi,

I think for VA rating the calculation is VA * 0.7, which would give 28VA. Or if you calculate 28/12, it's 2.3A.

 

[TunerPhish]

Much appreciated I've learned a lot from you. I'm on with my bridge selection now I've chosen a bridge rectifier rated at 5 Amps.