Power supply consumption

[Electronics4you]

I need to calculate the total power loss in my power supply to determine heat sinks and fans. Does a rule of thumb determine the typical power loss in rectifier bridge, capacitors and transformer? Maybe 10% power loss in transformer to the rated transformer power?

 

[dch222]

I guess a good rule of thumb is about 90% efficiency for transformer (maybe less for small ones) and 1.4 x average current for power dissipation in bridge rectifier. (Sorry that's two thumbs).

Dissipation in capacitor should be negligible unless you are talking about big power supplies or high ripple values.

 

[stevez]

You might use estimates for things like the transformer and rectifier however you'd be wise to understand the power losses in the switching or regulation circuitry - particularly at a variety of loads.

An example - if you were supplying 20 vdc from the rectifier/filter capacitor to a linear regulator and wanted to supply 12 vdc at 1 amp then the regulator would have to waste 8 vdc at 1 amp. Adjust this regulator down to an ouptut of 6 vdc at 1 amp and the power losses in the regulator would be 14 volts at 1 amp.

 

[Hero999]

Don't worry about the transformer too much but the efficiencty will vary depending on the power rating and whether it's an E-core or torroid.

Assuming it's a linear regulator, the power loss in the rectifier and regulator is the voltage drop times the current.