# Linear power supply vs SMPS (losses in transformer)

We know that, in a power supply, there are always losses taking place.

If we have a linear power supply, the major source of loss is the core loss in the transformer.

But in an SMPS, where we have a very high frequency voltage passing through the transformer, there are two main sources of loss

1. the switching element, usually a Mosfet of a power transistor and
2. the losses in the transformer.

To reduce the losses in the switching element, in this case being the Mosfet, there are two types of mosfet's : Trench type and Lateral type.

Now, coming back to the losses in the transformer, how do we calculate the different losses in the transformer, for a particular power supply (Linear power supply and SMPS).

Also, what are the different losses taking place in the transformer, when it is connected to a linear power supply, and when it is connected to an SMPS.

Also, how do we overcome these losses.

I read at a few different places stating two different things, being :

1. The core losses and the eddy current losses in a transformer are proportional to the frequency of the incoming wave, i.e., the core losses are directly proportional to the frequency, and the eddy current losses are directly proportional to the square of the frequency of the incoming frequency.
2. Another site states that, even though we can see that, from the equation of eddy current losses, that it is directly proportional to the square of the frequency of the incoming wave, but, it actually depends on the supply voltage.

So, what are the actual factors affecting the losses in a transformer, when we are connecting it to a linear power supply as compared to when it is connected to an SMPS.

Also, how do we decide about the trade off between the losses of the two systems, so as to select a particular supply for a particular application, considering the major factors, such as cost efficiency and and the efficiency of the system, given that the size is not an issue.

(NOTE : the SMPS in the above question is an Isolated toplogy, hence the transformer.)

Also, what are the different methods that can be used to isolate the supply, other than a transformer, if there is any better option.

Also, other than mosfet's and power transistors, (if any) is there any other switching device that can be used to provide switching to the SMPS. (which may have more or less switching losses).

• I think the major power loss in a linear power supply is in the series pass element in the regulator. Transformer losses should be quite small in comparison. – Peter Bennett Feb 8 '19 at 5:59
• Other switching devices: IGBT's e.g. Other ways of isolation: isolation is achieved by switching between energy domains, e.g. from electrical domain to magnetic domain, back to electrical (transformer), electrical to magnetical to mechanical domain and backwards (elec motor connected to generator), electrical to optical domain and backwards (optocoupler). All ways of isolation introduce powerloss; afaik the transformer is the most efficient way to isolate. – Huisman Feb 8 '19 at 6:58
• How to decide what to use? Calculate the power losses. – Huisman Feb 8 '19 at 7:00
• @PeterBennett The major power loss in a linear power supply may or may not be in the series regulator. it depends on the difference of the input voltage to the output voltage. the loss in a series regulator is given as [Power dissipation = (input voltage – output voltage) × load current]. if the difference in the input and the output voltage is very low, then the series regulator can be very efficient, and if the difference is high, then the losses would be very high. It all depends on the designer and the circuit designing. with proper circuit designing, it should not be a problem – APM Feb 8 '19 at 7:19
• If we have a linear power supply, the major source of loss is the core loss in the transformer. That would only be true under certain circumstances (as you point out in your comment). Under typical circumstances the series pass element is where most power is lost. there are two types of mosfet's : Trench type and Lateral type. I think that is irrelevant to the question, what matters is Rdson and maybe input capacitance. What type would fit best (trench/lateral/???) depends on what is needed like maximum voltages. – Bimpelrekkie Feb 8 '19 at 7:23