How to Select a Transformer

Before going to the discussion on how to select a transformer, let us cover first some of the transformer basic fundamentals. Transformer is could be a low frequency like those used in 50Hz or 60Hz line. Transformer is could be a high frequency one like those used in switching mode power supplies. Transformer will transfer power from the primary to the secondary in real time with no loss at all, ideally. Transformer is could be a voltage or current transformer. A voltage transformer will transform voltage; it could be a step-up or step-down voltage transformer. A current transformer on the other hand will transform current.

Transformer Non-Ideal Model

Below is the non-ideal model of a transformer. Rp is the primary winding resistance, Lp is the primary leakage inductance, Rc is the core resistance, Cp is the primary intra-winding capacitance and Lmag is the magnetizing inductance. On the other hand, Cs is the intra-winding capacitance in the secondary, Ls is the leakage inductance in the secondary and Rs is the secondary winding resistance.

The leakage inductance (Lp) is the result of the imperfect magnetic coupling between the windings. It is also dependent to the geometry of the core and the winding technique. There is energy stored to the leakage inductance and it will not transfer to the secondary. This can cause transformer performance issue or even failure when too much. The leakage inductance will be measured by shorting the secondary winding and putting an inductance meter across the primary winding.

The magnetizing inductance is the inductance that will be measured across the primary winding when the secondary winding is open.  This is the inductance responsible for the current seen by the core and will cause transformer saturation when not properly designed.

All the other parasitic parameters in the above diagram have importance but this will not cover in this article anymore.

Transformer Ideal Model

The ideal model of a transformer is below diagram.

For a voltage transformer,

Vp / Vs = Np / Ns

Where Np / Ns is the turns ratio (n), so

n = Vp / Vs

How to Select a Transformer for Switching Mode Power Supply

1. Primary and Secondary Voltages

Need to ensure that the primary winding is able to handle the application voltage. The primary voltage rating is provided in the datasheet. Likewise, the secondary winding must able to provide the intended application voltage. When shopping for off the shelf part, the primary and secondary voltage is clearly specified in the technical datasheet though. However, when designing an own transformer, there is a need to do calculation which involve turns ratio.

Specifications and drawings used here is from a Wurth Electronik part. Wurth Electronik is offering some off the shelf transformers and other magnetics. Visit them at

2. Wattage

Like other electronic devices, transformer is rated with power. For switching mode power supply applications, the power is given in terms of watts. If your power supply total output power is rated 500 watts, ensure the transformer is rated more than this. A utilization of 60% is a good number to ensure the longevity of the transformer. So for a 500 watts output, the transformer rating must be 500 watts/0.6 = 834 watts.

3. RMS and Saturation current

When you shop an off the shelf transformer, the primary and secondary windings are provided with the current rating. The application current must not exceed these specifications. Furthermore, saturation current is also provided. Saturation current is provided in order to consider the peak level of the actual current. Saturation current is provided for the primary winding. The transformer current may be a triangular in nature when used in SMPS. The RMS value of the triangular wave will be the one to compare to the transformer RMS current rating. On the other hand, the peak level of the primary current should be compared to the saturation current rating. The peak level should not exceed the saturation current rating.

Below is a sample transformer from Wurth Elektronik. The secondary winding current is provided in the drawing. On the other hand, the saturation current is provided in the table.

How to Select a Transformer

4. Magnetic Flux Density

When you design your own transformer, you will select a core material. The core material has a magnetic flux density rating. The computed or the actual application magnetic flux density should not exceed the magnetic flux density rating.

However, for an off the shelf transformer like above, a saturation current specification is already enough.

5. Inductance

Inductance is important for actual application. For instance, when the transformer is used in QR flyback converter, it is very important to set the right inductance as it has something to do with the converter correct operation.

6. Leakage inductance

For some switching converter topology, leakage inductance is very critical. In LLC converter, the leakage inductance will add up to the resonant inductance and if not considered, the converter will not function correctly.

7. Termination

Termination is important. You should know how many windings or how to connect the primary and secondary windings.

8. Turns ratio

Turns ratio is very important in order to compute the parameters of a converter when the transformer is used on it. For instance in LLC converter, the output voltage is dependent to the turns ratio. In below transformer, there are three sets of turns ratio.

9. Auxiliary

In SMPS, auxiliary winding is necessary to provide power to a certain circuit.

10. Step-up or step-down

Transformer is could be used for either step up or step down applications. Refer to the datasheet to ensure selecting the right transformer.

11. Frequency

When used in switching converter, the switching frequency could reach as high as khz range or even higher. It is important to choose a transformer that could support the actual frequency.

12. Operating temperature

This is one of the most important parameter to consider in selecting a transformer. Exceeding this rating will destroy the transformer immediately. To extend the transformer life, set the temperature to just 60%.

How to Select a Transformer for low frequency

Low frequency transformer is the one used in the AC line utility. In the US, it is 60 Hz while 50 Hz in the Europe. Here are the things you must consider on how to select a transformer for low frequency application.

1. kVA rating

kVA is the unit used to describe the transformer capacity. It is important that you will define or know your need. Sample sizes are 50kVA, 100kVA, 200kVA, etc.

Take note that the transformer rating is kVA. If your load is in watts, which is a real power, you need a power factor to get the kVA value. Supposing, a load of 300kW is not the same as 300kVA. To get the kVA rating, divide the kW with the power factor. For example, power factor is 90%, then the kVA equivalent of 300kW is 300 / 0.9 = 333kVA.

A common rule of thumb is to limit the transformer utilization to 80% maximum. For instance a 100kVA transformer must be loaded with 80kVA only.

2. Primary and Secondary Voltage

Define your requirement and ensure to select a transformer that can meet your specifications in terms of voltage.

3. Phase current

During the selection process, may also consider checking the phase current. Though phase current is already covered once the kVA and secondary voltage is defined.

4. Step-down or step-up

If you need to convert a 110V to 220V, then you need a step-up transformer. On the other hand, if you need to convert 220V to 110V, you need a step down transformer.

5. Connection or termination

You need to know if you need a three-phase or single phase connection. If three-phase, you need to consider if delta or wye termination. A delta termination is often a 4-wire connection; three lines and a single ground. On the other hand, a wye termination is a 5-wire termination; three lines, one neutral and ground. Neutral connection is only needed if there is a connection that requires line to neutral. A three-phase transformer is can be configured as delta-delta, wye-wye, delta-wye or wye-delta. For detailed explanation about this, read Easy Understanding of 3-phase transformer connections.

7. Frequency

Frequency is important. For US, it is 60Hz while 50Hz for Europe.

8. Cooling System

For a high kVA transformer, there is an option for forced air, natural cooling or whatever. If a transformer is designed as forced air or whatever cooling, it must be ensured to configure the cooling unit correctly so that it will function normally.

9. Operating Temperature

It is important to limit the transformer operation within its operating temperature. Operating the transformer into very high temperature will decrease its life or will damage immediately.

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