Transformer Design and Selection QR Flyback

Several factors to consider in order to perfectly arrived a good Quasi Resonant Flyback transformer design. It needs to consider the input voltage and the allowed level of the switch drain to source (or collector to emitter) voltage of the switch to define the level of the reflected voltage (Vref). It is also required to know the level of the output voltage in order to design the turns ratio. Those are basic requirements.

Aside from above, the designer needs also to consider the operation or the functionality. The operation is a quasi resonant flyback, hence, it is important to set the primary inductance correctly to ensure to stay in the discontinuous conduction mode (DCM). Quasi resonant flyback converter can operate in three operating modes. Read Quasi-resonant Flyback Operating Modes for more details.

The last thing to consider is the stress. The transformer core must be selected carefully to handle the stress. The minimum number of primary winding is one of the important keys to avoid saturation. The sizes of the wires or busbars used in the winding must also be carefully selected.

Design the Level of Reflected Voltage to the Primary (Vref)

Vref or the reflected voltage to the primary is equivalent to the primary voltage of a standard transformer.

Standard Transformer (Non-flyback)

[Vprimary / TurnsPrimary] = [Vsecondary / TurnsSecondary]

Flyback Transformer

[Vreflected / TurnsPrimary] =[Vsecondary / TurnsSecondary]

1. Define the Input Voltage

This is the voltage directly applied to the primary of the flyback converter. For instance, the flyback converter will get the power source from a 400Vdc line, then the input voltage we are talking here is 400Vdc.

2. Target Drain to Source Voltage, VDS (or Collector to Emitter Voltage)

This is mostly dictated to the available switching component. Mostly, MOSFETs have 600V-1200V capability. In my designs, I opt to set the voltage stress to 80%. Say, I going to select a MOSFET with 1000V rating, I will set my VDS to 0.8 X 1000V = 800V.

3. Include Spike Allowance

Switching converters are prone to voltage spikes due to switching effect and the parasitic element present in the power circuit. Thus, it is a good idea to consider adding margin during design stage. A good rule of thumb that I use in my designs in 20%-30% spike.

4. Compute the Reflected Voltage

Compute the reflected voltage using below equation.

Using the values above:

Vref = [800V / (1+0.3)] – 400V = 215.38V

Turns Ratio Selection

In order to define the turns ratio, the output voltage must be known. Supposing the output voltage is 12V, the turns ratio will be:

TurnsRatio = [Nprimary / Nsecondary] = Vref / Vsecondary

Considering the derived Vref above:

TurnsRatio = [Nprimary / Nsecondary] = 215.38V / 12V = 17.94

You may need to select a turns ratio that is easy to attain but do not go far from this. Perhaps you can make it 18.

Setting the Primary Inductance

For quasi resonant flyback, the primary inductance must be set accordingly in such a way the converter will operate in the DCM region. The maximum value of the primary inductance must be


Lpmax – maximum inductance to stay in DCM

Pin_max – corresponding power where the first minimum valley switching is expected to occur

Fsw – the desired switching frequency at the desired power

Vin – input voltage

Vref – reflected voltage

Cd – drain capacitance

There is a very good tool to set the primary inductance. You can see how the parameters in the above equation changes and you can decide where to set the operation of the quasi resonant flyback. Try it here.

Transformer Stresses

1. Minimum Number of Turns of Primary Winding

The idea is to arrive a maximum number of turns for the primary winding that will fit to the selected core, to maintain a low operating magnetic flux density. Below equation is very useful to set the minimum number of turns.


Ip_peak – peak short circuit current seen by the primary. The peak short circuit current is can be derived from the controller used.

Lp –  the computed primary inductance

Ae – effective cross-sectional area of the core

Bsat – saturation flux density of the selected core. In my designs, I use 60%-70% factor. For instance, the Bsat of the core is 100mT, then I only use 65%-70% of it and use in the equation above.

2. Winding Current Stress

Both primary and secondary windings must be sized according to the computed currents. If you want to know how to compute the primary DC and RMS current, read the article QR Flyback Primary Current Derivation. If you want to know how the secondary currents are derived, read the article QR Flyback Secondary Current Derivation.

3. Magnetic Flux Density Calculation

The steps given above may not ideal to all designers. Some will find it easy to supply predetermined values and compute how much margin they have. For instance, in saturation, designers may predetermine the turns ratio and the number of turns on the primary is already been fixed. The primary inductance is may also been defined. This is not an issue as long as you ensure that the magnetic flux density rating of the selected core is not exceeded. In my designs, I always limit the flux density stress to 60%-70%. For further details on how to determine transformer saturation, read QR Flyback Transformer Saturation Analysis.

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