Quasi Resonant Flyback Operation Checker and Design Tool – Excel

The main advantage of a quasi resonant flyback converter compared to the conventional flyback converter is the ability to switch on the lowest drain voltage of the primary MOSFET (or any semiconductor switch). This is called as switching on the lowest valley point. Read the article How Quasi Resonant Flyback Works – Detailed Operation for more details.

This advantage will not be realized if the parameters and devices are not set and selected correctly. The operation of a quasi resonant converter is dependent to the primary inductance of the transformer, stray capacitance of the circuit (mostly on the primary switch), reflected voltage to the primary winding during flyback action, turns ratio of the transformer, and the given voltages like input voltage, output voltage, output power and intended frequency of operation.

For a beginner or not technical person, these are too much to handle. It will take time even for a technician or an inexperienced engineer to digest all these. However, you don’t need to extend extra effort anymore. There is already a Quasi resonant flyback operation checker and design tool available. All you need to do is to play around the input variables.

How Quasi Resonant Flyback Operation Checker and Design Tool Works

Above is the snapshot of the quasi resonant flyback operation checker tool. The violet fields are values that to be entered to the tool. The yellow fields on the other hand are computation results. The switching frequency also (in green field) is can be changed.

Tool Legend

• Vin – input voltage feed to the primary winding
• Lp – primary inductance of the transformer
• Cd – total parasitic capacitance, this is mainly governed by the switch capacitance
• Vref – this is the reflected voltage to the primary winding during flyback action
• Vref = n X Vsec = n X ( Vout + Vdrop )
• n – turns ratio of the transformer
• Vout – output voltage
• Vdrop – output diode voltage drop
• Nvalley – this is the valley point where you desired to switch on the primary switch. For optimum result, set this to 0.5 for the first valley
• Fr – this is just a computed resonant frequency of the Lp and Cd. You may not need this.
• Input Power – this is the power measured or calculated to the input side
• Switching Frequency – this is the intended switching frequency where the user wants the QR flyback to operate. This must be within the frequency range of the controller. A nice move is to see the controller capability first. Another consideration in setting this frequency is possible EMI related noise. Setting this too high may create issue in few cases.

Tool Capability

As a Quasi Resonant Flyback Design Tool: Setting the Right Parameter and Values During Design Stage

During design stage, the know values are input voltage, output voltage, load and input power and switching frequency is could be pre-determined. With all these givens, you are going to design the transformer parameters and select parts.

1. Input the Desired Frequency on the Field

Example: 90000 Hz

2. Input all the Known Values

Example: Vin = 400V, Vout = 12V

3. Assumed the Value of Vref

During design stage, this can be pre-determined.

Example: 92.31V

4. Input the Turns Ratio

Since Vref is provided and Vout is also given, the turns ratio is can be computed.

n = Vref / (Vsec) = Vref / ( Vout +Vdrop) = 92.31V / ( 12V + 0V) = 7.6925

[I assumed here voltage drop (Vdrop) of zero]

Key in the turns ratio to the tool.

5. Play Around both the Lp and Cd and Monitor the Input Power

You may select the primary switch first. On the selected switch like MOSFET, you can find the output capacitance of the MOSFET on the datasheet. Input that value to the tool. Example: Cd = 1nF.

Now, your unknown value is only the primary inductance Lp. Start varying the Lp and monitor the corresponding input power. Once you get the desired input power you want, stop and record the value of the Lp. That should be the primary inductance of your flyback transformer to ensure operation at first valley with the desired power.

Checking the Switching Frequency of an Existing Quasi Resonant Flyback Converter

Since the quasi resonant flyback is already designed, you will have all the values already.

1. Supply All the fields with the Known Data

2. Vary the Switching Frequency

Start varying the switching frequency until you can see the input power on the yellow field matches to the actual input power. Stop the iteration and that is the actual switching frequency.

More Capabilities of this Quasi Resonant Flyback Operation Checker and Design Tool

There are many more uses of the tool. Like changing the input voltage, what will be the effect to the input power considering all other parameters are the same. Also changing the turns ratio, what will be the effect to the other parameter. You can also try changing the valley point and see what the effect to the other parameter. Lot of applications. Play around on the tool and enjoy.

Once you download the tool and run into trouble, send email to [email protected]. We are willing to help you. We are also open for consultation services.