Quasi Resonant Flyback Step by Step Design Guide

If you want to make a quasi resonant flyback but do not know where to start, therefore this Quasi Resonant Flyback Step by Step Design Guide is perfect for you. Follow below steps.

1. Set the Value of the Reflected Voltage, (Vref)

We will start this Quasi Resonant Flyback Step by Step Design Guide by setting the level of Vref. Below is the working equation to use.

a. VDS_target

This is the maximum anticipated level of the primary switch. Choose your desired component. For example, a MOSFET with an 800V rating, you may set the maximum VDS_target to 80% of this. This will give an 80% stress which is still a good margin.

Example: VDS_target = 0.8 X 800V = 640V

b. %spike

Can use 20%-30% rule of thumb here.

Example: %spike = 30%

c. Vin

This is the input voltage feed to the primary winding.

Example: Vin = 400V

Thus, Vref = [VDS_target / (1+%spike)] – Vin = [640V / (1+0.3)] – 400V = 92.31V

2. Compute Input Power, (Pin)

Pin = Pout / Efficiency

a. Pout

This is the total output power in case the secondary has multiple rails.

Example: Pout = 30W

b. Efficiency

This is an assumed desired efficiency during design stage.

Example: Efficiency = 90%

Thus,

Pin = Pout / Efficiency = 30W / 0.9 = 33.33W

3. Select the Primary Inductance to Stay in the DCM Region and First Valley Switching Assumption, (Lpmax)

I can say that the primary inductance is the most critical parameter in order to ensure proper operation of the quasi resonant converter. Below is the equation.

a. Pin_max

This is the maximum input voltage. The same with result of section 2 above.

b. Fsw

This is the switching frequency, can be referred to the capability of the selected controller.

Example: Fsw = 90kHz

c. Vin and Vref

Refer to section 1 above.

d. Cd

This is the total output capacitance. Think of a capacitor across primary switch.

Example: Cd = 1000pF

Thus,

Lpmax = 577.9uH

4. Select Transformer Core Material and Size

Example: Flux Density (Bsat) = 0.4 Tesla, Effective Area (Ae) = 50 sq. mm

5. Determine the Minimum Primary Winding Number of Turns, (Np_min)

Np_min = (Ipeak_short X Lpmax) / (Ae X Bsat)

a. Ipeak_short

This must be the peak level of the primary current during short circuit. This depends to the user setting.

Example: Ipeak_short = 2A

b. Lpmax

Refer to section 3 above.

c. Ae and Bsat

Refer to section 4 above.

Thus,

Np_min > (Ipeak_short X Lpmax) / (Ae X Bsat) = (2A X 577.9uH) / (50 sq. mm X 0.4 Tesla) = 58 Turns

You can increase this number of turns as long as will fit the core and practical enough to do so.

6. Determine the Transformer Turns Ratio

Turns Ratio = Vref / Vout

a. Vref

Refer to section 1 above

b. Vout

This is the output voltage.

Example: Vout = 12V

Thus,

Turns Ratio = Vref / Vout = 92.31V / 12V = 7.6925

7. Set the Secondary Winding Number of Turns

Secondary Turn = Np_selected / Turns Ratio

a. Np_selected

This is the actual chosen number of turns on the primary winding. Should be higher than Np_min on section 5 above.

Example: Np_selected = 70 Turns

b. Turns Ratio

Refer to section 6.

Thus,

Secondary Turn = Np_selected / Turns Ratio = 70 Turns / 7.6925 = 9 Turns

8. Checking the Parameters if Correct Based on the Template

What we have done so far in this Quasi resonant flyback step by step design guide are the basic ingredients to ensure operation in the first valley. It is time to double check it with a simple excel tool. All information above are inputted to the tool. The computed input power in yellow field is the same to the input power in section 2 above. This means above calculations are correct. Go to “http://electronicsbeliever.com/downloads/quasi-resonant-flyback-operation-checker-and-design-tool/” to download the tool.

9. Primary Switch Selection

You can use a BJT, MOSFET or IGBT for the primary switch. Whatever it is, you need consider the voltage rating, current rating, static and dynamic parameters, power dissipation, and operating temperature. In this quasi resonant flyback step by step design guide, I choses a MOSFET as a switch.

a. Voltage Rating

This is the drain voltage rating for MOSFET while collector voltage rating for BJT and IGBT. The device voltage rating must be higher than the actual voltage including the spikes. Follow section 1 above in order not to overstress the device.

Example: As with section 1 above, VDS rating of MOSFET = 800V

b. Current Rating

The primary switch current rating must be higher than the actual circuit current with ample of margin.

Peak Current, (Ipeak):

Ipeak = 2 X Pin / (Vin X Duty)

Pin – Input power, see section 2 above. Example: Pin = 33.33W

Vin – input voltage, see section 1 above. Example: Vin = 400V

Duty – this is the duty cycle of the quasi resonant flyback. For operation at DCM, it is:

Duty = [ Vref / (Vin + Vref) ] [1 – Fsw X Td ]

Vref – see section 1 above.

Example: Vref = 92.31V

Fsw – this is the switching frequency where the parameters are set.  Refer to section 3 above.

Example: Fsw = 90 KHz

Td – this is the dead time to ensure operating in the DCM region.

Td = π X √ ( Lpmax X Cd )

Lpmax and Cd – refer to section 3 above.

Example: Lpmax = 577.9uH, Cd = 1nF

Thus,

Td = π X √ ( Lpmax X Cd )=π X √ ( 577.9uH X 1nF ) = 2.4 usec

Thus,

Duty = [ Vref / (Vin + Vref) ] [1 – Fsw X Td ] = [ 92.31V / (400V + 92.31V) ] [ 1 – 90kHz X 2.4usec ] = 14.72%

Finally,

Ipeak = 2 X Pin / (Vin X Duty) = 2 X 33.33W / ( 400V X 0.1472 ) = 1.13A

DC Current, (Idc):

Idc = Duty X Ipeak / 2

Duty – see above. Example: Duty = 14.72%

Ipeak – see above. Example: Ipeak = 1.13A

Thus,

Idc = Duty X Ipeak / 2 = 0.1472 X 1.13A / 2 = 83.2mA

Ensure the selected has peak current rating higher than this value.

RMS Current, (Irms):

Irms = Ipeak X √ (Duty / 3)

Ipeak – see above.

Duty – see above.

Thus,

Irms = Ipeak X √ (Duty / 3) = 1.13A X √ (0.1472 / 3) = 0.251A

Ensure the selected has peak current rating higher than this value.

c. Static and Dynamic Parameters

Static parameters are can be referred to on-state resistance for MOSFETs (Rdson) or voltage drop for BJT and IGBT (VCEsat). The lower the Rdson or the VCEsat, the better.

For MOSFETs, dynamic parameters include input gate charge, input and output capacitance, rise time and fall time. Select the part with lower dynamic parameters.

d. Power Dissipation, (Pdiss_circuit)  

Pdiss_circuit = Pconduction + Pswitching

  • Where;
  • Pconduction – this is the power losses due to static parameters
  • Pswitching –  this is the power losses due to dynamic parameters

Considering as MOSFET Switch

Pconduction = Irms2 X Rdson

Irms – refer to above. Example: Irms = 0.251A.

Rdson – this is the on-state resistance of the MOSFET. Refer to the datasheet. Example: Rdson = 0.2 ohms.

Thus,       

Pconduction = Irms2 X Rdson = (0.251A)2 X 0.2 ohms = 12.6mW

Pswitching = Ploss_gatecharge + Ploss_Coss + Ploss_rise/fall_time               

Ploss_gatecharge = 0.5 X Qgtotal X Vdrive X Fsw                               

Ploss_Coss = 0.5 X Coss X VDS2 X Fsw                              

Ploss_rise/fall_time = 0.5 X ( trise + tfall ) X Irms X VDS X Fsw                               

Where;                               

Qgtotal – total gate charge, refer the MOSFET datasheet. Example: Qgtotal = 110nC                               

Vdrive – this is the applied voltage to the MOSFET gate-source. Example: Vdrive = 12V

Fsw – this is the switching frequency selected above. Example: 90kHz

Coss – this is the total output capacitance of the MOSFET. Refer to the datasheet.

Example: Coss = 420pF

VDS – this is the drain voltage. Refer to earlier derivation above. Example: VDS = 640V trise – MOSFET specified rise time in the datasheet. Example: trise = 79nsec

tfall – MOSFET specified fall time in the datasheet. Example: tfall = 45nsec

Irms – refer to above derivations. Example: Irms = 0.251A

Thus,

Ploss_gatecharge = 0.5 X Qgtotal X Vdrive X Fsw = 0.5 X 110nC X 12V X 90kHz = 0.059W

Ploss_Coss = 0.5 X Coss X VDS2 X Fsw = 0.5 X 420pF X (640V)2 X 90kHz = 7.741W

Ploss_rise/fall_time = 0.5 X ( trise + tfall ) X Irms X VDS X Fsw = 0.5 X ( 79nsec + 45 nsec) X 0.251A X 640V X 90kHz = 0.896W

Thus,                               

Pswitching = Ploss_gatecharge + Ploss_Coss + Ploss_rise/fall_time = 0.059W + 7.741W                               

+ 0.896W = 8.696W

Finally,               

Pdiss_circuit = Pconduction + Pswitching = 12.6mW + 8.696W = 8.709W       

The MOSFET must have a power dissipation rating higher than this value.       

e. Operating Temperature

Select a switch that has an operating temperature more than enough to accommodate the actual environment temperatures.

10. Diode Selection

a. Peak Reverse Voltage, (PRV)

This is also called peak inverse voltage (PIV) sometimes.

PRV = Vout + Vsec

Vout – this is the output voltage. Example: Vout = 12V

Vsec – this is the secondary voltage

Vsec = [ Nsec X Vin ] / Npri

Nsec – secondary winding number of turns. Refer to section 7 above.                               

Example: Nsec = 9 Turns                               

Vin – input voltage. Example: Vin = 400V                               

Npri – number of turns primary winding. Example: 70 Turns                               

Thus,                               

Vsec = [ Nsec X Vin ] / Npri = [ 9 X 400V / 70 ] = 51.42V                               

Finally,               

PRV = Vout + Vsec = 12V + 51.42V = 63.42V

b. Current Rating

Peak Current:

Ipeak diode = -2 X Idc / ( Duty + Fsw X Td – 1 )

Idc – this is the load current of the flyback. Example: 2.5A.

Duty – refer to section 9

Fsw – refer to section 3

Td – refer to section 9

 Thus,

Ipeak diode = -2 X Idc / ( Duty + Fsw X Td – 1 ) = -2 X 2.5A / ( 0.1472 + 90KHz X 2.4usec -1 ) = 7.839A

RMS Current:

Irms diode = [ sqrt ( -3 X ( Ipeak diode )2  X ( Duty + Fsw X Tdead – 1 ) ] / 3

Thus,

Irms diode = [ sqrt ( -3 X ( Ipeak diode )2  X ( Duty + Fsw X Tdead – 1 ) ] / 3 = [ sqrt ( -3 X ( 7.839A )2  X ( 0.1472 + 90kHz X 2.4usec – 1 ) ] / 3 = 3.614A

c. Forward Voltage, (Vf)

Low forward voltage is desirable.

Example: 0.7V

d. Power Dissipation, (Pdiss)

Pdiss = Vf X Irms diode

Thus,

Pdiss = Vf X Irms diode = 0.7V X 3.614A = 2.5298W

The selected diode must have power rating higher than this value.

e. Operating Temperature

Select a temperature range that can cover the actual operating temperature. Say -40’C to 150’C.

11. Output Capacitor Selection

Compute the Minimum Capacitance

Cout_min = Idc / ( Vout_ripple X Fsw )

Vout_ripple – this is the maximum allowed ripple voltage to the output. Example: Vout_ripple = 0.24V

Thus,

Cout_min = Idc / ( Vout_ripple X Fsw ) = 2.5A / ( 0.24V X 90KHz) = 115.74uF

Determine the Ripple Current

Iripple = sqrt ( Isec_rms2 – Idc2 )

Isec_rms – the same with Idiode_rms

Thus,

Iripple = sqrt ( Isec_rms2 – Idc2 ) = sqrt ( 3.6142 – 2.52 ) = 2.61A

The capacitor must capable to handle this current.

Determine the Maximum ESR

ESR_max = Vripple_out / Iripple

Thus,

ESR_max = Vripple_out / Iripple = 0.24V / 2.61A = 0.092 ohms

The capacitor ESR should be lower than this value.

12. Design Checking

All the calculations in this quasi resonant flyback step by step design guide is derived from engineering analysis and proven working for actual projects. You can check further your design using below design templates/tools.

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