Flyback converter

August 28, 2026
1 hour to complete
Workbench v1.9

Introduction

The objective of this experiment is to study the characteristics of a flyback converter. The circuit will be operated under both continuous conduction mode (CCM) and discontinuous conduction mode (DCM).

Theoretical background

The flyback converter is derived from the buck-boost converter shown below.

Buck-boost converter

The inductor in the buck-boost converter, shown in the above image with its low permeability core, is replaced with two mutually coupled coils as shown.

Flyback converter derived from buck-boost converter by replacing the inductor with mutually coupled coils.

The above circuit is typically drawn as shown below, with the two mutually coupled coils represented using a transformer.

Flyback converter

Turning on the transistor increases the primary current, thereby increasing the energy stored in the mutually coupled coils. During this period, the voltage across the transformer primary V_pri = V_in, and the secondary side voltage V_sec = -(N₂ / N₁) × V_in. The voltage across the diode V_D = V_o - V_sec = V_o + (N₂ / N₁) × V_in, which reverse biases the diode.

The transformer coils are oriented such that, when the transistor is turned off, the current switches from the primary to the secondary coil to maintain the flux in the core. This current forward biases the diode; thus, during the transistor off period, the voltage across the transformer secondary V_sec = V_o. During this period, the voltage across the primary V_pri = -(N₁ / N₂) × V_o and the voltage across the transistor V_sw = V_in + (N₁ / N₂) × V_o.

For a given transistor switching-function waveform q(t) with a switch duty ratio d in steady state, the converter voltage and current waveforms are as shown.

Flyback converter steady-state operation waveforms showing the gating signal, core flux, transformer primary and secondary voltage and current, power-pole voltage, and diode voltage

When the switch is ON, i.e., q(t) = 1, the change in the transformer flux Δφ_m is given by

\Delta \phi _{m} = \frac{V_{\mathrm{in}}}{N_{1}} \times d \times T_{s}\tag{1}

When the switch is OFF, i.e., q(t) = 0, the change in the transformer flux Δφ_m is given by

\Delta \phi _{m} = \frac{V_{o}}{N_{2}} \times (1 - d) \times T_{s}\tag{2}

At steady state under CCM, the changes in flux during the turn-on and turn-off periods must be equal. Thus, equating Eqns. 1 and 2 gives

V_{o} = \frac{N_{2}}{N_{1}} \times \frac{d}{1 - d} \times V_{\mathrm{in}}\tag{3}

Flyback-converter operation under DCM is quite similar to that of the buck-boost converter under DCM. In the case of the buck-boost converter, the boundary between CCM and DCM occurs when the inductor current briefly goes to zero. In the case of the flyback converter, the boundary occurs when the flux in the core briefly goes to zero. For a given transistor switching function waveform q(t), shown below, with a switch duty ratio d in steady state under DCM, the waveform of the voltage across the transistor V_sw, for an ideal flyback converter, is as shown below.

Ideal flyback converter steady-state operation waveforms showing the gating signal, transistor voltage, primary and secondary current, and the core flux

When the switch is ON, i.e., q(t) = 1, V_sw = 0. When the switch is OFF but the flux in the transformer core φ_m or the current in the secondary winding i_sec is not yet zero, V_sw = V_in + (N₁ / N₂) × V_o. This is because the diode is conducting during this period; hence, the output voltage is impressed across the transformer secondary winding. When the switch is OFF and the flux in the transformer core φ_m is zero, V_sw = V_in. This is because the diode is no longer conducting and blocks the output voltage across it.

Many flyback converters typically operate under DCM. This is because once the flux in the core goes to zero, the voltage across the switch is lower than when the flux was non-zero. Thus, turning on the switch under DCM leads to lower switching losses than under CCM. In practical flyback converters, this effect is more pronounced because parasitic components cause ringing in the voltage across the transistor, as discussed in Buck-boost converter - Discontinuous conduction mode (DCM) and shown below.

Practical flyback converter steady-state operation waveforms showing the gating signal and transistor voltage

When the flux goes to zero, the voltage across the transistor, ignoring damping, can fall as low as V_v = V_in - (N₁ / N₂) × V_o, clamped at 0 V. Practical flyback converters are typically designed to turn ON the transistor at this valley to significantly reduce the switching losses.

Preparing the Workbench model

Copy the folder containing the pre-built example project for this experiment, usually located at C:\Program Files (x86)\Sciamble\WorkBench v1\Examples\CUSPLab\BasicPowerElectronics\Experiment11, and paste it in a location where the user has permission to edit and save files, such as the Desktop folder.
Launch the Workbench application.
Pin () the Explorer dock on the right and the Toolbox dock on the left.
Click the second icon within the Explorer dock, to open the project.
Navigate to the folder where the pre-built example project was pasted in Step 1. Double-click the FlybackConverter_Si.project node within that folder to open the project.
Click the icon to explore the files within the project. Double-click the Flyback node in the Explorer dock to display the model file.
Ensure that the switching frequency is set to 200 kHz. To do this, double-click the project node, FlybackConverter_Si, in the Explorer. Go to the next page by clicking the icon in the property dock or choosing Device configuration from the drop-down menu. Check that the Frequency within PWM Configuration is set to 200000.
Ensure that the PWM channel is set to the Si power pole. To do this, open the properties of the tool labeled Si PWM in the model by double-clicking it. Check that the Channel is set to 2.
Open the Slider property and verify that its minimum, maximum, initial, and step values are 0, 0.6, 0, and 0.01, respectively.

Preparing the setup

Warning

Before proceeding, ensure that the isolated power supply is powered down and that the USB cable is disconnected.

Magnetics card connection: Replace the existing magnetics card with the Flyback magnetics card, making sure that all six pins are properly aligned and in contact with the power-pole board, adjusting the angle if necessary.

Prior to connecting the card, ensure that jumper ❻ (●) on the magnetics card is connected. This is to connect the snubber circuit across the transformer primary to clamp the ringing across the transistor. The theory of operation and the procedure for designing the snubber will be covered in the next experiment, Flyback converter snubber design.

Warning
Never leave the magnetics card unscrewed. If contact is lost while the converter is running, interrupting the inductor current can produce a very high voltage, potentially damaging the converter or creating a safety hazard.
Rheostat setting: Set the slider such that the resistance across the two closest rheostat terminals is 8 Ω.
Power connections:
1. Connect the I+/I- terminals to the isolated power supply:
  1. DC +ve: ● I+ (Red)
  2. ```
  DC −ve: ● I− (Black)
```
2. Connect the O+/O- terminals to the rheostat load:
  1. Rheostat rail: ● O+ (Red)
  2. ```
  Rheostat terminal closest to the rail: ● O− (Black)
```
3. Connect the power supply ground to the ground terminal on the board:
  
  Ground: ● GND (Green)
DSO connections:
1. Connect DSO channel 1 probe to I_i. Set the following options, if they are supported by the DSO:
  1. Set the probe to 1x.
  2. Set the measurement type as Current.
  3. Set the scaling factor to 4x.
  4. Set the offset at 6 A.
  5. Set termination to 1 MHz.
  6. Set the channel as inverted.
2. Connect DSO channel 2 probe to I_o. Use the same settings as Channel 1.
3. Connect DSO channel 3 probe to V_o. Set the following options, if they are supported by the DSO:
  1. Set the probe to 1x.
  2. Set the measurement type as Voltage.
  3. Set the scaling factor to 1x.
  4. Set the offset at 0 V.
  5. Set termination to 1 MHz.
  6. Set the channel as default/non-inverted.
4. Connect DSO channel 4 probe to S_a. Use the same settings as Channel 3.
Jumper settings:
1. Insert jumpers ❶ and ❺ (●) to bypass the external current-measurement resistors.
2. Insert jumper ❷ (●) between S_a and S_w. This connects the Flyback magnetics card to the midpoint of the Si power pole.
3. Jumper ❸ (●) for the GaN FET's external diode may remain inserted or removed, since this experiment does not use the GaN power pole.
4. Remove jumper ❹ (●) so that the high-side switch does not clamp the flyback switch-node voltage to the positive DC bus.
5. Insert jumper ❻ (●) on the Flyback magnetics card to connect the RCD snubber.
Connect the USB cable to the power-pole board and the computer.
DC power supply settings:
1. Make sure that the DC power supply is fully turned down to 0 V prior to turning on the supply.
2. Turn on the power supply and gradually ramp up the voltage from 0 V to 15 V.
3. If the option is available, set the power supply current limit at 4.5 A.

The final wiring should look similar to this:

Flyback converter wiring diagram

Warning

The input and output of a typical flyback converter are galvanically isolated. That is not the case in this lab kit. To enable both external measurement and measurement by the controller of the input, output, and switch voltages without separate isolated probes, the output negative is internally connected to the input negative. Thus the output is not electrically isolated. This does not alter the response of the converter in any manner.

Real-time open-loop control of flyback converter - with snubber

Running the setup:

Click on the icon in the top dock of Workbench to transition from the simulation mode to the real-time mode.
Click to run the control algorithm in real time.
If an undervoltage fault occurs, slightly increase the input voltage above 15 V but less than 16 V. Stop the model by clicking on and rerun it.
Gradually increment the duty cycle from 0 to 0.5 in steps of 0.05. If a fault occurs, stop the model by clicking the button.
Make the following measurements:
1. Observe the DSO waveforms and make a copy of the voltage across the switch (channel 4), the output voltage (channel 3), the input/primary current (channel 1), and the diode/secondary current (channel 2) for duty cycles of 0.1, 0.2, 0.3, 0.4, and 0.5 (or the highest possible value before a fault occurred). Adjust the time base to show between 4 and 10 switching cycles.
  
  Warning
  Do not exceed a duty cycle of 0.5. Even though the input and output voltages, which are monitored by the controller, will be within safe limits, the switch voltage, which is not monitored by the controller, could exceed the device rating at duty cycles greater than 0.5. This is because the voltage across the switch is the sum of the input voltage and the reflected output voltage.
2. For each duty-cycle step, record the values displayed by Ip Voltage, Op Voltage, Avg Ip current, and Avg Op Current in Workbench.
Click on to stop the model.

If required, repeat the same experiment using the GaN power pole instead of the Si power pole, as demonstrated in Switching characteristic of Si MOSFET/GaN FET and diode.

Lab report and reading assignment

Attach the DSO waveforms showing the voltage across the transistor, the output voltage, the secondary current, and the primary current for a switching frequency of 200 kHz and duty cycles of 0.1, 0.2, 0.3, and 0.4.
From these waveforms, calculate the transformer turns ratio N₁ : N₂.

For a switching frequency of 200 kHz and duty cycles varying from 0.1 to 0.4 in steps of 0.05, enter the measured values from the Workbench screen capture and calculate the following values:

d_set	V_in (V)	I_in,avg (A)	P_in (W)	V_o (V)	I_o,avg (A)	P_out (W)	d_act	Efficiency (%)

where d_set is the duty-cycle value set using Workbench and d_act is the duty cycle (d) calculated using Eqn. 3.

Plot the efficiency as a function of duty_act.

Plot the V_o as a function of duty_set.

From the plot of primary current at a switching frequency of 200 kHz and a duty cycle of 0.4 (or the maximum duty cycle before a fault), estimate the transformer magnetizing inductance.

Reference

"Power Electronics, A First Course," Ned Mohan and Siddharth Raju, Wiley Publication.