Forward converter

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

Introduction

The objective of this experiment is to study the characteristics of a forward converter. The circuit will be operated under continuous-conduction mode (CCM) and open-loop conditions (no feedback).

Theoretical background

The forward converter is derived from the buck converter shown below.

Buck converter

For isolation and to introduce a high voltage-transfer ratio, a transformer is placed between the buck-converter power pole and the output LC filter.

Forward converter derived from buck converter by introducing a three-winding transformer.

The above circuit is typically implemented with the switch on the low side, as shown below, to simplify the gate-drive circuitry.

Forward converter

Turning on the transistor leads to the input voltage appearing across the primary of the transformer. By transformer action, the secondary voltage is Vsec = (Ns / Np) × Vin. This charges the inductor at the output stage through diode D1 while diode D2 is reverse biased. Assuming that the transformer is ideal, i.e., it draws zero magnetizing current and has no leakage, the voltage across the primary winding during the transistor OFF period is Vpri = 0. During this period, the output stage inductor, previously charged during the ON cycle, discharges while freewheeling through diode D2. Thus far the operation is similar to that of a buck converter with the only difference being that the output is scaled by the transformer turns ratio.

\[V_{o} = \frac{N_{s}}{N_{p}} \times d \times V_{\mathrm{in}}\tag{1}\]

In the case of a real converter, the transformer is nonideal and has a finite magnetizing inductance. Thus, during transistor turn-on, not only does the energy stored in the output inductor increase, but so does the energy stored in the transformer magnetizing inductance. When the transistor is turned OFF, the core must be completely demagnetized, i.e., the energy stored in the magnetizing inductance must be completely discharged, thus requiring a tertiary winding.

During the turn-on period, diode D3 is reverse biased, thus preventing current from flowing in the tertiary winding. During the turn-off period, since flux cannot change instantaneously, the transformer forces a current to flow into the dotted terminal of the tertiary winding to maintain the same flux that existed just before the transistor was turned OFF. This results in -Vin being applied across the tertiary winding, causing the core flux to decline. During this period, the voltage across the secondary winding is Vsec = -(Ns / Nt) × Vin. This reverse biases diode D1, while the inductor current freewheels through diode D2. Thus, the presence of the tertiary winding does not alter the operation of the converter output stage, and Eqn. 1 still holds true.

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.

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

If the transformer is not fully demagnetized during the OFF period, the remanent flux builds up during each switching cycle until the core is saturated. To avoid this, the time required to demagnetize must be less than the OFF period.

\[T_{\mathrm{demag}} \le (1 - d) \times T_{s}\tag{2}\]

Typically, the primary and tertiary windings are tightly wound with exactly the same turns ratio, i.e., Np = Nt. This implies that the time for demagnetization is the same as the time for magnetization, which is

\[T_{\mathrm{mag}} = d \times T_{s}\tag{3}\]

Thus, the upper limit for the duty cycle is obtained from Eqns. 2 and 3:

\[T_{\mathrm{mag}} = T_{\mathrm{demag}} = d \times T_{s} \le (1 - d) \times T_{s}\]
\[\Rightarrow d \le 0.5\tag{4}\]
Preparing the Workbench model

  1. 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\Experiment13, and paste it in a location where the user has permission to edit and save files, such as the Desktop folder.

  2. Launch the Workbench application.

  3. Pin (Dock hide icon) the Explorer dock on the right and the Toolbox dock on the left.

  4. Click the second icon within the Explorer dock, Open simulation/real-time control project button to open the project.

  5. Navigate to the folder where the pre-built example project was pasted in Step 1. Double-click the ForwardConverter_Si.project node within that folder to open the project.

  6. Click the Project expand button icon to explore the files within the project. Double-click the Workbench model logo Forward node in the Explorer dock to display the model file.

  7. Ensure that the switching frequency is set to 200 kHz. To do this, double-click the project node, Workbench project logo ForwardConverter_Si, in the Explorer. Go to the next page by clicking the Property class next page icon 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.

  8. 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.

  9. Open the Slider property and verify that its minimum, maximum, initial, and step values are 0, 0.45, 0, and 0.01, respectively.

Preparing the setup
    warningWarning

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

  1. Magnetics card connection: Replace the existing magnetics card with the Forward 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.

    warningWarning

    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.

  2. Rheostat setting: Set the slider such that the resistance across the two closest rheostat terminals is 8 Ω.

  3. 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)

  4. DSO connections:

    1. Connect DSO channel 1 probe to Ii. 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 Io. Use the same settings as Channel 1.

    3. Connect DSO channel 3 probe to Vo. 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 Sa. Use the same settings as Channel 3.

  5. Jumper settings:

    1. Insert jumpers ❶ and ❺ (●) to bypass the external current-measurement resistors.

    2. Insert jumper ❷ (●) between Sa and Sw. This connects the Forward 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 forward-converter switch-node voltage to the positive DC bus.

    5. Insert jumper ❻ (●) on the Forward magnetics card.

  6. Connect the USB cable to the power-pole board and the computer.

  7. 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:

Forward converter wiring diagram

warningWarning

The input and output of a typical forward 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 forward converter
Running the setup:

  1. Click on the Numerical simulation to Real-time mode transition button icon in the top dock of Workbench to transition from the simulation mode to the real-time mode.

  2. Click Numerical simulation and real-time prototyping Run button to run the control algorithm in real time.

  3. If an undervoltage fault occurs, slightly increase the input voltage above 15 V but less than 16 V. Stop the model by clicking on Numerical simulation and real-time prototyping Stop button and rerun it.

  4. Gradually increment the duty cycle from 0 to 0.4 in steps of 0.05. If a fault occurs, stop the model by clicking the Numerical simulation and real-time prototyping Stop button button.

  5. 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 output-inductor current (channel 2) for duty cycles of 0.1, 0.2, 0.3, and 0.4. Adjust the time base to show between 4 and 10 switching cycles.

      warningWarning

      Do not exceed a duty cycle of 0.45 to ensure that the core is fully demagnetized at the end of each switching cycle.

    2. For each duty-cycle step, record the values displayed by Ip Voltage, Op Voltage, Avg Ip current, and Avg Op Current in Workbench.

  6. Click on Numerical simulation and real-time prototyping Stop button 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

  1. Attach the DSO waveforms showing the voltage across the transistor, the output voltage, the output-inductor current, and the input current for a switching frequency of 200 kHz and duty cycles of 0.1, 0.2, 0.3, and 0.4.

  2. From these waveforms, calculate the transformer turns ratio Np : Nt : Ns.

  3. 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:

    dset Vin (V) Iin,avg (A) Pin (W) Vo (V) Io,avg (A) Pout (W) dact Efficiency (%)

    where dset is the duty-cycle value set using Workbench and dact is the duty cycle (d) calculated using Eqn. 1.

    Plot the efficiency as a function of dutyact.

    Plot the Vo as a function of dutyset.

  4. From the plot of input current at a switching frequency of 200 kHz and a duty cycle of 0.4, estimate the transformer magnetizing inductance.

Reference

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



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