Forward converter

• November 28, 2022
• 1 hour to complete
• Workbench v1.7
• • 
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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 buck converter shown below.

For isolation as well as to introduce high voltage-transfer ratio, a tranformer is introduced between the buck converter power-pole and the output LC filter.

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

Turning on the transistor leads to the input voltage appearing across the primary of the transformer. By transformer action, the secondary voltage V_sec = (N_s / N_p) × V_in. This charges up 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 magentizing current and has no leakage, during the transistor OFF period, the voltage cross the primary winding V_pri = 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 = N_sN_p × d × V_in(1)

In case of a real converter, the transformer is non-ideal with a finite magnetizing inductance. Thus during transistor turn ON, not only the energy stored in the output inductor increases, 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., energy stored in the magnetizing inductance must be completely discharged, thus requiring a tertiary winding.

During the turn ON period, diode D₃ gets 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 so as to maintain the same flux that existed just prior to turning OFF the transistor. This results in -V_in being applied across the tertiary winding leading the core flux to decline. During this period the voltage across the secondary winding V_sec = -(N_s / N_t) × V_in. 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 otput 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 waveform of the converter voltages and currents are as shown.

If the transformer is not fully demagnetizied 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_demag ≤ (1 - d) × T_s(2)

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

T_mag = d × T_s(3)

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

T_mag = T_demag = d × T_s ≤ (1 - d) × T_s

⇒ d ≤ 0.5(4)

Preparing the Workbench model

1. Copy the folder where pre-built example project for this experiment is present, usually in 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, like the Desktop folder.

2. Launch Workbench application.

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

4. Click the second icon within the Explorer dock, to open the project.

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

6. Click the icon to explore the files within the project. Double click the Forward node in the Explorer dock to display the model file.

7. Ensure that the switching frequency is set to 100 kHz. To do this, double click on the project node, ForwardConverter, in the Explorer. Go to the next page by clicking the in the property dock or choosing Device configuration from the drop-down menu. Check that the Frequency within PWM Configuration is set to 100000.

8. Ensure that the PWM channel is set to the Si power pole. To do this, open the property of the tool labelled Si PWM in the model by double clicking on it. Check that the Channel is set to 1.

Preparing the setup

Warning

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

1. Magnetics card connection: Unscrew any existing magnetics card and replace it with the Forward magnetics card. Prior to connecting the card, ensure that the jumper on the bottom of the magnetics card is disconnected. Make sure that all the 6-pins of the magnetics card are making solid contact with the power-pole board.

   Warning

   Never leave the magnetics card unscrewed. If contact is lost while the converter is running, it has the potential to cause very high voltage due to interruption of inductor current, potentially damaging the converter, or worse, could lead to 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 Vin terminals to the isolated power supply:

      1. DC +ve: ● Vin+ (Red)

      2. Ground: ● Earth (Green)

      3. DC −ve: ● Vin− (Black)

      
      

   2. Connect the Vo terminals to the rheostat load:

      1. Rheostat rail: ● Vo+ (Red)

      2. Rheostat's other terminal that is closest to the rail: ● Vo− (Black)

      
      

4. DSO connections:

   1. Connect DSO channel 1 probe to Io. 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 5x.

      4. Set the offset at 1.5 V or 7.5 A.

      5. Set termination to 1 MHz.

      6. Set the channel as inverted.

   2. Connect DSO channel 2 probe to SWA. 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.

   3. Connect DSO channel 3 probe to Iin. Use the same setting as that of Channel 1.

   4. Connect DSO channel 4 probe to Vout. Use the same setting as that of Channel 2.

5. Jumper settings:

   1. Disconnect the Vdc jumper.

   2. Insert the GaN/Si jumper to the Si side.

   3. Insert the GaN switch's external diode jumper, labelled "Diode".

   4. The jumper on the bottom of the magnetics card was removed in Step 1, while inserting the 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:

Real-time open loop control of forward converter

Running the setup:

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

2. Click 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 and rerun it.

4. Compensate the offset in the current sensors. To do this, make note of the value, up to 3 decimal places, displayed on the Avg Ip Current and the Avg Op Current tool.

5. Click on to stop the model.

6. Open Sensor offset tool's property by double clicking on it and set the Offset value to the negative of the measured Avg Ip Current in the previous step.

   Open Sensor offset1 tool's property and set the Offset value to the negative of the measured Avg Op Current in the previous step.

7. Click to rerun model.

8. Once programmed, ensure that the Avg Ip Current and the Avg Op Current both initally display a value of 0 for up to 2 decimal places. If not, repeat the previous 3 steps.

9. Gradually increment the duty cycle from 0 to 0.4 in steps of 0.05. If a fault occurs, stop the model by clicking on button.

10. Make the following measurements:

    1. Observe the DSO waveforms and make a copy of the voltage across the switch (channel 2), the output voltage (channel 4), the input/primary current (channel 1), and the diode/secondary current (channel 3) for duty cycle of 0.1, 0.2, 0.3, and 0.4. Adjust the time base to show anywhere between 4-10 switching cycles.

       Warning

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

    2. Make a note of the voltage and current values displayed on Workbench, for each duty cycle step.

11. Click on to stop the model.

If required, repeat the same experiment using GaN power-pole instead of Si power-pole as demostrated 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 inductor current, and the input current for switching frequency of 100 kHz and duty cycles of 0.1, 0.2, 0.3, and 0.4.

2. From these waveforms calculate the tranformer turns ratio N_p : N_t : N_s.

3. For switching frequency of 100 kHz and duty cycle 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, d_set is the duty cycle value set using Workbench and d_act is the duty cycle (d) calculated using Eqn. 3.

   Plot the efficieny as a function of duty_act.

   Plot the V_o as a function of duty_set.

4. From the plot of input current at 100 kHz switching frequency, duty cycle of 0.4, estimate the transformer magnetizing inductance.

Reference

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