Switching characteristic of Si MOSFET/GaN FET and diode

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

Introduction

The objective of this experiment is to study the switching characteristics of power MOSFETs and power diodes using a buck converter. The circuit will be operated under open-loop conditions (without feedback). The primary objective is to understand the switching behavior of these two power devices.

Theoretical background

The fundamental building block of all the converters studied in this lab is the switching power-pole. It consists of two power electronic devices connected in series across a DC voltage source, as shown in the figure below.

Si MOSFET and GaN FET switching power pole

Either one or both of the two devices in the power pole may be active switches; when only one is active, the other is typically a diode. This allows control of the voltage at the midpoint of the power pole. The location of the switch and diode varies depending on the converter under consideration. For a buck converter, the device connected to the positive input voltage terminal is an active switch while the one connected to the negative input voltage terminal is the diode.

In the power electronics lab kit, both devices in the power pole are active switches, as shown in the figure above. For the Si power pole, the intrinsic-body diode of the Si MOSFET functions as the required diode without an additional external diode. For the GaN power pole, since the device does not have an intrinsic-body diode, an external low forward voltage drop Schottky diode is used in parallel to the GaN FET. Note that the GaN FET is capable of conducting reverse current while the gate is externally turned OFF, effectively functioning as a diode, although this comes at the cost of a large reverse voltage drop, typically 3.5 V compared to the 0.7 V of the Si MOSFET's intrinsic-body diode. Therefore, an external diode is required when the power pole is not used as a synchronous buck converter.

Calculating power loss:

The current flow through the diode results in a forward voltage drop Vfm across the diode. The average forward power loss Pdiode,f in the diode can be calculated as

\[P_{\mathrm{diode},f} = (1 - d) \times V_{\mathrm{fm}} \times I_{o}\tag{1}\]

where d is the FET duty ratio in the power pole; hence, the diode conducts for the (1 - d) portion of each switching period. Io is the output current, which is the same as the switching-cycle average inductor current.

In the on-state, the FET conducts a drain current for an interval Ton during every switching time-period Ts, with the switch duty-ratio \(d = \frac{T_{\mathrm{on}}}{T_{s}}\). Assuming this current to be at a constant Io during \(d\times T_{s}\), since it is zero during the rest of the switching time-period, the rms value of the switch current is

\[I_{T,\mathrm{rms}} = \sqrt{d} \times I_{o}\tag{2}\]

Hence, the average power loss in the on-state resistance RDS(on) of the FET is

\[P_{\mathrm{cond}} = R_{DS(\mathrm{on})} \times I_{T,\mathrm{rms}}^{2} = R_{DS(\mathrm{on})} \times d \times I_{o}^{2}\tag{3}\]
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\Experiment2, and paste it in a location where the user has permission to edit and save files, such as the Desktop folder.

  2. Click the Workbench icon icon on the desktop to launch the 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. This opens the file browser within the Explorer dock.

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

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

    Real-time open-loop control of buck converter Workbench model

  7. The above model is created by dragging-and-dropping and interconnecting the tools available in the toolbox dock. The purpose of each tool in the model is listed in the table below:

    Tool name Operation
    Slider Enables varying the power-pole duty cycle from 0 to 1 in real time.
    PWM Relays the duty cycle, selected through the slider, to the PWM controller which controls the gate signal for the Si power-pole.
    Vi Uses the ADC to read the voltage across I+ and I- and outputs the normalized value between ±1.
    Vi gain Scales the normalized value from the ADC back to the actual value. The gain is 42 because an ADC reading of 1 corresponds to 42 V.
    Vi filter Filters the Vi voltage measurement to remove noise. The low-pass filter bandwidth is set to 10 rad/s.
    Ip Voltage Displays the filtered input voltage.
    Vo Uses the ADC to read the voltage across O+ and O- and outputs the normalized value between ±1.
    Vo gain Scales the normalized value from the ADC back to the actual value.
    Vo filter Filters the Vo voltage measurement to remove noise. The low-pass filter bandwidth is set to 10 rad/s.
    Op Voltage Displays the filtered output voltage.
    Ii Uses the ADC to read the current from the I+ terminal into the Vdc terminal and outputs the normalized value between ±1.
    Ii gain Scales the normalized value from the ADC back to the actual value. The gain is 6 because an ADC reading of 1 corresponds to 6 A.
    Ii filter Filters the Ii measurement to remove both noise and switching ripple content. The low-pass filter bandwidth is set to 10 rad/s. High-frequency components, such as current ripple, cannot be observed using the Workbench scope because of the limited bandwidth of the USB connection. Hence, a DSO must be used to observe switching-frequency components.
    Avg Ip Current Displays the filtered average input current.
    Io Uses the ADC to read the current from the inductor output terminal Lo into the output capacitor terminal O+ and outputs the normalized value between ±1.
    Io gain Scales the normalized value from the ADC back to the actual value.
    Io filter Filters the Io measurement to remove both noise and switching ripple content. The low-pass filter bandwidth is set to 10 rad/s.
    Avg Op Current Displays the filtered average inductor/output current.
    Temperature Uses the ADC to read the temperature at the device-heatsink junction and outputs the normalized value between ±1.
    Temperature gain Scales the normalized value from the ADC back to the actual value. The gain is 125 because an ADC reading of 1 corresponds to 125 °C.
    Surface temperature Displays the instantaneous device-heatsink junction temperature.
    Over voltage Vi Status flag that outputs True when the instantaneous voltage across I+ and I- exceeds 25 V.
    Under voltage Vi Status flag that outputs True when the instantaneous voltage across I+ and I- falls below 14 V.
    Over voltage Vo Status flag that outputs True when the instantaneous voltage across O+ and O- exceeds 25 V.
    Over current Ii Status flag that outputs True when the instantaneous current from I+ terminal exceeds ±4 A.
    Over current Io Status flag that outputs True when the instantaneous inductor current exceeds ±4 A.
    Over temp Status flag that outputs True when the instantaneous device-heatsink junction temperature exceeds 85 °C.
    Input over voltage Green indicates that the input voltage is less than 25 V, i.e., the Over voltage Vi status flag is false. Otherwise, it turns red if the Over voltage Vi status flag is true.
    Input under voltage Green indicates that the input voltage is greater than 14 V, i.e., the Under voltage Vi status flag is false. Otherwise, it turns red if the Under voltage Vi status flag is true.
    Output over voltage Green indicates that the output voltage is less than 25 V, i.e., the Over voltage Vo status flag is false. Otherwise, it turns red if the Over voltage Vo status flag is true.
    Input over current Green indicates that the input current is less than ±4 A, i.e., the Over current Ii status flag is false. Otherwise, it turns red if the Over current Ii status flag is true.
    Inductor over current Green indicates that the inductor current is less than ±4 A, i.e., the Over current Io status flag is false. Otherwise, it turns red if the Over current Io status flag is true.
    Over temperature Green indicates that the device-heatsink junction temperature is less than 85 °C, i.e., the Over temp status flag is false. Otherwise, it turns red if the Over temp status flag is true.

  8. Ensure that the switching frequency is set to 100 kHz. To do this, double-click the project node, Workbench project logo BuckConverter, in the Explorer. This should bring up the project properties in the Properties dock on the left.

    Workbench project property

    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 100000.

    Workbench project device configuration

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 Buck/Boost/Buck-boost magnetics card, making sure that all six pins are properly aligned and in contact with the power-pole board, adjusting the angle if necessary.

    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 10 Ω.

  3. Power connections:

    1. Connect the I+/I- terminals to the isolated power supply:

      1. DC +ve: ● I+ (Red)

      2. Ground: ● GND (Green)

      3. 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)

  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 1.5 V or 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 Vi using 20:1 attenuator. 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 20x.

      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 using the 20:1 attenuator. Use the same settings as Channel 3.

  5. Jumper settings:

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

    2. Insert the GaN/Si jumper ❷ (●) to Sa/Sw. This connects the buck-converter inductor 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. Insert jumper ❹ (●) to connect the drains of the top switches to the positive DC bus (Vin+).

    5. Remove jumper ❻ (●) to disconnect the capacitors across the switches.

  6. Connect the USB cable to the power-pole board and the computer. Ensure the side of the cable marked with a white dot is facing up when plugged into the board.

  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:

Buck converter wiring diagram

Real-time switching characteristics measurement for Si MOSFET and diode
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 under-voltage 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 0.5. If a fault occurs, stop the model by clicking the Numerical simulation and real-time prototyping Stop button button and rerun the model.

  5. Make the following measurements:

    1. Observe and make a copy of the voltage across the diode (channel 4). Adjust the time base to show the switching details during turn-ON and turn-OFF.

      Measure the forward voltage drop across the diode.

    2. Observe the DSO waveforms and make a copy of the voltage across the MOSFET (channel 3 - channel 4). To do this, use the DSO's math operation to get the difference between the input voltage (channel 3) and the diode voltage (channel 4).

      Measure the forward voltage drop across the MOSFET during the ON period.

    3. Measure the average load current Io (channel 2), and the duty cycle of operation.

Real-time switching characteristics measurement for GaN FET and diode
Running the setup:

  1. Stop the model by clicking on Numerical simulation and real-time prototyping Stop button.

  2. Turn OFF the DC power supply and disconnect the USB cable.

  3. Change the PWM channel from the Si MOSFET to the GaN FET. To do this, open the properties of the tool labeled PWM in the model by double-clicking it. Change the Channel from 1 to 3.

    PWM property channel selection for controlling the GaN FET

  4. DSO setting: Connect DSO channel 4 probe to Sb instead of Sa. This measures the voltage at the midpoint of the GaN power pole with respect to I-.

  5. Jumper setting: Change the GaN/Si jumper ❷ (●) to Sw/Sb. This connects the buck-converter inductor to the midpoint of the GaN power pole.

  6. Reconnect the USB cable.

  7. Turn on the power supply and set the voltage to 15 V.

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

  9. Once programmed, gradually increment the duty cycle from 0 to 0.5.

  10. Repeat the earlier measurements made for the Si power pole.

  11. Click on Numerical simulation and real-time prototyping Stop button to stop the model.

  12. Turn OFF the power supply and disconnect the USB cable.

Lab report and reading assignment

  1. Attach the DSO waveforms for turn-ON and turn-OFF of the diode showing the switching details.

  2. Calculate the conduction loss of the diode using Eqn. 1.

  3. Attach the DSO waveforms for Si MOSFET and the GaN FET showing the switching characteristics.

  4. Estimate the Rds(ON) of the Si MOSFET and the GaN FET. Rds(ON) is determined by dividing the average voltage drop across the switch by the average inductor current, both measured during the turn-ON period.

  5. Calculate the conduction loss of the Si MOSFET and GaN FET using Eqn. 3.

Reference

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



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