Boost converter - Discontinuous conduction mode (DCM)

  • September 26, 2022
  • 1 hour to complete
  • Workbench v1.7

Introduction

In the previous experiment, Boost converter - Continuous conduction mode (CCM), was analyzed in continuous conduction mode (CCM), i.e., the inductor current was always greater than 0. In this experiment, the boost converter operation under discontinuous conduction mode (DCM) is anaylzed. Unlike the CCM, under DCM the inductor current briefly goes to 0 during each switching cycle.

Theoretical background

A boost converter is shown below, with the transistor and the diode making up the bi-positional switch of the power pole.

Boost converter

Similar to the CCM operation, under DCM, turning on the transistor increases the inductor current and when the transistor is turned off, the inductor current reduces. During the off period, as the energy stored in the inductor is transferred to the output capacitor, the inductor current continues to fall until it reaches zero. If the next switching cycle begins before the current goes to zero, the converter is said to operate under CCM. Else, if the current goes to zero and remains zero until the next switching cycle, the converter is said to operate under DCM. 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 vA at the current port, for an ideal boost converter, is as shown below

Ideal boost converter steady state operation waveforms showing the gating signal, power-pole voltage, inductor current, and diode current

When the switch is ON, i.e., q(t) = 1, vA = 0. When the switch is OFF but the inductor current iL is not yet zero, vA = Vo. This is because the diode is conducting during this period. When the switch is OFF and the inductor current iL is zero, vA = Vin. This is because the diode is no longer conducting and it blocks the difference between the output and input voltage across it.

Since under steady state the average current through the output capacitor is zero and the average diode current, Idiode, equals the output current, Io

Idiode = Io = VoR(1)

At the boundary of CCM and DCM, the average diode current, Idiode,crit equals

Idiode,crit = 1 - d2IL,pk(2)

During the switch ON time period d × Ts, the inductor current rises from 0 to IL,pk. During this period the voltage across the inductor is Vin = (1 - d) × Vo. Thus the peak inductor current is

IL,pk = (1 - d) × VoL × d × Ts(3)

From Eqns. 1 through 3, solving for the load resistance value at which the boost converter is at the border of CCM and DCM

Rcrit = 2LTs × d × (1 - d)2(4)

During CCM the relationship between the input and the output voltage is

Vo = Vin1 - d(5)

This relationship is not valid under DCM. The new relationship can be derived by assuming that the converter is ideal and that there is no power loss within the conveter. Therefore

Pin = Pout

⇒ Vin × Iin= Vo2R(6)

From the input/inductor current waveform above, the average input current is given by

Iin = 12 × (d + doff,1) × IL,pk(7)

From the inductor current waveform the peak inductor current is given by

IL,pk = d × Ts × VinL(8)

From the diode current waveform the average output current is given by

Io = VoR = 12 × IL,pk × doff,1(9)

Substituting Eqns. 7 through 9 into Eqn. 6

Vin × 12 × (d + doff,1) × d × Ts × VinL = Vo2R

⇒ Vin2 × d2 × Ts2L + Vo × VoR = Vo2R

⇒ Vo2 - Vin × Vo- M × Vin2 = 0

where, M = R × Ts2L × d2

Solving the above quadratic equation yields

Vo = Vin2 × (1 + √(1 + 4M)) = dcalc × Vin(10)

where,

dcalc = 1 + √(1 + 4M)2(11)

The above waveform was for an ideal boost converter. One stark difference between the ideal and actual response is the observed osciallation in vA voltage and the inductor current iL during the doff,2Ts time period, as shown below.

Practical boost converter steady state operation waveforms showing the gating signal, power-pole voltage, and inductor current

This is due to the fact that both the diode and the switch's parasitic capacitance along with the boost converter's inductor form an LC tank circuit as discussed in Buck converter - Discontinuous conduction mode (DCM).

Preparing the Workbench model

This model is exactly same as the one used earlier in Boost converter - Continuous conduction mode (CCM).

  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\Experiment6 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. Open the pre-built example project was pasted in Step 1.

  4. Open the Workbench model logo Boost model within the project.

  5. Ensure that the PWM switching frequency in Device configuration page in project properties, is set to 20 kHz.

  6. Ensure that the PWM channel is set to the Si power pole, i.e. Channel is set to 1.

Preparing the setup

This setting is exactly same as the one used earlier in Boost converter - Continuous conduction mode (CCM).

    warningWarning

    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 Buck/Boost/Buck-boost magnetics card. Prior to connecting the card, ensure that the jumper on the bottom of the magnetics card is removed. Make sure that all the 6-pins of the magnetics card are making solid contact with the power-pole board.

    warningWarning

    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 40 Ω.

  3. Power connections:

    Boost converter connections are the same as that of the buck converter where the input and output terminals are swapped.

    1. Connect the Vin terminals to the rheostat load:

      1. Rheostat rail: ● Vin+ (Red)

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

    2. Connect the Vo terminals to the isolated power supply:

      1. DC +ve: ● Vo+ (Red)

      2. DC −ve: ● Vo− (Black)

    3. Connect the power supply ground to the ground terminal on the board:

      Ground: ● Earth (Green)

  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 default/non-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 Vin. Use the same setting as that of Channel 2.

    please noteNote

    Since the input and output power terminals are swapped, Channel 1 which is connected to Io, actually measures the input/inductor current.Channel 3 which is connected to Iin, actually measures the diode current and Channel 4 which is connected to Vin, actually measures the output voltage.

  5. Jumper settings:

    1. Insert 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:

Boost converter wiring diagram

Real-time open loop control of boost converter - varying load resistance
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. Make note of the current sensor offset.

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

  5. Update the values in the Current A and Current B measurement offset.

  6. Click Numerical simulation and real-time prototyping Run button to rerun model.

  7. 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.

  8. Gradually increment the duty cycle from 0 to 0.2.

  9. Gradually reduce the load resistance until the peak inductor current exceeds 3 A or over current fault occurs, with at least 9 increments inbetween to make measurements - 4 in CCM, 1 at the border of CCM and DCM, and 4 in DCM.

    At the border of CCM and DCM, the inductor current (DSO channel 1), goes to zero just for an instant.

  10. Make the following measurements:

    1. Make a note of the voltage and current values displayed on Workbench, for each incremental change in the load resistance.

    2. Observe the DSO waveforms and make a copy of the voltage across the switch (channel 2), the output voltage (channel 4), the inductor current (channel 1) and the diode current (channel 3) for each incremental change in the load resistance. Adjust the time base to show anywhere between 4-10 switching cycles.

    3. Under DCM mode, make a note of the frequency of the ringing component in the measured voltage across the diode (channel 2).

  11. Click on Numerical simulation and real-time prototyping Stop button 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 diode, the output voltage, the inductor current, and the diode current for varying load resistance.

  2. For duty cycle of 0.2, enter the measured values from the Workbench screen capture and calculate the following values:

    Vin (V) Vo (V) Io (A) R (Ω) dcalc dact

    where, dcalc is 0.2 if in CCM and is given by Eqn. 11 if in DCM. dact = VoVin under both CCM and DCM. Load resistance R = VoIo.

    Plot dcalc and dact as a function of the output current Io.

  3. Compute the Rcrit using Eqn. 4. Use the inductance value estimated in Boost converter - Continuous conduction mode (CCM) experiment. Compare this result with that of measured value from the above table.

Reference

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



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