DC/DC Buck Converter

This experiment will utilize the DC-DC converter board provided by HiRel Systems. http://www.hirelsystems.com/shop/Power-Pole-Board.html

Information about the board operation can be found in this collections video “Introduction to the HiRel Board.”

The procedure shown here applies to any simple buck converter circuit that can be built on proto boards, bread boards, or printed circuit boards.

1. Board setup

1. Connect the ±12 signal supply at the "DIN" connector but keep "S90" OFF.
2. Make sure that the PWM control selector is in the open-loop position.
3. Set the DC power supply at 24 V. Keep the output disconnected from the board.
4. Before connecting the load resistor, adjusted it to 12 Ω.
5. Build the circuit shown in Fig. 2 by using the upper MOSFET, lower diode, and BB magnetic board. Record the inductance value shown in the board. Note that the BB magnetic board has an inductor with two terminals that plug in to the DC-DC converter (power-pole) board.
   1. Connect "R_L"across "V2+" and "COM."
   2. Make sure the switch array for MOSFET selection, PWM selection, and other settings are as shown in Fig. 2.

Figure 2. Buck converter circuit

2. Adjusting the Duty Ratio and Switching Frequency

1. Connect the differential probe across the gate to source of the upper MOSFET.
2. Turn ON "S90." A switching signal should appear on the oscilloscope screen.
   1. Adjust the signal time axis to see two or three periods.
   2. Adjust the frequency potentiometer to achieve a frequency of 100 kHz (period of 10µs).
   3. Adjust the duty ratio potentiometer to achieve a 50% duty ratio.

3. Buck Converter Testing for Variable Input

1. Connect the input DC power supply, which is already set at 24 V, to "V1+" and "COM."
2. Connect the differential probe across the gate to source of the upper MOSFET.
   1. Connect the other probe across the load. Make sure the ground connector is connected to "COM."
3. Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).
   1. Record the input current and voltage readings on the DC power supply.
4. Adjust the input voltage to 21 V, 18 V and 15 V, and repeat the above steps for each of these voltages.
5. Disconnect the input DC supply and adjust its output to 24 V.

4. Buck Converter Testing for Variable Duty Ratio

1. Connect the differential probe across the gate to source of the upper MOSFET.
   1. Connect the other probe across the load. Make sure the ground connector is connected to "COM."
2. Connect the input DC supply that is set to 24 V between "V1+" and "COM."
3. Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).
   1. Record the input current and voltage readings on the DC power supply.
   2. Adjust the duty ratio for three steps of your choice between 30% and 70%. Repeat the above steps for each of these three duty ratios.
4. Reset the duty ratio to 50%.
5. Disconnect the input DC supply.

5. Buck Converter Testing for Variable Switching Frequency

1. Connect the differential probe across the gate to source of the upper MOSFET.
2. Connect the other probe across the load. Make sure the ground connector is connected to "COM."
3. Connect the input DC supply to "V1+" and "COM."
4. Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).
   1. Record the input current and voltage reading on the DC power supply.
   2. Adjust the switching frequency for three steps of your choice between 5 kHz and 40 kHz. Repeat the above steps for each of these three duty ratios.
5. Turn OFF the input DC supply and "S90," and then disassemble the circuit.