Overview
Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.
While it is simple to step up or down AC voltages and currents using transformers, stepping up or down DC voltages and currents in an efficient and regulated manner requires switching power converters. The DC/DC buck converter chops the input DC voltage using a series input switch, and the chopped voltage is filtered through the L-C low-pass filter to extract the average output voltage. The diode provides a path for the inductor current when the switch is off for part of the switching period. The output voltage is this less than or equal to the input voltage.
The objective of this experiment is to study different characteristics of a buck converter. The step-down capability of the converter will be observed under continuous conduction mode (CCM) where the inductor current is non-zero. Open-loop operation with a manually-set duty ratio will be used. An approximation of the input-output relationship will be observed.
Principles
Linear regulators (series and shunt) can provide step-down capability, but are highly inefficient when the output-to-input voltage ratio is very low. Voltage dividers can also step down DC voltage, however, there is no regulation involved with variable loads. Buck converters thus present efficient and robust DC voltage step-down capabilities.
In order to construct a buck-converter, we can start with the circuit shown below in Fig. 1(a). When the switch is on for a portion (D) of the switching period (T), the output voltage (Vo) and input voltage (Vin) are equal. When the switch is off for a portion (1-D) of the period, the output voltage is zero. This produces a square-wave output voltage whose average (shown by brackets < >) is less than that of the input voltage: <Vo>=VoD+ Vo(1-D)= VinD + 0(1-D)= VinD.
In order to minimize the output current ripple, and thus the output voltage ripple with a resistive load, an inductor is added as shown in Fig. 1(b). The issue with an inductor is that it maintains current flow until all its stored energy is released, so if the switch turns off, a large dI/dt will occur across the switch since current has to flow. Therefore, a freewheeling diode is added to provide an inductor current path as shown in Fig. 1(c). However, the inductor's inductance will have to be very large in order to have very low output voltage ripple, and a capacitor must be added to reduce the inductor size and provide a clean DC voltage output at the load as shown in Fig. 1(d).
Figure 1. Steps to building a buck converter
As this experiment proceeds, it will be shown that the average output voltage will increase as the duty cycle, D, increases. With higher switching frequencies, the voltage ripple at the output will decrease since the voltage charging and discharging times at the capacitor become significantly shorter with a decreased switching frequency.
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Procedure
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
- Connect the ±12 signal supply at the "DIN" connector but keep "S90" OFF.
- Make sure that the PWM control selector is in the open-loop position.
- Set the DC power supply at 24 V. Keep the output disconnected from the board.
- Before connecting the load resistor, adjusted it to 12 Ω.
- 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.
- Connect "RL"across "V2+" and "COM."
- 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
- Connect the differential probe across the gate to source of the upper MOSFET.
- Turn ON "S90." A switching signal should appear on the oscilloscope screen.
- Adjust the signal time axis to see two or three periods.
- Adjust the frequency potentiometer to achieve a frequency of 100 kHz (period of 10µs).
- Adjust the duty ratio potentiometer to achieve a 50% duty ratio.
3. Buck Converter Testing for Variable Input
- Connect the input DC power supply, which is already set at 24 V, to "V1+" and "COM."
- Connect the differential probe across the gate to source of the upper MOSFET.
- Connect the other probe across the load. Make sure the ground connector is connected to "COM."
- Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).
- Record the input current and voltage readings on the DC power supply.
- Adjust the input voltage to 21 V, 18 V and 15 V, and repeat the above steps for each of these voltages.
- Disconnect the input DC supply and adjust its output to 24 V.
4. Buck Converter Testing for Variable Duty Ratio
- Connect the differential probe across the gate to source of the upper MOSFET.
- Connect the other probe across the load. Make sure the ground connector is connected to "COM."
- Connect the input DC supply that is set to 24 V between "V1+" and "COM."
- Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).
- Record the input current and voltage readings on the DC power supply.
- 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.
- Reset the duty ratio to 50%.
- Disconnect the input DC supply.
5. Buck Converter Testing for Variable Switching Frequency
- Connect the differential probe across the gate to source of the upper MOSFET.
- Connect the other probe across the load. Make sure the ground connector is connected to "COM."
- Connect the input DC supply to "V1+" and "COM."
- Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).
- Record the input current and voltage reading on the DC power supply.
- 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.
- Turn OFF the input DC supply and "S90," and then disassemble the circuit.
Buck converters generate a DC output voltage that is less than the DC input. In other words, buckling down or decreasing the supply voltage. Commonly used linear regulators step down voltage by dissipating power as heat in a resistor, which becomes very inefficient with large differences between the input and output voltages. While resistive components waste power through joule heating, buck converters use reactive components that ideally dissipate no power and consequently can efficiently decrease voltage with a corresponding increase in available current. In the buck converter, a switch traps the DC supply to create the AC input to a low pass filter. The low pass filter consist of an inductor and a capacitor and extracts the average voltage with only small losses due to parasitic resistances. The result is an output voltage less than or equal to the input voltage. This video will illustrate the construction of a buck converter and investigate how changing the converters operating condition affects its output voltage.
This buck converter circuit uses an electronic switch to connect and disconnect an inductor from the DC power supply. This switch maybe a bipolar transistor, a MOSFET or other similar electronic device. The inductor and a capacitor make up a low pass filter with a diode to provide a path for inductor current when the switch is open. The output of the low pass filter is connected to the load. A digital pulse train opens or closes the switch with a duty ratio, D, which is the ratio of the on-time to the period. When the switch is closed, the input to the low pass filter is connected to supply voltage, V in. The diode becomes reverse biased and does not conduct and current flows through the inductor. When the switch is open, this inductor current must continue in the same direction and the diode becomes forward biased to form a complete current loop. At the input to the low pass filter, this switch commutation produces a rectangular wave that oscillates between V in in approximately zero volts. Except for some ripple, the output of the filter is the average of the rectangular wave, which increases as the duty ratio increases. At sufficiently high switching frequencies, the capacitors charge and discharge times are short. So the voltage ripple becomes small and the result is a clean DC output stepped down from the DC input. Because the inductor and capacitor are reactive components, they ideally have no resistive power loss. The ideal LC filter then is able to pass power to the load with 100% efficiency. In reality, the wire resistance of the inductor and other parasitic resistances in the circuit, reduce efficiency to the range of 80 to 95%. Now that the basics of the buck converter had been discussed, let's take a look at how a buck converter steps down voltage and continue as conduction mode, also called CCM, a condition when the inductor operates at all times with non-zero current.
These experiments utilize the HiRel Systems power pole board which is designed for experimentation with various DC to DC converter circuit topologies. Begin by ensuring that the signal supply switch, S90 is turned off. Then plug the signal supply into DIN connector J90. Set the PWM control selection jumpers, J62 and J63 to the open-loop position. Adjust the DC power supply to positive 24 volts but do not connect the power supply output to the board. Build the circuit with the upper MOSFET, the lower diode and the BB magnetic board. Record the value of the inductor on the BB magnetic board. Load resistor RL is a power potentiometer. Use a multimeter to read its resistance while adjusting it to 12 ohms. Then connect the load resistor between terminals V2+ and COM. Set switch selector bank S30 as follows. PWM to upper MOSFET, use Onboard PWM and switched load off. Next, connect the oscilloscopes differential probe between terminal 15, which is the gate of the upper MOSFET and terminal 11, which is the source. Turn on signal supply switch, S90 and observe the pulse train that drives the MOSFET. Set the frequency adjustment potentiometer, RV60 to produce a switching frequency of 100 kilohertz. Set the duty ratio potentiometer, RV63 so the pulses have an on-time of five microseconds.
Keep the differential scope probe connected between terminals 15 and 11, which are the gate and source of the upper MOSFET respectively. To measure voltage across load resistor, RL, connect the other differential probe between terminals V2+ and COM. Connect the DC power supply to input terminals, V1+ and COM. Observe the triangular wave form for output voltage and the rectangular pulse train of the switching signal. The upward ramps of the output voltage occur when the buck converter switch is closed and the inductor is transferring energy to the capacitor and load. The downward ramps occur when the switch is open, the inductor is disconnected from the input voltage source and the capacitor is giving up some stored energy to the load. Next, measure the mean value of the output voltage and the on-time of the gate source voltage. Note the input current and voltage readings from the DC power supply. Repeat this test after adjusting duty ratio potentiometer, RV64 so the pulse train has duty ratios of 0.4, 0.6 and 0.7. As the duty ratio D increases, the average output voltage of the buck converter also increases. Ideally, if D has a value of 0.3, then an input of 24 volts generates an output of about 7.2 volts. Likewise, if D is 0.5, then output would be about 12 volts or if D is 0.7, then the output would be about 16.8 volts and so on.
Set the duty ratio to 0.5 and then connect the input DC supply to terminals V1+ and COM. Set RV60 to produce a switching frequency of 100 kilohertz. Like before, the output voltage waveform is a triangle wave resulting from the low pass filter acting on the rectangular wave input. The gate source voltage is a digital pulse train with a frequency of 100 kilohertz. A period of 10 microseconds and an on-time of five microseconds. Measure the mean value of the output voltage and the on-time of the gate to source voltage. Note the input current and voltage readings from the DC power supply. Repeat this test after adjusting RV60 to a switching frequency of 10, 20 and 40 kilohertz with the duty ratio fixed at 0.5. As the frequency increases, the output ripple decreases because the capacitor charge and discharge times also decrease. In general, the output voltage is in this experiment are less than expected from the ideal relationship. This deviation is the result of parasitic element such as wire resistance in the inductor and other resistances in the circuit, which create non-ideal voltage drops and unaccounted energy loss.
Buck converter provide well-controlled voltage regulation with an accompanying step up in current, making them crucial for applications concerned with minimum power loss in the conversion process. Power consumption in laptops has decreased greatly due to the development of microprocessors that operate with only 1.8 or 0.8 volts. Laptops and remote controlled devices use buck converters to reduce the voltage of lithium batteries to these low values, extending useful battery life and stepping up battery current to supply the needs of integrated circuits with millions of transistors. Electronic devices such as cellphones use lithium ion batteries with a nominal cell voltage, about 3.6 to 3.7 volts. However, standardized battery chargers with the USB connectors supply five volts. A buck converter in the electronic device steps down the USB output to the lower voltage required to charge the lithium ion battery.
You've just watched Jove's introduction to buck converters. You should now understand their operation and how the DC output depends on the duty ratio and switching frequency. Thanks for watching.
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Results
It is expected the output-input voltage relationship of an ideal buck converter to be related to the duty cycle or duty ratio D. If the input voltage is Vin and the output voltage is Vout, Vout/Vin = D, where 0≤D≤ 100%. Therefore, for an input voltage of 24 V, Vout≈ 12 V for D = 50%, Vout≈ 7.2 V for D = 30%, and Vout≈ 16.8 V for D = 70%. Nevertheless, the output voltage will be lower than expected from the ideal relationship, which is linear with the duty ratio, and the main reason is that the ideal buck converter model does not account for non-idealities and voltage drops in the converter.
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Applications and Summary
Buck converters are very common in electronic device chargers where they provide excellent voltage regulation required for battery charging. They are commonly used in power supplies that power computers, integrated circuits and electronic boards, as well as in renewable energy applications and battery fed systems.
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