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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.
