EEE 360 LAB 23

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Apr 3, 2024

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EEE 360 Lab 23 Experiment 23. Diode Bridge Rectifier cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-1

OBJECTIVE  To study the operation of a single-phase diode bridge rectifier. DISCUSSION Rectifiers are a class of power converters which convert alternating current (AC) to direct current (DC). Rectifiers are classified as either controlled or uncontrolled. Today, we will be exploring the uncontrolled, single-phase, full-wave, diode bridge rectifier. It should be noted that many of the concepts discussed can be extended to the three-phase power system. This bridge rectifier circuit is comprised of four diodes usually followed by a filter capacitor as illustrated in Figure 23-1. The diode is a two-terminal device, which permits current flow in a single direction. The forward current of a large diode can be can be several thousand amperes. The reverse current is only few milliamperes. The forward rated current produces potential drop of a few volts. In the reverse current direction, the increase of the voltage beyond the rated value produces breakdown and destruction of the device. The single-phase bridge type rectifier is widely used in low power appliances because of simplicity and low cost. A transformer is also usually placed before the rectifier to first step the voltage down. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-2

Figure 23-1. Bridge rectifier circuit with a filter capacitor. In many cases, the four diodes are contained in a single package as shown in Figure 23-2. Figure 23-2. Diode bridge components. The bridge rectifier, without a filter capacitor, functions as follows. When the AC voltage is positive relative to earth ground, current will flow through diode D1, through the load, and back through diode D4, as shown in Figure 23-3. Diodes D2 and D3 will be off during this first half of the cycle. Figure 23-3. The current through D1 and D4 in diode bridge rectifier. When the input swings negative, current will flow through diode D2, through the load, and back through diode D3, as shown in Figure 23-4. Diodes D1 and D4 will be off in this second half of the cycle. Current will always only flow in the same direction through the load. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-3

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Figure 23-4. The current through D2 and D3 in diode bridge rectifier. Figure 23-5 shows a typical voltage waveform applied to a resistive load without the filter capacitor. The DC output voltage is pulsed at twice the line frequency (120 Hz). Figure 23-5. Typical output voltage waveform without a filter capacitor. The average DC output voltage is related to the peak of the input AC waveform with the following expression: V DC = 2 V P π (1) With the filter capacitor included, the DC output waveform is smoother and has a greater average value as illustrated in Figure 23-6. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-4

Figure 23-6. Typical output voltage waveform with a filter capacitor. With the filter capacitor in place, there are two distinct time intervals: charging and discharging. While charging, the voltage source of the electric grid is charging the capacitor as well as feeding the load. When discharging, all of the diodes are off and energy stored in the capacitor is passed to the load. The peak-to-peak ripple voltage across the load can be approximated with Equation (2). This approximation is valid when the charging interval is small relative to the discharge interval. V pp = ( V P − 2 V ON ) T 2 RC (2)  V P is the peak voltage of the input sine wave  T is the period of the input sine wave  V ON is the diode voltage drop  R is the load resistance  C is the filter capacitance. This type of rectifier has a few peculiarities that need to be addressed. When the circuit is first attached to the grid, a large inrush current will occur because the electric grid voltage source is attached to the discharged capacitor through the diodes. This problem is usually solved with the inclusion of a negative temperature coefficient (NTC) thermistor between the electric grid voltage source and capacitor. This is illustrated as R1 in Figure 23-1. When cold, the NTC thermistor has a large resistance to limit inrush current. Once heated, from current passing through, the resistance reduces to minimize efficiency losses. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-5

Another peculiarity is that the rectified DC negative voltage node of the full bridge rectifier is at an average potential below earth ground! A short circuit will occur if the negative terminal is attached to earth ground! Furthermore, the discontinuous capacitor charging current causes a non-sinusoidal wave-shape and harmonic distortion of the current drawn from the grid. The bridge rectifier with filter capacitor causes a reduction in power factor through displacement and distortion of the AC current. The dominant distortion component can be analyzed in the frequency domain. We now define a quantity to evaluate the distortion of current fed by the grid. The Total Harmonic Distortion (THD) of a periodic current is as follows: THD = √ ∑ h = 2 ∞ ( I h 2 ) I 1 × 100% (3)  I 1 is the current amplitude at the fundamental frequency ( h =1) and  I h is the current amplitude of the harmonic h . IEEE has established standards that set permitted limits on the current harmonics drawn from the grid. A solution to harmonic distortion caused by rectifiers is Power Factor Correction (PFC). PFC is a circuit utilized in high-power rectifiers to draw a sinusoidal current from the grid while regulating the DC output. This leads to a brief discussion of controlled rectifiers. The diode bridge rectifier previously discussed is considered uncontrolled. The output voltage cannot be regulated. The thyristor controlled rectifier shown below is a traditional solution to regulate the output DC voltage in high-power applications. This is accomplished by controlling the firing angle of the Silicon Controlled Rectifiers (SCR). Figure 23-7. Thyristor controlled rectifier. A more efficient solution is the full bridge Pulse Width Modulated (PWM) rectifier (illustrated in Figure 23-8). The PWM rectifier is capable of regulating the output voltage and PFC. Furthermore, with appropriate controls, this circuit is capable of bi-directional power flow. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-6

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Figure 23-8. PWM rectifier. PROCEDURE The grading in this section is 2 points for each correct answer. 1. Open the Virtual Laboratory and from the Virtual Lab Welcome Window click on the Experiment 23 Procedure 1 button. Your screen should look similar to Figure 23-9. Wire the test circuit as shown in Figure 23-10. Attach a 32 Ω load to the output using the adjustment slider on the resistor. The oscilloscope can be used to observe and measure the output voltage waveform. Note that the oscilloscope is already connected to the DC terminal of the rectifier module. Connect the DC voltmeter across the output terminals of the PWM rectifier to measure the average voltage. If you are unsure how to make/remove connections, please refer to the Experiment 1 manual. Note : The DC Voltmeter will always measure the average value of voltage. The Oscilloscope will give an approximate RMS value of voltage. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-7

Figure 23-9. Screen capture for Procedure 1. Figure 23-10. Connection circuit for Procedure 1. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-8

Measurements/Calculations: a. Keep the “Capacitor on-off” switch, marked S1 in Figure 23-10, in the "OFF" position. Adjust the variable voltage slider to 30 V AC. Ensure that the 32 Ω load is attached. Then, click on the Run button. From the signal analyzer note the percentage of THD. From the oscilloscope, record the approximate RMS value of the output voltage, and from the voltmeter record the average DC output voltage. Also, calculate the average voltage (V dc ) based on Equation (1). Be sure to click on the “View Oscilloscope” button to look at the output voltage waveform and note down the peak value of the voltage from the waveform (you may find it helpful to zoom into the plot). To zoom the plot click on the magnifying glass with the “+” sign in the middle. Then, click and drag over the area in which you would like to enlarge). Table 23-1. Measurements without a capacitor filter Total Harmonic Distortion (%) 1.78 AC RMS voltage (V) 28.35 Average dc output measured (V) 25.26 Average dc output calculated (V) 25.78 b. Compare the calculated and the measured average output voltage. Is the relationship given in Equation (1) verified?  Yes  No c. Move the "Capacitor on-off" switch to the “ON” position in order to include the capacitor in the rectifier circuit. Adjust the variable output voltage slider for 30 V AC. Click on the Run button. Measure and record the THD and output voltages with two load configurations: 16 Ω and 32 Ω. To measure the voltage ripple, it will be helpful to zoom in on the oscilloscope plot (as described in Procedure 1a). Also, calculate the ripple voltage for each load condition using Equations (1) and (2). Assume a diode voltage drop of V ON = 0.6 V. If you receive abnormal/unexpected results, check your wiring and voltage magnitude. Once you have fixed your circuit, click the Run button again. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-9

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Table 23-2. Measurements with a capacitor filter Load 16 Ω 32 Ω Total Harmonic Distortion (%) 93.88 106.32 Measured average voltage (V) 33.97 36.02 Measured RMS voltage (V) 34.30 36.36 Measured ripple voltage (V) 5 3 Calculated ripple voltage (V) 8.1 4.17 REVIEW QUESTIONS 1. Discuss the advantages and disadvantages of diode bridge rectifiers with filter capacitor. (4) Advantages: The advantages include a smoother output voltage due to the reduction of output voltage ripple. Disadvantages: The disadvantages are that the THD is increased and a slightly increased design time because the correct capacitor needs to be chosen. Comment on the THD of the rectifier with and without the capacitor filter. (4) With Capacitor: Because the capacitors function as an energy storage device and cause the signal to no longer resemble a sinusoid, the THD is significantly larger when using capacitors. Without Capacitor: The THD is about 1-2% since there are no capacitors. The signal is much closer to a sinusoid. cf4c3cad91289812d1a9005f1afc056067777ff9.docx 23-10