Application of diodes Instructions for the practical exercises of subject AE4B34EM 1 Objective measurements 2 The purpose of this measure is the introduction to the features and applications of semiconductor diodes. During this measurement, you will be able to verify in practice the basic parameters of the diodes, try out their function and basic applications. 3 Preparing to exercise 4 Tasks 5 Theory Study the principle of metallurgical PN junction, Shockley equation and the effect of material structure on the resulting diode s threshold voltage. Study the parameters of ideal and real diodes. Study the function of diode rectifiers. Study how to use the measuring devices and read the catalog parameters of the diodes. Create a simple current source, measure its parameters to calibrate. Measure the voltage drop in the forward direction of the diodes on the calibrated values of the current, which supplies power source and draw the approximate VA characteristics of selected diodes. Measure the reverse recovery time t rr of KY708 diode. Connect single-pulsed rectifier with filter capacitor and load resistor. Display the output voltage and charging current for two different values of filter capacitor capacitance. Connect Greatz diode-bridge (two-pulsed rectifier) and display the output voltage. 5.1 Boards Evaluation boards on which we perform measurements contain all the necessary components for basic measurements on semiconductor diodes and diode rectifiers. Component leads are connected to the jacks, which are available on the front panel. For connecting devices use a thin connecting wires, to connect measuring devices use wires with connectors of a larger size. To connect the oscilloscope use the attached probes. Fig. 1 Evaluation board for diodes testing
Fig. 2 Diodes rectifier board On the first board it is possible to characterize different types of semiconductor diodes, the list of the basic parameters is given in the following chapter. The board consists of a power source, which is based on feedback voltage stabilizer type 7805. This stabilizer "guards" the voltage across the resistor plugged in the current branch. Since the voltage on this resistor is directly proportional to the current passed through, the current is also stabilized at a constant size by formula Iout = 5 / R [A, Ω]. This current source allows us to set the operating point of simple diodes and the possibility of measuring VA diode characteristics. Measurements using voltage source would be very complicated for precise voltage settings U F as diodes in their work areas have low differential resistance. The second board enables measurements on different types of diode rectifiers, including simulation of various load sizes. You can also choose different capacity and see the output voltage ripple. 5.2 Parameters of measured diodes Before we start connecting devices together, it is necessary to know the maxiaml parameters of several diodes, which must not be exceeded. The most restrictive parameters in the application of LEDs are mostly maximum forward current and maximum reverse voltage. Furthermore, we can meet the requirements of diode power dissipation, this is important in applications such as Zener diodes as the voltage stabilizer. The manufacturer gives a large number of parameters in the data sheets for their products, some are listed in Table 1 Tab 1 Diodes parameters Maximal values Nr. Typ I FAV [ma] I FSM [A] U RRM [V] U RSM [V] Notice 1 BB 204B 100 30 Varikap 2 KY 130/80 300 30 100 Rectifying p-n diode 3 ZD 5,1 Zener diode 4 BAT 45 30 0,06 15 Schottky diode 5 BP 104 30 Photodiode 6 LED R 20 5 Red LED 7 LED Y 20 5 Yellow LED 8 LED G 20 10 Green LED 9 LED INFRA 20 10 Infrared diode Heated 1 MBR 760 7500 150 60 Power Schottky diode Heated 2 KY 708 10000 80 90 100 Power P-N diode Notice: I FAV (Average forward current) I FSM (Forward surge maximum current) The maximum permissible peak forward current value, which cannot be repeated. To avoid damage to the diode, it can reapply only after remission of thermal effects. A typical example is a single overload caused by the diodes in the rectifier filter capacitor charging after applying the input voltage.
U RRM (Reverse repetitive maximum voltage) The maximum value of repeatable reverse voltage diode. This is a peak reverse voltage, because it decides on the structure breakdown diodes avalanche ionization mechanism. U RSM (Reverse surge maximum voltage) Unique maximum reverse voltage. The significance of this parameter is similar to the I FSM, the reverse voltage can be applied only once. A typical example is the overload overvoltage from electrical surges. 6 Practical measurement 6.1 Current source calibration In Figure 3 we can see the connection of single current source comprising a feedback voltage stabilizer. The principle of its action has already been introduced in Section 4.1, now proceed to verify its function and calibration. Resistor labeled R can be selected from four possible pieces varying its resistance value, the variability is achieved by current source output current. Inaccuracy of the resulting current is given by the instability of the feedback voltage stabilizer, inaccuracies of resistance R, and stabilizer s own consumption, which is expressed in current I Q. Use the schema shown in Figure 3. Before connecting measuring instruments set their measuring ranges as required. It is advisable to start higher range and refine it during the measurement. Prevent the destruction of the measuring device. Before connecting the power source, first set the output voltage to 12V and the selected output current limit to 500mA. Use all the values of resistors R to reach different output currents sources and write their measured values in Table 2 Fig. 3 Current source calibration Tab 2 - Current source calibration Output current I F [ma] Proposed value Measured value 6.2 V-A characteristics of diodes Take the previous schema and connect the voltmeter to read the forward voltage diode U F. Always connect the anode of the diode to the positive pole of the circuit, in this case the output current source. The cathode diodes (we know it as the bars of the diode schematic brand) connect to a common negative terminal of the circuit. Measure the voltage drops of diodes on the product outside the diodes placed on the hot plate for 4 different values of forward current. The measured values in the table and draw voltage characteristics of Schottky diodes, rectifier silicon diodes and one of the LEDs.
Fig. 4 Voltage drop measurement Tab 3 Voltage drops on diodes I F [ma] U F [V] BB204B KY 130 ZD 5,1 BAT 45 BP 104 LED-R LED-Y LED-G LED-IR Fig 5 - V-A characteristics 6.3 Reverse recovery time measurement One of the important parameters of a rectifier diode is reverse recovery time t rr. It is defined by the length of diode s commutation as the time interval between the zero crossing and the intersection of the straight line, led by 0.9 points and 0.25 I RRM with the axis. See Figure 6
Fig 6 Reverse recovery time definition Measurement of recovery will be performed on the diode KY 708, the schema can be seen in Figure 7. Current flowing through the diode is measured as a voltage drop across the resistor, whose resistance value is known. For measurements use a resistor in the upper right corner of the product, this has a resistance value of 100 Ω. According to Ohm's law we can therefore calculate the magnitude of the current passing through a U REZ /100 I R = [A, V]. Connected to the oscilloscope we can follow the output. Amplitude of the harmonic generator voltage is set to the highest possible value, its frequency gradually increase until it is clearly visible on the oscilloscope negative overshoot voltage. Subtract the size of the negative recovery period, as defined in Figure 6 and write it. Since this value is dependent on the size and speed of forward current commutation (the size of the stored charge and speed of clearing), subtract the following additional conditions and record them. Fig 7 Reverse recovery time - circuit Generator frequency f = Forward current peak I FM = Reverse recovery time t rr = 6.4 Single-pulsed rectifier measurement Use the schema, which is shown in Figure 8. Before connecting the measuring instruments set their measuring ranges as required. Both probes are connected to the oscilloscope ground terminals, this is due to oscilloscope design. In this case this means that the probe connected to channel 2 must be connected to the point with the lowest potential in the circuit. To display the progress of the voltage drop across the resistor RS in the correct direction, use the possibility to invert the waveform on the oscilloscope screen. Us the INVERT button on the oscilloscope. This voltage drop is again proportional to the current passed through, using Ohm's theorem we can convert it back. Due to the fact that the resistance value resistor RS is equal to 1 Ω, the voltage waveform scaled in mv is direct current waveform scaled in ma.
Use the output capacitor electrolytic capacitor with 1000 uf. Now display the waveforms of output voltage and charging current Iout capacitor I D for selected resistance load resistor. The displayed waveforms depend on the charging current, the size of the DC component and the output voltage s size of the curl. Use these data to calculate the ripple percentage. Fig 8 Output voltage and charging current circuit 1) R Z = 220 Ω C F = 100 µf U výst u výstšš = U OUT = U OUTPP I výstmax = I MAX p = u voutpp Uout. 100 =. 100 = (%) 2) R Z = 220 Ω C F = 1000 µf U výst u výstšš = U OUT = U OUTPP I výstmax = I MAX p = u voutpp Uout. 100 =. 100 = (%) 6.5 Double-pulsed (Graetz) rectifier measurement Build the Graetz bridge and make similar measurements as in the previous task. Using the same value of capacitance of filter capacitor and load resistor it drastically changes the output voltage ripple. See this situation on an oscilloscope screen and find out why the output voltage ripple is lower than in previous case.
Fig 9 Double-pulsed rectifier