Three-terminal regulator is a common component of air-conditioning controller. Its main function is to convert and stabilize DC voltage.
The crystal elements in the three-terminal voltage regulator are semiconductor materials. In the actual production process, electrostatic damage often occurs, which will affect the normal operation of the whole air conditioning controller. Therefore, it is particularly important to analyze and study the failure principle of three-terminal regulator, which can help us solve the failure problem from the root. In this paper, the failure of three-terminal regulator is analyzed in detail. According to the failure principle, three optimization directions are summarized: 1. enhance the anti-static ability of the damaged devices inside the regulator; 2. add a resistance inside the regulator to prevent static damage; 3. add a capacitor at the input end of the regulator to eliminate static electricity. At present, in order to realize intelligent control of air conditioning in enterprises, electronic controllers are needed. The weak voltage regulators commonly used in electronic controllers are three-terminal regulators, commonly used are 7815, 7812 and 7805 series. Because of the inconsistency of manufacturing process, design scheme and working environment of pressure regulator, the service life of pressure regulator is inconsistent. A large number of failure cases of three-terminal pressure regulator occur in the process of using, and the failure of three-terminal pressure regulator directly leads to the failure of air conditioning. Therefore, the failure principle of the three-terminal regulator in the use process is analyzed, the root causes are found and optimized, which can reduce the production process and after-sale use of the regulator offline, and help to produce manufacturing costs and after-sales maintenance costs.
This paper aims at the three-terminal voltage regulator which fails after sale.
By examining the operating environment of the voltage regulator and the anti-static ability of the device itself, the failure mode of the voltage regulator is analyzed, the similar failure parts are simulated, and the failure data are analyzed to give a reasonable optimization scheme. At present, the basic working principle of the three-terminal regulator designed by various manufacturers is much the same, but according to the abnormality of the three-terminal regulator after sale, some circuit optimization is made. By changing the parameters of some components, different voltage stabilization effects can be achieved and the test of various use environments can be resisted. The working principle of three-terminal regulator is shown in Fig. 1. Each manufacturer modifies circuit parameters and components according to actual needs. According to the schematic diagram, the functions of each module are divided into start circuit, reference voltage circuit, sampling comparison amplification and adjustment circuit, and protection circuit. The specific structure is shown in Figure 2. The following is a detailed analysis of the internal structure of the regulator. The starting circuit consists of T1, T2 and Dz1. When the input voltage Vi is higher than the stable voltage of the regulator Dz1, the current passes through T1 and T2, which makes the T3 base potential rise and turn on.
At the same time, the constant current source T4 and T5 also work. The collector current of T4 passes through Dz2 to establish the normal working voltage. When Dz2 and Dz1 reach the same constant voltage, the whole circuit enters the normal working state and the circuit starts up. At the same time, T2 is cut off because the voltage difference between base and emitter is 0, which cuts off the connection between start circuit and amplifier circuit, so as to ensure that the ripple and noise on the left side of T2 do not affect the reference voltage source. The reference voltage circuit is composed of T4. Dz2.
T3. R1. R2, R3, D1 and D2. The reference voltage can be compensated with the positive temperature coefficient of R1, R2, R3, Dz2 and the negative temperature coefficient of T3, D1 and D2 emission junctions in circuit design and process, so that the reference voltage can not change with temperature basically. At the same time, the constant current source is used to supply the voltage regulator Dz2, so as to ensure that the reference voltage is not affected by the fluctuation of the input voltage. Sampling comparison amplifier circuit and adjusting circuit are composed of T4T11, in which T10 and T11 constitute compound adjusting tube; R12 and R13 constitute sampling circuit; T7, T8 and T6 constitute differential amplifying circuit with constant current source; T4 and T5 constitute constant current source as its active load. As buffer circuits, T9 and R9 play a shunting role for T8.
The protection circuit is divided into current-reducing protection circuit and overheating protection circuit to prevent the damage of voltage regulator caused by high input voltage, fluctuation of input voltage and high operating temperature of voltage regulator. Decreasing current protection circuit T12, R11, R15, R14 and Dz3, Dz4 are composed of: R11 is a galvanometer resistance. The main purpose of protection is to enable the regulator (mainly T11) to work within the safety zone, and special attention should be paid to making its power consumption not exceed the rated value. Because the branches of Dz3, Dz4, R14, R15 and T12 limit the power consumption of T10 and T11 regulator circuits to the rated range and reduce the current flowing through R11 when the input voltage is high, they are called reduced current protection. (2) The overheat protection circuit consists of Dz2, T3, T14 and T13. At room temperature, the voltage drop on R3 is only about 0.4V, and T14 and T13 are cut off, which has no effect on the circuit operation. When the chip temperature rises to the limit value for some reason, the voltage drop on R3 increases with the increase of Dz2 working voltage. T14 and T13 are turned on. The base current of the adjusting tube T10 is shunted by T13, and the current flowing through R11 decreases to achieve the function of overheat protection. In the process of using three-terminal regulator in air-conditioning enterprises, it usually undergoes the tests of temperature, instantaneous power-on and power-off, electrical safety test and static electricity.
In the process of air-conditioning production, pressure regulator involves high temperature tests such as wave soldering and manual soldering, in which the maximum temperature of manual soldering can reach 400 C. Design experiment: Take three qualified products of pressure regulator, use soldering iron to contact the welding end of pressure regulator continuously, set 420 C, and continue high temperature welding for 1 minute, 2 minutes and 3 minutes respectively. After cooling, test whether the output voltage of pressure regulator meets the requirements. The experimental results are shown in Table 1. Taking 7805 series regulators as an example, the experimental results show that high temperature welding does not cause obvious damage to regulators. In the process of air-conditioning production, the voltage regulator needs to undergo on-off test. Design experiment: Take an electronic controller equipped with a regulator, connect the test points at the input end and the signal ground end of the regulator, simulate the process of plugging and pulling the power line, and use oscilloscope to measure whether the voltage at the input end of the regulator is abnormal. The experimental results are shown in Fig. 3. The maximum input voltage of 7812 regulator is 17V, and that of 7805 regulator is 12V. No overvoltage occurs at the input of 7812 regulator when power is switched on or off. There must be low frequency pulse voltage in the plug-in process of power supply line, but there is no pulse voltage at the input of voltage regulator. The reason is that the front of voltage regulator of air-conditioning electronic controller matches uF capacitor, which can filter out low frequency pulse voltage. Fig.
4.
For the application circuit of common regulator, matching C133 electrolytic capacitor can eliminate low frequency pulse voltage. In the process of air-conditioning production, voltage regulators need to undergo electrical safety testing. According to the testing principle of four basic indicators, such as AC/DC voltage withstanding, insulation resistance, leakage current and grounding resistance, the voltage regulator can be judged without influence (the testing principle is not described in detail here). Design experiment: Take an electronic controller equipped with a regulator, connect the test points at the input end and the signal ground end of the regulator, test the electrical safety of the controller, and use oscilloscope to measure whether the voltage at the input end of the regulator is abnormal. The experimental results are shown in Fig. 5,7812 and 7805 respectively. The maximum input voltage of the regulator is 6V and 3V, thermostatic element which will not cause damage to the regulator. The most common electrostatic release modes in air conditioning production are human body mode (HBM) and machine mode (MM). Design experiment: Take some qualified three-terminal regulators from a manufacturer, break down the regulator using human body mode (HBM) and machine mode (MM), five times per regulator, record the critical voltage of breakdown regulator. Figures 6 and 7. The capacitor is charged first, and then the capacitance is released to the input end and the ground end of the regulator. The experimental results are shown in Table 2. The breakdown voltage is 10 KV in HBM mode and 3.5 KV in MM mode according to the abnormal output voltage of the regulator. It can be judged that the regulator is easier to breakdown under the MM mode. The electrostatic discharge of MM mode in the air conditioning process is usually caused by injection molding and foam. The failure mode is judged from the basic attributes, heat dissipation, internal crystal failure point and power-on situation of the fault parts. Measurements of PN junction and resistance between pins of fault parts are shown in Table 3. It is found that the basic properties of fault parts are not different from those of qualified products. It is difficult to identify fault regulators in production process. The heat dissipation paste coating and radiator installation of the fault regulator and the qualified regulator are compared as shown in Figure 8. The heat dissipation condition is good. X-ray shows that the crystal elements of the faulty regulator and the qualified regulator are in good contact with the radiator block of the regulator, as shown in Figure 9.
The damaged point of the fault regulator is analyzed by scanning electron microscope, and the correctness of the damaged point is judged by designing experiments and analyzing data based on the damaged point. Scanning the internal crystal elements of a manufacturer’s fault regulator with an electron microscope, the close-up view shows that the fused silicon of Q7 transistor is caused by electrical damage. The internal schematic diagram of the manufacturer’s regulator is shown in Fig. 10. Only Q7 transistor is abnormal in voltage regulator. The circuit near Q7 transistor in FIG. 10 is simplified. The influence of static electricity or overcurrent on Q7 is analyzed when the voltage regulator input end and ground foot enter. The simplified circuit is shown in Fig. 12. Because of the protection of low-frequency capacitive filter at the input end, the regulator will not be damaged by EOS (Electrical Over Stress) when it is installed with PCB board. The maximum input voltage of the regulator is 35V. According to the production process, all EOS sources above 35V voltage will not be damaged by EOS when the regulator is installed with PCB board. The injury of Q7 was judged to be caused by ESD (Ele ctrical Static Discha rge). If static electricity is input from the foot, Ui output, D1 conduction, electrostatic voltage consumes Q7 base, which leads to Q7 breakdown from base to collector. Whatever the electrostatic intrusion mode, the Q7 damage is easily caused by the scheme involved in the regulator. According to Fig. 10, Q7 regulator is located in the start-up circuit. If it is damaged, the regulator will not be able to start and not work. For the input voltage of the regulator, the current flows through the way shown in Figure 13. The current flows from the red line to the blue line and the green line, and then the purple line flows out of the ground. When the D2 voltage is saturated, the output voltage Vout increases linearly with the input voltage because it only involves the resistive voltage divider. Design experiment: The input voltage of the fault regulator is continuously increased. Vout is recorded as table 4, and a linear graph is made as figure 14.
It is known that Vout will increase linearly with the input voltage, which accords with the fault phenomenon of Q7 damage. Analytical data: According to the data in Table 3, the basic attributes of the fault regulator PN junction and resistance value are not different from the qualified products. According to Fig. 10, we can see that the measurement of PN junction and resistance value of basic attributes is only to measure the attributes of some components on the right side of Fig. 10. The damage of Q7 does not affect the basic attributes, and it also shows that the device affecting Vout on the right side of Fig. 10 is not damaged. The results of design experiments and analysis data can judge that Q7 damage accords with the fault phenomenon. The damage point of the fault three-terminal regulator is the internal Q7 transistor. Because of the design reasons, the Q7 transistor of the three-terminal regulator is very vulnerable to electrostatic damage. The electrostatic protection of the three-terminal regulator is needed in production. The external circuit of the voltage regulator adds static electricity protection, such as adding capacitance (PF level) between the input end and the foot of the voltage regulator when the voltage regulator is used, which can be used to eliminate part of static electricity and reduce the possibility of static electricity damage.