New automatic voltage switching integrated circuit STR81145

    Abstract: S TR81145 is a new automatic voltage switching integrated circuit produced by Toshiba Corporation of Japan. It can be used to enable electronic equipment to automatically adapt to various electrical environments. The article introduces the necessity of voltage switching and the basic principle of automatic switching. Finally, an actual circuit of bridge/voltage doubler rectifier switching with overvoltage protection composed of STR81145 is given.

    Keywords: bridge/voltage doubler automatic switching STT81145

1 Introduction

    With the continuous development of international trade, some special requirements have been put forward for electronic products. The most prominent one is the requirement that modern electronic products can automatically adapt to the power grid requirements of different countries and regions (because the AC power supply voltage values ​​in different countries are different, some 110V, 127V, 220V, 240V, etc.); on the other hand, the power grid voltage in some areas is extremely unstable, and in severe cases, the power supply voltage fluctuates between 90 and 270V. This requires electronic equipment to have the ability to adapt to various power environments, which means that modern electronic equipment is required to work "all-weather". Although this requirement is somewhat harsh, it has been satisfactorily solved with the efforts of electronic engineering technicians. The solution to this problem is bridge/voltage doubling automatic switching technology.

2 Principle of bridge/voltage doubler switching

    The bridge/voltage doubler switching circuit is shown in the dotted line in Figure 1. "K" in the figure is the switching switch. If the switch "K" is out of service, D1~D4 form a bridge rectifier circuit. C1 and C2 are connected in series as the filter capacitor after rectification. R1 and R2 are voltage equalizing resistors; if the switch "K" When K" is closed, the circuit becomes a voltage doubler rectifier circuit. During the positive half cycle of the input AC power, current flows through D1, D2, and K, and C1 is charged; during the negative half cycle of the input AC power, the current flows through D4, C2, and K, and C2 Be charged. The voltage charged on C1 and the voltage on C2 are superimposed to form a double voltage output.

    From the above analysis, it can be seen that as long as an electronic switch "K" is set up, the bridge rectifier circuit can be easily converted into a voltage doubler rectifier circuit. Usually the switching point of the switch is defined by AC 145V. If the AC voltage is lower than 145V, the rectifier circuit works in the voltage doubler mode; if the AC voltage is higher than 145V, the rectified current works in the bridge mode. The current problem is that the switch "K" is required to automatically close when the input is below 145V, and automatically open when the input is above 145V. This requires detecting the level of the AC voltage, and then automatically controlling the switch "K" according to the detection structure. The STR81145 produced by Toshiba Corporation of Japan is a new integrated circuit developed to solve this problem.

3 Internal structure of STR81145

    The internal structure of STR81145 is shown in Figure 2. It includes three parts: voltage detection circuit, amplifier circuit and execution switch circuit. Figure 3 shows the internal circuit of STR81145. The detected voltage is from the internal circuit of STR81145. The detected voltage is input from pins 2 and 5 of STR81145. When the voltage divider composed of R1 and R2 is divided on R2 When the voltage is enough to turn on VDW1 and VT1, the integrated pole of VT1 is at a low potential, and VDW2 and VT3 are cut off. The control terminal of the silicon-controlled VS4 cannot be turned on because it does not get the control voltage, which makes pins 2 and 3 of the STR81145 In an open circuit state. When the voltage obtained from pins 2 and 5 is not enough to turn on VDW1 and VT1, the potential of VT1 and the collector increases, VDW2 breaks down, VT3 turns on, and the voltage formed on the collector of VT3 makes the thyristor completely On, pins 2 and 3 of STR81145 are short-circuited. It can be seen that the controllable switch state can fully meet the automatic switching of the bridge/voltage doubler detection process.

    The features of STR81145 are as follows:

    ●The circuit is simple and reliable;

    ●Can be controlled in both directions;

    ●Low power consumption;

    ●Efficient and safe;

    ●VDRM=±500V;

    ●TT(RMS): 5A

4 Applications of STR81145

    The bridge/voltage doubler automatic switching circuit composed of STR81145 is shown in Figure 4. The input AC voltage is added to the D3 rectifier circuit through T1, S1, and T2 (T1 and T2 are low-pass filtered). At the same time, after low-pass filtering, this AC voltage is also added to the half-wave rectifier circuit composed of D1 and D2, and A DC voltage is generated across C8. C8 forms a DC voltage. The DC voltage formed on C8 is added to pins 2 and 5 of STR81145. It is not only the working power supply of STR81145, but also used as a detection object to determine the level of AC voltage. When the input AC voltage is lower than 145V, the voltage formed at both ends of C8 after rectification is also low. This voltage is added between pins 2 and 5 of STR81145. Since the voltage between pins 2 and 3 is very low, the Under the control of the internal sampling amplifier circuit of STR81145, its internal electronic switch K is turned on (pins 2 and 3 are connected), that is, the two points A and B in Figure 4 are connected. At this time, the positive half cycle of AC is along D31→R5→C18 →Pin 3 of STR81145→Internal electronic switch K→Pin 2 of STR81145 returns to the other end of the input AC voltage and operates in a loop. At this time, the voltage charged at both ends of C18 is about 145 volts, and its polarity is up positive and down negative. . When the negative half cycle of the AC voltage arrives, the charging circuit is: STR81145 pin 2 → pin 3 → positive pole of C19 → D32 → input the other end of the AC voltage. This process can also be charged to a DC voltage of 145V on C19. , its polarity is also positive up and negative down. Finally, the voltage charged by C18 (145V) and the voltage charged by C19 (145V) are superimposed in series to form an output of 290V, which is supplied to the following switching power supply and becomes the input voltage of the switching power supply, thereby completing double voltage rectification.

    If the input AC voltage is higher than 145V, the DC voltage formed on C18 after rectification by D1 and D2 is also higher. After STR81145 determines, the internal electronic switch is disconnected and pins 2 and 3 are open circuit, which is equivalent to A in Figure 4 , point B is disconnected, and D3, C18, and C19 work in bridge rectification mode. Its output voltage (about 290V) also provides switching power supply to the following circuits through the same path.

    In addition, it must be noted that when the input AC voltage is higher than 145V, the electronic switch in the STR81145 must be in an open circuit state. Because if pins 2 and 3 of STR81145 break down or the electronic switch in pins 2 and 3 is mistakenly turned on and the rectifier circuit works in a voltage doubling state, the output DC voltage after doubling will be as high as 600V. Such a high voltage will It will also cause large area damage to the subsequent switching power supply circuit components. For this reason, an overvoltage protection circuit is also designed in the rectifier circuit in Figure 4. Among them, R8 is the overvoltage detection resistor. Under normal circumstances, the DC voltage at both ends of R8 is lower than 22V. This voltage is lower than the breakdown voltage of D4, 24V. D4 does not conduct, and the thyristor Q8 does not get the turn-on voltage. In the cut-off state, the protection circuit does not operate. However, when the rectified and filtered DC voltage is higher than the normal value and the divided voltage on R8 is higher than 22V, D4 breaks down and conducts. At this time, the trigger voltage of Q8 also conducts. It The conduction short-circuits the input AC voltage and fuse F1 blows, thereby protecting other components from being damaged.

Table 1 Application parameters of STR81145

    Note: Those marked with “*” indicate charging and discharging processes during measurement.

    Table 1 shows the circuit-to-ground (pin 5) voltage and open-circuit to ground (pin 5) voltage of each pin of STR81145 for reference.