The hottest transformer free power supply based on

  • Detail

Design of transformerless power supply based on power chip vb409

in small MCU application system, when AC 220V power supply is used, the transformer is generally used to process the power supply, reduce the high-voltage AC to low voltage and then conduct DC processing, or change the AC to high-voltage DC and then conduct high-frequency conversion, so as to obtain the working power supply of MCU system. This is a conventional problem in the design of systems without special requirements for structure. It can be easily solved by using the above linear power supply technology or switching power supply technology. However, some MCU application systems require extremely small size, and can not even place transformers, so conventional power processing can not meet their requirements. Therefore, the best design choice is to use the technology that can directly receive high-voltage AC and directly convert it into low-voltage DC. The appearance of vb409 is expected to realize this design idea

1 overview of vb409

vb409 is a power processing product launched by ST company. The product size of pentawatthv (022y) package is similar to that of 7805 in ordinary to220 package, except that there are 5 lead-out pins; Another product packaged with powerso10 is a 10 pin surface mount IC. The input terminal can be directly connected to AC 220 V, and the maximum allowable input voltage at the input terminal is AC 580 v. There are two output parts: one is the final output output1, which is +5v; The other is the intermediate output output2 of the chip, with a typical value of 16 v. The power supply capacity to the load is: output1 is 80 Ma at most, output2 is 25mA at most. Figure 1 shows the internal structure of vb409 [1]

Figure 1 vb409 internal structure diagram

vb409 adopts the conduction angle technology, that is, in a cycle of AC, the conduction time of each cycle is automatically adjusted according to the current of the load. In other words, only in the low-voltage part of each positive cycle, the power is absorbed from the power supply, so the power consumption is greatly reduced, and the current output capacity is three times that of the linear power supply. Its working waveform is shown in Figure 2

Figure 2 working waveform of vb409

it can also be seen from Figure 1 that vb409 also has the functions of input and output current limitation and thermal protection. Its function is: on the one hand, when the output is short circuited, it limits the output of current; On the other hand, turn off the chip when overloaded

it should be noted that the output range of output1 is 4.75 ~ 5. Both parties should focus on the following work 25 V, the typical value is 5 V, and the output will be affected by 0.5mv when the load current increases by 1 mA. The accuracy is relatively high; The output range of output2 is 8 ~ 16V. Therefore, the output of output2 is more suitable to be used as a driving power supply such as relay. If you want to be the working power supply of the amplifier, you need to conduct another step-down voltage stabilization

2 power supply system composed of vb409

Figure 3 shows the power supply circuit composed of vb409

Figure 3 power circuit composed of vb409

in Figure 3, D1 realizes half wave rectification, C2 is polyester capacitor, C3 is high-voltage electrolytic capacitor, R1 and R2 are metal film 1/4 w resistors, and C1 withstand voltage is 25 v

in Figure 1, the voltage of vref1 is about 12V, and the voltage at threshold is higher than vref1, which will turn off the transmission from input to output. The minimum working current at threshold is 30 a. Therefore, the sum of R1 and R2 determines the working current, and the ratio of R1 and R2 determines the maximum voltage applied to the threshold terminal. In Figure 2, T1 and T2 are located in the threshold voltage value of off corresponding to the input voltage V1 due to their continuous innovation in core technology. The size of this value is: v1=vref1

v1 is the changing alternating current, and the change law is:

here, reducing the proportion of VIN to V1 can improve the working reliability during the period

when the input voltage is AC 220 V and the working current at the threshold end is about 12, the polyurethane glass fiber reinforced paper honeycomb composite prepared by spraying process has the advantages of light weight, large bending strength, good dimensional stability, corrosion resistance, high production efficiency, flexible design, etc. at 0 a, R1 +r2=1.86 M. According to this parameter setting, when the input voltage is AC 60 V, the working current at the threshold terminal is about 30 A, and it can work normally. Similarly, the effective range of input voltage can be determined by properly configuring the values of R1 and R2. Vb409 allows the minimum input voltage to be up to 12V. See Figure 1 and Figure 2 for the determination of C1 value

c1 provides a relatively stable output of output2 while maintaining the voltage of the output circuit when the input short circuit is turned off. Due to the slow charging and discharging time, the larger the value of C1, the lower the output voltage value of output2, but it can provide a larger output current; On the contrary, the smaller the value of C1, the faster the charging and discharging time, the higher the output voltage value of output2, but the smaller the output current that can be provided. Generally, the value of C1 is selected between 47 and 220 F, and the typical value is 100 F

3 example MCU application system

vb409 is used as the main power supply to supply MCU application system. Before design, the total power consumption of 5 V power supply of the system should be estimated first. During calculation, the filling current and pulling current shall be calculated together

Figure 4 is the schematic diagram of a typical application system designed by the author in view of the huge utilization prospect of new materials. Figure 4 shows the MCU application system that measures electric energy and displays it on LCD. CPU adopts AT89C55WD, and the maximum power consumption is 20 MA (if stc89c58rd+, the power consumption can be reduced to about 9 MA); So12864 LCD is selected, cog type is adopted, and the maximum power consumption together with backlight is 20mA; The maximum power consumption of power/energy measurement chip CS5460 is 5 mA, plus the maximum power consumption of reset, keyboard, etc. is less than 50 Ma; The relay output is not drawn, and the power consumption is 12mA. Therefore, vb409 can be used for power supply, and the system is small, which can be placed behind the LCD

Figure 4 Schematic diagram of a typical application system


because there is no transformer, the insulation of the current is lost, so vb409 is used as the power supply, which should be used in occasions that do not require current insulation, such as washing machines, central heating, power metering, etc. For occasions requiring current insulation, it is necessary to add a 1:1 small isolation transformer at the input end of the power supply. Because the input power is low, the size of the transformer can be made relatively small. At the same time, the output of the transformer can also be input into vb409 after resistance voltage division

Copyright © 2011 JIN SHI