Design of transformerless power supply based on power chip VB409

In small MCU application systems, when AC 220V is used for power supply, a transformer is generally used to process the power supply, reducing the high voltage AC to low voltage before DC processing, or converting AC to high voltage DC before high frequency conversion to obtain the working power supply of the MCU system. This is a conventional problem in design for systems with no special structural requirements, and can be easily solved by using the above-mentioned linear power supply technology or switching power supply technology. However, some MCU application systems require extremely small size, and even transformers cannot be placed, so conventional power supply processing cannot meet their requirements. Therefore, using technology that can directly receive high voltage AC and directly convert it into low voltage DC is the best design choice. The emergence of VB409 is expected to realize this design idea.

1 VB409 Overview

VB409 is a power supply processing product launched by ST. Its PENTAWATTHV (022Y) package is similar in size to the ordinary TO220 package 7805, but with 5 lead pins; there is also a PowerSO10 package product that is a 10-pin surface mount IC. The input terminal can be directly connected to AC 220 V, and the maximum input voltage allowed by the input terminal is AC 580 V. There are two output parts: one is the final output OUTPUT1, which is +5 V; the other is the intermediate output OUTPUT2 of the chip, with a typical value of 16 V. The power supply capacity for the load is: OUTPUT1 is a maximum of 80 mA, and OUTPUT2 is a maximum of 25 mA. Figure 1 is the internal structure of VB409 [1].

Figure 1 VB409 internal structure

VB409 adopts the conduction angle technology, that is, in one cycle of AC power, it automatically adjusts the conduction time of each cycle according to the current size of the load. In other words, it absorbs electric energy from the power supply only in the low-voltage part of each positive cycle, thus greatly reducing power consumption, and the current output capacity is 3 times that of a linear power supply. Its working waveform is shown in Figure 2.

Figure 2 VB409 working waveform

It can also be seen from Figure 1 that VB409 also has input and output current limiting and thermal protection functions. Its functions are: on the one hand, it limits the output current when the output is short-circuited; on the other hand, it shuts down the chip when overloaded.

It should be noted that the output range of OUTPUT1 is 4.75~5.25 V, the typical value is 5 V, and the output is affected by 0.5mV for every 1 mA increase in load current, and the accuracy is relatively high; while the output range of OUTPUT2 is 8~16V. Therefore, the output of OUTPUT2 is more suitable for use as a driving power supply for relays. If you want to use it as the working power supply of the amplifier, you need to perform a step-down voltage regulation again.

2 Power supply system composed of VB409

Figure 3 shows the power supply circuit composed of VB409.

Figure 3 Power supply circuit composed of VB409

In Figure 3, D1 realizes half-wave rectification, C2 is a polyester capacitor, C3 is a high-voltage electrolytic capacitor, R1 and R2 are metal film 1/4 W resistors, and C1 has a withstand voltage of 25 V.

In Figure 1, the voltage of Vref1 is about 12V. The voltage at the Threshold end is higher than Vref1, which will shut down the transmission from input to output. The minimum operating current at the Threshold end is 30μA. Therefore, the sum of R1 and R2 determines the operating current, and the ratio of R1 to R2 determines the maximum voltage applied to the Threshold end. In Figure 2, the input voltage V1 corresponding to the position of t1 and t2 is the threshold voltage value for shutdown. The size of this value is: V1=Vref1.

V1 is a changing alternating current, and the changing rule is:

Here, reducing VIN to V1 in proportion can improve the working reliability during the period.

When the input voltage is AC 220 V and the working current of the Threshold terminal is about 120 μA, R1+R2=1.86 MΩ. According to this parameter setting, when the input voltage is AC 60 V, the working current of the Threshold terminal is about 30 μA, and it can still work normally. Similarly, by properly configuring the values ​​of R1 and R2, the effective range of the input voltage can also be determined. The minimum input voltage allowed by VB409 can be up to 12V. See Figures 1 and 2 for the determination of the C1 value.

C1 provides the voltage to maintain the output circuit when the input is short-circuited and shuts down, and provides a relatively stable output of OUTPUT2. As the charging and discharging time slows down, the larger the value of C1, the lower the output voltage value of OUTPUT2, but it can provide a larger output current; conversely, the smaller the value of C1, the faster the charging and discharging time, the higher the output voltage value of OUTPUT2, but the output current that can be provided becomes smaller. Generally, the value of C1 is selected between 47 and 220 μF, and the typical value is 100 μF.

3 Example MCU application system

uses VB409 as the main power supply for the MCU application system. Before designing, the total power consumption of the system's 5 V power supply should be estimated first. The sink current and the source current should be calculated together.
Figure 4 is a schematic diagram of a typical application system designed by the author. Figure 4 is an MCU application system that measures power and displays it on an LCD. The CPU uses AT89C55WD, with a maximum power consumption of 20 mA (if STC89C58RD+ is used, the power consumption can be reduced to about 9 mA); the LCD uses SO12864, which uses COG type, and the maximum power consumption together with the backlight is 20mA; the maximum power consumption of the power/power metering chip CS5460 is 5 mA, and the maximum power consumption plus reset, keyboard, etc. is less than 50 mA; the relay output is not drawn, and the power consumption is 12mA. Therefore, it is completely possible to use VB409 for power supply, and the system is small in size and can be placed behind the LCD.

Figure 4 Schematic diagram of a typical application system

Conclusion

Since there is no transformer, the current insulation is lost, so VB409 is used as the power supply, and it should be used in occasions where there is no requirement for current insulation, such as washing machines, central heating, power metering, etc. For occasions where current insulation is required, a 1:1 small isolation transformer should be added to the power supply input end. Because the input power is low, the size of the transformer can be made relatively small, and the output of the transformer can also be divided by resistors before being input into VB409.