High-performance isolated AC/DC power supply solution based on SE3910

At present, in power supply solutions below 100W, pulse width modulation (PWM) control chips are generally used to realize PWM modulation. The switch control mode has a high working efficiency relative to the DC working mode. The flyback offline working mode improves the safety of the system operation and is very suitable for application in portable charging devices and power adapters, such as mobile phone chargers, power adapters, etc. Therefore, the demand for AC/DC PWM switching power supply chips in the market is very large. However, traditional AC/DC power supply solutions all use the transformer secondary coil feedback mode (SSR). The transformer secondary feedback working mode requires a constant voltage-constant current control chip at the low voltage end to assist in completing the voltage conversion and realizing constant current. This type of application solution increases the complexity of the system application and also increases the design cost of the system solution. The AC/DC power supply control chip to be introduced in this article is SE3910 of Siwang Electronics, which is a PWM control chip of the transformer primary coil feedback mode (PSR).

SE3910 Technical Features

SE3910 is a green mode PWM controller chip, suitable for low-power AC/DC chargers, adapters and LED drive solutions; the chip is SOP-8 package, the switching frequency is fixed at 40KHz when working in PWM mode, and it integrates a constant voltage and constant current control module. The application solution uses the PSR mode, omitting the optocoupler, constant voltage/constant current control chip and its surrounding circuits in the traditional solution, which greatly simplifies the application cost of the chip and reduces the complexity of the system application.

EMI is specially taken into consideration when designing the chip. A frequency jitter function is specially designed for the switching frequency module. Eight different switching frequencies appear in the designed order within every 3.2ms cycle, shifting the electromagnetic interference spectrum to a relatively wide frequency bandwidth, thereby achieving the purpose of optimizing system EMI.

At the same time, the working state of SE3910 uses a multi-mode adjustment function. When it is no-load or light-loaded, the chip will automatically enter the PFM working mode to ensure the precise conservation of the input energy and output energy of the power supply system, and prevent excessive energy when it is light-loaded or no-loaded. When the load increases to the heavy-load design value set by the chip, the chip will control the system to automatically enter the PWM working mode, which greatly optimizes the system's working efficiency, so that the system efficiency can reach more than 80%, and also reduces the output ripple under no-load and light-load working conditions.

The chip is designed with a soft-start function, which effectively suppresses the high current when the system is powered on, protects the circuit board from damage, and reduces the impact of the high current when the system starts on the system power consumption; the chip also has power undervoltage protection, LEB function, over-temperature protection function, etc., which maximizes the reliability and safety of the chip during operation; the chip is suitable for use in power supply solutions of 5W and below.

Typical application solutions

SE3910 can be widely used in various low-power AC/DC switching power supply solutions, such as mobile phone chargers, power adapters, etc. In addition, since the chip integrates a constant current function, it can also be widely used in low-power LED drive solutions.

Figure 1 is the basic application circuit of SE3910, in which the negative feedback path is composed of transformer/output stage/R3/R4/SE3910, etc. The conversion energy of the transformer is controlled by adjusting the duty cycle of the switching signal at the GATE end, so that the system is stabilized in the set working state. The AC voltage first passes through a bridge rectifier circuit to convert the AC into a high-voltage DC signal. R1 and C2 form the system startup circuit. VIN is the startup PIN of SE3910. R5, C6 and C7 on the COMV PIN form the system compensation circuit to ensure that the system has a stable frequency response. FB is the output voltage detection PIN. By setting R3/R4, the voltage on the secondary side of the transformer can be adjusted. According to the principle that the transformer voltage ratio is proportional to the turns ratio, the DC output voltage can be adjusted; GATE is the PWM output PIN, which is used to control the power tube 13003 to control the peak current of the primary side of the transformer to achieve control of the transformer conversion energy. The CS PIN is used to detect the transformer peak current. When the system works in constant current mode, the voltage on the CS PIN will be fixed at the set maximum value, which determines the maximum peak current of the primary side of the transformer, thereby achieving output constant current. By adjusting the R6 resistor, the output constant current value can be flexibly adjusted.

Figure 2 shows some popular SE3910 application examples, one is a charger solution and the other is an LED driver solution. The key point in system design is the output constant voltage and constant current design.

Design of output constant voltage value:

The constant voltage realization principle of the system is to stabilize the sampling value of the output voltage signal of the transformer negative feedback on the chip FB PIN at the constant value of ref set by the chip through the operational amplifier in SE3910. Ref is a constant voltage of 1.5V output by a bandgap reference source module inside the chip, thereby achieving the purpose of stabilizing the output voltage.

Where is the number of turns of the secondary and secondary sides of the transformer, and is the conduction voltage of diodes D6 and D7. The voltage difference of the diodes can be ignored in rough design, so the above formula can be simplified to:

Output constant current value design:

The constant current design principle of the system is to set the peak current of the Primary Side to a constant value. When the CS pin of the chip is in constant current working mode, it will be fixed at the maximum value of 0.8V. 0.8V divided by the value of R6 can set the maximum peak current of the primary side of the transformer to the required value. The current ratio between the two sides of the transformer is equal to the inverse ratio of the number of turns. Multiplying it by the duty cycle during flyback can get a constant output current.

In the above formula, is the maximum peak current of the transformer primary side, which is 0.8V set by the chip.

When selecting the above parameters, the accuracy of the resistor and the turns ratio of the transformer must be high. These parameters directly affect the accuracy of the output constant voltage and constant current. The other components are basically universal and only need to follow the parameters provided by the manufacturer.

System Features:

The transformer primary feedback mode (PSR) is adopted, omitting the external optocoupler, constant voltage and constant current control chip and surrounding devices in the traditional solution, greatly reducing the system cost; the chip can work in multiple modes to ensure that the system achieves a high working efficiency; the system startup current is less than 50uA; it can achieve more accurate constant voltage and constant current values; the output has short-circuit protection; small standby power consumption (Standby Power); a wide AC voltage supply range (85V~275V).

The switching power supply solution designed based on SE3910 is generally only suitable for output solutions of 5W and below. This is mainly because when the output power is large, the transformer will work in continuous operation mode (CCM), resulting in a decrease in system efficiency and poor output constant current accuracy.

Conclusion

As portable electronic devices become more and more popular, portable chargers, adapters and other related accessories will become more and more popular among consumers. In the future, power solutions will inevitably develop in the direction of low cost, easy to carry, high performance and high reliability. The high-performance PWM controller chip SE3910 conforms to this trend. Using this chip, you can build an AC/DC converter solution that uses very few external components and maintains high performance within a wide AC voltage input range.