3V/4A high efficiency switching power supply made by TOP243

This low voltage, high current, wide input voltage switching power supply is designed for intelligent fast battery chargers. It can provide a maximum charging current of 1A for each of four AA batteries. The power supply uses the TOP series special switching power supply circuit TOP243Y, which is stable and reliable, has very few peripheral components, and basically does not require debugging.

Figure 1 is the basic application circuit of TOP243Y. TOP243Y is used in a sealed environment. Under an AC voltage of 85-265V, it can output a maximum power of about 15W, and under open conditions, the output power can reach 30W. The ultra-small power adapter we use here has a completely sealed working environment, so it is appropriate to choose TOP243, but a sufficiently large aluminum heat sink must be added to TOP243 in the plastic shell.

Figure 2 is the internal circuit diagram of TOP243Y. Below we briefly describe the working principle of this power supply.

The alternating current AC (ranging from 120V to 265V) is input from two AC contacts, and enters the rectifier circuit after the electromagnetic noise is suppressed by the EMI filter composed of C1 and L1. Since TOP243Y has the characteristics of frequency jitter, it can effectively suppress noise interference. Therefore, in a low-power switching power supply, only a simple EMI filter is needed, and reasonable grounding technology is adopted to meet the relevant electromagnetic compatibility requirements. BR1 is the rectifier circuit, and the fast recovery rectifier bridge 2KBP06M is selected here. The rectified pulsating DC is filtered by C1 and provided to the TOP243 switch modulation circuit.

TOP243 and peripheral components form a pulse width modulation circuit. R2 is an undervoltage or overvoltage detection resistor and provides voltage feedforward for the line to reduce the fluctuation of the switching frequency. Here, 2=1.8MΩ is taken. D1 and D2 form a drain clamp circuit to absorb the peak voltage generated by the primary leakage inductance of the high frequency device when the MOSFET is turned off, protecting the MOSFET from damage.

There are two secondary windings of the high-frequency transformer. One is the main winding, which provides the main energy of the power supply. The high-frequency voltage is rectified by the fast diode SB540 and filtered by the filter capacitors C4 and C5, and then output to the load through L2 to form a low-pass filter. L2 mainly suppresses the output of high-frequency noise to the load to prevent the load from being interfered by it. The electrolytic capacitor C6 at the output end is added to reduce the output AC ripple coefficient, which mainly reduces the AC ripple of the output DC voltage.
The other secondary winding constitutes the feedback voltage winding, which is rectified by the diode and added to both ends of the photosensitive tube. The output feedback voltage is added to the positive electrode of the optocoupler diode. The resistor R5 and the voltage regulator D5 form a reference voltage source to provide a reference voltage for the optocoupler. In this way, the luminous intensity of the diode in the optocoupler is controlled by the output voltage, and is coupled to the control end of TOP243 through the optocoupler, thereby realizing the controllable pulse width and achieving the purpose of voltage stabilization. Resistors R6 and C2 are compensation elements of the control loop.

The input voltage range of the power supply can reach 120-265VAC, the output voltage is 3V, and it can provide 4A current output. The load adjustment rate is ±4%, the power efficiency is about 84%, and the output ripple voltage is less than 120mV. The circuit uses the simplest design and can fully meet the requirements as an adapter power supply for the charger.