Low-cost switching power adapter designed with KA5M0365R

Probe domestic switch needle KG-300K needle head diameter is 3.0mm normally open switch needle

75V, 200mA, 1.25V, 3μs, 4A Low Leakage Switching Diode

Summary:

Power adapters made with switching power supplies (SMPS) are designed to deliver small, lightweight, and efficient performance. This paper introduces the design of a low-cost, high-performance AC-DC adapter using the KA5M0365R single-chip switching power supply chip produced by Fairchild Semiconductor. It focuses on the solution considerations, circuit structure and PCB design of the adapter.

Circuit configuration

The KA5M0365R is used in the circuit to form a continuous current mode (CCM) flyback switching power supply. After the power is turned on, the AC voltage is input from the line input terminal, and a DC high voltage is generated through a filter (C1, T1) for suppressing EMI, a bridge rectifier (D1 to D4), and a filter capacitor (C2). R8 is the starting resistor of U1. When the power is turned on for the first time, U1 operates in bypass mode and C8 is charged. Once the upper limit of the voltage lock threshold of 15V is reached, U1 starts to switch, its current demand increases, and VCC voltage begins to drop. However, assuming that the C8 capacity is large enough, the VCC voltage will remain at a lower level in the voltage lock threshold range. During normal operation, the feedback voltage provided by the feedback winding is rectified by D6 and C8, and a DC voltage is generated to start supplying power to U1.

The drain clamp protection circuit (Snubber Network) consists of C3, R7, and D5, which can limit the peak voltage generated in the primary due to the leakage inductance of the transformer when the drain is turned off to ensure that the switch will not be caused. The drain voltage exceeds its breakdown voltage. The secondary voltage is rectified and filtered by D7, C4, L1, and C5 to obtain a regulated output of +12V. The secondary high frequency rectifier D7 uses a high current, low dropout Schottky diode, model number MBR10100CT (10A/100V).

Adapter circuit composition

The adjustable precision shunt regulator TL431 constitutes an external error amplifier, which together with the optocoupler constitutes an optocoupler feedback circuit, and the feedback voltage is applied to the feedback terminal of U1. The voltage regulation principle is that when the output voltage V0 fluctuates, the sampling voltage obtained by dividing R5 and R6 is compared with the 2.5V reference voltage of the TL431, and an external error voltage is generated, and the feedback current of the FB is correspondingly changed by the optical coupler. And adjust the duty ratio to achieve the purpose of voltage regulation. R4 and C6 are the compensation resistors and capacitors of the control loop, and R2 is the current limiting resistor of the LEDs in the optocoupler. The coupling capacitance between the primary and secondary of the high-frequency transformer causes common-mode interference and can be filtered out by the safety capacitor C7.

See the uploading documentation for details (click to download)