12 W Wide Input Constant Voltage Adapter Using TinySwitch-III

The power supply is a wide voltage input flyback converter with 12 V and 1 A output, and uses the TNY278PN device in the TinySwitch-III series. Since many functions have been integrated into the device, only 31 direct-socket components are required (no surface mount components), which enables a simple single-sided PCB layout.

由于变压器设计中采用了E-shieldTM屏蔽技术、低噪声的漏极节点箝位（R2、C4及D5）、一个Y电容(C5)以及器件的开关频率调制特点，使得该设计只使用低成本的π型滤波（C1、L1及C2）就将传导EMI衰减到相对于EN55022的要求具有10 dB的裕量（见图4）。箝位电路中的稳压管(VR1)仅在开机时及最差的过载情况下起作用。最差的过载是指进入自动重启动状态之前的过载情况。尽管TinySwitch-III系列产品无需偏置绕组就可以工作，在此设计中仍然采用了偏置绕组，来实现输出OVP检测功能。如果环路开环引起输出端过压，VR2将会导通。当流入BP/M引脚的电流超过5.5 mA时，内部的锁存电路会被触发，从而禁止MOSFET开关操作（见图3）。此时AC输入电压必须重新上电来对锁存电路进行复位。电阻R1对流经VR2的电流加以限制。而R3和R7对偏置绕组上出现的漏感尖峰电压部分进行了衰减。

The current flows into the EN/UV pin of the IC to implement the input undervoltage lockout function and set the AC threshold for power supply startup at 65 V. The sum of the voltage drop across VR3, R6, and the LED of the U2 optocoupler determines the output voltage. The output voltage can be fine-tuned by adjusting the resistance values ​​of R4 and R6. If more precise output voltage regulation accuracy is required, VR3 can be replaced by a TL431.