High Efficiency 7.6 V 700 mA Isolated LED Driver Design

1. Design Features

1. Accurate primary side constant voltage/constant current controller (CV/CC) eliminates the optocoupler and all secondary side CV/CC control circuits, and can achieve the highest efficiency without current sensing resistors; it uses few components and low-cost solutions. 2. Automatic restart protection function can reduce the output power to less than 95%

under output short circuit or open loop conditions 3. Hysteresis thermal shutdown function prevents power supply damage 4. Meets CEC and Energy Star 2.0 efficiency requirements: 5. Green packaging: halogen-free and RoHS compliant

2. Working Principle

Figure 1 is a circuit diagram of a universal input 7.6 V, 700 mA constant voltage/constant current flyback power supply designed using the LinkSwitch-II device LNK606PG. This power supply is suitable for LED driver applications.

The LNK606PG (U1) integrates the power switch device, oscillator, CC/CV control engine, and startup and protection functions into one IC .

Diodes D1 to D4 rectify the AC input. Capacitors C1 and C2 filter the rectified AC. These capacitors, together with inductors L1 and L2, also attenuate differential-mode conducted EMI noise. Resistors R1 and R2 damp the resonant oscillations between these capacitors and the inductors. The above design, combined with Power Integrations’ Transformer E-sheild™ technology, allows this power supply to easily meet EN55015 Class B conducted EMI requirements with a margin of more than 10 dB and without the need for Y capacitors. Fusible resistor RF 1 is used to limit the inrush current generated at startup and acts as a fuse in the event of component failure due to excessive input current.

This power supply design utilizes the constant current feature integrated in U1 to drive the LED load and can operate at maximum output power in constant current mode. The constant voltage mode of IC U1 provides output overvoltage protection in the event of an open circuit failure of any LED.

When operating in the constant current phase, U1 adjusts the output current by changing the switching frequency of the MOSFET. As the output voltage increases, U1 increases its switching frequency. The output voltage is determined by the number of LEDs in the load. The values ​​of resistors R5 and R6 determine the maximum switching frequency and the output voltage. The inductance of the transformer ensures that the driver always operates at maximum power.

If an output fault occurs, the power supply will operate in constant voltage mode and use on/off control to regulate the output voltage. This provides automatic output fault protection and reduces power consumption in this situation. The auto-restart function of IC U1 provides output short-circuit protection.

IC U1 is completely self-powered through the BP (Bypass) pin and decouples capacitor C4, while also providing high-frequency decoupling. When the internal MOSFET is on, U1 uses the energy stored in C4; when the MOSFET is off, the internal 6 V regulator pulls current from the DRAIN pin. This eliminates the need for an external bias winding. Adding an external bias winding will further reduce no-load power consumption.

The rectified and filtered input voltage is applied to one side of the primary winding of T1. The MOSFET integrated in U1 drives the other side of the transformer primary winding. D5, R3, R4, and C3 form an RCD-R clamp circuit to limit the drain voltage spike caused by the leakage inductance. IC U1 automatically compensates for tolerance differences in the primary magnetizing inductance. The output power is directly proportional to the set primary inductance, and output power changes can be detected at the FB pin. When the output power changes, the switching frequency is adjusted to compensate for inductance fluctuations.

Diode D7 (a Schottky barrier diode for improved efficiency) is used to rectify the secondary output of the transformer, and C7 filters it. Resistor R8 and capacitor C6 eliminate high-frequency conducted and radiated EMI. When no load is connected, the dummy load resistor R9 acts as a bleed resistor for C7.

三、设计要点

1、U1上高压引脚与低压引脚之间的爬电距离非常大，可以避免产生电弧并进一步提高可靠性，这在高湿度或高污染条件下特别重要。

2、电容C7具有低效串联阻抗(ESR)，可降低输出电压 纹波 和省去LC后级滤波器。

3、反馈电阻R5和R6应具有1%的容差值，有助于将额定输出电压和恒流调节阈值严格控制在中心位置。
4、使用外部偏置绕组可进一步降低空载功耗。在 PCB 板上，将旁路引脚电容靠近U1放置。

5、减小箝位和输出二极管的环路面积，以降低EMI。

6、使AC输入和开关节点保持一定距离，降低可能会旁路输入滤波的噪声耦合。

7、确保U1D引脚的峰值漏极电压低于650 V，否则的话，需要通过降低R3的值来实现。