14W High Efficiency LED Driver Power Supply Design

1. Design Features

1. High operating temperature (75 degrees)

2. High energy efficiency 3. Meet EU CoC/CEC 2008/Energy Star 2.0 requirements, high efficiency in load mode (up to 86%, requirement is 79.6%); no-load input power

at 265 VAC input < 250 mW, requirement is 300 mW 4. Hysteresis overheat shutdown protection 5. Load disconnect protection 6. Meet EN55015B conducted EMI limit, EMI margin > 8 dB microvolt

2. Working Principle

Figure 1 shows a typical 20 V, 14 W constant voltage (CV), constant current (CV) output power supply circuit . The amount of light output from the LED array is proportional to the amount of current flowing through it. Therefore, the LED driver should have a constant current output instead of a constant voltage output. In this design, the DC output is not isolated from the AC input, so the LED array and housing should be safely isolated from the user.

The AC input is rectified and filtered by BR1, C1 and C2. Inductor L1 forms a π filter with C1 and C2 and provides EMI filtering. Fuse F1 provides protection in case of severe faults. In order for the power supply to operate normally without damage under no load, Zener diode VR2 is used for constant voltage regulation and keeps the voltage at about 21 V.

The constant current characteristic is achieved by detecting the voltage drop across the current sensing resistor R7. Shunt regulator IC U3, together with R9, R8, and R8A, generates a precise voltage reference of 0.07 V at the inverting input of operational amplifier U2. When the set current is reached, the voltage across R7 will exceed the reference voltage, which will increase the output of the operational amplifier. At this time, D4 ​​will be forward biased, driving the base of Q1, which in turn pulls current out of the EN/UV pin of U1. Capacitor C7 and resistor R11 provide loop compensation. Using the current limiting method of the operational amplifier minimizes the current sampling voltage, thereby reducing losses and maximizing efficiency.

Whenever the current drawn by the EN/UV pin exceeds 115 μA, the MOSFET in U1 is disabled on a cycle-by-cycle basis (on/off control). By adjusting the ratio of enabled to disabled switching cycles, the feedback loop can regulate the output voltage or current. The on/off control method also optimizes the converter efficiency under different load conditions to meet energy efficiency standards.

Due to the high ambient temperature, U1 will operate in a reduced current limit mode. This increases the overall efficiency of the power supply and improves its thermal performance. The primary clamp (D1, VR1, C3, and R3) controls the maximum peak drain voltage
below the 700 V BVDSS breakdown voltage of the internal MOSFET. Resistor R23 reduces high-frequency leakage inductance ringing, thereby reducing EMI. The output on the secondary side is rectified and filtered by diodes D2, D3, and C6.

3. Design points

1. Choose fast diodes instead of ultrafast diodes to improve efficiency by recovering some leakage inductance energy.
2. Capacitor C3 is used to improve EMI performance.
3. Select resistor R10 to provide 1 mA supply current to U3 at the minimum output voltage of 6 V.
4. The selectable current limit of U1 allows the current limit and device size to be optimized to suit the ambient temperature. For example, to reduce dissipation, the TNY280GN device can be used in the same design by changing C3 from 1 μF to 0.1 μF. Or, in an environment with higher heat dissipation performance, the TNY278GN device can be used by changing C3 from 1 μF to 10 μF.
5. The source works correctly when the LED string voltage is between 6 V and 20 V. However, since the output current is constant, the lower the string voltage, the lower the output power.