Low-cost non-isolated 12V LED driver design

1. Design Features

1. Accurate primary side constant voltage/constant current controller (CV/CC) eliminates optocoupler and all

2. Secondary side CV/CC control circuit

3. No current sense resistor is required to achieve the highest efficiency

4. Low-cost solution with few components (16 components)

5. Automatic restart for output short circuit and open loop protection

6. Extremely high energy efficiency

7. Full load efficiency is greater than 80% in the entire input voltage range

8. At 265 VAC input, no-load power consumption <200 mW

9. Easily meet EN55015 and CISPR-22 Class B EMI standards

10. Meet Energy Star's requirements for solid-state lighting (SSL) products

11. Green packaging: halogen-free and RoHS compliant

2. Circuit Schematic Diagram

Figure 1. 4.2 W LED driver designed using LNK605DG

3. Working Principle

Figure 1 is a circuit diagram of a universal input 12 V, 350 mA constant voltage/constant current LED driver power supply designed using the LinkSwitch-II device LNK605DG , which adopts a tapped inductor non-isolated buck converter structure.

The tapped buck topology is well suited for designing converters with high input voltage to output voltage ratios: it can provide current multiplication to the output, making it possible to use this new buck topology in applications requiring an output current more than twice the current limit of the device.

Converters using this topology have smaller PCB size, smaller inductor core size, and higher efficiency (80% at worst load condition) compared to isolated flyback converters. EMI filtering design is simplified because less common-mode noise is generated. This topology usually requires a clamp circuit on the primary side. However, since a 700 V MOSFET is integrated in U1, the clamp circuit can be omitted.

Integrated circuit U1 contains a power switch device (700 V MOSFET), an oscillator, a highly integrated CC/CV control engine, and startup and protection functions. The MOSFET can provide sufficient voltage margin for universal input AC applications including input surges.

Diodes D3, D4, D5, and D6 rectify the AC input, and then bulk capacitors C4 and C5 filter the rectified AC. Inductor L1, together with C4 and C5, forms a π-filter to attenuate differential-mode conducted EMI noise. This design easily meets EN55015 Class B conducted EMI requirements with a margin of 10 dB. Fusible flameproof resistor RF 1 provides severe fault protection.

When the switch in U1 is turned on, the current will increase and flow through the load and inductor. Capacitor C1 filters the load current, which eliminates the need for a switch. Diode D1 cannot conduct due to reverse bias. The current continues to increase until it reaches the current limit of U1. Once the current reaches the current limit, the switch will turn off.

After the switch is turned off, the energy stored in the inductor (T1) generates a current that flows into the output section: (pin 8 - pin 7). The current in the output winding increases by a factor of 4.6 (turns ratio) and flows from the output winding through the freewheeling diode D1 to the load. Since the leakage inductance (between the two parts of the inductor) is small, there is no need to use a clamp circuit to limit the peak drain voltage. Normally this would dissipate the leakage inductance energy, but in this design, the capacitance in the inductor winding and the MOSFET inductance (discharged during each switching cycle) are sufficient.

The LED is driven by a constant current , so U1 operates in constant current mode during normal operation. In constant current mode, the switching frequency is adjusted based on the output voltage (sensed at pins 5 and 6) to keep the load current constant.

The constant voltage feature automatically provides output when any LED fails open circuit or the load is disconnected.

4. Design points

1. Select T1's turns ratio (4.6) to ensure that the circuit operates in discontinuous mode (DCM) at low input voltage (85 VAC) and that D1's on-time is at least 4.5 μs.

2. Feedback resistors R1 and R2 should have a 1% tolerance value to help keep the rated output voltage and constant current regulation threshold tightly centered.

3. RF1 acts as a fuse: make sure it is rated to withstand transient dissipation when the power supply is first connected to AC. Use wirewound resistors or oversized resistors.

4. Dummy load resistor R4 maintains the output voltage under fault conditions (such as load disconnection). Output overvoltage protection.