5W dimmable non-isolated LED driver circuit design

This article introduces a non-isolated LED driver (power supply) designed using the LinkSwitch-PL series device LNK457DG. A single constant current output of 350 mA can be provided at 12 V and 18 V LED string voltages. The output current can be reduced to 1% (3 mA) using a standard AC mains thyristor dimmer, which will not cause unstable LED load performance or flicker. The circuit is compatible with both low-cost leading edge dimmers and more complex trailing edge dimmers.

The circuit is designed to operate over a universal AC input voltage range (85 VAC to 265 VAC, 47 Hz to 63 Hz), but will not be damaged over an input voltage range of 0 VAC to 300 VAC. This improves field application reliability and extends service life under line voltage sags and surge conditions. LinkSwitch-PL based designs offer high power factor (>0.9), helping to meet the requirements of all current international standards, allowing a single design to be used worldwide.

The form factor of the power supply was chosen to meet the requirements of a standard pear-shaped (A19) LED replacement lamp. The output is non-isolated, requiring the mechanical design of the enclosure to isolate the power supply output and the LED load from the user.

1. Circuit Schematic Diagram

Figure 1 Circuit diagram

2. Circuit Description

This circuit is a non-isolated, discontinuous conduction mode flyback converter circuit that drives an LED string with a voltage range of 12 V to 18 V with an output current of 350 mA. The driver is fully capable of operating over a wide input voltage range and provides high power factor. This circuit meets both input surge and EMI requirements, and its low component count allows the board size to meet the requirements for LED bulb replacement applications.

1. Dimming performance design guide

The requirement to provide output dimming using a low-cost thyristor leading-edge phase-controlled dimmer requires a comprehensive design trade-off. Due to the very low power consumption of LED lighting, the current drawn by the entire lamp is usually less than the holding current of the thyristor in the dimmer. This can cause undesirable conditions such as limited dimming range and/or flicker. Due to the relatively large impedance of the LED driver, severe ringing occurs when the thyristor turns on. At the moment the thyristor turns on, a very large inrush current flows into the driver's input capacitor, which excites the line inductance and causes current ringing. This also causes similar undesirable conditions because the ringing causes the thyristor current to drop to zero and turn off, causing the LED lamp to flicker. To overcome these problems, two circuit blocks are used in the circuit - an active damping circuit and a bleeder circuit. The disadvantage of these circuit blocks is that they increase power consumption, thereby reducing the efficiency of the power supply.

The damping and bleeder circuits in this design are chosen to allow one board to operate with most dimmers (up to 600 W and including low-cost leading-edge thyristor dimmers) over the entire input voltage range. This design allows flicker-free lighting when a lamp is connected to a dimmer at high input voltage. Operating a lamp at high voltage results in minimum output current and maximum inrush current (when the thyristor is conducting), which represents the worst case. Therefore, the role of active damping and bleeder circuits is very obvious: the bleeder circuit reduces the impedance and the damping circuit increases the impedance. However, this increases power dissipation, which reduces the efficiency of the driver and the effectiveness of the entire system.

Requiring multiple lamps to be connected to one dimmer for proper operation reduces the current required by the bleeder circuit, increasing the values ​​of R10 and R11 and reducing the value of C6.

If the fixture is operated only at low voltage (85 VAC to 132 VAC), the values ​​of R7 and R8 can be reduced as the peak current that occurs when the leading edge thyristor dimmer turns on is significantly reduced. Both changes will reduce dissipation and improve efficiency. For non-dimming applications, these components can simply be omitted and jumpers can be used in place of R7 and R8, which will improve efficiency without changing other performance characteristics.

2. Input EMI filtering and input rectification

The EMI filter is optimized to reduce the impact on dimming performance. Resistor R20 is a fusible resistor. Fusible resistors should be selected to fail open circuit if a component failure causes excessive input current. Thin film resistors (relative to wirewound resistors) are acceptable compared to non-PFC designs or passive PFC designs. This will reduce the instantaneous power dissipation when the input capacitors are charged, but a 2W rating is recommended for designs operating at high voltages. In addition, they can limit the inrush current generated when the phase leading thyristor dimmer is turned on and capacitors C4 and C5 are charged. The worst condition (inrush current is maximum) occurs when the thyristor is turned on at 90 degrees or 270 degrees, which corresponds to the peak of the AC waveform. Finally, they can attenuate any current ringing caused by the inrush current again between the AC input impedance and the power supply input stage when the leading thyristor is turned on.

Two π-type differential mode filter EMI stages are formed with C1, R2, L1 and C2 forming one stage and C4, L2, R9 and C5 forming the second stage. During testing, it was found that C1 was not required to meet the conducted EMI limits and was therefore not fitted. The AC input is rectified by BR1 and filtered by C4 and C5. The total equivalent input capacitance (the sum of C4, C5 and C6) is selected to ensure that the LinkSwitch-PL device has correct zero-crossing detection of the AC input, which is necessary to maintain normal operation and achieve optimal performance during dimming.

3. Active attenuation circuit

The active damping circuit is used to limit the inrush current, associated voltage spikes and ringing that occurs when the thyristor in the dimmer turns on. The circuit connects the impedance (R7 and R8) in series with the input rectifier for a brief period of each AC half cycle and is bypassed by a parallel SCR (Q3) for the remainder of the AC cycle. Resistors R3, R4 and C3 determine the delay before Q3 turns on.

4. Discharge circuit

Resistors R10, R11, and C6 form a bleeder circuit to ensure that the initial input current is sufficient to meet the thyristor's holding current requirement, especially when the thyristor conduction angle is not large enough. For non-dimming applications, both the active damping circuit and the bleeder circuit can be removed. To do this, delete the following components: Q3, R20, R3, R4, R10, R11, C6, and C3. Replace R7, R8, and R20 with 0 ohm resistors.

5. LinkSwitch-PL Primary

The LNK457DG device (U1) integrates power switching devices, oscillators, output constant current control, startup, and protection functions. The integrated 725 V MOSFET provides a wider voltage margin, ensuring high reliability even in the event of input surges. The device is powered from the bypass pin through the decoupling capacitor C9. After startup, C9 is charged by U1 from the internal current source and through the drain pin, and then powered by the output through R15 and D4 during normal operation. The rectified and filtered input voltage is applied to one end of the primary winding of T1. The MOSFET integrated in U1 drives the other side of the transformer primary winding. D2, R13, R12, and C7 form an RCD-R clamp circuit to limit the drain voltage spike caused by leakage inductance.

Diode D6 prevents the IC from generating negative ringing (drain voltage ringing below source voltage) when the power MOSFET is turned off due to the reflected output voltage exceeding the DC bus voltage, ensuring high power factor with minimum input capacitance.

6. Output Rectification

The transformer secondary is rectified by D5 and filtered by C11. Schottky barrier diodes are selected for efficiency. Since C11 provides energy storage during the AC zero crossings, its value determines the amplitude of the line frequency output ripple (2 x fL due to full wave rectification). Therefore, the value can be adjusted according to the required output ripple. For the 680 microF value shown, the output ripple is ±50% of IO. Resistors R17 and C10 are used to attenuate high frequency oscillations and improve conducted and radiated EMI.

7. Output feedback

The constant current mode set point is determined by the voltage drop across R18, which is then fed into the feedback pin of U1. Output overvoltage protection is provided by VR2 and R14 (R14 has a negligible effect on the current sense signal and can be ignored).