This article describes a non-isolated LED driver (power supply) designed using the LinkSwitch-PL family of devices, the LNK457DG. A 350 mA single constant current output is available at 12 V and 18 V LED string voltages. Using a standard AC mains thyristor dimmer reduces the output current to 1% (3 mA), which does not cause LED load performance to be unstable or flicker. The circuit is compatible with both low cost leading edge dimmers and more complex trailing edge dimmers.
This 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 cause damage over an input voltage range of 0 VAC to 300 VAC. This improves field application reliability and extends the life of online voltage dips and surges. The LinkSwitch-PL-based design provides a high power factor (>0.9) to help meet the requirements of all current international standards, making a single design universal.
The power supply is designed to meet the requirements of a standard pear-shaped (A19) LED replacement lamp. The output is non-isolated and requires a mechanical design of the housing to isolate the power output and LED load from the user.
First, the circuit schematic
Figure 1 circuit schematic
Second , the circuit description
This circuit is a non-isolated, non-continuous conduction mode flyback converter circuit that provides driving from an LED string of 12V to 18V with an output current of 350 mA. The driver is fully capable of operating over a wide input voltage range and provides a high power factor. This circuit meets both input surge and EMI requirements, and has fewer components, enabling the board size to meet the requirements of LED bulb replacement applications.
1, dimming performance design guide
For the use of low-cost thyristor leading edge phase-controlled dimmers to provide output dimming requirements, we need to make a comprehensive trade-off at design time. Since the power consumption of LED lighting is very low, the current absorbed by the whole lamp is usually smaller than the holding current of the thyristor in the dimmer. This can cause undesirable conditions such as limited dimming range and/or flicker. Since the impedance of the LED driver is relatively large, a very severe oscillation occurs when the thyristor is turned on. At the moment when the thyristor is turned on, a very large inrush current flows into the input capacitance of the driver, which excites the line inductance and causes current oscillation. This also causes a similar problem because the oscillation causes the thyristor current to drop to zero and turn off, causing the LED to flash. To overcome these problems, two circuit blocks are used in the circuit – an active attenuation circuit and a bleeder circuit. The disadvantage of these circuit blocks is that they increase power consumption and thus reduce the efficiency of the power supply.
In this design, the values ​​of the attenuation circuit and the bleeder circuit can make the majority of the dimmers (with a dimmer below 600 W and including a low-cost leading-edge TRIAC dimmer) across the entire input voltage. Working within the scope. This design enables a flicker to be connected to a dimmer for high-speed input. Operating a lamp at high voltage results in a minimum output current and maximum inrush current (when the thyristor is turned on), which represents the worst case. Therefore, the role of the active attenuation circuit and the bleeder circuit is very obvious: the bleeder circuit can reduce the impedance, and the attenuation circuit can increase the impedance. But this will increase power consumption, which in turn will reduce the efficiency of the drive and the performance of the entire system.
It is required to connect multiple lamps to one dimmer so that normal operation will reduce the current required by the bleeder circuit, in which case the values ​​of R10 and R11 can be increased and the value of C6 can be reduced.
If the luminaire is operated only at low voltages (85 VAC to 132 VAC), the values ​​of R7 and R8 can be reduced when the peak current that occurs when the leading edge TRIAC dimmer is turned on is greatly reduced. Both of these changes will reduce the consumption and increase efficiency. For non-dimming applications, these components can be omitted directly, replacing jumpers with R7 and R8 to increase efficiency without changing other performance characteristics.
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