Introduction: LED lighting will replace the mainstream of incandescent lighting and other lighting technology, occupy a dominant market position. But from the old technology to convert new technology also requires years of time. In the meantime, the LED lamp designer's challenge is how to ensure compatibility and reliability of the new design and development of the existing original incandescent lighting controller and wiring structure
LED lighting will replace the mainstream of incandescent lighting and other lighting technology, occupy a dominant market position. But from the old technology to convert new technology also requires years of time. In the meantime, the LED lamp designer's challenge is how to ensure compatibility and reliability of the new design and development of the existing original incandescent lighting controller and wiring structure. Introduced in this paper is also suitable for low power and high power LED lighting system solutions, it is proven and very mature.
Structure of LED lamp
A LED lamp comprises a dozen or more LED chips, they are usually connected in series. The brightness of each chip is decided by the size of the current through the. Because of the series connection in the bulb of each LED chip will automatically through the same current, but the voltage on each chip is different. The forward voltage drop of LED is usually 3.4V, but will be changed between 4.2V 2.8V. You can classify the LED to limit the amplitude of voltage fluctuation, but it will increase the cost, and the forward voltage drop will change with temperature and time. In order to provide uniform light output, LED lamp must be strictly regulated by the constant current source driver. As a substitute for LED incandescent lamp, the power supply must be integrated in the shell.
A typical integrated LED lamp comprises a drive circuit, and the LED cluster can provide mechanical protection and heat dissipation shell for driver and LED chip.
The LED driver is very strict. It must be efficient, must meet stringent EMI specifications and power factor, and can safely withstand various fault conditions. One of the most difficult requirements is to have the dimming function. Because there is a mismatch between the characteristics of LED lamp dimming controller and designed for incandescent lamp design, resulting in poor performance. The problem may be shown as the slow start, flicker, uneven illumination, or flicker in the adjustment of brightness. In addition, the performance of each unit is not consistent, as well as LED lights can be heard from the noise and other issues. These negative conditions are usually caused by the incorrect triggering or premature shutdown of the controller and the LED current control caused by improper.
Dimming controller
Lighting controller to line dimming or PWM dimming way to work. The most simple way to adjust the line is to control the front edge of the silicon controller. This is the most commonly used lighting control method, but unfortunately, the use of controlled silicon controller for LED lights dimming will produce a lot of problems. A more advanced line dimming device is the electronic frontier or the back edge of the light. PWM dimming device is used for professional lighting system.
When using the front silicon controlled rectifier, the dimming control is realized by changing the phase angle of each half cycle. The input power of the light bulb and the phase angle of the modulated light signal to a certain function relationship, the phase angle of the change range is between close to 0 degrees to 180 degrees.
One of the important parameters of silicon controlled silicon is maintaining current (IH). This is the minimum load required to maintain the conduction through the SCR without the use of gate drive. In order to maintain the stability of the controlled silicon, the current can not be zero, the typical value of 8mA is between IH to 40mA. Therefore, the phase angle of the incandescent lamp dimming device usually has a specified minimum load, 230V rated AC voltage is typically 40W. This is in order to ensure that the current flowing through the internal SCR is always higher than the required maintenance current threshold. As the power consumption of LED lighting is very low, maintaining the current will become a problem. Another potential problem is the surge current. Thyristor conduction, high surge current will flow into the LED lamp. The worst case is the phase angle of 90 degrees, while the AC input voltage peak. The incandescent lamp, the surge current is not a problem. But in the LED lamp in the input impedance and the capacitance drive will cause oscillation. Oscillation, thyristor current will be immediately reduced to maintain the current, so that the SCR conduction stop.
To solve these problems, it is necessary to modify the specification and design of LED driver.
Non isolated dimmable LED driver
Figure 1 shows the basic application circuit for non isolation type can be used to replace the incandescent lamp and the LED lamp dimmable LED driver. The following will introduce the drive function, in order to clarify the driver in question will appear when the load becomes controllable silicon dimmer.
The controller is Power Integrations (PI) LinkSwitch-PL devices launched. It integrates high-voltage power and MOSFET power switch controller in a single IC. The device provides a single-stage power factor correction (PFC) and LED current control. This circuit can be used as a non continuous mode, variable frequency and variable on-time flyback converter. The AC power is rectified by 725V input power MOSFET integrated by high-frequency transformer switch. The secondary winding of the voltage generated by the rectifier and smoothing in LED before the load will become. LED is the load current flowing through a detection resistor RSENSE. The voltage produced in the RSENSE (typical 290mV) will appear in the feedback through the RF pin (FB), which provides accurate constant current feedback control. DES and RES for LinkSwitch-PL power supply, DZOV and ROV provide overvoltage protection in the LED open.
The independent characteristic of output current and power transformer in the design of the. The inductance change of constant current.