04/16/2015 16:31
An led lamp contains a to a dozen or more led chips, they are usually in series. Each chip luminescence brightness by is determined by the current. Because use series connection mode, the light bulb inside each LED chip will automatically through the same current, but each a different voltage on a chip. LED the forward voltage drop of usually is 3.4 V, but will change between 2.8 V to 2.8 V. Can categorize T8 LED Tube Light to limit the amplitude of voltage changes, but this will increase the cost, and the forward voltage drop will change with the temperature and time. To provide a consistent light output, LED lights must be driven by the strict rules of high efficiency constant current power supply. As a substitute for incandescent bulb LED lights, the power must be integrated within the lamp shell.
The LED driver is very strict requirements. It must be of high efficiency and energy saving, specifications must meet stringent EMI and power factor, and can be safely tolerate various fault conditions. One of the most difficult is one of the requirements of the dimming function. Due to the nature of the LED and designed solely for incandescent lamp dimming controller between mismatch, so easily lead to poor performance. Problems may be slow to start, flicker, uneven illumination, or flickers when adjusting the brightness. In addition, there are various unit performance inconsistency and leds emit audible noise sound, etc. These negative conditions are often triggered by mistake or premature shutoff, improper control of controller and the LED current caused by factors such as joint.
Lighting controller circuit dimmer or PWM dimming manner. The simplest line move light way is frontier SCR controller. This is by far the most common way of lighting control, but, unfortunately, using SCR controller to move light leds would produce a lot of problems. More advanced circuit dimmer is electronic frontier or along the dimmer. PWM dimmer for professional lighting system.
Using cutting-edge LED dimming power supply of thyristor dimmer, dimming control is by changing the thyristor conduction of each half cycle phase Angle. The input power of the light bulb and the phase Angle of optical signals into certain function relation, the scope of the change of the phase Angle between close to 0°to 180°.
One of the important parameters of SCR is to maintain the current (IH). This is silicon controlled rectifier in the case of not use grid driver keep conduction must be maintained by the minimum load. To work in order to maintain the stability of the thyristor, the electric current cannot be zero, IH typical values between 8 ma ma to 40. Therefore, incandescent lamp phase Angle of the dimmers there is usually a specified minimum load, usually under rated voltage 230 v to 40 w. Internal SCR this is to ensure that the current is always higher than the threshold required to maintain current. As a result of the LED lighting power consumption is very low, to maintain the current will be a problem.
Another potential problem is the surge current. Thyristor turn-on, the high surge current will flow into the LED lights. Worst case is the phase Angle of 90 °, and AC input voltage peak. For incandescent lamp, the surge current won't be a problem. But in LED lights, drive in the input impedance and the capacitance can cause oscillation. Oscillation occurs, silicon controlled rectifier current will immediately to maintain current below, make the silicon controlled stop conduction.
To solve these problems, we must modify the specifications of the LED driver and design
Can be used to replace incandescent LED Light Bulbs of isolation type adjustable light LED drive circuit diagram of the basic application. The controller is Power Integrations (PI) launched LinkSwitch - PL device. It in a single chip IC used to high-voltage power MOSFET switch and power controller. The device provides single-stage power factor correction (PFC) and the LED current control. This circuit can be used as a discontinuous mode, variable frequency and variable conduction time flyback converter. After rectification of the ac power input by integration of the 725 v power MOSFET switch through the high-frequency transformer. Secondary windings on the resulting voltage before into the LED load will be rectifying and smooth. The LED load current is flowing through measuring resistance RSENSE. RSENSE on the resulting voltage (290 mv) typical values for in feedback by RF (FB) pins, so as to provide accurate constant current feedback control. DES and RES for power supply, LinkSwitch - PL DZOV and the ROV in LED provides overvoltage protection when open.
The design of the output current has nothing to do with the characteristics of the power transformer. Inductance change without any impact on characteristics of constant current. Therefore, it can make the constant current characteristic have very strict tolerance, this is very prominent in the single-stage converter. When performing the dimming control, LinkSwitch - PL device can detect the input voltage at the same time a zero and conduction Angle of thyristor dimmer. The input voltage of zero detection is done through the internal drain node. Control circuit can satisfy the needs of the data and set the feedback voltage, thus setting the LED load current.
