Design skills of the hottest LED lighting system

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Design skills of LED lighting system

LED lighting will replace the mainstream incandescent lighting and other lighting technologies and occupy the dominant market position. But the transformation from old technology to new technology will take many years. During this period, the challenge for LED designers is to ensure that the new design is compatible and reliable with the existing controller and wiring architecture originally developed for incandescent lighting. This paper introduces a solution that can be applied to both low-power and high-power LED lighting systems. It has been tested for a long time and is very mature

led bulb structure

an LED lamp contains one to a dozen or even more LED chips, which are usually connected in series. The luminous brightness of each chip is determined by the current passing through it. Due to the series connection, each LED chip in the bulb will automatically pass the same current, but the voltage on each chip is different. The forward voltage drop of LED is usually 3.4V, but it will vary from 2.8V to 4.2V. LEDs can be classified to limit the voltage variation, but this will increase the cost, and the forward voltage drop will still change with temperature and service time. In order to provide consistent light output, LED lamps must be driven by strictly regulated high-efficiency constant current power supply. As an alternative to incandescent LED lamps, the power supply must be integrated in the lamp shell

typical integrated LED lamps include drive circuits, led bundles, and enclosures that provide mechanical protection and heat dissipation for both drivers and LED chips

led driver is very strict. It must be energy efficient, meet strict EMI and power factor specifications, and can safely withstand various fault conditions. One of the most difficult requirements is to have a dimming function. Due to the mismatch between the characteristics of LED lamps and the dimming controller designed for incandescent lamps, it is easy to cause poor performance. The problem may be slow startup, flickering, uneven illumination, or flickering when adjusting the brightness. In addition, there are also problems such as inconsistent functions and requirements of each unit tested according to the requirements of flexible packaging film, and audible noise from LED lights. These negative conditions are usually caused by factors such as false triggering or premature shutdown of the controller and improper LED current control

dimming controller

the lighting controller works in the way of line dimming or PWM dimming. The simplest way of line dimming is the frontier thyristor controller. This is the most commonly used lighting control method at present, but unfortunately, there will be a lot of problems when using silicon controlled rectifier to adjust the light of LED lamps. More advanced line dimmers are electronic front or back edge dimmers. PWM dimmer is used in professional lighting system

when using the leading-edge thyristor dimmer, the dimming control is achieved by changing the phase angle of each half cycle of thyristor conduction. The input power of the bulb has a certain functional relationship with the phase angle of the dimming signal, and the variation range of the phase angle is close to 0 ° to 180 °

one of the important parameters of thyristor is maintaining current (IH). This is the minimum load that the thyristor must maintain to maintain continuity without using grid drive. In order to maintain the stable operation of SCR, the current cannot be zero, and the typical value of IH is between 8Ma and 40mA. Therefore, the phase angle dimmer of incandescent lamp usually has a specified minimum load, which is usually 40W at 230V rated AC voltage. This is to ensure that the current flowing through the internal thyristor is always higher than the specified maintenance current threshold. As the power consumption of LED lighting is very low, maintaining current will become a problem

another potential problem is surge current. When the thyristor is turned on, the high surge current will flow into the LED lamp. The worst case is that the phase angle reaches 90 ° and the AC input voltage reaches the peak. For incandescent lamps, surge current will not pose a problem. However, in LED lamps, the input stage impedance and line capacitance of the driver will cause oscillation. When oscillation occurs, the thyristor current will immediately fall below the maintenance current, so that the thyristor stops conducting

to solve these problems, we must modify the specification and design of LED driver

non isolated dimmable LED driver

Figure 1 shows the basic application circuit diagram of non isolated dimmable LED driver that can be used to replace LED lamps of incandescent lamps. The functions of the driver will be introduced below in order to clarify the problems that will occur when the driver becomes the load of the thyristor dimmer

this controller is a linkswitch PL device launched by power integrations (PI). It integrates high-voltage power MOSFET switch and power controller on a single chip IC. The device provides single-stage power at a reasonable price, including factor correction (PFC) and LED current control. The circuit can be used as a non continuous mode, variable frequency, variable conduction time flyback converter. The rectified AC power input is switched by the integrated 725v power MOSFET through the high-frequency transformer. The voltage generated on the secondary winding will be rectified and smoothed before it becomes an LED load. The LED load current also flows through the detection resistor RSENSE. The voltage generated on RSENSE (typically 290mv) will appear on the feedback (FB) pin through RF, providing accurate constant current feedback control. DES and res supply power to linkswitch PL, and dzov and ROV provide overvoltage protection when the LED is open circuit

the output current in this design has nothing to do with the characteristics of the power transformer. The change of inductance has no effect on the constant current characteristic. Therefore, this can make the constant current characteristic have a very strict tolerance, which is very prominent in the single-stage converter

when performing dimming control, linkswitch PL device will simultaneously detect the zero crossing point of input voltage and the conduction angle of thyristor dimmer. The zero crossing point of input voltage is detected through the drain node. The control circuit will process this data and set the required feedback voltage to set the LED load current

surge current

as shown in Figure 1, the driver forms a high impedance and large capacitance load on the SCR controller. In addition, there will be an input EMI filter circuit composed of capacitance and inductance. In each half cycle, surge current will be generated, resulting in oscillation (as described above)

to achieve trouble free dimming, the driver must be able to limit oscillation and prevent the thyristor current from falling below the maintenance current value. Figure 2 shows the complete circuit diagram of the driver with this function

Figure 2: circuit diagram of 5W, 15V thyristor dimming LED driver for A19 incandescent lamp replacement lamp

the circuit in Figure 2 provides 350mA single channel constant current output and 15V LED string voltage. Using a standard AC power thyristor dimmer can reduce the output current by 1% (3mA) without causing LED load instability or flashing. The driver can be compatible with low-cost silicon controlled dimmers and more complex electronic front and back edge dimmers at the same time

the function of the driver adds the unique components of input EMI filtering and three thyristor dimming: a passive attenuation circuit, an active attenuation circuit and a discharge circuit

input EMI filtering ensures compliance with IEC circular wave and en55015 conducted EMI regulations. However, the key point is that the linkswitch PL controller integrates the built-in frequency jitter characteristics. This feature can disperse the switching frequency and reduce the EMI peak value, so that the size of EMI filter circuit is far lower than the normal requirements. This helps greatly reduce the inductive load on the thyristor, thereby reducing the possibility of oscillation

resistor R20 forms a passive attenuation circuit. The active attenuation circuit connects the series resistors (R7 and R8) through the input rectifier tube in each AC half cycle, and bypasses the resistor through the parallel thyristor rectifier (Q3) in the remaining AC cycle. Resistors R3, R4 and C3 determine the delay time before Q3 turns on, and then short circuit attenuation resistors R7 and R8. Passive attenuation circuit and active attenuation circuit can jointly limit the peak surge current when the thyristor is turned on in each half cycle

resistors R10, R11 and C6 form a discharge circuit to ensure that the initial input current can meet the maintenance current requirements of the thyristor, especially when the conduction angle is small. For non dimming applications, passive attenuation circuit, active attenuation circuit and discharge circuit can be omitted

isolated LED driver

the driver in Figure 2 has been specially optimized for low-power, electrically non isolated integrated LED replacement lamps. PI introduced linkswitch pH controller for higher power LED lighting system requiring electrical isolation. Figure 3 (see our website for details) shows the circuit diagram of the isolated LED driver using linkswitch pH

Figure 3: circuit diagram of high power factor led driver with 14W thyristor dimming

this circuit can provide 0.5A driving current for the rated LED string voltage of 28V within the input voltage range of 90vac to 265vac. Its characteristics include ultra wide dimming range, flicker free operation (even if a low-cost AC input thyristor computer has a silicon dimmer that saves test results without time limit) and fast and smooth conduction

the topology used by it is an isolated flyback structure running in continuous conduction mode. The output current regulation is completely detected from the primary side, so there is no need to use the secondary feedback element. The single-stage internal controller adjusts the duty cycle of the high-voltage power MOSFET to maintain the input current as sinusoidal AC, thereby ensuring high power factor and low harmonic current

the function of this circuit is generally similar to that of the circuit in Figure 2. The most obvious difference is that this circuit adopts electrical isolation and does not use the detection resistance in series with the load. Feedback control is provided through the bias winding on the transformer. Feedback control has two functions: supplying power to linkswitch pH via bypass (BP) input and providing current feedback via feedback (FB) input. Another important input provided by linkswitch pH is voltage monitoring (V). This pin is connected to the interface of the external input voltage peak detector, which is composed of D1, C3, R1, R2 and R3. The applied current is used to control the stop logic of input undervoltage (UV) and overvoltage (OV), and provides a feedforward signal to control the output current and remote on/off function. The circuit integrates attenuation circuit and discharge circuit to ensure the operation of thyristor

in any LED lighting device, the performance of the driver determines the lighting experience of the end user, including start-up time, dimming, flicker free operation and consistency between units. The 14 w driver can be compatible with various dimmers at 115 VAC and 230 VAC at the same time, and is compatible with the widest possible dimming range. Therefore, the attenuation circuit and the discharge circuit will play a relatively positive role, but this will reduce the efficiency. Even so, the efficiency of the circuit can still be ≥ 85% at 115 VAC and ≥ 87% at 230 VAC with good corrosion resistance, thermal conductivity and non magnetism. If the dimming function is not required, the attenuation circuit and discharge circuit can be omitted, and higher efficiency can be achieved

with the continuous expansion of LED lighting market potential, the above design compromise highlights a series of philosophical problems. Since the power consumption of the new technology is only one tenth of that of the old technology, is it really necessary to be compatible with all the old thyristor controllers in the case of reducing efficiency (that is, increasing power consumption)? When using a 1000W thyristor controller with a minimum load specification of 40W to provide drive, can we make a 5W LED lamp work correctly? Yes, it can be done. Maybe it should be done as soon as possible. However, we must keep in mind that the ultimate goal of a complete lighting solution is to achieve maximum efficiency and minimum life cycle cost. (end)

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