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LED bulb construction
An LED light contains one to a dozen or more LED chips , which are usually connected in series. The luminance of each chip is determined by the amount of 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 the LED is typically 3.4V, but will vary from 2.8V to 4.2V. LEDs can be categorized to limit voltage fluctuations, but this adds cost and the forward voltage drop still varies with temperature and time of use. To provide consistent light output, LEDs must be driven by a highly regulated, constant current source. As an alternative to incandescent lamps, the lamp must be integrated into the lamp housing.
Typical integrated LED lamps include drive circuitry, LED bundles, and enclosures that provide both mechanical protection and heat dissipation for the driver and LED chips.
The requirements for LED drivers are very strict. It must be energy efficient, must meet stringent EMI and power factor specifications, and be safe to withstand various fault conditions. One of the most difficult requirements is to have a dimming function. Due to the mismatch between the characteristics of the LED lamp and the dimming controller designed for incandescent lamps, it is prone to poor performance. The problem may be slow start, flicker, uneven illumination, or flicker when adjusting the brightness. In addition, there are problems such as inconsistent performance of individual units and audible noise emitted by LED lamps. These negative conditions are usually caused by false triggering or premature shutdown of the controller and improper control of the LED current.
Dimming controller
The lighting controller works in either line dimming or PWM dimming. The simplest line dimming method is the leading edge thyristor controller. This is currently the most commonly used lighting control method, but unfortunately, the use of thyristor controllers to dim LED lights can cause a lot of problems. More advanced line dimmers are electronic leading or trailing edge dimmers. PWM dimmers are used in professional lighting systems.
When using a leading edge TRIAC dimmer, dimming control is achieved by changing the phase angle of the thyristor on each half cycle. The input power of the bulb is a function of the phase angle of the dimming signal, and the phase angle varies from approximately 0° to 180°.
One of the important parameters of thyristors is the holding current (IH). This is the minimum load that the thyristor must maintain to remain conductive without the use of a gate drive. In order to maintain stable operation of the thyristor, the current cannot be zero, and the typical value of IH is between 8 mA and 40 mA. Therefore, phase angle dimmers for incandescent lamps typically have a specified minimum load, typically 40W at 230V rated AC voltage. This is to ensure that the current flowing through the internal thyristor is always above the specified holding current threshold. Since the power consumption of LED lighting is very low, maintaining current will become a problem.
Another potential problem is the inrush current. When the thyristor is turned on, a high surge current flows into the LED lamp. The worst case scenario is that the phase angle reaches 90°, at which point the AC input voltage peaks. For incandescent lamps, inrush currents do not pose a problem. However, in an LED lamp, the input stage impedance of the driver and the line capacitance cause oscillation. When an oscillation occurs, the thyristor current will immediately drop below the holding current, causing the thyristor to stop conducting.
To solve these problems, you must modify the specifications and design of the LED driver.
Non-isolated dimmable LED driver
Figure 1 shows the basic application circuit diagram of a non-isolated dimmable LED driver that can be used to replace an incandescent LED lamp. The function of the drive will be described below to clarify the problem that will occur when the drive becomes a load on the TRIAC dimmer.
The controller is a LinkSwitch-PL device from Power Integrations (PI). It integrates high voltage power MOSFET switches and power controllers on a single IC. The device offers single stage power factor correction (PFC) and LED current control. This circuit can be used as a discontinuous mode, variable frequency, variable on-time flyback converter. The rectified AC power input is switched by an integrated 725V power MOSFET through a high frequency transformer. The voltage developed across the secondary winding is rectified and smoothed before it becomes an LED load. The LED load current also flows through the sense resistor RSENSE. The voltage developed on RSENSE (typically 290mV) is present on the feedback (FB) pin via RF, providing accurate constant current feedback control. DES and RES power the LinkSwitch-PL, and DZOV and ROV provide overvoltage protection when the LED is open.
The output current in this design is independent of the characteristics of the power transformer. The change in inductance has no effect on the constant current characteristics. Therefore, this enables the constant current characteristic to have a very tight tolerance, which is very prominent in a single-stage converter.
When performing dimming control, the LinkSwitch-PL device simultaneously detects the input voltage zero crossing and the conduction angle of the thyristor dimmer. The detection of the zero crossing of the input voltage is done internally through the drain node. The control circuit processes this data and sets the required feedback voltage to set the LED load current.
Inrush current
As shown in Figure 1, the driver forms a high-impedance, large-capacitance load on the thyristor controller. In addition, there will be an input EMI filter circuit composed of a capacitor and an inductor. In each half cycle, an inrush current is generated, causing oscillations (as described above).
For trouble-free dimming, the driver must be able to limit oscillation and prevent the thyristor current from falling below the holding current. Figure 2 shows the complete circuit diagram of the driver with this function.
The circuit in Figure 2 provides a single mA constant current output of 350 mA and a LED string voltage of 15 volts. Using a standard AC power thyristor dimmer reduces the output current by 1% (3mA) without causing the LED load to become unstable or flickering. The driver is compatible with both low-cost thyristor dimmers and more complex electronic leading edge and trailing edge dimmers.
The driver's functionality adds input EMI filtering and three thyristor dimming-specific components: a passive attenuation circuit, an active attenuation circuit, and a bleeder circuit.
Input EMI filtering ensures compliance with IEC ring wave and EN55015 conducted EMI regulations. However, the key point is that the LinkSwitch-PL controller integrates built-in frequency jitter characteristics. This feature disperses the switching frequency and reduces EMI peaks, making EMI filter circuits much smaller than normal. This helps to significantly reduce the inductive load on the thyristor, thereby reducing the possibility of oscillation.
Resistor R20 constitutes a passive attenuation circuit. The active attenuation circuit connects the series resistors (R7 and R8) through the input rectifier during each AC half cycle, and bypasses the resistor through the parallel thyristor (Q3) during the remaining AC cycles. Resistors R3, R4, and C3 determine the delay time before Q3 turns on, and then short-circuit the attenuating resistors R7 and R8. The passive attenuation circuit and the active attenuation circuit can collectively limit the peak surge current when the thyristor is turned on every half cycle.
Resistors R10, R11, and C6 form a bleeder circuit that ensures that the initial input current can meet the thyristor's holding current requirements, especially if the conduction angle is small. For non-dimming applications, passive attenuation circuits, active attenuation circuits, and bleeder circuits can be eliminated.
Isolated LED driver
The driver in Figure 2 has been optimized for low-power, electrically non-isolated integrated LED replacement lamps. PI introduced the LinkSwitch-PH controller for higher power LED lighting systems that require electrical isolation. Figure 3 (see our website) is a circuit diagram of an isolated LED driver using LinkSwitch-PH.
The circuit is capable of delivering 0.5A drive current to a 28V rated LED string voltage from 90VAC to 265VAC input voltage, including ultra-wide dimming range and flicker-free operation (even with low-cost AC input TRIAC dimming) And fast and smooth conduction.
The topology used is an isolated flyback structure that operates in continuous conduction mode. The output current regulation is completely detected from the primary side, eliminating the need for a secondary feedback component. A single-stage internal controller adjusts the duty cycle of the high-voltage power MOSFET to keep the input current sinusoidal, ensuring high power factor and low harmonic current.
The function of this circuit is similar to that of the circuit in Figure 2. The most obvious difference is that the circuit is electrically isolated and does not use a sense resistor in series with the load. Feedback control is provided by a bias winding on the transformer. Feedback control has two functions: powering the LinkSwitch-PH via a bypass (BP) input and providing current feedback via a feedback (FB) input. Another important input provided by LinkSwitch-PH is voltage monitoring (V). This pin is connected to an external input voltage peak detector interface consisting of D1, C3, R1, R2 and R3. The applied current is used to control the input undervoltage (UV) and overvoltage (OV) stop logic and provides a feedforward signal to control the output current and remote on/off function. The circuit integrates an attenuation circuit and a bleeder circuit to ensure thyristor operation.
In any LED lighting device, the performance of the drive determines the end user's lighting experience, including start-up time, dimming, flicker-free operation, and consistency between units. The 14 W drive is compatible with a wide range of dimmers at 115 VAC and 230 VAC and is compatible with the widest possible dimming range. Therefore, the attenuation circuit and the bleeder circuit play a relatively positive role, but this will reduce the efficiency. Even so, the efficiency of the circuit can be ≥ 85% at 115 VAC and ≥ 87% at 230 VAC. If the dimming function is not required, the attenuation circuit and the bleeder circuit can be omitted, resulting in higher efficiency.
As the potential of the LED lighting market continues to expand, the above design compromises highlight a number of philosophical issues. 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 when the efficiency is reduced (ie, the power consumption is increased)? Can we make a 5W LED light work correctly when using a 1000W thyristor controller with a minimum load specification of 40W? Yes, this can be done, maybe it should be done as soon as possible. But we must keep in mind that the ultimate goal of a complete lighting solution is to achieve maximum efficiency and lowest life cycle costs.
LED lighting system design skills
LED lighting will replace mainstream incandescent lighting and other lighting technologies, occupying a dominant position in the market. But the transition from old technology to new technology will take many years. During this time, the challenge for LED light designers was to ensure that the new design was compatible and reliable with existing controllers and cabling architectures originally developed for incandescent lighting. This article describes a solution that can be applied to both low-power and high-power LED lighting systems. It is proven and mature.