Lighting accounts for about 17.5% of global electricity consumption. As the global trend from incandescent lamps to solid-state lighting (SSL) technology intensifies, it is of great concern to utilities and government regulators around the world, as switching to solid-state lighting by such a large consumer base will increase infrastructure costs. cost. This is caused by the reactive nature of LED-based solid-state lighting, which places higher demands on the grid due to the adverse effects of power factor (PF) resulting in higher distribution currents.
Regulators have been working with utilities to develop stringent standards to control the impact of solid-state lighting technology on the grid (Figure 1). Since LED-based solid-state lighting can greatly reduce actual power consumption, it can significantly reduce the carbon footprint of the grid. However, if the power factor is not managed, the grid will still need to provide much higher power levels than the actual load requires, negating a large part of the benefits of moving to solid-state lighting.
Previous incandescent lamps had a near-perfect power factor. As a result, solid-state lighting has always adhered to much higher power factor standards than traditional AC and DC power supplies. In most cases, they are not subject to any power factor standard for power supplies under 75W. For solid state lighting, however, power factor limitations need to be considered starting from as low as 5W or even lower.
To efficiently design LED-based light sources, designers need to understand power factor, the impact of LED drivers on power factor, and the different techniques for integrating cost-effective power factor correction in LED driver designs.
Power Factor
Power factor is a simple ratio of active power to apparent power, with no units. Active power is the power used at a load and is measured in kilowatts (kW). Apparent power is the measured power in volt-amps (VA) supplied by the grid to the system load. In a high reactance system, currents and voltages with angular parameters can be highly out of phase with each other. This will result in the grid needing to provide much more reactive power than true real power at any given point in time.
Solid-state lighting with power factor correction technology can reduce the impact of changing from incandescent to LED lighting by adding circuitry to the LED driver that corrects the reactive input impedance to increase the power factor closer to unity.
LED Drivers And Power Factor Correction
LEDs and their drivers have non-linear impedances, so inherent power factor is very low. To solve this problem, the driver needs to compensate for the power factor, bringing it as close to unity as possible. For industrial warehouses or large shopping malls, the effect is very insignificant when just considering one LED light and its effect on the total power factor, but in a large commercial space the sum of all lighting units will significantly affect the total power factor, it is therefore necessary to implement power factor correction for each individual lamp or each ballast driving those lamps.
There are two methods of power factor correction, active and passive. Passive power factor correction solutions typically consist of passive input filters, offering some degree of cost benefit, but since passive power factor correction is only optimized for specific input voltage and current conditions, when these conditions change, the power factor will also decline. In dimmable lighting fixtures, passive power factor correction is unacceptable because the power factor varies significantly over the entire dimming range of the lamp. This is where active power factor correction is required to maintain a high enough power factor under various load and line conditions.
With active power factor correction, there are methods that use the mains conversion circuit to compensate for power factor (single stage), and methods that use a separate pre-regulator to provide power factor correction (two-stage). Both methods have their own benefits. Most obviously, the cost minimization can be achieved with a single-stage approach, since part of the power factor correction is done in the main power conversion circuit. Determining which topology is best for the end application requires a more in-depth analysis of each converter.
With single-stage LED drivers, the main power stage circuit converts the input voltage into a usable DC voltage and current, which is then used to drive the LEDs. Since there is only one power stage, the drive of the main power stage needs to be managed to increase the power factor close to unity. Since the power factor measurement depends on how linear the driver input appears to the mains input voltage, the modulation topology determines what impedance the converter input impedance presents relative to the mains supply.
The best way to maximize power factor is to use the fixed on-time method, which effectively creates a voltage-controlled current source, or an input impedance that appears to be highly resistive. The peak current through the transformer primary is directly proportional to the on-time of the primary drive, so this approach inherently has a high power factor close to unity. While a fixed on-time architecture can provide the benefit of high power factor, the trade-off is often unacceptable. When operating normally in DCM mode, the peak current is very high, with two main consequences, high stress on the passive components and high current ripple on the LEDs. High stress on the input capacitors can reduce the life of these critical components, resulting in shorter lamp operating life. Excessive current ripple on the output will reduce the quality of the output light, because the ripple current will flow through the output capacitor, the output light will increase the flicker noise, reduce the luminous efficiency, and increase the self-heating loss.
Using constant current mode operation instead of fixed on-time reduces output ripple current and reduces stress on passive components in the circuit, but the power factor is significantly reduced due to the inherent reactive nature of the input impedance.
Finding alternatives that combine high power factor and low ripple current while minimizing the impact on passive external components is the key to finding the best low-cost solution for single-stage LED drivers.
The iW3626 (Figure 4) is an example of a single-stage high-power-factor LED driver that is not only a high-power-factor driver, but also minimizes output ripple. The implementation technology is at the heart of the digital engine, capable of monitoring the input voltage and current as well as the output status by monitoring the primary of the power transformer. The digital core allows modulation of the drive signal to the main power transistor, in this case the power bipolar junction transistor. The proprietary modulation technique in this example allows the end user to program a desired minimum power factor of 0.7, 0.8 or 0.9, or no power factor at all. Along with the minimum power factor is presented the corresponding output ripple. This flexibility allows designers to optimize circuits for output ripple (without power factor) or for power factor (with moderate output ripple), or for balanced high power factor and low output ripple designs.
Another important LED driver characteristic in solid-state lighting, which is also regulated by the international lighting standards shown in Figure 1, is total harmonic distortion (THD). In general, when the total harmonic distortion is small, the power factor is usually high (>0.9). However, when using single-stage conversion techniques, there is often a trade-off between output ripple, total harmonic distortion, and power factor. The iW3626 uniquely combines low output ripple and high power factor with acceptable total harmonic distortion for most applications. The US market is particularly interested in total harmonic distortion, while Europe uses the IEC61000-3-2 standard to specify harmonic requirements for power supplies. The two-stage approach can be used when the end application requires low total harmonic distortion, high power factor, and low output ripple.
The biggest difference between the single-stage and the two-stage approach is that the latter significantly adds a second-stage conversion circuit. Additional conversion stage circuits are used not only for power factor correction, but also to minimize total harmonic distortion. The initial conversion stage not only removes the line frequency from the main output voltage and flicker noise from the output light, but also adds flexibility in integrating dimming techniques and reducing inrush current, benefiting passive components at the input. The first stage of the two-stage approach can use a boost converter or a simple chopper circuit. Compared to a simple chopper circuit, a complete boost converter can provide higher efficiency, higher power factor correction and lower total harmonic distortion over a wider range of line voltages.
The iW3630 is an example of a two-stage LED driver that can be used in inherent lighting applications requiring high power factor (>0.95), low total harmonic distortion (<15%), and low output ripple (Figure 5). The first stage circuit is a complete boost converter operating in a constant on-time architecture to optimize power factor and minimize total harmonic distortion. This stage converts the mains voltage to an intermediate voltage, decoupling the output from the mains voltage and frequency.
Another important LED driver characteristic in solid-state lighting, which is also regulated by the international lighting standards shown in Figure 1, is total harmonic distortion (THD). In general, when the total harmonic distortion is small, the power factor is usually high (>0.9). However, when using single-stage conversion techniques, there is often a trade-off between output ripple, total harmonic distortion, and power factor. The iW3626 uniquely combines low output ripple and high power factor with acceptable total harmonic distortion for most applications. The US market is particularly interested in total harmonic distortion, while Europe uses the IEC61000-3-2 standard to specify harmonic requirements for power supplies. The two-stage approach can be used when the end application requires low total harmonic distortion, high power factor, and low output ripple.
The biggest difference between the single-stage and the two-stage approach is that the latter significantly adds a second-stage conversion circuit. Additional conversion stage circuits are used not only for power factor correction, but also to minimize total harmonic distortion. The initial conversion stage not only removes the line frequency from the main output voltage and flicker noise from the output light, but also adds flexibility in integrating dimming techniques and reducing inrush current, benefiting passive components at the input. The first stage of the two-stage approach can use a boost converter or a simple chopper circuit. Compared to a simple chopper circuit, a complete boost converter can provide higher efficiency, higher power factor correction and lower total harmonic distortion over a wider range of line voltages.
The iW3630 is an example of a two-stage LED driver that can be used in inherent lighting applications requiring high power factor (>0.95), low total harmonic distortion (<15%), and low output ripple (Figure 5). The first stage circuit is a complete boost converter operating in a constant on-time architecture to optimize power factor and minimize total harmonic distortion. This stage converts the mains voltage to an intermediate voltage, decoupling the output from the mains voltage and frequency.
The second-stage main power conversion circuit then converts the intermediate voltage into a DC voltage and current for driving the LEDs. This stage circuit can be isolated or non-isolated, depending on the needs of the end application. With the iW3630, whether the application is isolated or non-isolated, there is no need for an optical feedback device because the primary feedback originates from the primary of the transformer using iWatt’s PrimeAccurate technology.
The Summary Of This Article
As more regulations impose power factor requirements for solid-state lighting, designers need to incorporate power factor correction circuits into driver designs. A clear understanding of the final requirements based on the target lighting application can determine the type of power factor correction that needs to be implemented. Whether it’s a single-stage cost-driven solution for residential lighting or a two-stage performance-driven solution for commercial and industrial applications, there are proven driver technologies today to help enable a brighter, greener future.
