LED Driver GreenPoint Reference Design

With the comprehensive improvement of high brightness light emitting diodes (HB-LED) in terms of light output, energy efficiency and cost, combined with many advantages such as compactness, low voltage operation and environmental protection, LED lighting (also known as solid-state lighting (SSL)) is setting off a lighting revolution. Under the trend of energy saving and environmental protection, LED lighting has naturally become the target of many regulatory agencies. For example, the 1.1 version of the solid-state lighting standard of the "Energy Star" project of the US Department of Energy has been in effect since February 2009. China's China National Institute of Standardization is also taking the lead in working with relevant agencies to prepare to release the Chinese version of the LED lighting energy efficiency standard in 2010.

As for the new version of the solid-state lighting standard of the "Energy Star", an important feature of this standard is that it requires a minimum power factor of 0.7 for a variety of residential lighting products, some of which are typical products such as portable table lamps, cabinet lights and outdoor corridor lights. The power of this type of LED lighting application is generally between 1 and 12W, which is a low-power application. The most suitable power supply topology for this type of low-power application is the isolated flyback topology. Unfortunately, the standard design techniques used to design these power supplies typically result in a power factor (PF) in the range of 0.5 to 0.6. This article will analyze the reasons why the existing designs have low power factors, explore the techniques and solutions to improve the power factor, introduce the relevant design process and share some test data to show how this reference design can easily meet the power factor requirements of the "Energy Star" solid-state lighting specification for residential LED lighting applications.

Design Background

A typical offline flyback power converter uses a full-wave bridge rectifier and a large capacitor in front of the switching regulator. This configuration is chosen because the line power decreases to zero every 2 line cycles and then rises to the next peak. The large capacitor acts as an energy storage element to fill the corresponding missing power and provide a more constant input to the switching regulator to maintain power flow to the load. This configuration has a low power utilization or power factor of the input line waveform. The line current is consumed in large and narrow pulses close to the peak of the voltage waveform, introducing interfering high-frequency harmonics.

There are many passive power factor correction (PFC) solutions in the industry, which usually use more additional components. One of the solutions is the valley-fill rectifier, in which the combination of electrolytic capacitors and diodes increases the line frequency conduction angle, thereby improving the power factor. In fact, this process uses high line voltage to charge the series capacitors with low current, and then discharges the capacitors to the switching regulator with a larger current at a lower voltage. Typical applications use 2 capacitors and 3 diodes, while to further enhance the power factor performance, 3 capacitors and 6 diodes are used.

Figure 1: Typical valley-fill circuit.

Although the valley-fill rectifier improves the utilization of the line current, it does not provide a constant input to the switching regulator. The power delivered to the load will have a large ripple, up to twice the line frequency. It should be noted that four diodes are still required to rectify the line power, bringing the number of diodes used in this solution to seven or ten. These diodes and multiple electrolytic capacitors increase the solution cost, reduce reliability, and occupy considerable board area.

Another solution is to use an active PFC stage before the flyback converter, such as the NCP1607B. This solution provides excellent power factor with typical performance of better than 0.98, but increases component count, reduces efficiency and increases complexity, and is best used at power levels much higher than the power level of this application.

Solution

High power factor usually requires a sinusoidal line current with a very small phase difference between the line current and voltage. The first step in modifying the design is to obtain very low capacitance before the switching stage, resulting in a more sinusoidal input current. This allows the rectified voltage to follow the line voltage, resulting in a more ideal sinusoidal input current. Thus, the input voltage to the flyback converter follows the rectified sinusoidal voltage waveform at twice the line frequency. If the input current is maintained at the same waveform, the power factor is high. The energy delivered to the load is the product of the voltage and current, a sine-squared waveform. Due to this sine-squared energy transfer, the load will see a ripple at twice the line frequency, similar in nature to the ripple seen in valley-fill circuits.

As mentioned above, the input current must be maintained at a nearly sinusoidal waveform to achieve a high power factor. The key to a high power factor is to not allow the control loop to correct for output ripple by maintaining the feedback input at a constant level related to the line frequency. One option is to significantly increase the output capacitance, thereby reducing the amount of 120Hz ripple, which may be required for some applications. LEDs for general lighting applications are more tolerant of ripple if the frequency is above the range of visible light perception. A more compact and inexpensive solution is to filter the feedback signal back to the PWM converter to establish a nearly constant level. This level fixes the maximum current in the power switch. The current of the power switch is determined by the applied transient input voltage divided by the transformer primary inductance multiplied by the length of the power switch on time.

ON Semiconductor's NCP1014LEDGTGEVB evaluation board is optimized to drive 1 to 8 high-power, high-brightness LEDs, such as CreeRebel, SeoulSemiconductorZ-Power®, or OSRAMGoldenXR-E/XP-E, Luxeon™XLAMP®Dragon™. The design is based on the NCP1014, a compact fixed-frequency pulse-width modulation (PWM) converter that integrates a high-voltage power switch with internal current limiting. Because the NCP1014 operates at a fixed frequency, the current cannot rise above a certain point; this point is determined by the input voltage and the primary inductance before the end of the switching cycle or on-time. Due to the on-time limitation, the input current will follow the waveform of the input voltage, providing a higher power factor. The relevant circuit diagram is shown in Figure 2.

Figure 2: NCP1014LEDGTGEVB circuit diagram