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PFC sheds light on THD
The importance of power factor correction (PFC) in offline power supplies to lower transmission line losses in order to conserve dwindling energy resources is being addressed worldwide through legislation. Examples of this legislation are IEC61000-3-2 input current harmonic emission specification from the European Union and ANSI C82.77-2002 harmonic emissions specifications for lighting in the United States. Energy Star has expanded its PF requirements in 2009 for computing power to include power supplies for solid-state lighting (SSL).
These requirements have forced power-supply manufacturers to have some type of PFC in their designs. Some offline LED ballast designers have a misconception that to meet these specifications requires an input current total harmonic distortion (THD) of less than 10 percent. A ballast can be designed for extremely low input current THD with a traditional two-stage approach. However, as this article will show, in low-power LED lighting applications it is not necessary to have an extremely low-current THD to meet PF and current harmonic specifications. This can be achieved with a THD as high as 28 percent. In these low-power applications the harmonic requirements can be achieved with a single-stage transition mode PFC flyback power converter.
The traditional two-stage offline power converter as mentioned previously can be used to design a LED ballast for an extremely low THD (< 10 percent) and high power factor (>0.98). A functional schematic of this power converter is presented in Figure 1. Stage 1 consists of an EMI filter, bridge rectifier and PFC boost pre-regulator used to shape the input current and provide high PF. Stage 2 could be a flyback or forward converter designed to regulate the current through the LEDs (Figure 1). In these designs, Stage 1 is typically 94 to 96 percent efficient, while Stage 2 is typically 85 percent efficient. This system’s overall efficiency is generally between 80 and 82 percent efficient. However, to meet the input current harmonic specifications mentioned in this article, this approach is not necessary. It is neither as efficient nor as cost-effective as a single-stage transition mode PFC flyback converter.
Figure 1: Two-Stage LED Ballast with PFC
Before reviewing the single stage transition mode PFC flyback converter, we will study the input current emission, THD and PF requirements for low-power, off-line (less than 150 W) lighting power converters presented in Table 1. From this table you can see that IEC61000-3-2 does not have a PF or a current THD requirement, but only a harmonic input current requirement. The PF and THD requirements are unique to the ANSI and Energy Star SSL specifications. To meet the low-power PF and THD requirements of the ANSI and Energy Star SLL, a PF of > 0.9 and a current THD of less than 32 percent is required.
Table 1: Input Current Harmonic and PF Specifications
A functional schematic of the single-stage transition-mode PFC flyback converter used in an LED ballast application is presented in Figure 2. This topology requires much fewer components than the more traditional two-stage approach that was discussed earlier. It is also slightly more efficient than the two-stage approach. When designed correctly this single-stage PFC power converter can be designed for a full-load efficiency of 85 percent or greater.
This type of converter is also referred to as a constant on-time control, or boundary mode PFC flyback back converter. It works by turning the FET on when zero energy is detected in the transformer, and turning off the FET when it has reached a peak current that is forced to track the line voltage. For example, the UCC28810 PFC controller is designed to specifically work in a single-stage transition mode PFC flyback for LED lighting applications.
Figure 2: Single-Stage LED Ballast with PFC
To show the performance benefits of the single-stage transition mode flyback, a 25-W LED ballast using this topology was evaluated. This converter was designed for a universal input and had a full-load efficiency of greater than 85 percent. In these converters the highest power factor and lowest THD can be achieved by designing for a narrow input range of +/–10 percent. However, this is not necessary to meet harmonic current and PF specifications. The scope plot in Figure 3 shows the input current (IIN) and rectified line voltage (VREC) of this converter at full load with an input voltage of 115 V RMS. The PF is 0.97 while the input current THD is 22 percent, which easily meets ANSI’s THD and Energy Star’s PF requirements.
Figure 3: IIN and VREC at 25W, 115V RMS
The scope plot in Figure 4 is taken at high line (230V) RMS on this 25-W single-stage PFC flyback ballast. Even though the input current waveform looks distorted, the PF is 0.93 and the THD is 28 percent. Still it easily meets the PF and THD requirements listed in Table 1.
Figure 4: IIN and VREC at 25W, 230V RMS
Not only does the distorted input current wave presented in Figure 4 meet PF and input current THD requirements, it also meets IEC61000-3-2 Class C input current harmonic content requirements. The bar graph presented in Figure 5 shows the input current harmonic content amplitudes of this waveform; as well as the IEC61000-3-2 input current harmonic limits for this power converter.
Figure 5: Input Current Harmonic Content
Some ballast designers have a misconception that an input current THD of less than 10 percent is required to meet PF, input current THD and input current harmonic requirements. Even though this low THD requirement is achievable with a two-stage power converter, it is not necessary or cost-effective.
As this article points out, these specifications can be met using single-stage transition-mode PFC flyback with a THD as high as 28 percent. In general, these single-stage PFC converters are roughly three to five percent more efficient than the traditional two-stage off-line power converter. In addition to the single-stage PFC discussed in this article as being more efficient than the two-stage approach, it also uses fewer components, which, in the end, also makes it more cost-effective.
References:
Find out more about power factor correction at: www.ti.com/pfc-ca2.
To learn more about LED solutions, visit: www.ti.com/led-ca2.
Download a datasheet for the UCC28810 transition mode PFC flyback controller here: www.ti.com/ucc28810-ca2.
About the author:
Michael O’Loughlin is an applications engineer with the Power Supply Control Products group at Texas Instruments. He specializes in offline and isolated power supply design and has authored numerous articles on power factor correction and power supply design related topics. Michael received his Bachelor of Science degree from the University of Massachusetts. Michael can be reached at ti_mikeoloughlin@list.ti.com
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