Innovative PFC-driven, high-voltage, high-brightness LED lighting solution

Technology News | November 27, 2011

By eeNews Europe

The development of environmentally-friendly, cost-effective, lighting solutions is becoming increasing important as more and more governments around the world introduce legislation to phase out/ban the use of incandescent light bulbs and encourage the use of more energy-efficient alternatives, in particular compact fluorescent lamps (CFLs) and light-emitting diodes (LEDs). Although both approaches are feasible, there are significant differences between them (Table 1).

COMPACT FLUORESCENT LAMPS	LIGHT-EMITTING DIODE
Slow turn-on time	“Snap-on” characteristics
Poor dimming performance	Good diming performance
Lower cost	Higher cost

Table 1. Comparison of CFL and LED lighting solutions.

The implementation of LED solutions has been slow owing to the higher costs, however, the availability of new, cost-effective LED driver components, as well as high-brightness LEDs with increased luminous efficiency that require less power, has made them a more viable option. This has led to innovations such as high-voltage, high-brightness (HVHB) LEDs, which present exciting new lighting solution opportunities, as well as challenges i.e. the much higher voltage required to initiate light emission.

AC-driven HVHB LEDs

Initially, HVHB LEDs were designed to be self-driven directly from an AC line, however, this leads to a number of issues that limit their wider application, including low efficiency, low power factor, and poor dimming performance. Figure 1 shows the characteristics of AC-driven HVHB LEDs. Reduced efficacy is a major issue. This is because in order to maintain the voltage range (90-135 Vac or 207-253 Vac), the forward voltage drop (Vf) must be set at the lowest AC voltage. This means that when the voltage is at the upper limit, the voltage falls across the current limiting resistor thus reducing efficiency. This also generates heat, which can reduce LED life span.

“Turn-on” time is also an issue (Figure 1) with AC-driven HVHB LEDs. Power factor is very low because only a small proportion of the overall AC period (the peak voltage) is used to create light (LEDs only conduct when the Vf has been met or exceeded). Consequently, AC-driven LEDs cannot be used in residential or commercial lighting in many countries including the US and European Union (EU) unless much more power is supplied by the power companies. Furthermore, dimming can only be achieved during the brief time the LEDs are conducting (the light will be completely on or off over 90% of the dimming range), as most dimmers chop the lamp’s AC waveform.

Figure 1. HVHB LED characteristics without driver.

PFC-driven HVHB LEDs

The solution is to drive HVHB LEDs directly from a boost power factor correction (PFC) supply. Figure 2 shows a simple boost design using a PFC device that guarantees a power factor (≥0.97) that surpasses that required (0.7-0.9) to market lighting in the US and across the EU. This is achieved by matching the current consumption with the AC waveform, which controls the power delivered to the load.

Figure 2. Simple boost design using an active PFC device. For better resolution, click here.

Boosting the input voltage generates a high voltage. The benefits of this are 2 fold: 1) it allows a single power supply design to be used (irrespective of input voltage), and thus a universal input LED (90-253 Vac); 2) it requires a low current for a given output power (e.g. only 42 mA for a 16W LED light source using four 4W LEDs in series/parallel with a total Vf of 380 Vdc).

The Vf ranges of LEDs (e.g. Everlight HiVo series) cover 110Vac and 220 Vac voltages. By using two 220 Vac connected in series, all input voltages (90-277 Vac) can be handled by the active PFC boost design. Flexibility to encompass many power and voltage configuration is achieved by connecting them in series or parallel.

Although a universal design enables both 120 and 220 Vac applications to be implemented using a single stock item, it results in larger components and a higher overall cost, compared with separate 120 and 220 Vac solutions. This is because the single design must be able to cope with the higher voltage and current that is needed for 220 Vac operations.

The advantage of low current is that longer lived and less expensive ceramic capacitors can be used as there is less storage capacitance. This allows removal of electrolytic caps thus increasing the light sources’ life. So much so that LEDs can last as long as 50,000 hours, compared with 20,000 hours (or significantly less if overheating occurs) for typical “long-life” electrolytics. This is particularly useful for applications that have high ambient temperature, including street lighting and high bay lighting.

With a switch mode LED driver, ≥90% efficiencies can be achieved that vary little over the VIN range. The LED switching power supply (IC), drive FET, and rectifier can reduce efficiency, however, by keeping an acceptable switching frequency (e.g. 150 KHz) switching losses in the FET can be minimised. In addition, the low current results in minimal power loss in the rectifier.

Power factor correction supply

PFC can be achieved using several methods although there are drawbacks with some approaches (Table 2). To use the technique shown in Figure 1 for driving low-voltage LEDs, the input voltage for the PFC must first be boosted and then the voltage bucked down to the LED string Vf drop. This is complex and costly, and converting the high PFC voltage to LED current can require additional power stages.

Method	Solution	Drawbacks
Actively	IC with a built-in algorithm for matching current consumption with input voltage	–
Valley-fill	Routing diodes and storage caps to supply current when the AC input is in transition	– Limited driver life as requires low-life electrolytics
Natural/flyback	Switched-mode power supply or SMPS, in a flyback configuration running discontinuously	– Expensive transformer required – Creates considerable electromagnetic interference (EMI) radiation (through the flux loss of the transformer) – Creates considerable conduction (through the high voltage and current spikes caused by discontinuous operation, as well as the reflected voltage, equal to VIN + VOUT)

Table 2. Methods of power factor correction.

A more usual approach is to use a low-voltage transformer-based LED driver as shown in Figure 3. This involves using a power supply in a flyback topology running in a discontinuous mode, with the transformer’s turn ratio helping to bridge the VIN/Vf voltage differential. There are, however, a number of downsides to using this topology:

Increased cost owing to greater design complexity

An universal AC input cannot be accepted

Reflected voltage and transformer flux loss increases EMI conduction and radiation

Requires high-voltage, shorter-lived bulk capacitors

Higher currents (resulting from lower voltages) increases temperature, power path component size, and limits the LED output

Figure 3. Low-voltage, transformer-based LED driver. For full resolution click here.

Using existing dimming solutions

An important attribute that separates PFC-driven LEDs from self-driven LEDs and CFLs is the ability to incorporate existing TRIAC-based dimming solutions, and dim over the entire range of the TRIAC span. “Decoding” the AC signature and converting the resultant into either a voltage or current equivalent achieves dimming. The voltage or current can be inserted directly into the power supply, or converted into a pulse width modulated (PWM) signal which enables the LED “on” time to be adjusted in line with the dim percentage. To keep the dimmer in a conductive mode, sufficient load at light dim must be provided, and TRIAC misfiring (flicker) should be prevented. The 60 Hz waveform can affect the LED drive voltage, so AC line filtering is imperative. Filtering will also limit propagation of the conductive EMI back onto the input waveform.

The cheapest approach to achieving dimming, that requires minimal components, is to insert an analogue voltage or current in the driver feedback path. Drawbacks include:

Compromised loop stability in a PWM SMPS Oscillations/ringing during voltage transients leads to LED flickering Analog dimming reduces the current through the LED, causing colour changes during dimming (e.g. a blue-white LED at full current but yellow-white at low currents)

It is possible to keep the LED colour constant over the dimming range by using digital dimming (e.g., PWM). The current through the LEDs remains constant but the LEDs appear to dim as they are lit for a portion of the time. This is achieved by turning the LED strings on or off by “shorting” them with a parallel FET, or by disconnecting them from ground by paralleling a FET with the current sense resistor.

In conclusion, PFC-boost driven HVHB LEDs, provide a viable alternative to the CFL lighting solutions that are currently available. The benefit of a simple, small size, low lost design with high PFC and efficiency ensures a competitive lighting solution.

Further reading

Everlight Electronics Co. Ltd. provides additional information on HVHB LEDs,

For more information about TI’s LED solutions go to: www.ti.com/led-ca.

About the author:

Dave Priscak is a System Applications Manager with the Worldwide End-Equipment Solutions Marketing Team at Texas instruments. Dave received his BSET degree from ETI Technical College, Cleveland, Ohio/USA.