EEbeat

In today’s green-centric world, designers and manufacturers are constantly looking for ways to improve energy efficiency in their products. From cell phone manufacturers to PC makers, new industry standards are emerging that will significantly impact the rate of power consumption in electronic devices. In the lighting industry, tremendous energy savings can also be realized with a change from incandescent or fluorescent technologies to LED lighting.

Many national governments, including those of the U.S., China, Japan, Taiwan, and South Korea, UK, Germany, Ireland and Australia are actively promoting the development of LED lighting. In
China alone, authorities estimate that by moving one-third of their lighting market to LEDs, they will save 100 million kilowatt hours of electricity and reduce carbon dioxide emissions by 29 million tons each year.

LED technology has become a serious candidate for lighting applications thanks to the improvement in the lumens-per-watt of high-brightness LEDs. In addition to the environmental benefits of moving to a more efficient alternative, LEDs are becoming increasingly desirable due to rising energy costs. Along with very low energy consumption, LED lighting offers long life and low maintenance. Back on the green theme, LEDs are completely free of dangerous or environmentally-harmful chemicals.

As Figure 1 shows, the expected growth in LED utilization offers spectacular opportunities for lighting and power supply manufacturers. 
As LED lighting moves into mainstream use, government agencies are firming up their specifications to ensure that the expected energy cost improvements are realized. Efficiency is the key driver in successfully meeting these standards.

Several different power supply implementations are needed for LED lighting. The power required can range from 0.5 W to over 50 W, and isolated supplies with dimming features are needed for external supplies. A range of power supply designs for LED applications has been developed by the California chip supplier Power Integrations (PI) utilizing its 700 V integrated MOSFET technology.

An LED luminary generally contains a number of LEDs, usually connected in series. Higher power lamps can include several series strings of LEDs that are then connected in parallel. In all cases, the LED load must be driven by a constant current supply. Other power supply requirements include protection against short- and open-circuit fault conditions, overheating and minimal generation of harmonics and electromagnetic compatibility (EMC). Table 1 lists the PI product families most appropriate for LED driving, highlighting their suitability for given applications.

PI’s LinkSwitch-II(1) product family offers many features aimed at producing the ideal LED driver for loads up to 10 W, 36 V, 350 mA for isolated configuration, or up to 15 W in tapped buck configuration. The device incorporates a 700 V power MOSFET, a novel On/Off control state machine, a high-voltage switched-current source for self biasing, frequency jittering, cycle-by-cycle current limit, and hysteretic thermal shutdown circuitry.

As described below, two LinkSwitch-II LED driver applications clearly illustrate the challenges of producing high-performance solutions in extremely cost-sensitive devices. Both applications are constant current/constant voltage (CV/CC) circuits capable of producing up to 4.2 W at 12 V. The first example is an isolated flyback converter ideal for external drivers, and the second is a low component count buck converter suitable for fitting within the lamp envelope. Figure 2 shows the schematic for the flyback converter. This application is described in PI Design Idea DI-185(2).

The circuit operates by switching the DC voltage presented at terminal 1 of transformer T1 through primary winding (Pin 1 – Pin 4) via the integrated 700 V MOSFET in the LinkSwitch-II controller U1. U1 applies a combination of adjustments to both the switching frequency and primary current limit, in addition to skipping switching cycles to achieve either CV or CC control. Tight feedback control is achieved via the FB input connected to the bias winding (Pin 2 – Pin 3). As the output voltage across winding (Pin 6 – Pin 10) increases, a corresponding increase in flyback voltage occurs across the bias winding. Each switching cycle, this voltage is sampled by U1 2.5 µs after turnoff of the high-voltage switch. In addition to eliminating the optocoupler and other feedback circuitry, a key advantage of this control mechanism is that all manufacturing and temperature-dependent tolerances of the transformer are automatically compensated via adjustments to the switching frequency.

While operating in constant voltage mode, U1 maintains the output voltage level by skipping switching cycles as needed, and maintains regulation by adjusting the ratio of enabled cycles to disabled cycles. This also optimizes efficiency of the converter over the entire load range. At light loads, the current limit is reduced to decrease the transformer flux density, which reduces audible noise and switching losses. Any increase in load demand beyond 350 mA causes the circuit to transition to constant current mode. The current demand above 350 mA creates a drop in output voltage. U1 detects this drop and responds by linearly reducing the switching frequency, hence ensuring constant output current.

Figure 3 is the schematic for the buck converter. This design provides the same CV/CC output characteristics as the flyback converter in Figure 2, yet uses extraordinarily few components (16). The complete application is small enough to fit comfortably within the base of a light bulb.
 
In this circuit, when U1 turns on, current ramps up and passes through the load and the inductor (T1). The current continues to ramp up until it reaches U1’s current limit, then U1 turns off. When the switch turns off, the energy stored in the inductor induces a current to flow in the output section (Pin 8 – Pin 7). The current in the output winding steps up by a factor of 4.6 (the turns ratio) and flows from the output winding through freewheeling diode D1 and to the load. The bias winding (Pin 5 – Pin 6) provides feedback control, as before. Further details of the circuit operation are published in PI Design Idea DI-186(3).

Despite its small size and simplicity, the circuit is capable of exceptional performance, and no compromise has been made in security or reliability. Figure 4 compares the full load efficiency over input voltage range for a tapped buck circuit vs. a flyback converter circuit.

 
Arguably the most important feature of both circuits is full compliance with EMI standards. For LED lights to be used in the U.S. and Europe, EMI specifications must be met. LinkSwitch-II incorporates automatic frequency jittering that simplifies EMI filtering and reduces cost. The flyback circuit also employs additional winding in the transformer (using PI’s E-Shield™ technology) that eliminates the need for copper shielding. Both circuits meet EN55015 and CISPR 22 Class B EMI standards with 10 dB margin.

Other features provided by LinkSwitch-II include auto-restart protection for output short-circuit and control loop faults, hysteretic thermal shutdown and no load consumption <200 mW at 265 V AC. These features enable the system designer to produce a highly efficient driver solution for any LED application.

By Silvestro Fimiani, Product Marketing Manager
Power Integrations, Inc.

 References
1. LNK603-606/613-616 LinkSwitch-II Family. Energy Efficient, Accurate CV/CC Switcher for Adapters and Chargers. Power Integrations Inc.
2. DI-185 Design Idea. LinkSwitch-II. Low Component Count, Isolated 350 mA, 4.2 W LED Driver. Power Integrations Inc.
3. DI-186 Design Idea. LinkSwitch-II. High Efficiency Low Cost, Non-Isolated 350 mA, 4.2 W LED Driver. Power Integrations Inc.