Power management

The Challenges of Choosing Off-Line LED Driver Topologies

When considering LED driver topologies to convert AC input voltage to a constant regulated current source for the LED loads, it is helpful to view LED applications in three general power levels: low-power applications which require less than 20W input, mid-power up to 50W; and high-power requiring higher than 50 W input. The designer faces a different mixture of challenges from cost, space, efficiency, design complexity, power factor, and reliability - to name but a few. Basic topologies within these basic power ranges are recommended to satisfy the design challenges.

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The low-power solutions targeting small lamp sizes require small design volumes to fit the LED driver, consistent light radiation by controlling the current through the LED, efficiency, and low cost. In order to be compliant with Energy Star Program Requirements for Luminaires, the light fixture must have a power factor ≥ 0.7 for residential applications and ≥ 0.9 for commercial applications for input power greater than 5 watts. [1] If isolation from the LED driver is not required, a buck regulator topology is the lowest BOM and therefore a low cost solution to consider. Figure 1 is a non-isolated buck topology example which also includes power factor correction and dimming capability. There is only one magnetic component which is a simple inductor and a single MOSFET/Diode pair for buck power conversion. This topology choice is best selected when the input voltage is larger than the output voltage required from the LED load.

Figure 1 Non-isolated Buck Converter with PFC

When isolation from the LED driver is required, a good topology choice is the primary-side regulated (PSR) flyback topology; Figure 2 is an example of a PSR flyback LED driver. Reduced cost is achieved by not requiring secondary side feedback so the topology has a smaller component count to achieving good constant current regulation. The MOSFET can be integrated into the controller to reduce BOM count and reduce the printed wiring board space. The reliability of the PSR flyback is improved without the use of an opto-isolater for secondary feedback.

Figure 2 Primary-Side Regulated Converter

Discontinuous Conduction Mode (DCM) is the preferred operating mode for the PSR flyback topology since it allows for better output regulation. The typical waveforms are shown in Figure 3.

Figure 3 Waveforms of DCM Flyback Converter

When operating a PSR LED driver in constant voltage regulation mode, during the inductor current discharge time TDIS, the sum of the output voltage and diode forward-voltage drop is reflected to the auxiliary winding side. Since the diode forward-voltage drop decreases as the current through the output diode decreases, the auxiliary winding voltage reflects the output voltage at the end of diode conduction time TDIS. By sampling the auxiliary winding voltage at the end of the diode conduction time, the output voltage information is obtained.

When operating in constant current regulation mode, the output current can be estimated using the peak drain current IPEAK and the inductor current discharge time TDIS since the output current is the same as the average of the diode current in steady state. With Fairchild's innovative TRUECURRENT™ technology, constant current output can be precisely controlled.

Efficiencies for a PSR topology can achieve 85%. As an example, consider an 8.4 watt application, the total power loss in the LED driver was measured at 1.32 watts at 85 VAC input. The largest contributors to the loss budget start with the transformer estimated at 0.55 watts loss followed by the snubber (Shown in Figure 2 with the series combination diode connected to the parallel resistor and capacitor across the transformer primary winding) at 0.31 watts, the MOSFET at 0.26 watts, and the output rectified and bridge rectifiers total together 0.20 watts. [2] The transformer and snubber are generally the worse case power dissipation components and it is the leakage inductance from the transformer that requires a snubber to prevent voltage stressing the MOSFET. The printed wiring board and input EMI filter can also become significant sources of power dissipation sources if attention is not paid to these two areas of the design.

The total loss of 1.32 watts may not seem like a significant source of power loss but in a low-power LED driver, the LED loads are close to the driver so it is the total load power plus driver losses heat the design. Heat transfer does not have as an option forced cooling airflow so the example cited would have to use a light fixture capable of conducting efficiently 8.4 watts away from the semiconductor and electrical devices in order to maintain reliability. The use of electrolytic capacitors can lower the mean-time-to-failure of the design if the thermal solution is not efficient in removing this power to keep component temperatures low.

Mid-power solutions still require small design volumes and power factor correction is required. Efficiency and reliability are also important design constraints in this power range. A good topology to use is the single stage power factor correction flyback topology as shown in Figure 4.

Figure 4 Single-Stage PFC Flyback Converter

A one stage design reduces component count and does not require the input bulk capacitor saving not only volume design space but reduces cost as well. The flyback is controlled for power factor correction and secondary feedback is present. With these mid-power flyback topology designs, efficiencies up to 84% can be achieved. Since the topology is a flyback, the transformer and use of a snubber are still the major power dissipation losses in this LED driver design. The higher power level in this mid-power range increases the snubber power loss since snubber loss is proportional to the transformer leakage inductance times the square of the peak current in the MOSFET. In this mid-power design, the transformer is increasing in size and the peak current in the MOSFET is increasing.

High-power solutions focus on the best efficiency and reliability at reasonable cost using a low BOM count. A two-stage LED driver is recommended. The first stage is used for power factor correction followed by a DC-DC conversion stage to regulate the constant current output. The first stage can be the same PFC single stage flyback converter as used in the mid-power range. In order to reduce component cost in this two-stage approach, the controller integrates components and features on the first stage.

There are two recommended second stage DC-DC converter choices: a quasi-resonant (QR) flyback for under 100 watt applications or the LLC topology above 100 watt applications. A flyback achieves reasonable efficiency and is a less complex topology versus the LLC topology choice. The QR topology reduces switching losses associated with the MOSFET output capacitance by reducing the turn-on starting capacitance voltage. The QR topology MOSFET soft switching also reduces EMI. However, for the LLC topology, better efficiency is due to the MOSFET zero voltage switching and a small holdup capacitor can be used. Up to 92% efficiency can be achieved in this two-stage approach. Figure 5 and Figure 6 show the QR and LLC topologies.  Note the LLC circuit in Figure 6 uses the leakage and magnetizing inductance of the transformer to create the LLC resonant circuit.

Figure 5 Two-Stage PFC + QR Flyback Example

 Figure 6 Two-Stage PFC + LLC Example

The high-power applications generally use multiple strings of LEDs. Figure 6 shows the use of a secondary controller to balance current through the different LED string loads.

Conclusion

Different topology recommendations are presented in three different power ranges of off-line LED driver applications. In increasing LED load power levels; the buck, PFC single-stage flyback, and two-stage PFC flyback followed by either the QR flyback or LLC are proposed. Each one of these topology recommendations best satisfies constraints based on design space available to fit the LED driver, efficiency requirements, reliability, cost, and design complexity.

[1] United States Department of Energy, Energy Star Program Requirements Product Specifications for Luminaires (Light Fixtures) Version 1.0, Effective date: Oct 1, 2011

[2] Fairchild Reference Design RD-L015, 8.4W, FL103 Low Power PSR Flyback, http://www.fairchildsemi.com/applications/diagrams/lighting_low_power.html

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