The ENERGY STAR® Regulation for LED drivers requires a Power Factor (PF) of 0.7 or better for residential applications and 0.9 for commercial applications.
This article describes a cost-effective single-stage power factor correction (PFC) circuit for LED drivers using Power Integrations’ TOPSwitch®-GX products. Using a single-stage approach in this design keeps the driver cost low and improves efficiency.
To understand the importance of PFC, consider this: A typical LED lamp has a PF of 0.5 and an input current with a high harmonic content. Incandescent lamps have a PF of 1. If all incandescent lamps in use today were suddenly replaced by their low-power, long-life LED equivalents, much more reactive power would be needed to drive the
LEDs because of their low PFs and associated high harmonic currents. This means periods of heavy load could cause black-outs.
ENERGY STAR foresaw this potential issue and created the Program Requirements for Solid State Lighting Luminaries1 which requires LED drivers to have a 0.7 PF for residential uses, and 0.9 PF for commercial applications. While the PF requirements remove the problem of potential power grid overload, they present the LED driver designer with new challenges. Not only must a power supply design be inexpensive and highly efficient, it must also comply with the new ENERGY STAR requirements.
PFC Correction – three approaches, presented here and commonly used for driving loads with low PFs, are: power supplies with valley-fill correction circuitry, dual-stage power supplies with front-end PFC followed by a high-voltage DC-to-DC stage, and single-stage flyback power supplies with minimal input capacitance which inherently give high PF. The valley-fill method is a passive PFC technique and provides very good power factor correction (0.9 or more is common). For power supplies delivering less than 20 W, this technique presents an inexpensive solution. The lossy nature of the valley-fill technique limits its applications to designs delivering below 20 W and only addresses the PF. It does not typically meet the harmonic requirements of standards such as EN61000-3-2.
To drive loads requiring more than 20 W, a dual-stage power supply with front-end PFC works well. These have better regulation, low ripple in the output, and exhibit better transient response than single-stage supplies or valley-fill solutions. The dual-stage supply eliminates harmonic currents and performs PFC in the front end (first stage). The power supply’s second stage converts the front end’s high output DC to the lower voltage needed by the LED load. However, this is a more costly solution. A dual-stage solution naturally requires more components. Dual-stage solutions should be used for loads needing higher bandwidth, better transient response, and tight regulation.
A third alternative, the single-stage power supply with PFC, drives loads requiring from 20 W to 75 W, provided the loads function well with lower bandwidth. Such loads must be of the type that provide a static load on the converter. When valley-fill and dual-stage solutions are inefficient or would not warrant the extra cost incurred, single-stage PFC design presents a viable alternative. One example of such a load would be an LED string, which is used here to illustrate the use of PFC in a power supply designed by Power Integrations.
Power Integrations’ Solution – provides a PF > 0.9, with high efficiency (> 85%), with a Single-stage PFC power supply design, using TOP250YN = Farnell Order Code 9921443.
No Input Capacitance - a flyback converter operating in discontinuous conduction mode (DCM) with a constant duty cycle naturally provides PFC because the input current’s envelope follows the input voltage. This occurs because in DCM, with no input capacitance to filter the rectified AC, the current envelope must follow the input voltage. In this design, the only input capacitance, provided by C3, is very low. Capacitor C4 is in place at startup, and D5 isolates it from the circuit once the power supply enters steady-state operation. Inductors L2, L3, and L4 provide necessary filtering; L2 provides common-mode filtering, and L3 and L4 provide differential-mode filtering as well as surge protection. Capacitor C4 is required to limit the maximum voltage across the DC bus. Operating in DCM results in higher peak drain currents and, therefore, an increase in the primary RMS current. To support the larger RMS current, this design uses a larger TOPSwitch device (the TOP259YN) than would have been
chosen without PFC, to ensure reduced RDSON-related conduction losses, providing higher efficiency and reduced power dissipation.
Input Current – holding the converter duty cycle constant over the AC line cycle in this design is an important factor in achieving PFC. Given that the voltage across the transformer’s primary side is described by the equation V = L x (di/dt), by keeping both the primary inductance and the duty cycle fixed, the peak primary current stays proportional to the input voltage. This ensures the peak drain current envelope follows the voltage as closely as possible, providing a high PF.
Keeping the operating duty cycle constant requires holding the current into U1’s Control (C) pin constant. This can be done easily by choosing a sufficiently large value for C5. However, the value needed for high PF also increases the start-up time. The increased start-up time causes the LED load to blink as the supply goes through a restart cycle, instead of turning on cleanly. To resolve this, C5 is kept small to give acceptable startup performance while a discrete circuit formed by emitter-follower transistor (Q1) and its associated components transforms the effective impedance at U1’s C Pin to keep the current constant over an AC cycle (needed for high PF). Looking into the emitter of Q1, C10 looks larger (C10 x Q1hfe) and R6 looks smaller (R6/Q1hfe) than they actually are.
Dominant Pole Correction for Bandwidth Limiting - resistor R7 performs loop compensation by creating a zero at approximately 200 Hz, giving additional phase at frequencies of approximately 20 Hz and above, for better phase margin at gain crossover (35 Hz to 40 Hz). The intentional bandwidth limitation in this design (and all designs using PFC) keeps the input current’s third harmonic (180 Hz) low and keeps the PF high. Diode D8 prevents reverse currents through Q1 during startup. Optocoupler U2B provides feedback from the secondary, modulating the base voltage of Q1 and changing the current into U1’s control pin as needed to maintain output regulation.
Additional Design Notes
Using Power Integrations’ solution centered on the TOP250YN enables designers to easily meet high-PFC requirements. The design proposed here provides a high PFC, as well as other advantages; resistors R11, R12, R13, Q2, Q3, and Q4, with the LED in U2, form a low-drop CC-sensing circuit, and program the average output current to stay at 3.1 A (+10%). Resistor R16 and VR2 limit the output voltage when the load is removed. Capacitor C5 and diode D8 isolate U1 from the rest of the circuit’s feedback components before regulation is reached. An optional soft-start circuit consisting of D12, C15, R18, R19, R20, C16, and Q5 may be used to charge C10 before the output reaches regulation and to prevent output overshoot. Diodes D10 and D11 rectify the secondary winding voltage. Parallel secondary windings and diodes distribute dissipation and improve efficiency.
For more information about this design, refer to DER-136 on the Power Integrations website www.powerint.com.
Features & benefits:
- Softstart feature
- External current limiting, programmable
- Line OV/UV shutdown, programmable
- Thermal shutdown
- Eliminates/reduces external discrete components
- Saves costs on transformer & other power components
- High- efficiency universal power supply units & adapters
- Multi-output power supply units
- Processor controlled power lines
Product information table:
|Mftrs. Part No.||Product Description|
OFF LINE SWITCHER, DIP8, 242
OFF LINE SWITCHER, DIP8, 244
OFF LINE SWITCHER 20W, DIP8, 245
OFF LINE SWITCHER, DIP8, 246
OFF LINE SWITCHER, TO-220-6, 247
OFF LINE SWITCHER, TO-220-6, 242
OFF LINE SWITCHER, TO-220-6, 243
OFF LINE SWITCHER, TO-220-6, 244
OFF LINE SWITCHER, TO-220-7, 259
OFF LINE SWITCHER, TO-220-6, 245
OFF LINE SWITCHER, TO-220-6, 246
OFF LINE SWITCHER, TO-220-6, 249
OFF LINE SWITCHER 180W, TO-220