As a solid state light source, light-emitting diodes (LEDs) have been widely used due to their superior longevity, excellent efficacy, and friendly environmental attributes.
Today, LEDs are replacing existing lighting sources such as incandescent lamps, fluorescent lamps, and HID lamps. To light LEDs, they need be operated with constant current and the ballast must have a high power factor. While new EnergyStar directives for solid-state lighting require a power factor greater than 0.9 for power levels above 3W, the ballast input line current harmonics also need to meet the requirement set by IEC61000-3-2 Class C regulations.
To achieve these LED lighting application requirements, a single stage Flyback converter with PFC is typically used for low power (<25W) LED lighting application. Furthermore, among various circuitries of Flyback topology, a Primary-Side-Regulated (PSR) Flyback can be the most cost-effective solution. By using single-stage topology with primary-side regulation, an LED lighting board can be implemented with few external components and minimized cost, without requiring an input bulk capacitor and feedback circuitry. Figure 1 shows a single-stage PSR Flyback LED driver circuit.
Generally, discontinuous conduction operation mode (DCM) is preferred for primary-side regulation because it offers very precise output regulation . To achieve high power factor and low total harmonic distortion (THD), constant on-time control usually is adopted for DCM Flyback converter with fixed switching frequency. Figure 2 shows the typical theoretical waveforms of the primary-side switch current, the secondary-side diode current and MOSFET switch gate signal.
With constant on-time, the average input current can be represented by following equation.
Here, D is the switching duty cycle of the converter, Lm is the Flyback transformer primary winding inductance. The above equation indicates that the input current waveform is always following the input voltage. Therefore, the converter achieves unity power factor.
The rms input current then can be given as:
To stay in DCM, the maximum duty cycle D has to be :
VR is the reflected voltage that occurs across the primary side of the transformer while the secondary diode is conduction.
To guarantee the Flyback converter operates in DCM for unity power factor and low THD, a transformer with a relatively small turn ratio usually is used. Such a Flyback transformer will lead to a small switching duty cycle resulting in higher peak and RMS current through the MOSFET switch and transformer, therefore more power losses. A relatively large EMI filter also will be required due to high peak switching current.
With its zero-voltage turning-on nature that minimizing the switching losses, Flyback with boundary conduction operation mode (BCM) is often used as a single-stage PFC converter. The operation principles of single-stage PFC Flyback converter with BCM operation are presented in detail in Reference . Unlike DCM operation, the BCM Flyback is controlled with constant on-time and variable switching frequency. The BCM Flyback for PFC is usable for many applications that need a relatively high PF but not a low THD lower than 10%. Below Figure 3 shows its theoretical primary-side switches current, the secondary-side diode current and gating signal.
As Reference  explained in detail, the average input current can be expressed as below:
The rms input current is given as:
It is quite disappointing that the denominator of the above input current equation makes the current shape clearly non-sinusoidal, unless the ratio:
is made made very small. Figure 4 below shows the input current shape of BCM Flyback with RVR as a parameter . Harmonics analysis of the input current shape shows that it is difficult to get a THD lower than 10% with an RVR of 2.
Figure 4: Shape of the input current of BCM Flyback with RVR as a parameter.
During the off-time of the switcher, the maximum voltage across the switcher is the peak input voltage plus the reflected voltage VR . So, due to the voltage rating limit of MOSFET switcher, the feasible values for RVR are only in the range of 1 for US and 2 to 3 for European input voltages. For lighting application with universal input voltage, to achieve a relatively low THD, an 800 V or even a 1000 V MOSFET must be used to allow for a small enough RVR ratio. Its switching frequency also can go very high, especially for LED dimming application under high input AC voltage.
A careful review of the equations above lead to the following conclusions:
1. The input voltage as a reference for the MOSFET peak drain current is not needed. If the on-time is made constant over one half-cycle, the peak drain current will follow the input voltage.
2. The main reason for the non-ideal shape of the input current is the variable frequency or rather the variable duty-cycle. With the identical drain current shape, if the duty-cycle were kept constant over a half-cycle, the input current would be sinusoidal.
Want to explore Flyback LED Driver operation basics in minutes? Try the on-line simulation of our Power Supply WebDesigner tool (You will have to register)
Want to learn more about Flyback with PFC? Download our applications notes:
. Fairchild Semiconductor Inc. AN-9750 High-Power Factor Flyback Converter for LED Driver with FL7732 PSR Controller .
. Michael Weirich, Fairchild Semiconductor Inc. AN-9750 A High Power Factor Flyback with Constant Current Output for LED Lighting Application . (You will have to register)