LED driver design steps with high power factor

Since LED lamps are widely used in a variety of applications, they are also available in a variety of shapes and sizes to suit a particular fixture. In addition, LED drivers should be designed to accommodate a wide range of LEDs and custom specifications. Because of this, the LED voltage may vary over a wide range depending on the number of LEDs connected in series or temperature. However, regardless of the design, the LED current should be kept constant because the total lumen is proportional to the current. At the same time, high power factor (PF) and low total harmonic distortion (THD) are key design requirements for LED drivers. As a result, drivers for a wide range of output voltages help increase flexibility and are compatible with a wide range of LED characteristics. This article describes a PWM controller with integrated primary-side regulation (PSR) technology and a single-stage flyback converter design guide for a wide range of output voltages.

1. Primary side regulation controller and its operating mode

The primary driver side (PSR) solution for LED drivers enables solid state lighting (SSL) products to comply with international regulations such as Energy Star. The PSR precisely controls the output current based solely on the information at the primary side of the power supply, eliminating the output current sensing losses and eliminating the need for a secondary feedback circuit. This allows the use of driver circuits in small retrofit luminaires as well as compliance with international regulations without excessively increasing the cost of SSL applications. Fairchild's FL7733 Pulse Width Modulation (PWM) PSR controller helps simplify design to meet SSL requirements without the need for external components. The FL7733 provides high-precision output current regulation to handle changes in transformer magnetizing inductance, input and output voltage information, and provides powerful protection for system reliability.

Figure 1. Primary-side regulated flyback converter and critical waveforms

Mode I

During the MOSFET turn-on (tON), the input voltage (VIN) is applied across the primary inductance (Lm) of the transformer. Then, the MOSFET's leakage current (IDS) increases linearly from zero to the peak value (IDS.PK), as shown in Figure 1. During this time, electrical energy is taken from the input and stored in the inductor.

Mode II

When the MOSFET (Q) is turned off, the stored energy in the transformer forces the rectifier diode (D) to turn on. When the diode is turned on, the output voltage (VOUT) and the diode forward voltage drop (VF) are applied to the secondary side of the transformer, and the diode current (ID) linearly decreases from the peak (IDS.PK·NP/NS) to zero. At the end of the inductor current discharge time (tDIS), all energy stored in the transformer is transferred to the output.

Mode III

When the diode current reaches zero, the transformer auxiliary winding voltage begins to oscillate due to the resonance between the primary side inductance (Lm) and the effective capacitance loaded on the MOSFET (Q).

The output current can be estimated by the peak leakage current and the inductor current discharge time because the output current is the same as the average of the diode current at steady state. The leakage current peak is determined by the CS peak voltage detector, and the inductor current discharge time is detected by the tDIS detector. Based on peak leakage current, inductor current discharge time, and duty cycle information, the innovative TRUECURRENT calculation module estimates the output current as follows:

(1)

(2)

Figure 2. DCM control

The DCM should be guaranteed to achieve a high power factor in the flyback topology. To maintain DCM over a wide range of output voltages, the switching frequency is linearly adjusted by the output voltage in linear frequency control. The output voltage is sensed by the auxiliary winding and a resistor divider connected to the VS pin, as shown in Figure 2. As the output voltage decreases, the secondary diode turn-on time increases and DCM control extends the switching period to keep the DCM running over a wide range of output voltages.

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