NCL30000 데이터 시트보기 (PDF) - ON Semiconductor

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NCL30000

Power Factor Corrected Dimmable LED Driver

ON Semiconductor 

NCL30000

THEORY OF OPERATION

High power factor requires generally sinusoidal line

current and minimal phase displacement between the line

current and voltage. Normally this is not the case with a

traditional isolated flyback topology so the first step to

achieve high power factor is to have minimal capacitance

before the switching stage to allow a more sinusoidal input

current. A simplified block diagram is illustrated in

Figure 23. Since the input bulk capacitor has virtually been

eliminated except for a small capacitor, the voltage to the

flyback converter now follows a rectified sine shape at twice

the line frequency. By employing a critical conduction mode

control technique such that the input current is kept to the

same shape, high power factor can be achieved. The

NCL30000 is a voltage mode, fixed on-time controller

specifically intended for such applications.

AC

Line

Input

EMI

Filter

Primary

Ï

Secondary

Zero

Current

Detect

& Bias

Winding

NCL30000

Controller

CC/CV

Control

Figure 23. Simplified Block Diagram

Since the input voltage waveform to the flyback is

sinusoidal, with a fixed on-time control scheme, the current

through the transformer primary will increase directly with

the line voltage and the average current drawn from the line

will have a sinusoidal shape. When the switch is turned off

the energy from the primary will be transferred to the

secondary. By monitoring the auxiliary winding the

controller can detect when the secondary current reaches

zero and restart the switching cycle to transfer additional

energy to the load. The current in the primary of the

transformer starts at zero each switching cycle and is directly

proportional to the applied voltage times the on-time. One

of the primary benefits of this CrM approach is that we can

operate with zero current switching which results in a very

efficient architecture for low to medium power applications.

A secondary side control loop monitors the average LED

current and adjusts the on-time to maintain proper

regulation. To achieve high power factor, the control loop

bandwidth must be sufficiently low such that the on-time is

constant across a line half cycle. Since the off time varies

depending on the energy transferred through the transformer

and the load, the switching frequency varies with load and

line. Figure 24 illustrates the theoretical current waveform

through the primary and secondary transformer windings.

The energy delivered to the load through the transformer

will follow the product of voltage and current which is a

sine-squared shape. As a result of this sine-squared energy

transfer, the load will experience ripple at twice the line

frequency, either 100 or 120 Hz depending on the source.

The delivered power through the transformer starts at zero,

rises to a peak and returns to zero following the shape of the

rectified input line. The 100/120 Hz ripple is superimposed

on the normal switching waveform of the PWM converter.

The maximum on-time must be set such that the maximum

power is delivered at the minimum required operation

voltage. The LED current required for a particular

application is generally specified as an average value. LEDs

can tolerate ripple current as long as the ripple frequency is

above the visible range of the human eye and the peak

current does not exceed the rating of the LEDs. Just like a

standard flyback, the output capacitors filter the pulsing

power from the transformer to match the average current

required by the LED and must be sized appropriately to limit

the peak current through the LEDs.

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