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NCL30002

Power Factor Corrected Buck LED Driver

ON Semiconductor 

NCL30002

THEORY OF OPERATION

High power factor, high efficiency, and small size are key

parameters for LED drivers in the incandescent replacement

market. The NCL30002 has all the features required to

accomplish that is in a compact SOIC-8 package. Power

factor is broadly defined as:

PF

+

Pin(avg)

Vin(rms) Iin(rms)

This differs from the classical definition where there is a

phase angle difference between the input voltage and

current. However, the underlying concept of optimizing

power delivery by minimizing line current is the same.

Ideally, current would be directly proportional to the voltage

which is the case when the load is a resistor. Offline power

converters are active devices which are not purely resistive,

capacitive, or inductive often drawing distorted current

waveforms from the power lines. This distortion reduces

power factor by increasing input RMS current.

Preregulators using boost converters are the most common

method to correcting the distortion and making the offline

power supply appear to be a resistor as far as the power line

is concerned. Their performance is excellent achieving

power factor greater than 0.99. Regrettably, this two stage

approach negatively impacts efficiency and board area.

Fortunately, power factors greater than 0.9 are acceptable in

the general lighting market and in some applications like US

Energystart Integral LED bulbs, the minimum acceptable

power factor is 0.7. So a certain amount of distortion can be

accepted while maintaining high power factor. This buck

topology meets the requirements for PF greater than 0.9 and

regulate LED current in a single power stage. Unlike the

boost converter, the NCL30002 buck controller operates in

several different modes over the line cycle.

Buck Modes

1. “Zero” Input Current (Iin=0) - Buck converters

cannot deliver power when Vin is less than Vout.

The “dead time” where no current flows around

the zero crossing is dependent on the line voltage

and the load voltage.

2. Constant On-Time (Ton = constant) - This is the

same as the boost converter. Constant Ton forces

the peak current to be proportional to the input

voltage which is key to improved PF.

3. Constant Peak Current (Ipeak = constant) – The

NCL30002 limits the peak inductor and thus the

LED current. In this region, the unique nature of

the CrM buck means that the average output

current is equal to half the peak current. Also the

off time is fixed is this mode since the peak current

and the output voltage are virtually constant.

In the example below (Figure 23) in spite of the distortion,

the power factor is 0.97. The corresponding pre-filtered

output current is shown in Figure 24.

Mode 3 Ipeak = Constant

Mode 2 ton = Constant

Mode 1 Input Current = 0

Figure 23. Theoretical Average Input Current over one half line cycle (conduction angle)

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