NCL30002
http://onsemi.com
12
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 +
P
in
(avg)
V
in
(rms) I
in
(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 (I
in
=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 (T
on
= constant) - This is the
same as the boost converter. Constant T
on
forces
the peak current to be proportional to the input
voltage which is key to improved PF.
3. Constant Peak Current (I
peak
= 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.
Figure 23. Theoretical Average Input Current over one half line cycle (conduction angle)
Mode 3 I
peak
= Constant
Mode 2 t
on
= Constant
Mode 1 Input Current = 0
