NCL30001
www.onsemi.com
18
DETAILED DEVICE DESCRIPTION
Introduction
The NCL30001 is a highly integrated controller
combining PFC and isolated step down power conversion in
a single stage, resulting in a lower cost and reduced part
count solution. This controller is ideal for LED Lighting
applications with power requirements between 40 W and
150 W with an output voltage greater than 12 V. The single
stage is based on the flyback converter and it is designed to
operate in CCM mode.
Power Factor Correction (PFC) Introduction
Power factor correction shapes the input current of
off−line power supplies to maximize the real power
available from the mains. Ideally, the electrical appliance
should present a load that emulates a pure resistor, in which
case the reactive power drawn by the device is zero. Inherent
in this scenario is the freedom from input current harmonics.
The current is a perfect replica of the input voltage (usually
a sine wave) and is exactly in phase with it. In this case the
current drawn from the mains is at a minimum for the real
power required to perform the needed work, and this
minimizes losses and costs associated not only with the
distribution of the power, but also with the generation of the
power and the capital equipment involved in the process.
The freedom from harmonics also minimizes interference
with other devices being powered from the same source.
Another reason to employ PFC in many of today’s power
supplies is to comply with regulatory requirements. Today,
lighting equipment in Europe must comply with
IEC61000−3−2 Class C. This requirement applies to most
lighting applications with input power of 25 W or greater,
and it specifies the maximum amplitude of line−frequency
harmonics up to and including the 39
th
harmonic. Moreover
power factor requirements for commercial lighting is
included within the ENERGY STAR® Solid State Lighting
Luminaire standard regardless of the applications power
level.
Typical Power Supply with PFC
A typical power supply consists of a boost PFC
preregulator creating an intermediate X400 V bus and an
isolated dc−dc converter producing the desired output
voltage as shown in Figure 45. This architecture has two
power stages.
Figure 45. Typical Two Stage Power Converter
Rectifier
&
Filter
PFC
Preregulator
DC−DC
Converter
with isolation
AC
Input
V
out
A two stage architecture allows optimization of each
individual power stage. It is commonly used because of
designer familiarity and a vast range of available
components. But, because it processes the power twice, the
search is always on for a more compact and power efficient
solution.
The NCL30001 controller offers the convenience of
shrinking the front−end converter (PFC preregulator) and
the dc−dc converter into a single power processing stage as
shown in Figure 46.
Figure 46. Single Stage Power Converter
Rectifier
&
Filter
NCL30001 Based
Single−Stage
Flyback Converter
AC
Input
V
out
This approach significantly reduces the component count.
The NCL30001 based solution requires only one each of
MOSFET, magnetic element, output rectifier (low voltage)
and output capacitor (low voltage). In contrast, the 2−stage
solution requires two or more of the above−listed
components. Elimination of certain high−voltage
components (e.g. high voltage capacitor and high voltage
PFC diode) has significant impact on the system design. The
resultant cost savings and reliability improvement are often
worth the effort of designing a new converter.
Single PFC Stage
While the single stage offers certain benefits, it is
important to recognize that it is not a recommended solution
for all requirements. The following three limitations apply
to the single stage approach:
• The output voltage ripple will have a 2x line frequency
component (120 Hz for North American applications)
that can not be eliminated easily. The cause of this
ripple is the elimination of the energy storage element
that is typically the boost output capacitor in the
2−stage solution. The only way to reduce the ripple is to
increase the output filter capacitance. The required
value of capacitance is inversely proportional to the
output voltage. Normally the presence of this ripple is
not a issue for most LED lighting applications.
• The hold−up time will not be as good as the 2−stage
approach – again due to the lack of an intermediate
energy storage element.
• In a single stage converter, one FET processes all the
power – that is both a benefit and a limitation as the
stress on that main MOSFET is relatively higher.
Similarly, the magnetic component (flyback
transformer/inductor) can not be optimized as well as in
the 2−stage solution. As a result, potentially higher
leakage inductance induces higher voltage spikes (like
the one shown in Figure 47) on the MOSFET drain.
This may require a MOSFET with a higher voltage
