NCP1606
http://onsemi.com
9
Introduction
The NCP1606 is a voltage mode power factor correction
(PFC) controller designed to drive cost effective
pre−converters to meet input line harmonic regulations.
This controller operates in critical conduction mode
(CRM) for optimal performance in applications up to about
300 W. Its voltage mode scheme enables it to obtain unity
power factor without the need for a line sensing network.
The output voltage is accurately controlled by a high
precision error amplifier. The controller also implements a
comprehensive array of safety features for robust designs.
The key features of the NCP1606 are as follows:
• Constant on time (Voltage Mode) CRM operation.
High power factor ratios are easily obtained without
the need for input voltage sensing. This allows for
optimal standby power consumption.
• Accurate and Programmable On Time Limitation. The
NCP1606 using an accurate current source and an
external capacitor to generate the on time.
• High Precision Voltage Reference. The error amplifier
reference voltage is guaranteed at 2.5 V ±1.6% over
process and temperature. This results in very accurate
output voltages.
• Very Low Startup Consumption. The circuit
consumption is reduced to a minimum (< 40 mA)
during the startup phase which allows fast, low loss,
charging of V
CC
. The architecture of the NCP1606
gives a controlled undervoltage lockout level and
provides ample V
CC
hysteresis during startup.
• Powerful Output Driver. A −500 mA / +800 mA totem
pole gate driver is used to provide rapid turn on and
turn off times. This translates into improved
efficiencies and the ability to drive higher power
MOSFETs. Additionally, a combination of active and
passive circuitry is used to ensure that the driver
output voltage does not float high while V
CC
is below
its turn on level.
• Programmable Overvoltage Protection (OVP). The
adjustable OVP feature protects the PFC stage against
excessive output overshoots that could damage the
application. These events can typically occur during
the startup phase or when the load is abruptly
removed. The NCP1606B gives a lower OVP
threshold, which can further reduce the application’s
standby power loss.
• Protection against Open Loop (Undervoltage
Protection). Undervoltage protection (UVP) disables
the PFC stage when the output voltage is excessively
low. This also protects the circuit in case of a failure in
the feedback network: if no voltage is applied to FB
because of a bad connection, UVP is activated and
shuts down the pre−converter.
• Overcurrent Limitation. The peak current is accurately
limited on a pulse by pulse basis. The level is
adjustable by modifying the switch sense resistor. The
NCP1606B uses a lower overcurrent threshold, which
can further reduce the application’s power dissipation.
An integrated LEB filter reduces the chance of noise
prematurely triggering the overcurrent limit.
• Shutdown Features. The PFC pre−converter can be
easily placed in a shutdown mode by grounding either
the FB pin or the ZCD pin. During this mode, the I
CC
current consumption is reduced and the error amplifier
is disabled.
Application information
Most electronic ballasts and switching power supplies
use a diode bridge rectifier and a bulk storage capacitor to
produce a dc voltage from the utility ac line (Figure 20).
This DC voltage is then processed by additional circuitry
to drive the desired output.
Figure 20. Typical Circuit without PFC
Load
ConverterRectifiers
Bulk
Storage
Capacitor
+
AC
Line
This simple rectifying circuit draws power from the line
when the instantaneous ac voltage exceeds the capacitor
voltage. Since this occurs near the line voltage peak, the
resulting current draw is non sinusoidal and contains a very
high harmonic content. This results in a poor power factor
(typically < 0.6) and consequently, the apparent input
power is much higher than the real power delivered to the
load. Additionally, if multiple devices are tied to the same
input line, the effect is magnified and a “line sag” effect can
be produced (see Figure 21).
Figure 21. Typical Line Waveforms without PFC
Line
Sag
Rectified DC
AC Line Voltage
AC Line Current
0
0
V
pk
Increasingly, government regulations and utility
requirements necessitate control over the line current
harmonic content. To meet this need, power factor
correction is implemented with either a passive or active
circuit. Passive circuits usually contain a combination of
large capacitors, inductors, and rectifiers that operate at the
ac line frequency. Active circuits incorporate some form of
