NCL30000
http://onsemi.com
11
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.
EMI
Filter
AC
Line
Input
CC/CV
Control
NCL30000
Controller
Zero
Current
Detect
& Bias
Winding
Primary Secondary
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.
