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NCL30051
PFC Output Capacitor - Cbulk
The bulk capacitor is one of the most critical components
in the PFC design. High value, high voltage capacitors are
expensive and take up a large space. In traditional PFC
applications, the voltage rating of this capacitor is about
450 V (some designers cut it to 420 V for cost savings), but
for 277 Vac lighting applications, 450 V rating is not
sufficient. As shown in the table above, if the output
voltage is allowed to vary, the bulk voltage can go even
higher. Availability of bulk capacitors above 450 V is
limited. One solution is to take two capacitors and put them
in series. The effective value of two series capacitors is
lower, but for low-medium power applications, this should
not be a big issue. For 600 V maximum bulk voltage, two
400 V capacitors need to be used, but for 90 ? 135 Vac only
applications, lower rated capacitors can be used. When
putting capacitors in series, it is required to have a parallel
high value resistor pair in order to ensure voltage sharing.
The effective bulk capacitance value also depends on the
application requirements. Normal rule of thumb for
traditional PFC circuits is to use around 1 m F/W to achieve
desired hold-up time and ripple performance. In this
approach, due to absence of a regulated second stage, it
may be prudent to increase the capacitance value if low
ripple or fast transient response is required. Another factor
in selecting the capacitor is that it handles high ripple
current due to the CrM topology implemented here. The
equations for ripple current through the capacitor are
derived in ON Semiconductor application note AND8123
and should be used to determine that the selected capacitor
can handle the ripple current without overheating or
lifetime degradation.
PFC Diode (D BST )
The PFC diode provides the rectification function and
has to be rated above the peak value of Vbulk. In the CrM
operation, with the diode current going to zero every cycle
prior to its turn-off, the reverse recovery is not that
prominent and an ultrafast diode can be used. In addition,
there is little or no overshoot caused by the reverse
recovery, so the FET voltage is also well contained. In most
cases a 600 V diode is sufficient depending on the derating
criteria.
PFC Switch (Q BST )
Typically, the PFC switch is a MOSFET rated anywhere
from 500 V to 650 V. Better commercial availability of
higher voltage rated FETs in recent years has meant that the
QBST is not a major constraint in implementation of
variable Vbulk approach offered by NCL30051. However,
depending on derating guidelines and practices, the 600 V
rating of the FET may not be sufficient. In that case, a
higher voltage FET is required.
PFC Inductor (L BST )
The PFC inductor is designed using the standard CrM
design equations. When the output voltage goes up from
390 V to 540 V, there is about 20% increase in value of
inductance required. Thus, variation in PFC voltage results
in higher boost inductor value and size (and/or higher
ripple current when the output voltage is higher).
HBR Converter Design
The half-bridge resonant converter utilizes an LLC
resonant circuit to achieve the ZVS of the primary switches
and also to reduce the transition losses in the secondary.
Additionally, this circuit offers a major benefit wherein the
output inductor can be eliminated.
In traditional LLC approaches, when the second stage
converter is regulated, the switching frequency of the HBR
converter has to be varied to respond to load or line
changes. As a result, operation near the resonant frequency
is not always guaranteed and the efficiency takes a hit.
Also, varying the frequency imposes additional design
burden on the designer to ensure that the control circuit is
stable and provides desired results over the full load and
line range. The feedback design and loop closure is more
challenging in this type of converter.
By keeping a constant switching frequency, not only is
the control circuit simplified, the magnetics design also
becomes easier. The transformer size can be reduced as it
is designed for a single frequency and full optimization is
available. Studies have shown that this approach leads to
about 25-40% reduction in total magnetics area-product.
Other design considerations for the LLC resonant
converter remain the same as given in ON Semiconductor
application note AND8311 and are not repeated here.
The power conversion architecture of the NCL30051 is
ideal for many LED Lighting applications since it provides
higher efficiency and power factor correction. Since
hold-up time and output ripple are not major considerations
in these applications, NCL30051 fits in very well. This
means that it is ideal for fixed output voltage LED power
supplies (ex: 24 Vdc and 48 Vdc) as well as constant current
schemes where the output voltage varies depending on the
number of LEDs and the variation of the LED forward
voltage. This topology is best suited for applications where
the output voltage variation is constrained to a ratio of
about 1.5 for designs that require operation at 230 Vac.
Supply Sequencing
The error amplifier of the PFC controller is enabled once
V CC reaches V CC(on) and the PFB voltage exceeds
V PUVP(high) , typically 290 mV. Once enabled, the PControl
voltage starts rising and when it exceeds V EA(OL) and V CC
is above V CC(enable) , the first PFC drive pulse is generated.
The half-bridge driver is enabled after the first PFC drive
pulse is generated. This ensures a monotonic output
voltage rise as the input voltage to the half bridge stage is
regulated.
In the event that V CC falls below V CC(enable) before the
control voltage exceeds V EA(OL) , the error amplifier will
remain on and V CC will fall to Vcc (OFF) at which time the
HV startup circuit will be enabled and a new startup
sequence will be initiated.
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