PFS7323-7329
Design, Assembly, and Layout Considerations
A suitable NTC thermistor should be used on the input side to
provide inrush current limiting. Choice of this thermistor should
be made depending on the inrush current specification for the
power supply. NTC thermistors may not be placed in any other
location in the circuit as they fail to limit the stress on the part in
the event of line transients and also fail to limit the inrush current
in a predictable manner. Example shown in Figure 14 shows
the circuit configuration that has the inrush limiting NTC thermistor
on the input side which is bypassed with a relay after PFC
start-up. This arrangement ensures that a consistent inrush
limiting performance is achieved by the circuit.
Power Table
The data sheet power table as shown in Table 2 represents the
maximum practical continuous output power based on the
following conditions:
For the universal input devices (PFS7323L-PFS7329H):
1. An input voltage range of 90 VAC to 264 VAC.
2. Overall efficiency of at least 93% at the lowest operating
voltage.
3. 385 V nominal output.
4. Sufficient heat sinking to keep device temperature ≤100 ºC.
Input EMI Filter
Operation beyond the limits stated above will require derating.
The variable switching frequency of the HiperPFS-2 effectively
modulates the switching frequency and reduces conducted EMI
peaks associated with the harmonics of the fundamental
switching frequency. This is particularly beneficial for the
average detection mode used in EMI measurements.
Use of a nominal output voltage higher than 390 V is not
recommended for HiperPFS-2 based designs. Operation at
voltages higher than 390 V can result in higher than expected
drain-source voltage during line and load transients.
The PFC is a switching converter and will need an EMI filter at
the input in order to meet the requirements of most safety
agency standards for conducted and radiated EMI. Typically a
common mode filter with X capacitors connected across the
line will provide the required attenuation of high frequency
components of input current to an acceptable level. The
leakage reactance of the common mode filter inductor and the
X capacitors form a low pass filter. In some designs, additional
differential filter inductors may have to be used to supplement
the differential inductance of the common mode choke.
HiperPFS-2 Selection
Selection of the optimum HiperPFS-2 part depends on required
maximum output power, PFC efficiency and overall system
efficiency (when used with a second stage DC-DC converter),
heat sinking constraints, system requirements and cost goals.
The HiperPFS-2 part used in a design can be easily replaced
with the next higher or lower part in the power table to optimize
performance, improve efficiency or for applications where there
are thermal design constraints. Minor adjustments to the
inductance value and EMI filter components may be necessary
in some designs when the next higher or the next lower
HiperPFS-2 part is used in an existing design for performance
optimization.
A filter capacitor with low ESR and high ripple current capability
should be connected at the output of the input bridge rectifier.
This capacitor reduces the generation of the switching frequency
components of the input current ripple and simplifies EMI filter
design. Typically, 0.33 mF per 100 W should be used for
universal input designs and 0.15 mF per 100 W of output power
should be used for 230 VAC only designs.
Every HiperPFS-2 family part has an optimal load level where it
offers the most value. Operating frequency of a part will
change depending on load level. Change of frequency will
result in change in peak-to-peak current ripple in the inductance
used. Change in current ripple will affect input PF and total
harmonic distortion of input current.
It is often possible to use a higher value of capacitance after the
bridge rectifier and reduce the X capacitance in the EMI filter.
Input Fuse and Protection Circuit
The input fuse should be rated for a continuous current above
the input current at which the PFC turns-off due to input under
voltage. This voltage is referred to as the brown-out voltage.
Regulatory requirements require use of a discharge resistor to
be connected across the input (X) capacitance on the AC side
of the bridge rectifier. This is to ensure that residual charge is
dissipated after the input voltage is removed when the
capacitance is higher than 0.1 mF. Use of CAPZero integrated
circuits from Power Integrations, helps eliminate the steady-
state losses associated with the use of discharge resistors
connected permanently across the X capacitors.
The fuse should also have sufficient I2t rating in order to avoid
nuisance failures during start-up. At start a large current is
drawn from the input as the output capacitor charges to the
peak of the applied voltage. The charging current is only limited
by any inrush limiting thermistors, impedance of the EMI filter
inductors, ESR of output capacitor and the forward resistance of
the input rectifier diodes.
Inductor Design
It is recommended that the inductor be designed with the
maximum operating flux density less than 0.3 T and a peak flux
density less than 0.39 T at maximum current limit when a ferrite
core is used. If a core made from Sendust or MPP is used, the
flux density should not exceed 1 T. A powder core inductor will
have a significant drop in inductance when the flux density
approaches 1 T.
A MOV will typically be required to protect the PFC from line
surges. Selection of the MOV rating will depend on the energy
level (EN1000-4-5 Class level) to which the PFC is required to
withstand.
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