LCS700-708
the PIXls spreadsheet. LPRI is the primary inductance of an
integrated transformer (high leakage inductance), or in the case
of the use of an external series inductance, the sum of this
inductance and the transformer primary inductance.
Key Design Details
The LLC converter is a variable frequency resonant converter.
As input voltage decreases, the frequency must decrease in
order to maintain output regulation. To a lesser extent, as load
reduces the frequency must increase. When the converter is
operating at the series resonant frequency, the frequency changes
very little with load. The minimum operating frequency required
occurs at brownout (minimum input voltage), at full load.
Leakage Inductance
The parameter KRATIO is a function of leakage inductance:
LPRI
LRES
KRATIO
=
- 1
The recommended KRATIO is from 2.5 - 7. This determines the
acceptable range of leakage inductance.
Operating Frequency Selection
For lowest cost, and smallest transformer size with the least
amount of copper, the recommended nominal operating frequency
is ~250 kHz. This allows the use of low-cost ceramic output
capacitors in place of electrolytic capacitors, especially at
higher output voltages (≥12 V). If the core and bobbin used
exhibits too much leakage inductance for 250 kHz, operation at
180 kHz also results in excellent performance. For optimal
efficiency at 250 kHz, AWG #44 (0.05 mm) Litz is recommended
for the primary, and AWG #42 (0.07 mm) for the secondary
winding. Thicker gauge lower cost Litz can be used at the
expense of increased copper loss and lower efficiency. Litz
gauge (AWG #38 or 0.1 mm) is optimal for very low frequencies
(60-70 kHz), requires much larger transformers and greater
lengths of Litz wire.
LRES is the leakage inductance in an integrated transformer; if a
separate series inductor is used, it is the sum of this inductance
and the leakage inductance of the transformer.
A low KRATIO (high leakage inductance) may not be capable of
regulation at the minimum input voltage, and may show
increased transformer copper losses due to the leakage flux. A
high KRATIO (low leakage inductance) will have high peak and
RMS currents at low-line, and require a lower primary inductance
to achieve ZVS operation over a suitably wide range, which
increases the resonant circulating current, reducing efficiency.
The core and bobbin designs available to the designer may limit
the adjustability of leakage inductance. Fortunately, excellent
performance can be achieved over a relatively wide range of
leakage inductance values.
For nominal operating frequencies even as low as 130 kHz, the
use of PC44 or equivalent core material is recommended for
reduced losses. For a given transformer design, shifting the
frequency up (by substituting a smaller resonant capacitor), will
reduce core loss (due to reduced AC flux density BAC) and
increase copper loss. Core loss is a stronger function of flux
density than of frequency. The increased frequency increases
copper loss due to eddy current losses.
The KRATIO directly affects the frequency range that the LLC needs
to operate in order to maintain regulation over the input voltage
range. Increasing KRATIO increases this frequency range,
lowering fMIN
.
A low fMIN is only a potential problem for low frequency designs
which typically run at higher nominal BAC. This may allow the
core to reach saturation when operating at fMIN. Operating at
fMIN occurs when the input voltage is at a minimal (input
brown-out).
Nominal operating frequencies >300 kHz start to lose significant
efficiency due to increased eddy current losses in the copper,
and due to the fact that a more significant percentage of time is
spent on the primary slew time (ZVS transition time) which
erodes the percentage of time that power is transferred to the
secondary.
For a design with a separate resonant inductor, running the
inductance on the low side of the range (KRATIO = 7), minimizes
the size and cost of the inductor.
Resonant Tank and Transformer Design
Please refer to the Application Note AN-55 for guidance on
using the PIXls HiperLCS spreadsheet which assists in the
entire design process.
Adjusting Leakage Inductance
Sectioned bobbins (separated primary and secondary) are
commonly used for LLC converters. Increasing or decreasing
both primary and secondary turns (while maintaining turns ratio)
will change the leakage inductance proportionally to the square
of primary turns.
Primary Inductance
The optimal powertrain design for the HiperLCS uses a primary
inductance that results in minimal loss of ZVS at any steady-
state condition. Some loss of ZVS during non-steady-state
conditions is acceptable. Reducing primary inductance
produces higher magnetizing current which increases the range
of ZVS operation, but the increased magnetizing current
increases losses and reduces efficiency.
If the leakage inductance is too high, one possible solution is to
use a 3-section bobbin, where the secondary is in the middle
section, and the primary winding is split into 2 halves connected
in series.
The calculation of the primary inductance to be used for a
first-pass design is based on device size, rated load, minimum
input voltage, and desired operating frequency. It is provided in
Lastly, if the leakage inductance is too low an external inductor
may be added.
11
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