20-08-2013, 04:55 PM
Designing an LLC Resonant Half-Bridge Power Converter
Designing an LLC Resonant .pdf (Size: 1.19 MB / Downloads: 36)
ABSTRACT
While half-bridge power stages have commonly been used for isolated, medium-power applications,
converters with high-voltage inputs are often designed with resonant switching to achieve higher efficiency,
an improvement that comes with added complexity but that nevertheless offers several performance
benefits. This topic provides detailed information on designing a resonant half-bridge converter that uses
two inductors (LL) and a capacitor ©, known as an LLC configuration. This topic also introduces a
unique analysis tool called first harmonic approximation (FHA) for controlling frequency modulation.
FHA is used to define circuit parameters and predict performance, which is then verified through
comprehensive laboratory measurements.
Introduction
Higher efficiency, higher power density, and
higher component density have become common
in power-supply designs and their applications.
Resonant power converters—especially those with
an LLC half-bridge configuration—are receiving
renewed interest because of this trend and the
potential of these converters to achieve both higher
switching frequencies and lower switching losses.
However, designing such converters presents many
challenges, among them the fact that the LLC
resonant half-bridge converter performs power
conversion with frequency modulation instead of
pulse-width modulation, requiring a different
design approach.
Brief Review of Resonant Converters
There are many resonant-converter topologies,
and they all operate in essentially the same way: A
square pulse of voltage or current generated by the
power switches is applied to a resonant circuit.
Energy circulates in the resonant circuit, and some
or all of it is then tapped off to supply the output.
More detailed descriptions and discussions can be
found in this topic’s references.
Among resonant converters, two basic types
are the series resonant converter (SRC), shown in
Fig. 1a, and the parallel resonant converter (PRC),
shown in Fig. 1b. Both of these converters regulate
their output voltage by changing the frequency of
the driving voltage such that the impedance of the
resonant circuit changes. T
LCC and LLC Resonant Converters
To solve these limitations, a converter
combining the series and parallel configurations,
called a series-parallel resonant converter (SPRC),
has been proposed. One version of this structure
uses one inductor and two capacitors, or an LCC
configuration, as shown in Fig. 2a. Although this
combination overcomes the drawbacks of a simple
SRC or PRC by embedding more resonant fre
quen
cies, it requires two independent physical
capacitors that are both large and expensive
because of the high AC currents. To get similar
characteristics without changing the physical com
ponent count, the SPRC can be altered to use two
inductors and one capacitor, forming an LLC reso
nant converter (Fig. 2b). An advantage of the LLC
over the LCC topology is that the two physical
inductors can often be integrated into one physical
component.
Operation At, Below, and Above f0
The operation of an LLC resonant converter
may be characterized by the relationship of the
switching frequency, denoted as fsw, to the series
resonant frequency (f0). Fig. 4 illustrates the
typical waveforms of an LLC resonant converter
with the switching frequency at, below, or above
the series resonant frequency. The graphs show,
from top to bottom, the Q1 gate (Vg_Q1), the Q2
gate (Vg_Q2), the switch-node voltage (Vsq), the
resonant circuit’s current (Ir), the magnetizing
current (Im), and the secondary-side diode current
(Is). Note that the primary-side current is the sum
of the magnetizing current and the secondary-side
current referred to the primary; but, since the
magnetizing current flows only in the primary
side, it does not contribute to the power transferred
from the primary-side source to the secondary-
side load.
Peak-Gain Curves from a Bench Test
The attainable peak-gain curves shown in Fig.
12a were made from the gain function described
by Equation (23), but Equation (23) was developed
with approximations. These approximations allow
errors in the peak-gain curves, causing accuracy
concerns. To test the accuracy, a comparison was
made between the gain function of Equation (23)
and a bench test with a 135-kHz series resonant
frequency. The peak-gain values were found to
differ from those shown in Fig. 12a and from the
gain curves shown in Fig. 6.
Conclusion
This topic has presented comprehensive
considerations for designing an isolated LLC
resonant half-bridge converter. It has been shown
that the FHA method is especially effective for
initiating a new design of this type. A step-by-step
design example has also been presented to
demonstrate how to use this method.