AN2450 Datasheet, PDF (1/35 Pages) STMicroelectronics – LLC resonant half-bridge converter design guideline

AN2450
Application note
LLC resonant half-bridge converter design guideline
Silvio De Simone
Introduction
The growing popularity of the LLC resonant converter in its half-bridge implementation (see
Figure 1 on page 4) is due to its high-efficiency, low level of EMI emissions, and its ability to
achieve high power density. Such features perfectly fit the power supply demand of many
modern applications such as LCD and PDP TV or 80+ initiative compliant ATX silver box.
One of the major difficulties that engineers are facing with this topology is the lack of
information concerning the way the converter operates and, therefore, the way to design it in
order to optimize its features.
The purpose of this application note is to provide a detailed quantitative analysis of the
steady state operation of the topology that can be easily translated into a design procedure.
Exact analysis of LLC resonant converters (see 1. of Section 9: Reference on page 34)
leads to a complex model that cannot be easily used to derive a handy design procedure.
R. L. Steigerwald (see 2. of Section 9: Reference) has described a simplified method,
applicable to any resonant topology, based on the assumption that input to output power
transfer is essentially due to the fundamental Fourier series components of currents and
voltages.
This is what is commonly known as the “first harmonic approximation” (FHA) technique,
which enables the analysis of resonant converters by means of classical complex ac circuit
analysis. This is the approach that has been used in this paper.
The same methodology has been used by T. Duerbaum (see 3. of Section 9) who has
highlighted the peculiarities of this topology stemming from its multi-resonant nature.
Although it provides an analysis useful to set up a design procedure, the quantitative aspect
is not fully complete since some practical design constraints, especially those related to soft-
switching, are not addressed. In (see 4. of Section 9) a design procedure that optimizes
transformer's size is given but, again, many other significant aspects of the design are not
considered.
The application note starts with a brief summary of the first harmonic approximation
approach, giving its limitations and highlighting the aspects it cannot predict. Then, the LLC
resonant converter is characterized as a two-port element, considering the input
impedance, and the forward transfer characteristic. The analysis of the input impedance is
useful to determine a necessary condition for Power MOSFETs' ZVS to occur and allows the
designer to predict how conversion efficiency behaves when the load changes from the
maximum to the minimum value. The forward transfer characteristic (see Figure 3 on
page 10) is of great importance to determine the input to output voltage conversion ratio and
provides considerable insight into the converter's operation over the entire range of input
voltage and output load. In particular, it provides a simple graphical means to find the
condition for the converter to regulate the output voltage down to zero load, which is one of
the main benefits of the topology as compared to the traditional series resonant converter.
March 2014
DocID12784 Rev 6
1/35
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