20-09-2012, 01:55 PM
LLC Resonant Converter
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Introduction
In previous chapters, the trends and technical challenges for front end DC/DC
converter were discussed. High power density, high efficiency and high power are
the major driving force for this application. Hold up time requirement poses big
penalty to the system performance. Two methods were proposed in chapter 2 to
solve this problem and improve the efficiency. Range winding solution could
improve the performance at high input voltage significantly, but with extra
devices, windings and control circuit. Asymmetrical winding solution provides a
simpler solution, but could only apply to asymmetrical half bridge topology. Also
it introduced other problems like discontinuous output current and unbalanced
stress.
To catch up with and move ahead of the trend, higher switching frequency,
higher efficiency and advanced packaging are the paths we are taking now.
Within all these issues, a topology capable of higher switching frequency with
higher efficiency is the key to achieve the goal.
Three traditional resonant topologies
In this part, these three topologies will be evaluated for front end DC/DC
application. The major goal is to evaluate the performance of the converter with
wide input range. For each topology, the switching frequency is designed at
around 200kHz.
Series resonant converter
The circuit diagram of a half bridge Series Resonant Converter is shown in
Figure 4.1 [B8]-[B13]. The DC characteristic of SRC is shown in Figure 4.2. The
resonant inductor Lr and resonant capacitor Cr are in series. They form a series
Bo Yang Chapter 4. LLC Resonant Converter
resonant tank. The resonant tank will then in series with the load. From this
configuration, the resonant tank and the load act as a voltage divider. By changing
the frequency of input voltage Va, the impedance of resonant tank will change.
This impedance will divide the input voltage with load. Since it is a voltage
divider, the DC gain of SRC is always lower than 1. At resonant frequency, the
impedance of series resonant tank will be very small; all the input voltage will
drop on the load. So for series resonant converter, the maximum gain happens at
resonant frequency.
Parallel resonant converter
The schematic of parallel resonant converter is shown in Figure 4.4 [B14]-
[B17]. Its DC characteristic is shown in Figure 4.5. For parallel resonant
converter, the resonant tank is still in series. It is called parallel resonant converter
because in this case the load is in parallel with the resonant capacitor. More
accurately, this converter should be called series resonant converter with parallel
load. Since transformer primary side is a capacitor, an inductor is added on the
secondary side to math the impedance.