A high value of M indicates high magnetizing inductance and low circulating energy, but the achievable gain is limited. For lower M values, high gains can be achieved in a narrow frequency range. The resulting magnetizing inductance is relatively small, and the associated circulating current and losses are relatively high.
A value between 6 and 10 is sufficient to change from [6] Start. Select Q according to the ZD gain requirement at full load. If the gain is not enough, the value of M must be reduced. The gain range should be realized over the entire load range or Q range.
The frequency range of the relative gain should be small, and the small Z frequency should have little effect on the size and loss of the magnet. Reiterate the design of Q and M to meet gain and frequency range standards.
The LLC converter has bidirectional power flow capability. But in the discharge mode, the magnetizing inductance appears directly at both ends of the battery, followed by Lr and Cr, resulting in a series resonant converter type configuration.
The discharge curve shows that the converter has no voltage gain, which will cause the output to be unstable. In battery charging mode, the boost stage is bypassed by the relay. However, this method increases component cost and system size. The resonant tank is symmetrical, and the converter has approximately similar gain curves in charging and discharging modes. The secondary resonant components are all called primary, and the equivalent circuit produces a transfer function.
In order to simplify the design steps, assume that the reflection Lrs and Lrp are the same, and set the ratio of reflection Crs to Crp. When determining the value of M, ensure that the gain curve is monotonously decreasing, without multiple peaks, so as to achieve linear control within the entire operating frequency range. However, Crs needs to be used to adjust the gain curve of the discharge mode. If the transformer leakage inductance is also to be used, the equivalent configuration becomes the CLLLC type.
CLLLC with variable DC link voltage The frequency change of the output regulation causes the converter to deviate from resonance, the point where the converter is optimized. In order to keep the frequency swing at the low Z limit, the DC bus voltage will vary according to the required output voltage.
Adjust the transformer ratio to make the Z-small output voltage correspond to the V DC bus, and then the DC link changes linearly according to the set output reference. With modern wide band gap devices, designers can easily achieve high efficiency at high frequencies. The article describes popular resonant converter configurations based on LLC converters. The design method suitable for bidirectional OBC specification in the literature is introduced. Skip to content. Typical OBC power system Figure 1.
LLC converter power stage Figure 2. These techniques process power in a sinusoidal manner and the switching devices are softly commutated. Therefore, the switching losses and noise can be dramatically reduced. Conventional resonant converters use an inductor in series with a capacitor as a resonant network.
Two basic configurations are possible for the load connection; series connection and parallel connections. For the series resonant converter SRC , the rectifier-load network is placed in series with the L-C resonant network as depicted in Fig. From this configuration, the resonant network and the load act as a voltage divider. By changing the frequency of driving voltage Vd, the impedance of the resonant network changes. The input voltage will be split between this impedance and the reflected load.
At light load condition, the impedance of the load will be very large compared to the impedance of the resonant network; all the input voltage will be imposed on the load. This makes it difficult to regulate the output at light load.
Theoretically, frequency should be infinite to regulate the output at no load. For parallel resonant converter, the rectifier-load network is placed in parallel with the resonant capacitor as depicted in Fig.
Half-bridge parallel resonant PR converter In order to solve the limitations of the conventional resonant converters, the LLC resonant converter has been proposed []. The LLC-type resonant converter has many advantages over conventional resonant converters. First, it can regulate the output over wide line and load variations with a relatively small variation of switching frequency.
Second, it can achieve zero voltage switching ZVS over the entire operating range. Finally, all essential parasitic elements, including junction capacitances of all semiconductor devices and the leakage inductance and magnetizing inductance of the transformer, are utilized to achieve ZVS. This paper presents an analysis and design considerations for a half-bridge LLC resonant converter.
Using the fundamental approximation, the voltage and current waveforms are analyzed and the gain equations are obtained. Fairchild Power Seminar II. In Fig. Operation of the LLC resonant converter is similar to that of the conventional LC series resonant converter. Since the magnetizing inductor is relatively small, there exists considerable amount of magnetizing current Im as illustrated in Fig.
In general, the LLC resonant topology consists of three stages as shown in Fig. The square wave generator stage can be built as a full-bridge or half bridge type. The resonant network filters the higher harmonic currents. Thus, essentially only sinusoidal current is allowed to flow through the resonant network even though a square wave voltage is applied to the resonant network.
As can be seen in Fig. The rectifier network can be implemented as a full-wave bridge or center-tapped configuration with capacitive output filter. Typical waveforms of half-bridge LLC resonant converter The filtering action of the resonant network allows us to use the classical fundamental approximation to obtain the voltage gain of the resonant converter, which assumes that only the fundamental component of the square-wave voltage input to the resonant network contributes to the power transfer to the output.
Because the rectifier circuit in the secondary side acts as an impedance transformer, the equivalent load resistance is different from actual load resistance. The primary side circuit is replaced by a sinusoidal current source, Iac and a square wave of voltage, VRI appears at the input to the rectifier. With the equivalent load resistance obtained in 5 , the characteristics of the LLC resonant converter can be derived.
Using the AC equivalent circuit of Fig. In previous research, the leakage inductance in the transformer secondary side was ignored to simplify the gain equation [].
However, as observed, there exists considerable error when ignoring the leakage inductance in the transformer secondary side, which generally results in an incorrect design. One is determined by Lr and Cr while the other is determined by Lp and Cr. In actual transformer, Lp and Lr can be measured in the primary side with the secondary side winding open circuited and short circuited, respectively.
It should be noticed that the peak voltage gain does not occur at fo nor fp. The peak gain frequency where the peak gain is obtained exists between fp and fo as shown in Fig. As Q decreases as load decreases , the peak gain frequency moves to fp and higher peak gain is obtained. Meanwhile, as Q increases as load increases , the peak gain frequency moves to fo and the peak gain drops.
Thus, the full load condition should be the worst case for the resonant network design. Another important factor that determines the peak gain is the ratio between Lm and Llkp which is defined as k in 9.
Even though the peak gain at a given condition can be obtained by using the gain in 8 , it is difficult to express the peak gain in explicit form. Moreover, the gain obtained from 8 has some error at frequencies below the resonant frequency fo due to the fundamental approximation. In order to simplify the analysis and design, the peak gains are obtained using simulation tool and depicted in Fig.
It appears that higher peak gain can be obtained by reducing k or Q values. With a given resonant frequency fo and Q value, decreasing k means reducing the magnetizing inductance, which results in increased circulating current. Accordingly, there is a trade-off between the available gain range and conduction loss. This is a distinct advantage of LLC-type resonant converter over the conventional seriesresonant converter.
Therefore, it is natural to operate the converter around the resonant frequency to minimize the switching frequency variation at light load conditions.
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