The next regional and regional work on the LLC resonant converter design
Recently, with LED LCD TV backlight module market began to surge, many manufacturers have committed to developing a more slim and efficient solutions. Various types of LEDTV have introduced the main power stage topology, such as asymmetrical half-bridge converter, two- Switch Forward converter and LLC resonant converters. Which, LLC resonant converter though compared to other converter has more advantages, but complex and difficult because of its design, so little attention in the past. However, these few years, IC The manufacturer has developed for LLC resonant converter controller, and published a number of related technical specifications and design tools allowed the design easier, and allow this technology to get more attention. Now, LLC resonant converter has become the most popular LEDTV main power stage topology.
LLC resonant converter has outstanding advantages: (1) in the entire load range (including the light-load) are all under the ZVS (zerovoltageswitching, zero voltage switching) conditions, thus achieving high efficiency; (2) relatively narrow frequency range, to facilitate high-frequency Transformer And input filter design; (3) used for the primary-side switch voltage stress is clamped at the input voltage, while the two diodes on the secondary side voltage is always equal to the output voltage center-tapped transformer twice.
LLC resonant converter can operate in two different types of ZVS within the region. One is called "the area (aboveregion)" (or the resonant region), where the primary side of the circulation decreases, but the secondary side of the diode to hard switching. Another is the "area under" (or work under the resonance region), then the secondary side of the diode can achieve soft switching. This article briefly describes the work of LLC resonant converter principle and work area, the design steps will also be discussed.
Figure 1 LLC resonant converter basic circuit
LLC resonant converter works Figure 1 shows the LLC resonant converter basic circuit. LLC resonant converter with MOSFET generally includes a controller, a resonant network and a rectifier network. 50% duty cycle controller to turn provided for the two MOSFET gate signal change with the load change frequency, adjust the output voltage Vout, this is known as pulse frequency modulation (PFM). Resonant network includes two resonant inductor and a resonant Capacitance . Resonant inductor Lr, Lm and resonant capacitor Cr primarily as a divider, the impedance change with frequency (see type 1), to obtain the required output voltage. In practical design, the resonant network shown in Figure 2 may be a use of the sub-frame (sectionalbobbin) of the integrated transformer magnetizing inductance and leakage inductance Lm Llk form. The rectifier network resonant network is rectified sine wave generated, and then transferred to the output stage.
(1) The formula, Vd is approximately equal to the basic Vin / 2, and Rac is approximately equal to the basic 8n2Vout/2 Iout.
Type (2) shown in Figure 2 are given by the real transformer, LLC resonant converter voltage conversion ratio. In equation (2) can be observed in the two resonant frequencies. Lp and Cr by a decision recorded as p, another decision by the Lr and Cr, denoted r. Using this formula, the availability of LLC resonant converter with frequency and load changes in the gain characteristic curve, shown in Figure 3.
Figure 2 sub-frame with integrated transformer (a), transformer equivalent circuit (b)
Figure 3 LLC resonant converter gain curves and work area
Figure 3, each curve with symbol '+' mark of the highest value is called 'peak gain', in two between the resonant frequency p and r. When the output load increasing, the peak gain decreases, its location to a higher frequency. Meanwhile, a symbol '×' mark the r gain resonance when it is fixed, do not change with output load change. Gain curve shows the ZVS condition, with the resonant frequency of the network increases, the gain decreases, the output voltage decreases.
(2) Type of . LLC resonant converter working area
Shown in Figure 3, LLC resonant converter working area can be marked as "+" peak gain and marked as "×" the resonant frequency is divided into three parts. First, the peak point as the boundary, the left is the ZCS (zero current switching) area (or capacitor area), the right is the ZVS (zero voltage switching) area (or inductor area). In the ZVS area, the resonant frequency is the next area to the left r (belowregion), the right is the area (aboveregion). When the LLC resonant converter working in ZCS areas, in large reverse recovery switching transient current flows through the MOSFET, so the work of LLC resonant converter in ZVS areas should make full use of the minimum operating frequency band limits prevent the LLC resonant MOSFET ZCS converter into the area.
As mentioned above, according to the operating frequency is greater than r or less than , LLC resonant converter can operate in the region or regions under. It also depends on the different characteristics of two modes. When the LLC resonant converter is designed to work on the region, the flow circulation is less than under the MOSFET resonant work, MOSFET conduction loss therefore reduced, thereby enhancing efficiency. However, the secondary side of the diode for the hard-switching, it must be Schottky or UF (ultra fast recovery) diode to prevent a serious reverse recovery current. In view of this, as portable equipment LCD's Power supply This low-voltage applications is sometimes considered the resonant work. On the other hand, the work of the next resonant case, the circulation flow MOSFET resonant than the larger work. However, resonant work permitted under the secondary side of the diode for soft turn-on / off, so that you can use ordinary fast recovery diode. Under the resonance is high-pressure applications such as LED or PDPTV preferred. These applications, the output voltage slightly higher, and therefore can not use the low-rated voltage Schottky diode .
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