LLC resonant converter improves DC-DC efficiency-EEWORLD

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In recent years, the increasing demand for power has directly led to the latest trend of using digital control for AC-DC and DC-DC power conversion. Digital control provides design flexibility, high performance and high reliability. In order to achieve more efficient power supplies, different topologies are being considered for DC-DC conversion. This article will discuss the digital control of inductor, inductor, capacitor (LLC) resonant converters, the advantages of resonant converters, and the overall advantages of digital control. Digital Control Addresses the Demand for Power Since many power supplies operate at loads far below the maximum load or the load at which they operate most efficiently most of the time, improved efficiency is often required in normal mode and low-power modes. For example, the 80 PLUS program requires 115V power supplies to achieve at least 80% efficiency at 20%, 50% and 100% of rated load. Achieving higher efficiency at these operating points can result in bronze, silver, gold or platinum ratings. For 230V power supplies, the minimum bronze standard requires efficiency of 81% at 20% load, 85% at 50% load and 81% at 100% load. The U.S. Department of Energy has extended its push for more efficient products to data centers through the ENERGY STAR Data Center Energy Efficiency Program. The program is designed to address all energy-intensive aspects of a facility, including information technology (IT) equipment and supporting infrastructure such as uninterruptible power supplies (UPS). Many procurement specifications require that purchased products meet these standards or be certified to other recognized energy efficiency standards, forcing suppliers to meet these levels or lose market share. Therefore, achieving higher efficiency is urgent. Reducing operating costs alone is enough to drive energy efficiency improvements. Applications in the medium and high power range (200 to 1000W), such as telecommunications, are increasingly implementing lower power supplies to control operating costs for powering and cooling equipment. To achieve the highest efficiency, many designers are turning to digital control, which also provides design flexibility, high performance, and high reliability. With low-pin-count digital signal controllers (DSCs), such as Microchip Technology's dsPIC DSCs, complex control can be achieved through the digital signal processing (DSP) capabilities and intelligent power peripherals of these devices. Before adding digital control, it is necessary to understand the basic principles of resonant converters. Advantages of Resonant Converters Converters that operate in resonant mode (where the impedance between the input and output of the circuit is minimal) offer higher efficiency. With resonant converters, the power dissipation in the MOSFET is greatly reduced by providing a sinusoidal voltage or current to the MOSFET and switching it close to the zero crossing of the sinusoidal voltage or current. Switching the MOSFET when the drain-source voltage is close to zero (i.e., zero voltage switching, ZVS) and transitioning the MOSFET from one state to another when the current through the switch is zero (i.e., zero current switching, ZCS) minimizes the MOSFET switching losses. This soft switching approach also reduces noise in the system and provides enhanced immunity to electromagnetic interference (EMI). ZVS is the preferred choice for high-voltage, high-power systems. In a resonant switching converter, reactive elements (capacitors and inductors) are added around the switch to generate a sinusoidal voltage or current. The three main categories of resonant converters are: series resonant converter (SRC), parallel resonant converter (PRC), and a combination of the two, series-parallel resonant converter (SPRC). Figure 1 shows a block diagram of an advanced resonant converter and the three types of resonant tanks. Figure 1 : Advanced resonant converter structures have many different forms of resonant tanks. As the name implies, in a series resonant converter, the load is in series with the resonant inductor and capacitor. The gain of the resonant tank is ≤ 1. When the SRC operates at no load, its output voltage cannot be regulated. For ZVS, the circuit needs to operate above the resonant frequency in the inductive region. At low line voltages, the SRC operates close to the resonant frequency. In a PRC, the load is connected in parallel with the resonant capacitor. The PRC can operate at no load output, but unlike the SRC, its output voltage can be regulated at no load. For ZVS, the PRC also needs to operate above the resonant frequency in the inductive region. Similar to the SRC, the PRC operates close to the resonant frequency at low line voltages, but the PRC differs in that it has larger circulating currents. The series inductor and shunt capacitor provide inherent short-circuit protection. In an SPRC, the resonant circuit is a combination of series and shunt converters, which can be either LCC or LLC configurations. Similar to the SRC and PRC, the SPRC LCC design cannot be optimized at high input voltages. Therefore, the LLC is the preferred solution for many applications. The LLC resonant tank is shown in Figure 1. The LLC converter can be operated at the resonant frequency at the nominal input voltage and is capable of operating at no load. In addition, it can be designed to operate over a wide input voltage range. Zero voltage and zero current switching are achieved throughout the entire operating range. The performance of a resonant converter can be measured by several parameters. The quality factor (Q) of a resonant circuit is a dimensionless parameter that describes the amount of damping in the circuit. It is defined as the ratio of the power stored to the power dissipated in the circuit. A higher Q value indicates a narrower bandwidth of the resonant tank. Quality is a key parameter for the gain of a resonant circuit, also known as the voltage conversion ratio or M. By considering a series of M curves generated when λ, the normalized frequency, or the Q value is varied, an indication of the performance of the resonant converter can be obtained before all parameters are calculated. M is defined as follows: M(fsw)=f(fn,λ,Q) Where, fn=normalized frequency, f/fr; λ=inductance ratio, Lr/Lm; Q=quality factor, a function of the output impedance. As shown in Figure 2, the LLC circuit with Q as a parameter actually has two resonant frequencies, one determined by the series inductor Lr and capacitor Cr (Q is 0.5) and the other determined by the parallel inductor Lm. Lr and Cr have a resonant frequency when fn=1 (fr), and Lm+Lr and Cr have a resonant frequency when fn is approximately equal to 0.5. Figure 2: Depending on the quality factor (Q), different gains can be obtained from the resonant tank. The Y-axis is the resonant tank gain (M). All Q curves intersect at the resonant frequency (fn=1). The different operating modes of the LLC include: at the resonant frequency, below the resonant frequency, and above the resonant frequency. When operating at the resonant frequency, the MOSFET switches at the resonant frequency within a very narrow timing window (determined by the selected components). The losses generated at this time are very low. When operating below the resonant frequency, the circuit characteristics are similar to those when operating at the resonant frequency, but the loop current is limited by the magnetizing current for a period of time during the cycle. If a MOSFET is used instead of a diode on the secondary side for synchronous rectification, the gate must be turned off at the appropriate time. This usually requires current sensing techniques, such as measuring the voltage drop across the MOSFET. When operating above the resonant frequency, the loop current is greater than the magnetizing current and is no longer limited by the magnetizing current. In this region, the synchronous switch can be turned on and off at the same time as the primary side switch, simplifying their control. Due to the use of zero voltage switching, an inherent advantage of LLC resonant power supplies is that electromagnetic interference and radio interference are very low. Efficient digital control topology uses current digital signal controllers to easily implement full digital control of power conversion and system management functions of LLC resonant converters.

The actual LLC circuit components and parts, in addition to those shown in Figure 1, include the DC input, switching network, LLC resonant tank, transformer, rectifier, filter, and load. Figure 3 shows the digital control added to the LLC resonant converter. This design represents a design that can be specified for telecom circuits. In these applications, LLC converters are widely used as DC/DC converters following the power factor correction (PFC) circuit in AC-DC systems. The typical PFC output voltage is about 400V and can be directly fed into the LLC converter. The wide input range allows the use of large capacitance capacitors with small size. The design specifications are summarized in Table 1. Figure 3: The reference design high-level block diagram illustrates how the digitally controlled feedback loop is added to the LLC resonant converter. Table 1: The reference design specifications meet the needs of many telecom power supplies. The dsPIC33FJ GS provides digital computing capabilities for the resonant converter. Its 40MIPS performance and intelligent power peripherals make it an ideal choice for this application. Peripherals include high-speed PWM (16-bit, cycle resolution of 1ns) and phase-shiftable outputs. The switching circuit in the reference design uses a half-bridge topology, so the half-bridge voltage swings between 0V and Vd=400Vdc nominal. The resonant tank consists of capacitors, inductors, and the magnetizing inductance of the isolation transformer. Because the design uses the magnetizing inductance of the transformer, no external inductor is required, reducing system cost. The design also uses the leakage inductance of the transformer as the secondary inductor, eliminating the need for an additional external inductor, saving further cost. If the resonant tank is properly tuned to the switching frequency, the resonant tank presents a finite impedance to the fundamental frequency and very high impedance to all other harmonic frequencies. The impedance of the tank causes a phase shift between the voltage and current, allowing zero voltage switching. Zero voltage switching of the primary MOSFET is shown in Figure 4. Figure 4: In this reference design, the half-bridge MOSFET switches without any conduction losses due to the phase shift between the resonant tank current and the MOSFET voltage. Synchronous rectifiers are used in the secondary side design instead of diodes to reduce conduction losses on the secondary side. This reduces the losses caused by the forward resistance (Rf) and the forward voltage of the diode. Figure 5 shows the switching waveforms of the synchronous rectifier. Figure 5: To eliminate turn-off losses in the secondary-side (synchronous) rectifier, the MOSFET drain-source voltage is increased after the MOSFET current reaches zero. The legend is the same as Figure 4, i.e., green line = MOSFET gate-source voltage, purple line = MOSFET drain-source voltage, and yellow line = MOSFET current. For synchronous rectification, digital control performs the MOSFET switching without the need for current sensing circuitry on the secondary side. This allows full-wave rectifier designs to be more efficient and less expensive. Figure 6 shows the efficiency over a range of load currents. Additional benefits include increased flexibility in compensator design, as the DSC also implements duty-cycle controlled soft-start. Figure 6: LLC efficiency at two different operating voltage inputs shows its insensitivity to input voltage. Efficiency of more than 80% is achieved at output load currents below 2A. Maximum efficiency is 95% at higher loads, and the efficiency curve is extremely flat from 7 to 17A of output load current. Because power conversion control can be implemented in easily reprogrammable software, digital solutions give designers the freedom to innovate and easily modify or adapt their designs. In addition to enabling the addition of cost-effective, value-creating new features, precise digital control can also improve the reliability of the power supply. Using reference designs can reduce development time, time to market, and mitigate manufacturing issues that may arise from the beginning of the design. This article summarizes the performance advantages of LLC resonant converters, making this design method an ideal choice for improving energy efficiency in medium and high power telecommunications applications. At the same time, adding digital control also provides electronic systems with the design flexibility, high performance and high reliability that designers expect. To easily achieve the above two points, reference designs provide the easiest way to evaluate the system and shorten the time to market, or more appropriately, shorten the time to achieve higher efficiency.

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