Optimization of LLC Resonant Converters: State-trajectory Control and PCB based Magnetics

With the fast development of information technology (IT) industry, the demand and market volume for off-line power supplies keeps increasing, especially those for desktop, flat-panel TV, telecommunication, computer server and datacenter. An off-line power supply normally consists of electromagnetic interference (EMI) filter, power factor correction (PFC) circuit and isolated DC/DC converter. Isolated DC/DC converter provides input to output isolation and output voltage regulation for off-line power supply. It occupies more than half of the volume in an off-line power supply, so isolated DC/DC converter is the key aspect to improve the overall performance and reduce the total cost for off-line power supply. On the other hand, of all the power supplies for industrial applications, those for the data center servers are the most performance driven, energy and cost conscious due to the large electricity consumption. The total power consumption of today’s data centers is becoming noticeable. In 2014, data centers in the U.S. consumed an estimated 70 billion kWh, representing about 1.8% of total U.S. electricity consumption. Moreover, with the increase in cloud computing and big data, energy use of data centers is expected to continue rapidly increasing in the near future. Isolated DC/DC converters in datacenters are required to provide low-voltage high-current output and fast transient response, and those requirements make the design of isolated DC/DC converters even more difficult. The LLC resonant converters have been widely used as the DC-DC converter in off-line power supplies and datacenters due to its high efficiency and hold-up capability. With the property of zero-voltage-switching (ZVS) for the primary switches and zero-current-switching (ZCS) for the secondary synchronous rectifiers (SRs), using LLC converters can also reduce electromagnetic interference. Almost all the high-end offline power supplies employs LLC converters as the DC/DC converter. But there are three major challenges in LLC converters. Firstly, the control characteristics of the LLC resonant converters are complex due to the dynamics of the resonant tank. The statetrajectory analysis and control is a powerful tool to solve this challenge, but it requires highperformance digital controller together with analog control circuitry. This dissertation proposes a method to implement state-trajectory control with a low-cost microcontroller (MCU). And further efforts have been made to integrate all the state-trajectory control function into one MCU for highfrequency LLC converters, including start-up and short-circuit protection, fast transient response, light load efficiency improvement and SR driving. Secondly, most power supplies in IT industry are required to provide low-voltage high-current output, but the transformer is very bulky and it is very challenging to design due to large conduction losses. By pushing switching frequency up to MHz with gallium nitride (GaN) devices, the magnetics can be integrated into printed circuit board (PCB) windings. This dissertation proposes a novel matrix transformer structure and its design methodology, to integrate four elemental transformers into one magnetic core with much reduced winding loss and core loss. On the other hand, to suppress the CM noise, shielding layers can automatically be inserted into the PCB windings between the primary and secondary windings in the fabrication process. A novel shielding technique is proposed to utilize half of the shielding as the primary winding, which not only suppresses CM noise, but also improves the efficiency. The proposed transformer design and shielding technique is applied to an 800W 400V/12V LLC converter design. Peak efficiency of 97.7%, power density of 900W/inch and CM noise reduction of 30dB are achieved. Thirdly, the LLC converters have sinusoidal current shape due to the nature of resonance, which has larger root mean square (RMS) of current, as well as larger conduction loss, compared to pulse width modulation (PWM) converter. This extra conduction loss compared to PWM converter is considered circulating energy in resonant converter. Three-phase interleaved LLC converters can reduce the circulating energy by inter-connecting the three phases in certain way, but it suffer from bulky and numerous magnetic components. This dissertation proposed a novel magnetic structure to integrated three inductors and three transformers into one magnetic core. By pushing switching frequency up to 1MHz, all the magnetics can be implemented with 4-layer PCB winding. Additional 2-layer shielding can be integrated to reduce CM noise. A 1MHz 3kW 400V/48V three-phase LLC converter with proposed magnetic structure is designed, and peak efficiency of 97.5% and power density of 600W/inch are achieved. This dissertation solves the challenges in analysis, digital control, magnetic design and EMI in high-frequency DC/DC converters in off-line power supplies and datacenters. With the academic contribution in this dissertation, GaN devices can be successfully applied to highfrequency DC/DC converters with MHz switching frequency to achieve high efficiency, high power density, simplified but high-performance digital control and automatic manufacturing. The cost will be reduced and the performance will be improved significantly. Optimization of LLC Resonant Converters: State-trajectory Control and PCB based Magnetics Chao Fei General Audience Abstract With the fast development of information technology (IT) industry, the demand and market volume for off-line power supplies keeps increasing, especially those for desktop, flat-panel TV, telecommunication, computer server and datacenter. The total power consumption of today’s data centers is becoming noticeable. Moreover, with the increase in cloud computing and big data, energy use of data centers is expected to continue rapidly increasing in the near future. The efficiency of off-line power supplies is very critical for the whole human society in order to reduce the total electricity consumption. And the cost is also a key driving force for the development of novel technology in off-line power supplies due to the large market volume. An off-line power supply normally consists of electromagnetic interference (EMI) filter, power factor correction (PFC) circuit and isolated DC/DC converter. Isolated DC/DC converter occupies more than half of the volume in an off-line power supply and takes the most control responsibilities, so isolated DC/DC converter is the key aspect to improve the overall performance and reduce the total cost for off-line power supply. Among all the DC/DC converter topologies, the LLC resonant converters have been most widely used as the DC/DC converter due to its high efficiency and hold-up capability. But there are three major challenges in LLC converters. Firstly, the control characteristics are very complex due to the dynamics of the resonant tank. To achieve good control performance, very complex and expensive digital controller has to be employed. Secondly, the magnetic components are very bulky, and it is expensive to manufacture them. Thirdly, there is circulating energy in LLC converters due to the nature of resonance, which increases the total loss. To solve these challenges, this dissertation proposes to implement a special control method, state-trajectory control, with a low-cost microcontroller (MCU). All the control functions can be integrated into one simple, low-cost MCU to replace the previous complex and expensive controller. By pushing switching frequency up to MHz with next generation power devices, this dissertation proposes a novel magnetics structure that can be integrated into printed circuit board (PCB) windings to achieve low-cost and automatic manufacturing. Furthermore, this dissertation employs three-phase interleaved LLC converters topology to reduce the circulating energy, and proposed a novel magnetic structure to integrated three inductors and three transformers into one magnetic core with simple 4-layer PCB winding. All the proposed technologies have been verified on hardware prototypes, and significant improvements over industrial state-of-art designs have been demonstrated. To sum up, this dissertation solves the challenges in analysis, digital control, magnetic design and EMI in DC/DC converters for off-line power supplies. With the academic contribution in this dissertation, the cost can be reduced due to the simplified control and automatic manufactured magnetics, and the efficiency can be improved with proper utilization of next generation power devices. This dissertation will improve future DC/DC converter for IT industrial in the three most important aspects of efficiency, power density and cost.