In high-voltage power conversion circuit design, issues such as power supply noise, switching frequency, switching loss, power supply volume, reliability, etc. have always been key. Compared with other high-voltage topologies, LLC converters have been favored by designers for their high efficiency and low power supply. However, their design is very difficult.
Power Integrations' (PI) recently introduced the HiperLCS family of LCS700-708 high-voltage LLC power ICs that combine variable frequency controllers, high-side and low-side drivers, and two MOSFETs into one low-cost package for superior design flexibility. Its maximum load efficiency exceeds 97%, and the use of high-frequency switches to reduce the size of the transformer and the footprint of the output capacitor, thereby reducing the size of the power supply. Figure 1 shows a simplified circuit diagram of the HiperLCS power stage with the LLC resonant inductor integrated into the transformer. The variable frequency controller switches the power MOSFET through zero voltage (ZVS) to eliminate switching losses and achieve high efficiency.
Design structure and key features
Figure 1 Schematic diagram of the HiperLCS power stage
The LLC converter design is complex, and the biggest challenge is the need to properly configure all circuits to eliminate cross conduction and provide precise control of various parasitic components. According to Andrew Smith, PI Product Marketing Manager, HiperLCS integrates key components, including multi-function controllers, high-voltage and low-side gate drivers, and LLC half-bridge power stages for two power MOSFETs (up to 30 peripheral components can be eliminated) And fine-tuned during the manufacturing process to optimize the pairing of MOSFETs in the driver. The variable frequency controller in this device provides zero voltage switching, eliminating switching losses.
HiperLCS devices have a maximum operating frequency of 1MHz and a nominal steady-state operating frequency of up to 500kHz, allowing the use of low-cost SMD ceramic capacitors in the output loop, replacing bulky, less reliable electrolytic capacitors while reducing the need The size of the core. It also achieves excellent transformer utilization at a peak switching frequency of 750kHz. In addition, HiperLCS devices have precise duty cycle symmetry to balance the output rectifier current for improved efficiency. Typical values ​​at 50 kHz are 50% ± 0.3%.
HiperLCS devices are used in two ways, depending on the design requirements. For high-efficiency designs, the resonant control circuit provides extremely low power losses, enabling designs to achieve efficiencies greater than 97% at 66kHz rated switching frequency. If the cost and size determine the design criteria, a high switching frequency can be preferred. In the latter case, higher efficiencies can still be achieved, for example, up to 96% at 250 kHz (the frequency at which the maximum power is obtained). In addition, the HiperLCS device allows the user to achieve optimal design by setting critical circuit parameters such as dead time and soft start for improved design. During the design process, design engineers can use PI's real-time design and modeling tool PI Xls to simplify power supply design.
Figure 2 Comparison of 70KHz switching frequency of traditional LLC transformer and 250KHz high frequency switch of HiperLCS in power supply size
Protection and Encapsulation Like other PI power ICs, HiperLCS has comprehensive fault handling and current limiting, including programmable voltage ramp/drop threshold and hysteresis, undervoltage (UV) and overvoltage (OV) protection, programmable Stream Protection (OCP), Short Circuit Protection (SCP) and Over Temperature Protection (OTP). On the package, the HiperLCS series is available in a high-power and high-frequency ultra-thin eSIP-16C package that reduces assembly costs and reduces the loop area of ​​the PCB layout, allowing quick mounting to the heat sink with a clip. Multiple TO220 packages for external MOSFETs can be eliminated (Figure 2). The exposed heat-dissipating metal portion is connected to the ground potential so that there is no need for a Silpad thermal insulating spacer between the package and the heat sink. The eSIP-16C's package pins are staggered to simplify the routing path of the PCB and meet the high-voltage creepage requirements. The introduction of HiperLCS completes the PI end-to-end complete power supply, including EMI filter circuit, PFC stage and main power supply and standby. Level power supply. "The design of the HiperLCS device easily exceeds the performance benchmarks set by ENERGY STAR 4.0 and 5.0, as well as the 80PLUS Gold and Silver PC energy efficiency standards." Andrew said, "HiperLCS can be designed with HiperPFS PFC and other products to be fully functional. High efficiency, low component count power supply for 75W~440W high voltage power supplies such as servers, industrial control, LCD TVs, LED street lights and outdoor lighting, printers and audio amplifiers."
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