AC / DC power factor correction (PFC) rectifiers make great use of GaN FETs. They have very simple topologies. The inductance is the only magnetic part and this is usually a constant frequency inductor – Continuous conduction mode (CCM). The effect of GaN FETs on the performance of the PFC rectifier can be directly demonstrated. The lower parasitic capacitances of 650-V GaN FETs reduce switching losses. Moreover, 650-V GaN FETs have a lower resistance (Ron) within the same chip size compared to 650-V Si MOSFETs, and GaN FETs eliminate the reverse recovery loss. Peak power supply switching efficiency increased to 99% with GaN FETs. 1-4 Although GaN costs remain an obstacle to widespread industry adoption, the performance that can be achieved with GaN FETs, including efficiency and density improvements Finally, it has a positive effect on the total cost of switching power supply solutions. This article examines GaN-based PFCs in detail, and reviews the control topology and GaN performance without a bridge. GaN PFC topology conventional PFC booster uses only one active switch, the superconductor Si MOSFET is usually 650V. Most of today’s traditional switching power sources use booster PFCs, adding to their simplicity, low cost, and reliability. While replacing the 650-V Si MOSFET with the 650-V GaN FET can reduce switching loss, this efficiency improvement is minimal – usually, only about 0.1% to 0.15%. However, replacing the fast recovery diode with another 650-V GaN FET will reduce the loss considerably because the diode conduction loss is eliminated as the Ron FET decreases, and the GaN FET eliminates the reverse recovery loss. This change can save about 0.25% efficiency improvement. The huge conduction loss caused by the diode bridge is another major source of switching loss. Replacing the diode bridge with low-rune Si MOSFETs can improve the efficiency by about 0.4%. The diode bridge can also be replaced with a hybrid device chassis that includes a diode bridge and Si MOSFETs. 5. The hybrid device can reduce conduction loss from light load to heavy load with low cost. Figure 1: The topologies of the non-bridged PFC rectifier of GaN include (a) a bridgeless PFC reinforcement, (b) a PFC binary without a bridge, and (c) a PFC totem electrode. (Photo: University of Texas at Austin) Double reinforcement PFC without bridges is another common topology used in switching power supplies. Again, the Si MOSFETs can be replaced with 650-V GaN FETs to obtain an efficiency improvement of 0.1% to 0.15%, and the replacement of the fast recovery diode can lead to an efficiency improvement of approximately 0.25%. Finally, replacing the low-frequency diodes with low Ron Si MOSFETs or hybrid MOSFETs can increase the efficiency by about another 0.25%. However, double-reinforcement PFC, having two alternating phases, has a low use of devices and inductors. The PFC topology with GaN totem column contains two GaN FETs, two Si MOSFETs (or hybrid switches) and one inductor. This architecture uses fewer both bridgeless reinforced PFC and double reinforced PFC and achieve better hardware and inductor utilization. The efficiency and density can also be slightly higher with totem pole PFC compared to double reinforcement PFC, and the cost is lower. GaN PFC control GaN PFC control can be summed up based on these modulation strategies: continuous conduction mode (CCM), critical conduction (CRM) mode, and quasi-square wave (QSW) mode. For CCM, the switching frequency is constant, and a high switching loss results in a relatively low switching frequency; Conventional medium current control, which is commonly used to enhance PFC, can be used for GaN PFC in this case. For CRM, traditional peak current control and continuous on time control, which are also used to enhance PFC, can be used. The traditional CRM control also integrates discontinuous conduction mode (DCM) control, which can limit the peak switching frequency. The operation and control of the QSW mode for GaN PFC is often discussed because eliminating the operating loss results in a much higher switching frequency, which could reduce the transformer size. To achieve the QSW process, control strategies based on zero-crossing detection (ZCD) are discussed. 3,4,6 The main concept is that after receiving a ZCD signal, the controller will extend the connection time of the synchronous rectifier (SR) switch to achieve zero voltage switching ( ZVS) for the active key. The digital extended controller calculates the exact time based on average input, output voltage, and current information. However, this method is very difficult due to the need for a fast and accurate current sensor or ZCD, especially when the switching frequency extends to several MHz. This control method is challenging even when multi-stage interleaving is required in the system. Another control method is based on variable frequency pulse width modulation (PWM). 7 This method uses the core portion of the conventional current control average for CCM PFC stimulation. The innovation here is that the frequency of the triangular carrier signals can be changed based on the perceived input, the output voltage and the current information. Changing the frequency of the triangular carrier changes the switching frequency. The average current control loop determines the duty cycle. The main concept of this control method is that for a QSW process the duty cycle and frequency of the PWM bus are two independent degrees of freedom. This method eliminates the high-speed sensor current or the ZCD step. Multiphase interleaving can be easily achieved by variable frequency PWM because PWM carriers are always synchronous. Table 1: Comparison of the performance of GaN PFC rectifiers (Source: The University of Texas at Austin) GaN PFC rectifiers have demonstrated the performance of GaN PFCs with success in academia and industry. Table 1 summarizes the performance achieved by the various institutions and companies. In general, the peak efficiency of 99% – a new high level for PFC power source conversion – can be achieved. This efficiency performance elevates the PFC power supply conversion efficiency to a new level. Some solutions can achieve the highest efficiency up to 99.2%. Usually, higher efficiency is sacrificed by lower frequency, resulting in lower density. Another efficiency performance advantage of CCM GaN PFC is that the heavy load efficiency of the topology is not much lower than its peak efficiency, because CCM is better than QSW in reducing the current RMS value, especially high frequency AC RMS. Usually QSW GaN PFC modifiers have higher power density because their switching frequency is much higher, but the decrease in efficiency from peak to heavy load value is steeper for QSW than for CCM. The multi-level GaN PFC is an attractive solution for achieving increases in efficiency and density. 12,13 The multi-level process reduces the second voltage on the inductor and the equivalent operating frequency to enable a significant reduction in the size of the inductor. The size of other negative components will also be reduced. The CCM process and the low ripple current also lead to low conduction loss, especially for high frequency alternating current conduction. Low switching voltage is also a factor in switching loss reduction. Conclusions Power electronics designers can achieve a low switching loss and zero reverse recovery loss by using 650-V GaN FETs. Among the structures discussed in Fig. 1, GaN totemic pole PFCs have the fewest switches, exhibit symmetric operation between switches, and enable better instrument and inductor use. PFC totem pole GaN can achieve 99% peak efficiency via CCM or QSW operation. QSW process eliminates operating loss, which is the dominant part of total switching loss; So QSW results in much higher switching frequency and higher power density compared to CCM process. The challenge of ZVS variable frequency control for QSW operation can be solved by using variable frequency PWM, which replaces the traditional PWM fixed frequency bus with variable frequency carrier. This PWM approach eliminates high-speed current sensing or ZCD and solves the problem of multi-phase variable-frequency interleaving control. Multilevel technology can be applied to GaN PFC to achieve both high efficiency and high density using the CCM process. References 1 liter. Zhou, Y. Wu, J. Honea, and Z. Wang. High Efficiency True Bridgeless Totem Pole PFC Based on GaN HEMT: Design Challenges and Cost Effective Solutions. Proceedings of PCIM Europe 2015; International Exhibition and Conference on Energy Electronics, Smart Mobility, Renewable Energy and Energy Management, 1-8. 2015. 2F.C. Lee, Q. Li, Z. Liu, Y. Yang, C. Fei, and M. Mu. GaN Hardware Application of 1 Kw Server Power Supply with Integrated Magnets. CPSS Transactions on Power Electronics and Applications, Vol. 1, No. 1, 3–12. December 2016. 3Z. Liu, FC Lee, Q. Li, and Y. Yang. Design of GaN-GHz-based PFC totemic rectifier. IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol. 4, No. 3, 799-807. September 2016. Fourth Quarter. Huang, RU, Kyu Ma, and Aiyu Huang. 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Written by Qingyun Huang, research associate at the University of Texas, Austin, Qingchuan Ma, graduate researcher at the University of Texas, Austin, Alex Kyu Huang, professor at the University of Texas, Austin. .
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