Energy recovery circuit and its detection based on soft switching technology

   Minimizing power consumption and increasing power density while saving costs are important challenges faced by modern high-efficiency switching power supplies. The design goal of a switching power supply is to reduce power conduction loss and switching loss [1].
    It is difficult to achieve the goal of optimizing power conduction loss without affecting power density and cost, because this requires a lot of materials and components, various chips, or increasing the copper wire area. Unlike conduction loss, it is easier to reduce power switching loss without significantly increasing power supply cost. The circuit discussed in this article uses a soft switching method, and its energy efficiency ratio is better than that of silicon carbide diodes.
1 Energy recovery circuit
    This circuit is designed with reference to the requirements of soft switching [2], as shown in Figure 1. In order to recover the energy stored in coil L, two diodes D1 and D2 are newly added near the boost coil LB, and there are also two auxiliary coils NS1 and NS2.

1.1 Concept Description
    When transistor TR is turned on, coil NS1 can restore the reverse recovery current IRM flowing through boost diode DB [3]. The AC input voltage also modulates the boost diode current IDB and its associated reverse recovery current IRM. This modulation process causes the reverse recovery current IRM flowing through coil L to be reset by coil NS1. When the transistor is turned off, the auxiliary coil NS2 injects the additional current of the small coil L into the output capacitor. The current flowing through the small coil L disappears in the bulk capacitance through diode D2. When the dI/dt slope is low, such as in the case of a discontinuous switching converter, the additional coils NS1 and NS2 will affect the turn-off diodes D1 and D2; the diode reverse recovery current IRM will not affect the circuit characteristics.
1.2 Phase Timing Description
    The transformation ratios m1 and m2 are the ratios of coils NS1 and NS2 to NP, respectively.
    Before t0, the characteristics of the recovery circuit are the same as those of a conventional boost converter.
    At t0, the power transistor is turned on and the current in DB is equal to I0. At t0+, current soft switching starts with no switching losses. After t0, the current flowing through DB decreases linearly to -IRM.
    At t1+, the boost diode DB is turned off. Due to the low reflected voltage VNS1, it is necessary to keep dI/dt_D1 with a low slope to eliminate the adverse effects of the reverse recovery current on the diode D1. However, during this phase, a high reverse voltage is applied to the boost diode DB. This characteristic requires the addition of a diode to this application so that the diode reverse recovery current IRM is accurately balanced with the breakdown voltage.
    At t2, the current on the diode D1 is 0 A, and the recovery circuit becomes a more traditional power boost converter.
    At t3, the power transistor is turned off. At the same time, the voltage polarity on the main winding also changes until the DB diode is turned on again.
    At t4, the current on the diode D2 reaches 0 A, and the recovery circuit becomes a traditional power boost converter again, with only the boost diode DB turned on.
    The circuit requires a special diode with a breakdown voltage higher than 600 V. In addition, the reverse recovery current of this diode needs to be optimized to prevent the power transistor TR from being hit by a higher current during the phase sequence from t1 to t2.
1.3 Calculation of transformation ratios of m2 and m1
    In order to comply with the discontinuous mode during the phase sequence of t1-t2 and t3-t4, the time td1 and td2 shown in Figure 2 should be positive. According to the principle of continuous conduction mode (CCM) power factor correction and the results of tD1_ON and tD2_ON, the transformation ratios m1 and m2 can be determined.

Where PIN is the input power of the power factor correction circuit (PFC) [4], FS is the switching frequency; VmainsRMSmax is the maximum circuit voltage; IRMmax is the maximum reverse recovery current under the conditions of on-current change rate and maximum operating junction temperature.
2 Energy recovery circuit of 450 W power factor correction circuit
    In order to demonstrate the advantages of the recovery circuit, a general series 450 W power factor corrector with a VmainsRMS of 90-260 V was fabricated. This series of products adopts hard switching mode and a standard current sharing PWM controller. The energy recovery circuit and the silicon carbide Schottky diode are compared in terms of conduction conditions, energy efficiency comparison, and heat measurement.
2.1 Design of the recovery circuit
    When measuring the energy recovery circuit, specific diodes are used. In Figure 1, DB uses STTH8BC065DI, D2 uses STTH8BC060D, and D1 uses STTH5BCF060.
2.2 Typical waveforms of the recovery circuit
    Figure 3 shows the typical waveforms of the energy recovery circuit of the power factor correction circuit at 200 kHz. Each time the power transistor is turned on, a current soft switching operation occurs. This curve highlights that the two diodes D1 and D2 are always in a discontinuous state; D1 recovers the IRM current of DB; and D2 sends the current stored in the coil L through the body capacitance of the power factor correction circuit. During the phase sequence of t0~t1 and t4~t5, once D2 is turned off, the drain voltage of the power transistor will immediately decrease, and the turn-off loss will be eliminated.

2.3 Energy Efficiency Comparison
    The energy efficiency of the energy recovery circuit and the SiC Schottky diode is compared under the same Vmains voltage and the same switching frequency of 140 kHz, as shown in Figure 4 and Figure 5. When the power supply voltage is 230 VRMS, the recovery circuit saves about 2.25 W of power compared to the 8 A SiC diode under full load conditions, and saves about 1 W of power when the load is 100 W.

    Under low load conditions, the reflected voltage generated by NS2 can still improve the energy efficiency of the energy recovery circuit because the turn-off loss of the recovery circuit is lower than that of the SiC diode. However, if the power factor correction circuit operates in discontinuous mode (<100 W), the energy recovery circuit will have the same energy consumption as the SiC diode, as shown in Figure 4.
    At a voltage of 90 VRMS, the advantages of the soft switching method and the energy saved by the discharge of the power transistor body capacitance COSS further highlight the advantages of the energy recovery circuit. When the output power reaches 450 W, the energy recovery circuit saves about 5.4 W compared with the SiC diode; under low load conditions, due to the lack of turn-off loss, the energy recovery circuit saves about 1.7% of the power compared with the SiC diode. The advantages of the soft switching method energy recovery circuit and COSS discharge in reducing energy consumption are strengthened, especially under low load conditions. This advantage will be more obvious.
2.4
    The soft switching method of thermal measurement current can reduce the power loss of the power transistor. Figure 6 shows the temperature difference (18 ℃) generated by the energy recovery circuit solution and the SiC diode on the power transistor in a power factor correction circuit. If the PN junction temperature of the power transistor is the same, the energy recovery circuit should be able to further reduce the size of the heat sink. In this way, the space saved offsets the space occupied by the micro coil L of the energy recovery circuit. In addition, the recovery circuit has the same power density as the SiC diode.

    Although thermal optimization techniques are used, the energy efficiency of the energy recovery circuit will be reduced if the RDS(on) of the power transistor causes the PN junction temperature to rise to 90°C, but it is still higher than that of the SiC diode. Therefore, in the 90 VRMS energy efficiency comparison shown in Figures 5 and 6, 0.75 W must be subtracted from the saved power Pout×[1/(SiC_efficiency)-1/(BC2_efficiency)]=5.4 W. In summary, the energy saving effect and power density of the energy recovery circuit are better than those of the SiC diode. The energy recovery
    circuit uses a current soft switching method, which can help power supply designers achieve the goal of improving energy efficiency through a unique lossless recovery circuit. The use of a dedicated diode can improve the performance of the power factor correction circuit in the continuous conduction mode.
References
[1] Luo Ping, Li Qiang, Xiong Fugui, et al. Key technologies of new switching power supplies [J]. Microelectronics, 2005, 35(1): 63-66.
[2] Qi Qun, Zhang Bo. A review of the development of soft-switching PWM converters [J]. Journal of Circuits and Systems, 2000(3): 50-56.
[3] Li Siqi, Guo Ben, Jiang Xiaohua, et al. Research on dynamic dead-time suppression of MOSFET reverse recovery current [J]. Power Electronics Technology, 2010, 44(7): 91-93.
[4] Lin Weiming, Wang Jinghui, Huang Junlai, et al. A high-efficiency voltage-doubling and boost-type soft-switching power factor correction circuit [J]. Proceedings of the CSEE, 2008(36): 62-67.