Efficiency analysis of flyback converter based on resonant working mode of UCC28600

Abstract: This paper analyzes in detail the losses of key components of a 65W resonant working mode flyback converter within the full voltage input range, and gives the calculated and measured curves of the converter efficiency within the full voltage working range, which has guiding significance for theoretical analysis of converter efficiency and improving the efficiency of resonant working mode converters.

1. Input and output electrical parameters of the converter:
    The analysis and design of this paper is based on a 65W output laptop adapter, with an input DC voltage Vin of 100~370V DC and an output DC voltage and current of 18V/3.6A. According to the input and output conditions, the low voltage full load is set to a 65KHZ operating frequency. According to the conventional design of the converter, Np: Ns=6:1, Lp=290uH is obtained. For other relevant design parameters and schematic diagrams, please refer to Reference 2 and the Appendix.

2. Loss analysis of main components
2.1. Changes in duty cycle and operating frequency at full range input voltage
    The converter operates in the quasi-resonant valley switching mode at full range, so the basic output formula of Flyback is satisfied. The change law of duty cycle at full load at full voltage input can be calculated according to the following formula, where D(Vin) is the function relationship between duty cycle D and Vin, and its change law is shown in Figure 1, where Vo is the output voltage and Vd is the conduction voltage drop of the secondary rectifier Schottky:

    The change law of the converter operating frequency at full voltage input can be obtained by the following calculation method as shown in Figure 2, where Lp is the primary inductance of the transformer, Ipk_p(Vin) is the function relationship between the primary peak current and the input voltage, f(Vin) is the function relationship between the converter operating frequency and the input voltage, and Ton and Toff are the turn-on and turn-off times respectively.

2.2． Mosfet loss analysis

    As shown in Figure 3, the working waveform of Vds when the converter actually works in the resonant mode. It can be seen that when the Mosfet is turned on, the primary inductance Lp of the transformer resonates with Cds. When turned on, the voltage resonates to Vin-(Vout+Vd). At this time, the current increases from zero, which greatly reduces the loss when turned on. This is the advantage of the resonant working mode.

     The loss of Mosfet is divided into four parts: turn-off loss, conduction loss, turn-on loss and driving loss. The SPP11N60C3 Mosfet used in this design can calculate and analyze the loss distribution of the full input range of Mosfet according to the transformation law of voltage and current when the Mosfet is in the full input range. Among them, the driving loss is mainly related to the operating frequency. The conduction loss needs to estimate the influence of its thermal effect. It is estimated based on the junction temperature of 100⁰C. It can be seen from the figure that the turn-off loss increases with the increase of input voltage.

2.3. Analysis of rectifier bridge and secondary side rectifier Schottky losses

   In this design, STPS20120CT is selected as the secondary rectifier Schottky. At 100⁰C, VF=0.6V. Figure 5 shows the changes in the losses of the rectifier bridge and Schottky at full input range, where BD(Vin) and D(Vin) are the losses of the rectifier bridge and Schottky, respectively.

2.4． Transformer loss analysis
   In this design, the RM10 core is used. The effective cross-sectional area of ​​the core is large and the leakage flux is small. Under the condition of satisfying the saturation flux margin, the design turns ratio is N1: N2 = 36T: 6 T. It can be found that the variation law of the full range maximum flux can be obtained according to the following formula as shown in Figure 6. It can be seen that when the input voltage is the lowest, the flux is the maximum value. Therefore, when designing, it is necessary to ensure that when the full load input voltage is the lowest, the flux is less than the saturation flux and a certain margin is left.

   On the basis of satisfying the skin effect and proximity effect, the sandwich winding method is used, the leakage inductance is small, the primary side is Φ0.3*3, and the secondary side is Φ0.5*4. Figure 7 shows the change law of its loss under the full range input. Among them, Pwinding is the copper loss of the transformer, and Ploss_core is the iron loss. The iron loss is mainly related to the material of the magnetic core.

2.5. Input filter capacitor and primary current detection resistor loss analysis
   As shown in Figure 8, the input capacitor and primary current detection resistor loss changes when the full range voltage is input at full load. The loss of the primary resistor is mainly related to the effective value of the current passing through the resistor, and the loss of the input capacitor is related to the effective value of the current flowing through it.

2.6. Other loss analysis
    The other losses of the converter are mainly divided into: primary current detection resistor loss, output filter LC loss, input capacitor loss, RCD clamp absorption loop loss, IC power supply loss, EMI filter loss and PCB routing loss. The following Figure 9 is a summary of these losses, among which the EMI filter loss and PCB routing loss are mainly resistive losses.

3. Comparative analysis of theoretical analysis and measured efficiency
    Based on the above analysis, the total loss of the converter can be obtained as follows. By adding up these losses, the variation law of all losses within the full range of input voltage can be obtained as shown in Figure 10, and the variation law of efficiency at the full range of input voltage can be obtained as shown in Figure 11.

    Based on the above design and TI's resonant controller UCC28600EVM, the actual converter efficiency is tested as shown in Figure 12. It can be seen that the calculated efficiency change law is basically the same as the measured efficiency curve, which more realistically reflects the theoretical calculation of the converter. The closer the working model established in the actual calculation is to the actual working model, the more accurate the calculation result will be. The efficiency of the converter can be optimized in the design according to the above analysis method, among which the transformer and switching devices are the focus of optimization.

4. Conclusion
Through the above analysis and testing, the following conclusions can be drawn:
   1. For the flyback converter in resonant working mode, when the voltage input is the lowest, the efficiency of the fully loaded converter is the lowest and the magnetic flux is the maximum. It is necessary to evaluate the saturation magnetic flux of the transformer for the lowest input voltage and leave a certain margin.
   2. Since the converter efficiency is the lowest at the lowest input voltage, the converter loss is the largest at this time. In actual design, the thermal design of the entire system can be evaluated based on the loss at this time.
   3. The calculated efficiency result is close to the measured efficiency. Using the loss calculation method in this article is an effective means of evaluating efficiency. The efficiency of the converter can be optimized according to the above analysis method.

5. References:

1. UCC28600 QUASI-RESONANT FLYBACK CONTROLLER datasheet
Texas Instruments http://www.ti.com.cn/cn/lit/ds/slus646j/slus646j.pdf

2. Quasi-Resonant Flyback Converter Universal Off-Line Input 65-W Evaluation Module
Texas Instruments http://www.ti.com.cn/cn/lit/ug/sluu263c/sluu263c.pdf