Dual transformer LLC Resonant Converter

Dual transformer LLC Resonant Converter

Kavana E 1, Rudranna Nandihalli 2

1 Student, Dept. of EEE, RV College of Engineering, Karnataka, India,

2 Dept. of EEE, RV College of Engineering, Karnataka, India

Abstract – This paper presents the Inductor- Inductor Capacitor (LLC) resonant converter with series transformers that are connected in series suitable for high input and high output voltage applications. The converter has two transformers connected in series. The leakage inductances are used as LLC circuit, resulting in resonant series inductance circuit while the other one is used as magnetizing element for each transformer. External capacitance

1. is included in the circuit to complete the LLC structure. These transformers induce voltages into the LLC network and enhance the voltage gain, while the other works as a regular LLC converter transformer. Proposed topology has multiple advantages: a) high voltage gain is achieved with reduced switching losses, b) higher efficiency at higher voltage. The simulation results of the converter prove that efficiency is higher, obtained for broad range of voltages.

   Key Words: LLC converter, dual-transformer, resonant circuit.

   1. INTRODUCTION

      Resonant converter is electric power converters that consists of resonant tank mainly inductors and capacitors are tuned at a particular frequency [1]. There are different types of resonant converter namely series resonant converter, parallel resonant converter and series parallel resonant converter. LLC resonant converter is a type of DC/DC converter that allows soft- switching operation. Soft-switching operation is that the pulses are turned on or turned off at Zero Voltage Switching (ZVS) [2-4]. So the LLC resonant converter circuit reduces the switching losses through ZVS. The LLC resonant converter keeps the output voltage regulated even under light load condition [5].

      LLC converter mainly includes three set of components namely the switch network, the resonant tank and also rectifier network. The block diagram of the LLC resonant converter is as shown in the Fig.1 [1]. Some of the important characteristics of LLC DC/DC converters are soft-switching capabilities, lesser electromagnetic interference (EMI) and simple in structure [6-7]. Series parallel or LLC resonant converter is commonly used in applications that demand high input output voltage ratio, high power density, renewable energy power generation applications and electric vehicle on-board chargers [9].

      Fig.1. Block Diagram of LLC Converter

   2. PROPOSED CONFIGURATION

      The proposed topology is as shown in Fig.2. The ideal transformer includes the leakage inductances and the magnetizing inductance consisting of a primary switch network, LLC resonant circuit, a diode rectifier and a full bridge. Primary switch network consists of inner full bridge converters and outer full bridge converter is built with switches S1, S2 ,S3 ,S4. The inner full bridge converter is made up of S5, S6 ,S7 ,S8. The LLC resonant circuit consists of two numbers of high frequency transformers Tr1 and Tr2 having transformation ratios equal to n1 (ns1/ np1) and n2 (ns2/ np2) for Tr1 and Tr2 respectively. There are two transformers and both of them are connected in series, and they have magnetizing inductances Lm1 and Lm2 for Tr1 and Tr2 respectively. Leakage inductances Ll1 and Ll2 for Tr1 and Tr2 respectively are used as series resonant inductance Ll1+Ll2, while Lm2 is the magnetizing inductance element. External capacitance Cr is included to complete the LLC resonant circuit structure. The full bridge diode rectifier consists of diodes D1, D2, D3, D4. The output voltage ripple is reduced by adding capacitor Co to the output of the diode rectifier. The load resistance is RL.

      .

      Fig.2 Proposed Converter Includuing Transformers Modelled by Magnetizing and Leakage Inductances

   3. MODELLING

      The voltage gain is obtained by analysing fundamental or first harmonic analysis (FHA) [2].Voltage gain is used to obtain resonant frequency and load quality factor. The magnetics ratio of the proposed converter is in equations respectively. The equivalent circuit of the converter is obtained using the

      fundamental components Vbc, Vad as depicted in Fig.3 and the

      simplified equivalent circuit of the converter is shown in Fig. 4. n =

      Fig.3 Equivalent Circuit of Resonant Tank using FHA.

      The design of the proposed dual transformer LLC resonant converter system and the design parameters are shown in Table 1.

      Fig.4. Simplified Equivalent Circuit (of Fig. 3.)

      Table 3.1: Parameters considered for simulation of proposed converter

      Design parameters	Specifications
      	50V
      	60uF
      	0.73uF
      	250
      	250V
      	12.2uH
      	1.1uH
      	1
      	2:5

      Transformer turns ratio,

      Here, n is transformer turns ratio Vdc is the supply voltage

      Vout is the output voltage

      Voltage gain is provided by the equation

      Here, k is magnetics ratio

      is external capacitance is resonance frequency

      is normalized switching frequency

      Load quality factor Q is

      Q =

      Lm1 and Lm2 are magnetizing inductances RL is load resistance

   4. SIMULATION RESULTS

      The dual transformer LLC resonant converter is simulated with the designed values. The simulations are performed in the PSIM software. This section includes the simulation of the circuit and gives the results obtained from the simulations. The parameters values used for the simulations are as in the Table.1. The circuit diagram of proposed converter is as shown in Fig.5. The input and the output voltages are as shown in Fig.6. Fig.7 represents the current across the load resistor varying with time.

      Voltages V12, V23 ,V34 ,V14 varying with time is as shown in Fig 8.

      Fig.5 Dual transformer LLC resonant converter

      Fig.8. Voltages V12, V23 ,V34 ,V14 varying with time

      Fig.6. A plot of input voltage and output voltage varying with time

      The proposed LLC resonant converter is simulated using PSIM software with the following parameters as shown in Table I. The supply voltage (Vdc) of the proposed converter is 50V and the resonant frequency (fr) is 111 kHz. The two transformers Tr1 and Tr2 has the same magnetizing inductances Lm1 = Lm2, primary transformer with the turns ratio = 1:1 and secondary transformer with turns ratio = 5/2 respectively. The filter capacitor Co connected across the output is large enough to obtain a constant output voltage Vo. The converter operates at a switching frequency that is equal to the resonance frequency. The maximum possible efficiency and the duty cycle is set to 50%. The output voltage Vo and output current Io waveform is obtained. The output voltage Vo is approximately five times more than the input voltage. The supply voltages V12, V23, V34, V14 are depicted in the Fig.8. The voltages are fluctuating between -50V to +50V is injected into the LLC resonant circuit to maximize the voltage gain without changing the switching frequency. Fig. 9 represents the closed loop output voltage varying with time using PI controller.

      Fig.7. Current across load resistor varying with time

      [7]	R Liu and C Sagdi, Q Lee Design and analysis of LLC-type series resonant convertor, IEEE Tansactions on Industrial Electronics, vol. 15, no. 424(24):1517-1519, May, 2017.
      [8]	C Fei, Q Li and F C Lee, Digital implementation of adaptive synchronous rectifier (SR) driving scheme at high-frequency LLC converters with microcontroller, IEEE Transactions on Power Electronics, vol. 50, no. 433(6): 5351-5361, January, 2017.
      [10]	G Zhang, Z Y Li, W Suig and N Zhang, Power Electronics converters: Past, present and future, Renewable and Sustainable Energy Reviews, vol. 50, no. 477 81(2): 2028-2044, June, 2016.
      [11]	M A Saket, John and M Ordonez, LLC converters with the planar transformers: Issues and mitigation, IEEE Transactions on Power Electronics, vol. 13, no. 4, 32(6): 4524-4542, October, 2017.
      [12]	H Park, M Kim and J Jung. Spread spectrum technique to reduce EMI emission for LLC resonant converter using hybrid modulation method, IEEE Transactions on Power Electronics, vol. 7, no. 41, 33(5): 3717- 3721, September, 2018.
      [13]	M Mohammadi, M Kim John and M Ordonez, Synchronous rectification of LLC resonant converters using homopolarity cycle modulation, IEEE Transactions on Industrial Electronics, vol. 8, no. 41, 66(3): 1781-1790, July 2019.

      Fig.9 Closed loop output voltage varying with time using PI controller

   5. CONCLUSION

The dual-transformer Inductor Inductor Capacitor (LLC) resonant converter is applicable in applications that demands high output voltage range. The leakage inductance (LI1, LI2) of the transformer uses as series resonant element, reduces high conversion efficiency, external inductance, during the comparison of the conventional LLC converter with one transformer. The additional switches and transformer is justified by using the high conversion efficiency at high voltage gains.

The proposed converter is simulated using PSIM software. The dual transformer LLC resonant converter is designed for supply voltage of 50V. The simulation results provide an output voltage of 250 V, the output voltage is almost five times more than the supply voltage.

REFERENCES