VERSATILE CONTROL USED FOR LLC RESONANT … with PSIM 6.0 software tool as shown in Fig.2. Fig .2...

http://www.iaeme.com/IJEET.asp 35 [email protected]

International Journal of Electrical Engineering & Technology (IJEET)

Volume 6, Issue 9, Nov-Dec, 2015, pp.35-46, Article ID: IJEET_06_09_005

Available online at

http://www.iaeme.com/IJEETissues.asp?JType=IJEET&VType=6&IType=9

ISSN Print: 0976-6545 and ISSN Online: 0976-6553

© IAEME Publication

___________________________________________________________________________

VERSATILE CONTROL USED FOR LLC

RESONANT CONVERTER SUITABLE FOR

PORTABLE APPLICATIONS

Kowstubha. P and Krishnaveni K

Department of Electrical and Electronics Engineering,

Chaitanya Bharathi Institute of Technology, Hyderabad, India

Ramesh Reddy K

Department of Electrical and Electronics Engineering,

G. Narayanamma Institute of Technology and Science, Hyderabad, India

ABSTRACT

Conventional voltage mode control with one of the Pulse Analog Control

Schemes called Pulse-Position Modulation (PPM) offers only limited

performance for LLC resonant DC/DC converters undergoing wide variations

in operating conditions. This paper presents current mode control with PPM

that could consistently provide good dynamic performance for LLC resonant

DC/DC converters irrespective of wide variations in operating conditions. The

proposed versatile control scheme employs additional feedback from the current of the resonant tank network with an integrator-type compensation

amplifier which improves the dynamic performance, enhances the noise

immunity and the reliability of feedback controller to overcome the limitation

of the existing voltage mode control. Significance of current mode control over

voltage mode control is presented by providing necessary design guidelines

and simulation results incorporated with PPM. The design of LLC resonant

converter uses Fundamental Harmonic Approximation (FHA) with maximum

gain adjustment.

Key words: Closed-Loop Performance, Control Design, Dynamic Analysis,

LLC Series DC/DC Resonant Converters, Pulse-Position Modulation.

Cite this Article: Kowstubha. P, Krishnaveni K and Ramesh Reddy K.

Versatile Control Used For LLC Resonant Converter Suitable For Portable

Applications. International Journal of Electrical Engineering & Technology,

6(9), 2015, pp. 35-46.

http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=6&IType=9

Kowstubha. P, Krishnaveni K and Ramesh Reddy K


1. INTRODUCTION

LLC resonant DC/DC converters [1]- [4]

are widely adopted in consumer electronics

due to their advantages over the remaining load resonant converter topologies. But, in

these applications LLC resonant DC/DC converters often undergo wide variations in

operating conditions. For example, when these converters employed as an off-line

power supply, they receive the input from a power factor corrected rectifier whose

output voltage could vary substantially due to line voltage variations.

With reference to [1]- [3],

it is clear that the small-signal performance of LLC

resonant DC/DC converters drift substantially as the operating condition is altered. So

at this juncture, a control scheme should be designed which could offer consistently

the desired dynamic performance for the entire operational range, regardless of any

changes in operating conditions.

Conventionally, LLC resonant DC/DC converters are controlled by voltage mode

control where output voltage is employed as a feedback signal. As illustrated in [1] - [4]

,

the performance of voltage mode control is directly influenced by the changes in the

operating conditions of the converter, making the control scheme inappropriate for

practical LLC resonant DC/DC converter. [1]

Therefore, in this paper, a current mode

control for LLC resonant DC/DC converters is proposed that offers good closed loop

performance for the entire operational region. In this control scheme, an additional

current feedback from the resonant tank circuit in addition to the output voltage

feedback is employed. The aim of applying current feedback is to have a composite

feedback signal which minimizes the influence of potential changes in power stage

dynamics.

The performance of LLC resonant DC/DC converters in closed loop configuration

i.e. with PPM using current mode control and voltage mode control schemes is being

carried out with simulation using PSIM 6.0 software tool. The concept of the

proposed current mode control and its performance over voltage mode control scheme

is illustrated. Then, design considerations and the references for the proposed current

mode control are discussed and formulated. Finally, the performance of the proposed

current mode control is analyzed in comparison with the conventional voltage mode

control. The analysis includes primary side conduction and switching losses, stress on

primary side capacitor, secondary side diode loss and closed loop performance.

2. DESIGN / OPERATIONAL GUIDE LINES AND SIMULATION

CIRCUITS

2.1. Design Guide Lines of LLC Resonant DC / DC Converter in Open

Loop Configuration

A LLC resonant DC/DC converter with 60W/12V output has been selected as a

design example. The design specifications are as follows:

For the design specifications mentioned, the final resonant network parameter

values were designed and tabulated in Table 1 on referring to the design guide lines

given in [5]

Versatile Control Used For LLC Resonant Converter Suitable For Portable Applications


Table 1 Designed Values of LLC Resonant DC/DC Converter.

Parameter Designed Values

Magnetizing Inductance Lm 1924µH

Series Inductance Lr 481 µH

Series capacitance Cr 5.26nF

Resonant frequency fr 100kHz

Inductance ratio m 4

Quality factor Q 0.48

M @f0 (Gain) 1.1

Minimum frequency 75kHZ

Turns ratio of transformer 18:1

2.2. Design Guide Lines / Operation of LLC Resonant DC / DC Converter

using Voltage Mode Control with PPM

In this section, the design guide lines of LLC resonant DC/DC converter for closed

loop configuration with one of the Pulse Analog Control Schemes named PPM using

with voltage mode control is presented. Based on this design, a complete simulation

diagram is developed with PSIM 6.0 software tool is as shown in Fig.1.

Figure 1 Closed loop configuration of LLC Resonant DC/DC Converter with PPM

using Voltage Mode Control.



Fig.1 represents the simulation circuit diagram of LLC resonant DC/DC converter

with PPM using Voltage Mode Control in closed loop configuration. Each circuit of

Fig.1 is explained in detail as follows. The values of all parameters shown in LLC

Resonant DC/DC converter circuit are taken from Table 1. In this circuit, the two

MOSFETs of half bridge inverter are gated by a square pulse with a dead time of

370ns to avoid the cross conduction of two MOSFET switches, which is provided by

the monostable multivibrator circuit. In the voltage feedback circuit, the sensed output

voltage of LLC resonant DC/DC converter is compared with a reference voltage of

12V leading to an error voltage. Proportional Integration controller is implemented on

this error voltage that leads to ramp signal. The obtained ramp signal is given to the

constant gain amplifier circuit. The constant gain amplifier is a common base

amplifier circuit which gives a constant dc output, which is considered as the

modulating signal. Now a Pulse-width modulated (PWM) signal is generated by

comparing this modulating signal with a carrier signal i.e. is a triangular signal with a

carrier frequency equal to switching frequency. The obtained PWM signal acts as the

input for the Pulse-Position Modulation circuit, which consists of a differentiator,

rectifier and a monostable multivibrator. A pulse, with a dead time of 370ns, is

generated at the output of the monostable multivibrator. The input to differentiator is a

PWM waveform. In PPM, the position of a pulse relative to its un-modulated time of

occurrence is varied in accordance with the message signal i.e. the sensed output. If a

PWM signal is differentiated then a pulse train is obtained. It consists of both positive

and negative going narrow pulses corresponding to the leading and trailing edges of

pulses respectively. If the position corresponding to the trailing edge of an un-

modulated pulse is counted as zero displacement then the other trailing edges will

arrive earlier or later. Thus these pulses will have time displacement proportional to

the instantaneous value of sensed output signal voltage. Thus a change in output

voltage is controlled by controlling the position of occurrence of gate pulse. The

design aspects of PI controller, differentiator and monostable multivibrator are taken

from [6] - [8]

.

2.3. Design Guide Lines / Operation of LLC Resonant DC / DC Converter

using Current Mode Control with PPM

In this section, the design guide lines of LLC resonant converter for closed loop

configuration with one of the Pulse Analog Control Schemes PPM using current mode

control is presented. Based on this design, a complete simulation diagram is

developed with PSIM 6.0 software tool as shown in Fig.2.

Fig .2 represents the simulation circuit diagram of LLC resonant converter for

current mode control with PPM in closed loop configuration. Each circuit of Fig.2 is

explained in detail as follows. In this circuit diagram an additional feedback circuit

shown as current feedback circuit is considered. The operation/function of remaining

all circuits is same as mentioned in section 2.1 as in voltage mode control with PPM.

The resonant tank current is a pure sinusoidal AC waveform and hence it should be

filtered adequately before being utilized as the final current feedback signal. The

current mode control scheme [9]

employs a simple rectification circuit, which acts as a

current sensing circuit (CSC). A constant gain amplifier acts as a current feedback

circuit is connected to the output of CSC. Though the rectification circuit of CSC can

provide some filtering, still the current feedback signal contains an AC component.

Therefore, the under-filtered AC component could destabilize the output of PWM



block and could become a threat to frequency modulation control for LLC resonant

DC/DC converters.

Figure 2 Closed loop configuration of LLC Resonant DC/DC Converter using

Current Mode Control with PPM.

To resolve the above cited problem, in the proposed control scheme, an integrator-

type compensation amplifier is provided that gives required filtering and boost the

gain of the current feedback circuit. The integrator-type compensation amplifier

eliminates the left out AC component from the current feedback signal and also

improves the performance of the original voltage mode control of the converter while

greatly enhancing the noise immunity and reliability of frequency modulation control

through PPM. The design aspects of PI controller, differentiator and monostable

multivibrator are taken as in the case of voltage mode control. The concept of PPM is

referred from [10]

.

3. PERFORMANCE ANALYSIS

In this section, Performance Analysis of LLC resonant DC/DC converter is carried

out both for voltage mode and current mode controls with PPM. The performance

analysis includes primary side conduction and switching losses, stress on primary side

capacitor, secondary side diode loss and closed loop performance. In both the

controls, input voltage is considered as 300V, as it refers the worst situation of LLC

resonant converter due its high rms current on primary side. The simulation diagrams

shown in Fig.1 and Fig.2 were run for a simulation time of 100ms.



3.1. Primary Side Conduction Loss

Fig. 3(a) and Fig. 3(b) represents the simulated waveforms of primary side inductor

currents iLm and iLr for both voltage mode and current mode control schemes given in

amps.

(a)

(b)

Figure 3 Waveforms of iLm and iLr of LLC Resonant DC/DC Converter with PPM

using (a)Voltage Mode Control (b) Current Mode Control.

The summary of primary side currents and the corresponding switch conduction

loss (PConduction ) and switching loss (PSwitching) for a single MOSFET switch at the

worst operating condition of low input voltage Vin=300V are tabulated in Table 2 for

both the control schemes.

Table 2 Summary of Primary Side Currents and the Corresponding losses for both the

Control Schemes

Voltage mode control Current mode control

Resonant tank current iLr in amps 0.640 0.446

Switch turn off current iLm in amps 0.234 0.209

PConduction in watts for single switch 0.098 0.047

PSwitching in watts for single switch 0.221 0.198

The conduction loss given in Table 2 is calculated with the formula

(1)



where D=Duty Cycle, iLr rms = resonant tank current and RDSON= On state

Drain to Source resistance of MOSFET which is taken as 0.48 Ω.

ZVS can be achieved for both the control schemes with the energy stored in the

resonant tank. So with the proper design of resonant tank and dead time, the

MOSFETs turn on resonantly (full ZVS).Therefore, turn-off loss contributes

switching loss for the LLC resonant converter. Turn-off loss can be approximated by

(2)

iLm = switch turn off current

The observations made from the Table 2:

Conduction and Switching losses are less for Current mode control compared to

Voltage mode control

3.2. Stress on resonant capacitor

Fig. 4(a) and Fig. 4(b) represents the simulated waveforms of voltage drop of resonant

capacitor (VCr) with PPM using both voltage mode and current mode control schemes

in volts.

(a)

(b)

Figure 4 Simulated Waveforms of VCr for LLC Resonant DC/DC Converter with

PPM using (a) Voltage Mode Control (b) Current Mode Control.

The observations made from Fig.4:

The minimum voltage drop or stress is observed for Current mode control compared

to Voltage mode control scheme.

The values being 505V and 488V for both the control methods.

3.3. Secondary Side Diode Conduction Loss

Fig.5 (a) and Fig.5 (b) represents the current through both the diodes on secondary

side with PPM using both voltage mode and current mode control schemes in amps.



(a)

(b)

Figure 5 Simulated Waveforms of current through rectifier diodes ID for LLC

Resonant DC / DC Converter with PPM using (a) Voltage Mode Control (b) Current

Mode Control.

By witnessing Fig.5 for both the control schemes, the summary of secondary side

diode current and the corresponding diode conduction loss (PConduction) for a single

diode switch is tabulated in Table 3.

Table 3 Summary of Secondary Side Diode Currents and the Corresponding Conduction loss

for both the Control Schemes


Secondary side diode current ID in amps 9.34 8.03

PDconduction (Diode conduction) in watts 1.634 1.405

On realizing each of diode carrying half the corresponding output current, the

conduction loss for a single rectifier diode given in Table 3 is calculated from the

formula

(3)

where ID =average diode current

and VfD1 = forward voltage drop in volts taken as a reasonable value of 0.35V for

schottkey diodes.

The observations made from Fig.5 and Table 3:

Less Discontinuity is observed for Current mode control compared to Voltage mode

control scheme.

Less diode conduction loss is observed for Current mode control compared to

Voltage mode control scheme.



3.4. Closed Loop Performance

3.4.1. Full load regulation

Fig. 6(a) and Fig. 6(b) represents the output voltage of LLC resonant DC / DC

converter on full- load simulated for both voltage mode and current mode control

schemes in volts.

(a)

(b)

Figure 6 Simulated Waveforms of Output Voltage for LLC Resonant DC/DC

Converter on full- load with PPM using (a) Voltage Mode Control (b) Current Mode

Control.

It is clear from Fig. 6(a) and Fig. 6(b) , both the control schemes are providing

Full load regulation with current mode control giving less ripple and more overshoot

compared to voltage mode control scheme.

3.4.2. No Load Regulation

Fig.7(a) and Fig.7(b) represents the output voltage of LLC resonant DC/DC converter

on no- load simulated with PPM using voltage mode and current mode controls.

(a)



(b)

Figure 7 Simulated Waveforms of Output Voltage for LLC Resonant DC/DC

Converter on no- load with PPM using (a) Voltage Mode Control (b) Current Mode

Control.

It is clear from Fig. 7(b), that the current mode control is providing no load

regulation compared to the one given for voltage mode control scheme shown in Fig.7

(a).

4. INFERENCES DRAWN FROM SIMULATION STUDIES

The inferences drawn from the simulation studies for LLC Resonant Converter using

conventional voltage mode control and current mode control are presented in Table 4.

Table 4 Inferences drawn for voltage and current mode control schemes for LLC Resonant

Converter


Conduction and Switching losses in watts more less

Stress on capacitor more less

Discontinuity of secondary diode current more less

Diode conduction loss in watts more less

Full load regulation with less ripple and more

overshoot

No load regulation Better regulation

5. CONCLUSIONS

A versatile current mode control scheme that overcomes the limitations of voltage

mode control for LLC resonant DC/DC converter with PPM was presented in this

paper. The current mode control scheme, with the focus on benefits of tank current

feedback is discussed. In the proposed control scheme, the current feedback signal is

compared with that of the voltage mode control to highlight/emphasize the filtering

capacity of integrated compensation amplifier. This integrator-type compensation

amplifier in current feedback circuit improves the performance of the converter by

enhancing the reliability and noise immunity of the controller. Guidelines for

selecting the parameters of the current and voltage feedback circuits are also

provided.



ACKNOWLEDGEMENTS

The authors wish to thank Chaitanya Bharathi Institute of Technology and G.

Narayanamma Institute of Technology and Science authorities for permitting to

publish.

REFERENCES

[1] J. Jang, M. Joung, B. Choi, S. Hong, and S. Lee, Dynamic analysis and control

design of optocoupler-isolated LLC series resonant converters with wide input

and load variations, IET Power Electron., Vol. 5, pp. 755-764, Jun. 2012.

[2] B. Yang, Topology investigation for frontend DC/DC power conversion for

distributed power systems, Ph. D. dissertation, Virginia Polytechnic Institute and

State University, Blacksburg, VA, 2003.

[3] B. Yang and F. C. Lee, Small-signal analysis for LLC resonant converter, CPES

Seminar, S 7.3, pp. 144-149, 2003.

[4] E. X. Yang, Extended describing function method for small signal modelling of

resonant and multi resonant converters, Ph. D. dissertation, Virginia Polytechnic

Institute and State University, Blacksburg, VA, 1994.

[5] B. Yang, F. C. Lee, A. J. Zhang, and G. Huang, LLC resonant converter for front

end DC/DC conversion, in Proc. IEEE Appl. Power Elec. Conf. and Expo, Mar.

2002, vol. 2, pp. 1108 -1112.

[6] Benjamin C. Kuo, Automatic Control Systems, 7th ed.,2003, PHI publications

[7] Katsuhiko Ogata, Modern Control Engineering, 4th ed.,2003, Pearson Education

Asia Publications

[8] Ramakant A. Gayakwad, Op-Amps and Linear Integrated Circuits , 4th ed.,2010,

PHI publications

[9] J. Jang, M. Joung, S. Choi, Y. Choi, and Byungcho Choi, Current mode control

for LLC series resonant dc-to-dc converters, IEEE Applied Power Electronics

Conf., pp. 21-27, Mar. 2011.

[10] Kennedy, Electronic communication System, 4th ed.,2008, Tata McGraw-Hill

publications

[11] Ramjee Prasad Gupta, Dr. Upendra Prasad. Design of A PWM Based Buck Boost

Dc/Dc Converter with Parasitic Resistance Suitable For Led Based Underground

Coalmines Lighting System. International Journal of Electrical Engineering &

Technology, 3(3), 2012, pp. 175-186.

[12] Kanade Jyoti Suresh and Kulkarni Vishwashri Amrut. A Versatile

Microcontroller Based Semiconductor Device Tester. International Journal of

Electronics and communication Engineering & Technology, 5(2), 2014, pp. 42-

49.



AUTHORS DETAILS

KOWSTUBHA. P received her four-year B.Tech degree from Sri Venkateswara

University in 1995 and M.E from Bangalore University in 2003. She has 12years of

teaching experience and presently pursuing PhD. Her fields of interest are Power

Electronics and Integrated circuits.

KRISHNAVENI. K received her four-year B.Tech degree from Nagarjuna

University in 1993, M.Tech and Ph.D. from JNTU Hyderabad in 2002 and 2009. She

has 22 years of teaching experience. Her fields of interest are Power Electronics and

FACTS. She has published over 15 papers in national & international conferences and

technical journals. She is a member of IEEE and MIE. Presently seving as a professor

in the department of Electrical and Electronics Engineering.

RAMESH REDDY. K received his four-year B. Tech degree from Nagarjuna

University, M. Tech from REC, Warangal and Ph.D. from S. V. University, Tirupathi

in the years 1985, 1989 and 2004. He has 28 years of teaching experience. His fields

of interest are Power Quality, Power Harmonics & Custom Power Devices. He has

published over 80 papers in national & international conferences and technical

journals. He received “Best Engineering Teacher Award” from ISTE. He serves as a

reviewer for the National journal of Institution of Engineers (India), Kolkata &

International journal of IEEE transactions on Power Delivery, Journal of Power

Electronics, South Korea and IET, South Korea and International Journal of Power

Electronics & Drives, UK. Presently he is serving as principal G Narayanamma

Institute of Technology and Science, Hyderabad.

VERSATILE CONTROL USED FOR LLC RESONANT … with PSIM 6.0 software tool as shown in Fig.2. Fig .2...

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