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Fuzzy Logic Controller for Fast Transient Response DC-DC Converter

1A.Rajkumar and 2B.Karthikeyan

1Associate Professor, Department of Electronics and Communication Engineering, Srinivasan Engineering College, Perambalur, Tamilnadu.

2Prof/Principal, Dhanalakshmi Srinivasan Institute of Research and Technology, Siruvachur, Perambalur, Tamilnadu, India.

A R T I C L E  I N F O

Article history:

Received  3 September 2014

Received in revised form 30 October 2014

Accepted 4 November 2014

Keywords:

DC-DC buck converter, Fast transient response, Fuzzy logic controller (FLC).

A B S T R A C T

A new low output voltage and fast transient response dc-dc converter is designed in order to feed devices such as microprocessors and DSPs. It is a fast response DC-DC buck converter (FRBC) comprising of two buck converters connected in shunt and controlled by Fuzzy logic controller. First the Linear Control DC-DC converter is designed using MATLAB. Next the same DC-DC converter is designed with Fuzzy logic controller (FLC). Both results are compared .By using FLC, quick settling time is achieved even if there is a variation in supply side or load side. The results on simulation model with Fuzzy logic are obtained using FLC.

© 2014 AENSI Publisher All rights reserved.

To Cite This Article: A.Rajkumar and B.Karthikeyan., Fuzzy Logic Controller for Fast Transient Response DC-DC Converter. Adv. in Nat. Appl. Sci., 8(21): 65-71, 2014

INTRODUCTION

Advanced microprocessors and DSPs require power supplies with low output voltage and, as main challenge, fast transient response. Different techniques have been used in order to improve the transient response of this type of power supplies. One of the existing topology is voltage regulator module (VRM). (S.Goodfellow et al). The voltage regulator module converters are having a fast evolution since they present important features such as robustness. But it cannot provide fast response, since it is having large size of inductor and capacitors to filter out the ripples. Most of the today‟s VRMs use conventional buck topology. In order to meet the voltage regulation of feature requirements during the transients, more output filter capacitors and decoupling capacitors will be needed which in turn increases the space of the VRMs there by increasing the cost. This proposed topology is a new DC/DC switching converter, the fast transient response converter, which belongs to those converters with modified topology and controlled by FLC (A.Barroda, et al - 2002,).

Limitations of the VRM Topology:

It has some limitations in transient response and efficiency. They are

a) Transient limitation

b) Efficiency limitation

Today‟s VRM uses the conventional buck converter using MOSFET &diode with large size output filter inductance ranging from 2 to 4 µH.

Conventional VRM inductor design is according to

L ≥ (1)

Where

D- Duty cycle

- input voltage

- Output voltage

- Full load current

- switching frequency

Corresponding Author: A.Rajkumar, Associate Professor,Dept of ECE,Srinivasan Engineering College Perambular,

Tel: 9790631364; E-mail:[email protected],

66 A.Rajkumar and B.Karthikeyan,2014

Advances in Natural and Applied Sciences, 8(21) Special 2014, Pages: 65-71

As the VRM having large output inductance during the transient this large inductor limits the energy transfer speed and the capacitors have to store or discharge all from load. With large output filter inductances the VRM requires large filter capacitor to reduce the ripples. The need of a large quantity of inductor makes the VRM impractical for feature microprocessor. (J.wei P et al-2001)

METHODS AND MATERIALS

The following block diagram shows the FRBC which is composed of two buck converter connected in parallel.

Fig. 1: Basic structure of Fast response buck converter.

Both converters have to be designed with different purposes. The main switching converter must be designed to work in steady and a low output voltage ripple, but consequently with slow response. On the contrary, the auxiliary switching converter must be designed to work in transient operation. The main aim of this converter is to provide the required high current slew rate and fast transient response. (D.Briggs et al -2000)

Control Strategy:

This control is based on the utilization of a threshold band (2∆Vo). This band presents two limits, a higher threshold (HT, Vo+∆Vo) and a lower threshold (LT, Vo-∆Vo). If the output voltage is within the threshold band only the main switching converter operates in this case like typical buck converter with linear control.

If output voltage goes out the threshold band, the auxiliary switching converter is connected. i.e If the output of the converter is above threshold limit, the auxiliary converter is connected in parallel with the main converter in order to take out the extra current by forcing the duty cycle 1.

If the output of the converter is below threshold limit, the auxiliary converter is connected in parallel with the main converter in order to inject the additional current by setting the duty cycle 0. But the duty cycle remains unchanged, if the output voltage level is within the threshold limit. (R.Miftakhudtniov et al -2001).

Fig. 2: Block diagram of fuzzy control buck converter.

In this Fig., two-buck topologies have been used to design the auxiliary and main converter. Both of them feed the output in parallel furthermore, a FLC block can be noticed.

Main and Auxiliary converter circuit:

The auxiliary converter acts as simple switch. The inductor connected with the auxiliary converter gets charged and discharged based on the condition of output voltage level. Inductor gets charged, when the output voltage exceeds beyond the upper threshold limit. i.e. the extra current is used for charging the inductor, thus the auxiliary converter helps to take out the extra current. Inductor gets discharged, when the output voltage level goes below the lower threshold limit. i.e. the additional current required is supplied by the inductor when it discharges. Thus the voltage level is maintained within the threshold limit by the auxiliary converter action.

The duty cycle saturation & reset logic block forces to „1‟ the duty cycle if lower threshold is surpassed, or it set the duty cycle to „0‟ if output voltage is above the high threshold. In short, and from a topological point of

67 A.Rajkumar and B.Karthikeyan,2014

Advances in Natural and Applied Sciences, 8(21) Special 2014, Pages: 65-71

view, the principle operation is based on keeping the main switch converter operating all time; connect the auxiliary switching converter only at the edges of the load current steps. In this way, the auxiliary converter completes the main converter performance in order to fulfill the requirements.

Fig. 3: Converter waveforms.

Fig. 4: Main and Auxiliary converter circuit.

Design Parameters of the Proposed Topology:

Table I: Design Parameters of The Proposed Topology.

Switching Frequency 250kHZ

Main Inductance 18 Micro Henry

Auxiliary Inductance 18 Micro Henry

Input Voltage 5V

Output Voltage 1.5V

Output Capacitor 7000 Micro Farad

Maximum Load Current Step 10A

Output Voltage Equation:

(2)

Current during the ON-state is given by

(3)

Conversely, the decrease in current during the Off-state is given by:

(4)

(5)

(6)

(7)

(8)

Hence, (9)

Basics of Fuzzy Controllers:

68 A.Rajkumar and B.Karthikeyan,2014

Advances in Natural and Applied Sciences, 8(21) Special 2014, Pages: 65-71

Fuzzy logic control is a new addition to control theory. Its design philosophy deviates from all the previous methods by accommodating expert knowledge in controller design. FLC is one of the most successful applications of, fuzzy set theory. Its major features are the use of linguistic variables rather than numerical variables [Wing-Chi So et al]. Linguistic variables, defined as variables whose values are sentences in a natural language (such as small and large), may be represented by fuzzy sets. FLC‟s are an attractive choice when precise mathematical formulations are not possible. [Liping Guo et al -2006, M. Hellmann et al, W. C. So et al]

Fig. 4: Fuzzy controller.

The general structure of an FLC is represented in Fig.4 and comprises four principal components:1) fuzzification 2) a knowledge base; 3) a decision making logic 4) defuzzification. Design of fuzzy logic or rule based non -linear controller is easier since its control function is described by using fuzzy sets and if-then predefined rules rather than cumbersome mathematical equations or larger look-up tables; it will greatly reduce the development cost and time and needs less data storage in the form of membership functions and rules. It is adaptive in nature and can also exhibit increased reliability, robustness in the face of changing circuit parameters, saturation effects and external disturbances and so on. [Paolo Mattavelli et al]. Design of fuzzy controllers is based on expert knowledge of the plant instead of a precise mathematical model. The first step in the design of a fuzzy logic controller is to define membership functions for the inputs. Fuzzy levels or sets are chosen and defined by the following library of fuzzy-set values for the change in threshold voltage.

The derivation of the fuzzy control rules is based on the following criteria:

a) When the output of the converter is above threshold limit, the auxiliary converter is connected in parallel with the main converter in order to take out the extra current by forcing the duty cycle 1.When the output of the converter is below threshold limit, the auxiliary converter is connected in parallel with the main converter in order to inject the additional current by setting the duty cycle 0.

b) When the output of the converter is below threshold limit, the main converter only being operated whereas the auxiliary converter is kept in off position. But the duty cycle remains unchanged.

Controller Action:

Table II: Controller Action.

Voltage level / Duty Cycle Main Converter Aux Converter

Switching Action

Below Threshold Reset On On

Above Threshold Set On On

Lies Between Threshold No Change On Off

Far from the set point:

When the output voltage is far from the set point (e, is PB or NB), the corrective action must be strong

meaning F, should be NB or PB, while should be zero.

IF e, is PB AND is NORM, THEN is PB  AND, is ZE.

IF e, is NB AND is NORM THEN , is NB AND is ZE.

This shows that far from the set point, the control action is denoted by the output voltage error, provided the existence of the current limit.

Close to the Set Point:

The current error must be taken properly into account in order to ensure stability and speed of response. The goal in this region is centered in achieving a satisfactory dynamic performance with small sensitivity to parameter variations. The control rules are according to energy balance and inductor current is far from the limit.

IF e, AND  are both Zero,  AND   must be zero too (steady state condition).

69 A.Rajkumar and B.Karthikeyan,2014

Advances in Natural and Applied Sciences, 8(21) Special 2014, Pages: 65-71

IF the output voltage error e, is Negative AND inductor current is greater than the reference value ( < 0), and   should be negative.

IF output voltage error is Positive AND the inductor current is greater its reference value, THEN  and  

must be kept to zero to prevent undershoot and overshoot.

IF the output voltage is Positive AND the current is lower than its reference value ( > 0),   and  must

be positive, the system energy increases in this condition.

Conservation of the fuzzy to crisp or non-fuzzy output is defined as De-fuzzification. In the defuzzification operation a logical sum of the inference result from each of the four rules is performed. This logical sum is the fuzzy representation of the change in duty cycle (output).

Removal of Output Harmonics:

In addition to the desired dc output voltage Vo the switch waveform also contains undesired harmonics of the switching frequency. In most applications, these harmonics must be removed, such that the converter output voltage is essentially equal to the dc component. A low-pass L-C filter is employed for this purpose as shown in the main block diagram of converter.

The filter has corner frequency fo= . The corner frequency fo is chosen to be sufficiently less

than the switching frequency f, so that the filter essentially passes only the dc component. To the extent that the inductor and capacitor are ideal, the filter removes the switching harmonics without dissipation of power. Thus, the converter produces a dc output voltage whose magnitude is controllable via the duty cycle D, using circuit elements that (ideally) do not dissipate power .

Simulation Diagram for Dc-Dc Converter without Fuzzy Logic:

Fig. 5: Simulation diagram without FLC.

Simulation Diagram for Dc-Dc Converter with Fuzzy Logic:

Fig. 6: Simulation diagram for DC-DC converter with Fuzzy logic.

Fig. 7: Simulation results of linear control and proposed fuzzy control converters under 10 A positive load Current step.

Fig. 8: Simulation results of linear control and proposed fuzzy control converters under 10 A Negative load Current step.

70 A.Rajkumar and B.Karthikeyan,2014

Advances in Natural and Applied Sciences, 8(21) Special 2014, Pages: 65-71

RESULTS AND DISCUSSION

Table III: Comparative Simulation results of linear control and proposed fuzzy control converters.

Converter Parameters Type of Load Current step Settling Time (us) Variation in Voltage

(mV)

Linear Controlled 100 151

Converter

Positive

Fuzzy Controlled 12 120

Converter

Linear Controlled 100 -118

Converter

Negative

Fuzzy Controlled 6 -54

Converter

From the above simulation result; it is observed that the o/p voltage of fuzzy control converter has less variations and the recovery time during load current step.

As it has an additional mechanism using fuzzy logic to suppress the voltage variations, so the recovery time and the variation in voltage are highly minimized.

Conclusion:

The new fast response DC-DC converter comprising of two buck converters connected in shunt and controlled by the fuzzy logic controller which is developed based on the linguistic description of the system and not its mathematical model .The fuzzy controllers were designed based on the in-depth knowledge of the converter using simulation model. First the DC-DC converter is designed using MATLAB for linear converter . Next the same DC-DC converter is designed with Fuzzy logic controller (FLC).

The results of linear converter and the converter with fuzzy logic were compared. The solution reduces the recovery time, settling time and variation of the o/p voltage in comparison with conventional linear control buck converter. As fuzzy control converter has a mechanism using fuzzy logic to suppress the voltage variations, so the recovery time, settling time and the variation in voltage are highly minimized. Since a fuzzy controller is a nonlinear controller. Therefore it is rapidly increasing interests in digital control from the industry in order to achieve small space and high efficiency in DC-DC converters.

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