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Application of Digital Slope Compensation in Peak Current Mode Control of Buck- Boost Converter

Nikhila N,A. Rajalakshmi
Department of EEE, Rajalakshmi Engineering College, Anna University, Thandalam, Chennai, India
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Abstract

Peak current mode control of a buckboost converter along with the load and transient analysis using a PIC microcontroller is done. The new elucidation includes a discrete time controller using digital slope compensation technique which removes all the subharmonic oscillations using a discrete staircase ramp. Combining a digital controller with peak current control gives a number of benefits which is implemented by a new single chip solution with on chip comparators without any external analog ICs. Digital slope compensation is implemented within the PIC controller as a digital staircase ramp using the on board modules.

Keywords

DC–DC switch mode power supplies (SMPS), peak current mode control, digital slope compensation, transient analysis and load variation

INTRODUCTION

Voltage mode control and current mode control are the two most popular control methods used in DC-DC converters. For controlling switched mode power supplies, current mode control is the commonly used method because of its advantageous features like better dynamic performance and natural protection from short circuit. This method can again be classified into average and peak current mode control. A control reference is used to regulate the peak current in peak current mode control which has benefits likeimproved load line regulation, better flux balancing, inherent cycle by cycle current limiting and protection. This control is commonly used in converters operating in continuous conduction mode. Here, when the duty cycle becomes more than 0.5, the feedback loop becomes highly unstable and this instability is called as sub harmonic oscillations. By using slope compensation technique an external ramp is added to reduce this oscillation.
In continuous conduction mode, the current will flow continuously throughout the switching period and one of its advantages over discontinuous conduction mode is that the dc conversion ratio is independent of the load, making the analysis simpler. Also peak currents are higher in discontinuous conduction mode which leads to heavy losses in the path of conduction affecting the efficiency and leading to switching stresses with higher ripples. A switched mode power supply (SMPS) is an electronic power supply that includes a switching regulator that converts electric power skillfully and this high efficiency is one of its important benefits. Source to load power flow is the main application of SMPS; (for eg: from the main power supply to any household appliance). During this transfer voltage and current characteristics are adjusted frequently. SMPS dissipates no energy when it is ideal and is the best alternative for linear regulator that controls the output voltage by dropping power frequently in the pass transistor because it reduces the unwanted wastage of energy and is highly efficient and is adaptive to usage when smaller size and lighter weights are considered.
Peak current mode control is mainly required in order to save the devices from damaging. In this control, two rapidly changing ramp signals are compared and thepeak current limit is said to be reached when these two ramps are equal. The two ramp signals considered are the slope compensated reference voltage signal and sensed output inductor current. Digital control of peak current with the implementation of slope compensation had been a very tedious task till recent days because of the unique features of the control technique and its need for extra support circuits which made it very expensive. But nowadays we can see that with the arrival of digital signal processors and microcontrollers with strong performance graphs, use of digital architecture for control has become very simple. Microcontrollers available today are very cheap, easier to get hold of and are of very high accuracy. So these are used in large number of applications that include power systems, industrial production and supply, rectifier supplies, medical applications etc [3].

BUCK-BOOS T CONVERTER

A. Block Diagram showing Peak Current Mode Control
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The input voltage source is connected with the buck-boost converter and has a resistive load connected in a closed loop circuit. Functions of the microcontroller are,
• Analog to Digital conversion
• Digital Slope Compensation
• Digital to Analog conversion
• PWM generation
When peak current problem arises, the output voltage as well as the input voltage are sensed by the microcontroller and based on the ‘C’ language coding, it compensates the ramp signals and incorporates the digital slope compensation technique. The inbuilt comparators of the microcontroller compares the output signal with the reference signal as well as the input signal and as a result PWM is generated which thereby turns on or turns off the switch of the converter so as to control the peak current.

PEAK CURRENT MODE WITH DIGITAL SLOPE COMPENSATION

The control circuit of the buck-boost converter when digital slope compensation is used is revealed in Fig.2. The basic principles of peak current mode control and the advantages of slope compensation are thoroughly discussed in this part.
image
In situations without use of slope compensation, some instabilities may occur when the duty cycle go beyond 50%.This can be reduced by adding a compensation ramp with slope to the switch off threshold. This compensation technique has now become easier to be controlled digitally with the advent of microcontrollers that use analog comparators (eg: PIC, Piccolo etc) which makes it feasible for conversion of the threshold value to analog voltage which represents the current threshold of the on chip comparators [6].
At the start of every cycle DPWM unit is used to turn on the switch and to restrict the duty cycle value within the maximum tolerable limit. Here the output from the comparator is connected with the DPWM generator and hence it compels the DPWM output to be switched off.But still with the use of slope compensation an appropriate control technique has to be developed for the digital control of peak current. An evident answer to this task is to implement slope compensation using analog technique and by adding ramp to the inductor current signal[6]. So it is evident that for the digital implementation of the ramp compensation, the discrete threshold value should be reduced permanently for every switching cycle with least possible step size.
image
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So the threshold value of the comparator current can be measured by considering the reference value i* which we get from the voltage controller and this is now transferred to the digital control through the analog to digital converter (ADC). Choosing of the compensating factor should be appropriate in order to adjust the obtained current threshold value with the slope compensation technique used [6].

DESIGN EXAMPLE

A. Calculation Of Duty Cycle
After choosing the various parameters of the converter the next priority objective is to obtain the duty cycle for the buck mode which is minimum and for the boost mode which is maxima [10].
image
Similarly in the boost modethe following equation is a fine [10] estimate for the appropriate inductance value:
image
When the ripple current shown in the inductor is 20or 40 percentage of the output current it is noted to be a proper calculation.

SOFTWARE SIMULATION

A. Simulation Circuit of the Buck-Boost Converter with Peak Current in Open Loop
The peak current in a buck-boost converter can be measured by an open loop circuit without using the digital slope compensation technique which forms the feedback.
image
B. Simulation Values of the Circuit Elements in Open Loop
Fig.4 shows the open loop circuit of the buck-boost converter (boost mode) showing peak current simulated in MATLAB. In the circuit the peak current and peak voltagebetween the load and the source that may occur due to the fluctuations in the load demand can be obtained and can be measured. In the boost mode the input is 12 V and the duty cycle is 62.5%. The switching frequency is 20 kHz.
image
Table.I shows the specifications of the buck-boost converter in open loop circuit. Using these values the simulation has been done and the waveforms are analyzed.
C. Simulation Circuit of the Buck-Boost Converter in Closed Loop using Digital Slope Compensation
The peak current in the open loop can be reduced using the digital slope compensation technique which is applied in the form of a feedback loop.
image
Fig.5 shows the circuit for the peak current mode control of the buck- boost converter (boost mode) simulated in MATLAB. In the circuit the voltage between the load and the source is continuously monitored by a feedback loop. Based on the comparator output the PWM is generated to update the duty cycle by which peak current appearance in the circuit is eliminated. For the boost mode the input is 12 volt and the reference voltage is 20 volts.
D. Simulation Values of the Circuit Elements in Closed Loop
image
Table. II shows the specifications of the buckboost converter in closed loop circuit. Using these values the simulation has been done and the waveforms are analyzed.

SIMULATION RESULTS

A. Output Current Waveforms of the Buck-Boost Converter with and without Digital Slope Compensation
In the closed loop operation digital slope compensation technique is used to reduce the peak current in the converter that is present in open loop. Hence the amount of peak current reduced can be measured. Also sub harmonic oscillations can be condensed using this technique easily
image
In open loop the current goes to maximum overshoot and then settle down to the required level.
Peak current in open loop = 0.145A
Peak current in closed loop = 0.0991A
Amount of peak current reduced is 45.9Ma
B. Output Voltage Waveforms Showing Peak Voltage in Open Loop Boost Mode And closed Loop Boost Mode
image
In open loop the voltage goes to maximum and then settles down to the required level.
Peak voltage in open loop = 29V
Peak voltage in closed loop=19.96V
Amount of Peak voltage reduced =29-19.96=9.04V
C. Transient Analysis
When there is any disturbance in the input voltage, the transients developed and the changes in the output voltage and output current waveforms are analyzed. Here the input voltage is changed from 12 V to 14 V and the stability analysis for both open loop as well as closed loop operation of peak current mode control of buck-boost converter is done.
image

CONCLUSION

Thus the simulation of peak current mode control of the buck-boost converter using discrete staircase slope compensation is discussed in this paper.The peak current and voltage waveforms with the transient analysis for open loop and closed loop circuits have been analyzed and the amount of peak current reduced is obtained.A novel digital staircase ramp is used to implement the digital slope compensation which guarantees a dynamic performance of the current control loop with less overshoot. The availability of less costly PIC microcontrollers with on chip comparators makes it feasible for digital slope compensation to be implemented with less effort and less cost than a DSP processor.

ACKNOWLEDGMENT

The authors wish to acknowledge the management of Rajalakshmi Engineering College and especially the faculties of EEE department for their full support during the course of this project phase. Also gratefully thank the helpful comments and suggestions of the reviewers which have greatly improved the presentation.

References

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