ISSN (PRINT) :2320 – 8945, Volume -1, Issue -2, 2013
33
An Isolated Cuk Converter with Multiple Outputs Using PWM Controller
Kavya Shree G V1, Eranna2 & K Chandra Mohan Reddy3
1&2Dr. AIT, Bangalore-56, 3ISRO, LEOs, Bangalore
E-mail : [email protected]1, [email protected]2, [email protected]3
Abstract – Cuk converter is a combination of buck & boost topologies , provides an output voltage that is less than or greater than the input voltage & the output voltage polarity is opposite to that of input voltage.
The basic cuk converter is derived from the boost & buck converter topology, combining the characteristic low input current ripple of the boost converter with the low output current ripple of the buck converter. The main applications of this circuit are in regulated dc power supplies, where a negative polarity output may be desired with respect to the common terminals of the input voltage and the the average output is either higher or lower than the dc input voltage.
The dc-dc converter converts the input dc from one level to another level. This is accomplished by high switching , using frequency isolation transformer. The obtained output is unregulated one, it is regulated by means of a feedback control that employs a pwm controller , where the control voltage is compared with a saw tooth waveform at the switching frequency, the error signal is processed in the error amplifier & with change in error signal the drive circuit will control the duty ratio of the switching element to keep the output constant at all the times.
Keywords – cuk converter, pulse width modulation, voltage mode control, feedback loop.
I. INTRODUCTION
Dc-dc power converters are employed in a variety of applications, including power supplies for personal computers, office equipment, spacecraft power systems, laptop computers, and telecommunications equipment, as well as dc motor drives. The input to a dc-dc converter is an unregulated dc voltage Vg. The converter produces a regulated output voltage V, having a magnitude (and possibly polarity) that differs from input voltage.
Regulated DC power supplies are needed for most analog and digital electronic systems. Most power
supplies are designed to meet some or all of the following
requirements.
Fig 1: Block Diagram
a. Regulated output: The output voltage must be held constant within a specified tolerance for changes within a specified range in the input voltage and the output loading.
b. Isolation: The output is required to be electrically isolated from the input.
c. Multiple outputs: There will be multiple outputs (positive and negative) that will differ in their voltage and current ratings. Such outputs are isolated from each other. In addition to these requirements, common goals are to reduce power supply size and weight and improve their efficiency.
CUK CONVERTER
The typical schematic circuit for the Cuk Converter is as shown in Fig. 1. The capacitor C1 acts as a primary means to store and transfer the power from input to output. The voltage vc1 is always greater than either input or output voltage.
The average output to input relations are similar to that of a buck-boost converter circuit. The output
ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
ISSN (PRINT) :2320 – 8945, Volume -1, Issue -2, 2013
34 voltage is controlled by controlling the switch-duty cycle. The ratio of output voltage to input voltage is given by:
Vo/Vin= D/(1- D)=Iin/Io
Where, Vo and Vin are the output and input voltages, respectively. The term Io and Iin are the output and input currents, respectively. The term D is the duty ratio and defined as the ratio of the on time of the switch to the total switching period. This shows the output voltage to be higher or lower than the input voltage, based on the duty-ratio D.
Fig 2: CUK Converter II. ISOLATED ĆUK CONVERTER
Fig 3: Isolated CUK Converter
In the majority of applications, it is desired to incorporate a transformer into the switching converter, to obtain dc isolation between the converter input and output.
A single power supply unit needs to give several different voltage outputs. The individual output voltages may have different ratings in terms of output current, voltage regulation and ripple voltages. These outputs may need isolation between them. Generally a common high frequency transformer links the input and output windings and in spite of output voltage feedback all the outputs cannot have same regulation because of different loads connected to different outputs and hence different ohmic (resistive) drops in the output windings (loads are generally variable and user dependent). Also the coupling between the different secondary windings and the primary winding may not be same causing
different voltage drops across the respective leakage inductances.
When the switch S1 turns on, inductor L1 will be charged and capacitor Ca discharges energy through S1
simultaneously. On the secondary side, the current will be negative and the capacitor Cb and inductor L2 will damp their energy to the load together schematic of Isolated Ćuk Converter.
When the switch S1 is on, inductor L1 and the DC source will charge the capacitor Ca. On the secondary side, the current i_Sec will positive and the current of capacitor Cb flows via diode D1. Inductor L2 will release its energy to the load at the same time.
ADVANTAGES
This circuit has a very low output current ripple due to the presence of L2 in the output circuit, similar to the Buck regulator. It also has a very low ripple current in the input circuit due to the presence of L1, similar to the Boost regulator. A further advantage is that only one switching device is required, with no base drive coupling problems as in the bridge and half-bridge circuits.
Another advantage of Cuk converter is capacitive isolation, the failure of the switching transistor will short the input and not affect the output.
The two inductors L1 and L2 can be coupled together on one core.
The converter can be both Boost and Buck the output voltage.
DISADVANTAGES
In spite of advantages of Cuk converter, there are significant disadvantages which prevent its wide spread use.
The converter is difficult to stabilize. Complex compensation circuitry is often needed to make the converter operate properly. This compensation also tends to slow down the response of the converter, which inhibits the dimming capability of the converter.
An output current controlled cuk converter tends to have an uncontrolled and undamped resonance due to an L-C pair (L1 & C1). The resonance of L1 &
C1 leads to excessive voltage across the capacitor, which can damage the circuit.
Another disadvantage of the Cuk regulator is that the series capacitor is the main energy storage/transfer component, instead of an inductor as in most other circuits. This capacitor, therefore,
ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
ISSN (PRINT) :2320 – 8945, Volume -1, Issue -2, 2013
35 has to be relatively large and capable of handling high r.m.s. currents, so a low ESR is essential.
CIRCUIT DIAGRAM
Fig 4: Circuit Diagram
SMPS are rapidly replacing linear regulated power supplies in most of the consumer electronic applications due to their advantages like higher efficiency, better output voltage regulation, compact size and capability to provide isolation between multiple outputs . Linear power supply basically consists of a mains transformer and a dissipative series regulator. This means a large and heavy 50/60 Hz transformer is used at the front end that increases the cost, size and weight of the power supply. The efficiency of a linear power supply is very poor and is of the order of 30%-60%. SMPS typically uses switching frequencies of the order of a few tens of kHz and hence the size of associated filtering components and the transformer is reduced drastically.
Personal Computers and embedded system based control applications require power supplies with multiple outputs delivering stiffly regulated and isolated DC voltages at different levels such as ±15V.
III. VOLTAGE MODE CONTROL
In the direct duty ratio PWM control if the input voltage changes, an error is produced in the output voltage, which gets eventually corrected by the feedback control. This results in a slow dynamic performance in regulating the output in response to the changes in input voltage. If the duty ratio could be adjusted directly to accommodate the change in the input voltage, then the converter output would remain unchanged. This can be accomplished by feeding the input voltage level to the PWM IC. The PWM strategy is similar with the direct duty ratio PWM control except that the ramp does not stay constant but varies in direct proportion to the input voltage results in a decreased duty ratio.
Fig 5:Voltage Mode Control
The SG1825C is a high-performance pulse width modulator optimized for high frequency current-mode power supplies. Included in the controller are a precision voltage reference, micropower start-up circuitry, softstart, high-frequency oscillator, wideband error amplifier, fast current limit comparator, full double- pulse suppression logic, and dual totempole output drivers. This device can be used to implement either current mode or voltage-mode switching power supplies. It also is useful as a series-resonant controller to frequencies beyond 1MHz.
PROJECT SPECIFICATIONS AND FEATURES Cuk Converter Design Specification
Input voltage: Vin= 110 Volts Output voltages : Vout = +15v,-15v Output current : Io = 1 amps Switching Frequency : fs=250kHZ Duty ratio : D= 0.25
Core material : ferrite Efficiency : >76.4%
ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
ISSN (PRINT) :2320 – 8945, Volume -1, Issue -2, 2013
36 EXPERIMENTAL TEST READINGS
IV. CONCLUSIONS
This proposed work provides the regulated power supply for the SMPS application. An effective design and exhaustive performance analysis of a multi-output CUK converter applied to an SMPS system has been carried out in this work. SMPS is an electronic power supply unit (PSU) that incorporates a switching regulator. While a linear regulator maintains the desired output voltage by dissipating excess power in a pass power transistor, the switched-mode power supply switches a power transistor between saturation (full ON) and cut off (completely OFF) with a variable duty cycle whose average is the desired output voltage. The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller size and lighter weight and lower heat generation due to higher efficiency. Disadvantages include greater complexity, the generation of high amplitude, high frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency. For the required design specification Cuk Converter topology is selected.
The main applications of this circuit are in regulated dc power supplies, where a negative polarity output may be desired with respect to the common terminals of the input voltage and the average output is either higher or lower than the dc input voltage.
V. REFERENCES
[1] R.D. Middle brook and S. Cuk, “Isolation and Multiple Output Extensions of a New Optimum Topology Switching DC-to-DC Converter,” in Proc. of IEEE PESC 1978, pp. 256-264.
[2] C.A. Canesin and I. Barbi, “A Unity Power Multiple Isolated Outputs Switching Mode Power Supply Using a Single Switch,” in Proc. Of IEEE- APEC’91, pp. 430 -436.
[3] Abraham I. Pressman, Switching Power Supply Design, 2nd ed., McGraw Hill, New York.
[4] R.W. Erickson, Fundamentals of Power Electronics, Kluwer Academic Publishers, Massachusetts, 1999.
[5] Jutty, M.K.; Kazimierczuk, M.K, “Efficiency of the transformer Ćuk PWM converter”; Aerospace and Electronics Conference, NAECON 1993, Proceedings of the IEEE 1993 National 24-28 May 1993 Page(s):639 - 644 vol.
Input voltage
Input current
Output voltage
Output current
Efficien cy
Total width
On time 60V 91mA 14V 149mA 76.4% 4us 0.9us
