DC/DC Converter Reference Lander, C. W. , Power Electronics, McGraw-Hill, Third Edition,1993. Ned Mohan, Power Electronics, Third edition, 2003 DC-DC CONVERTER

Switch Mode D.C to D.C Converters Switch-mode DC to DC converters convert one DC voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. This conversion method is more power efficient (often 75% to 98%) than linear voltage regulation (which dissipates unwanted power as heat). This efficiency is beneficial to increasing the running time of battery operated devices.  DC-DC CONVERTER

Introduction to D.C to D.C Converters (Choppers) DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply. They are also widely used in switch mode power supply(SMPS), dc-motor drive applications and renewable applications(solar power). Often input to these converters is an unregulated dc voltage, which is obtained by rectifying the line voltage. DC-DC CONVERTER

Structure Study on types of DC DC converters Buck converter Boost converter Cuk Converter Theory and operation Design Simulation DC-DC CONVERTER

Step Down (Buck) Converter Low pass filter low-pass filter is to reduce output voltage fluctuations As name implies a step-down converter produces a lower average output voltage than the dc input voltage E. Application: regulated switch mode dc power supplies dc motor drives Watch Video on BUCK CONVERTER DC-DC CONVERTER

Step Down (Buck) Converter Condition – Switch ON When switch S is turned on, current begins flowing from the input source to S and L, and then to C and the load. The diode is reverse biased due to the positive terminal of input voltage to cathode. The inductor voltage VL=E-Vo =L 𝑑𝑖 𝑑𝑡 Then magnetic field is built up in L and energy is stored in the inductor with the voltage drop across L opposing input voltage. DC-DC CONVERTER

Step Down (Buck) Converter Condition – Switch OFF When S is turned off, the inductor opposes any drop in current by suddenly reversing its EMF, and supplies current to the load. The diode is ON due to the polarity of inductor voltage. The current flows though the inductor to load and capacitor and back though the diode. The inductor voltage VL=-Vo = -L 𝑑𝑖 𝑑𝑡 DC-DC CONVERTER

Step Down (Buck) Converter- waveforms Switch state Inductor current Switch current Diode current Switch voltage Diode voltage Output voltage DC-DC CONVERTER

Output Voltage The DC output load voltage is a proportional to the input voltage and the ratio depends on d, duty cycle. 𝑉 𝑜 𝐸 = D or Vo = E x D where D is the duty cycle, and equal to Ton/T, where T is switching period, the inverse of the operating frequency fs. So by varying the duty cycle, the buck converters output voltage can be varied as a fraction of the input voltage. Thus:- The output voltage can be regulated by means of the duty cycle of the switch using PWM (pulse width modulation) DC-DC CONVERTER

Inductor current in buck converter Continuous Mode: Given the same duty cycle, an increase in the load current Io, resulting from a lower load resistance, would result in the inductor current always being greater than zero. Discontinuous Mode: A reduction in the load current would result in the inductor current falling to zero before the end of the off-period and hence being discontinuous. DC-DC CONVERTER

Inductor current in buck converter Just Continuous : The ideal is for the inductor current to be just continuous; discontinuous current creates problems in maintaining stability, but if the current is always greater than zero it leads to excessive losses and slows the response to transient load changes. DC-DC CONVERTER

buck converter- design Calculate D to obtain required output voltage 𝑉 𝑜 𝐸 = D Calculate Io from power output and output voltage requirement Calculate Inductor L The minimum value of inductor required to keep the converter operation in continuous-conduction mode can be calculated using DC-DC CONVERTER

buck converter- design Calculate Capacitor C C is calculated using equation of peak to peak ripple in the output ∆ 𝑉 𝑂 The output voltage ripple factor The filter capacitor C is practically very large for achieving nearly constant output at the load with minimum ripple. DC-DC CONVERTER

Psim simulation –buck converter DC-DC CONVERTER

DC-DC CONVERTER

Step up (Boost) Converter Step-up converter produces a higher average output voltage than the dc input voltage E. Applications: regulated switch mode power supplies DC drive Watch Video on BOOST CONVERTER DC-DC CONVERTER

Step up (boost) Converter-operation Switch ON The diode is reverse biased by the potential of the inductor voltage, so the current through the diode is zero. When S is switched on, current flows from the input source to L and S, and energy is stored in the inductor’s magnetic field. IL increases linearly and inductor voltage VL=E Load current is supplied by the charged capacitor C. DC-DC CONVERTER

Step up (boost) Converter-operation Switch OFF When S is turned off, the inductor opposes any drop in current by suddenly reversing its EMF. The inductor voltage adds to the source voltage and boost the voltage across the load through E L and D Voltage across inductor VL= E-V0 Capacitor C will be charged at this mode of operation DC-DC CONVERTER

BOOST Converter-waveforms Inductor current iL flows continuously Average inductor voltage over a time period must be zero Filter capacitor is very large to ensure constant output voltage DC-DC CONVERTER

Output Voltage-BOOST converter Output voltage always higher than the input voltage where D is the duty cycle, is fraction between 0-1 and D= Ton/T, where T is switching period T=1/fs fs is switching frequency DC-DC CONVERTER

BOOST converter-waveforms Switch state Inductor Current Switch Current Diode Current Switch Voltage Diode Voltage DC-DC CONVERTER

BOOST converter- design Calculate D to obtain required output voltage Calculate Io from power output and output voltage requirement Calculate Inductor L The minimum value of inductor required to keep the converter operation in continuous-conduction mode can be calculated using DC-DC CONVERTER

bOOST converter- design Calculate Capacitor C C is calculated using equation of peak to peak ripple in the output ∆ 𝑉 𝑂 The output voltage ripple factor The filter capacitor C is practically very large for achieving nearly constant output at the load with minimum ripple. DC-DC CONVERTER

CUK Converter Watch Video on CUK CONVERTER The Ćuk converter (pronounced Chook) is a type of DC/DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is essentially a boost converter followed by a buck converter with a capacitor to couple the energy. It uses a capacitor as its main energy-storage component, unlike most other types of converters which use an inductor Applications: regulated power supply Watch Video on CUK CONVERTER DC-DC CONVERTER

CUK Converter The circuit configuration is like a combination of the buck and boost converters and it delivers an inverted output. It combines the good characteristics of buck and boost converter(in terms of filtering on input and output sides) The output current must pass through C1 in the two modes of operation and it has an energy storing function, usually has high value. It has a function of transferring energy from input to output. Two inductors on input and output ensures the currents are non pulsating (smooth) DC-DC CONVERTER

CUK Converter Condition – Switch ON When S is turned on, current flows from the input source to L1 and S, storing energy in inductor’s magnetic field. Thus the current through the inductor rises. The diode is reverse biased by the polarity of C1 capacitor voltage. The capacitor C1, by the stored energy from the previous cycle will force current to the capacitor C2 and load via the switch. Both the input and output side currents flows through the switch S, in this cycle. DC-DC CONVERTER

CUK Converter Condition – Switch OFF S is turned off and diode is forward biased due to polarity of C1 Both the inductor current of L1 and L2 flows through the diode D. The inductor L1 current charges the capacitor C1 in this phase. The current in inductor L2 flows through the diode to capacitor C2 and load. In this mode both input and output current flows though Diode. At the condition of Switch ON and OFF, notice that the current though the capacitor C2 and load is from bottom to top, so the polarity of load voltage Vo is opposite to that of the source E. DC-DC CONVERTER

Output Voltage-CUK converter Output voltage to input voltage ratio for CUK Converter where d is the duty cycle, equal to Ton/T, where T is switching period DC-DC CONVERTER

Summary Buck converter: step-down, has one switch, simple, high efficiency greater than 90%, provides one polarity output voltage and unidirectional output current Boost converter: step-down, has one switch, simple, high efficiency, provides one polarity output voltage and unidirectional output current, requires a larger filter capacitor and a larger inductor than those of a buck converter Cuk converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal, additional capacitor and inductor needed DC-DC CONVERTER

Conclusions In many industrial applications, it is required to convert fixed dc voltage into variable dc voltage Various types of dc-to-dc converters Operation of dc-to-dc converters The step-down, step-up, and Cuk converters are only capable of transferring energy only in one direction A full-bridge converter is capable of a bidirectional power flow Like ac transformers, dc converters can be used to step-up or step-down a dc voltage source Applications: electric automobiles, trolley cars, marine hoists, mine haulers, etc. Also used in regenerative braking of dc motors to return energy back into the supply –energy savings for transportation systems with frequent stops Reference Power Electronics, C.W. Lander, McGraw-Hill, Third Edition,1993. Power Electronics, Ned Mohan, 3rd Edition,2003 DC-DC CONVERTER