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

2. 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

3. 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

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

5. 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

6. 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

7. 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

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Psim simulation –buck converter
DC-DC CONVERTER

15. DC-DC CONVERTER

16. 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

17. 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

18. 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

19. 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

20. 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

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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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