1. Switching Power Supplies Week 6

2. Buck Converter

3. Buck Converter
A buck converter is a voltage step down and current step up converter. The simplest way to reduce the voltage of a DC supply is to use a linear regulator (such as a 7805), but linear regulators waste energy as they operate by dissipating excess power as heat.

4. Step-Down (Buck) Converter
converts dc from one level to another
the average output voltage is controlled by the ON-OFF switch
pulse-width modulation (PWM) switching is employed
lower average output voltage than the dc input voltage Vd depending on the duty ratio, D
D=ton/Ts
Average output:
Applications:
regulated switch mode dc power supplies
dc motor drives
low-pass filter: to reduce output voltage fluctuations
diode is reversed biased during ON period, input provides energy to the load and to the inductor
energy is transferred to the load from the inductor during switch OFF period
in the steady-state, average inductor voltage is zero
in the steady-state, average capacitor current is zero

5. Converts dc from one level to another
The average output voltage is controlled by the ON-OFF switch
Pulse-width modulation (PWM) switching is employed
Lower average output voltage than the dc input voltage Vd depending on the duty ratio, D
D=ton/Ts
Average output:
Applications:
regulated switch mode dc power supplies
dc motor drives

6. Step-Down (Buck) Converter: Continuous current conduction mode
Inductor current iL flows continuously
Average inductor voltage over a time period must be zero
Assuming a lossless circuit
Buck converter is like a dc transformer where the turns ratio can be controlled electronically in a range of 0-1 by controlling D of the switch

7. Example…..
For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio and “on” time of the switch.

8. Solution….
D = V0 /Vd = 5/40 = = 12.5%
Ts = 1/fs = 0.25 ms = 250 ms
Ton = DTs = ms
Toff = Ts – ton = ms
When the switch is “on”: VL = Vd - V0 = 35 V
When the switch is “off”: VL = -V0 = - 5 V
I0 = IL = V0 / R = 5 A
Id = D I0 = A

9. Voltage and current waveforms in a buck converter

10. Waveforms of the capacitor current and output voltage in a buck converter

11. Boost Converter

12. Voltage and Current Waveforms in a Boost Converter

13. Voltage and Current Waveforms in a Boost Converter

14. Impact of the inductor resistance on the voltage gain of a boost converter

15. Buck-boost Converter

16. Voltage and Current Waveforms in a Buck-boost Converter

17. 𝐂 uk Converter

18. Waveforms of voltage and current in a 𝑪 uk converter

19. SEPIC (a) and Zeta (b) Converters

20. Voltage and Current Waveforms in SEPIC and Zeta Converters

21. Voltage and Current Waveforms in SEPIC and Zeta Converters

22. Vorperian’s Switch Model

23. Averaged models of switched-mode dc-to-dc converters with Vorperian’s Switch Model: (a) Buck, (b) Boost, (c) Buck-boost, (d) 𝑪 uk

24. Waveforms of the output voltage and current in a buck converter: solid lines – actual converter, dashed lines – averaged model

25. Comparison of Converters
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
Buck-boost converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal
Cuk converter: step-up/step-down, has one switch, simple, high efficiency, provides output voltage polarity reversal, additional capacitor and inductor needed
Full-bridge converter: four-quadrant operation, has multiple switches, can be used in regenerative braking

26. 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, buck-boost 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