High efficiency and high power factor single-stage balanced forward-flyback converter

1. High efficiency and high power
factor single-stage
balanced forward-flyback converter
By,
Kaushik K Naik
naikkaushik93@gmail.com
1
MTECH TECHNICAL SEMINAR
Contact for detailed report and ppt files

2. CONTENTS
• Introduction
• Conventional flyback converter
• Conventional forward converter
• Comparison of flyback and forward
converter for power factor correction
• Proposed flyback-forward converter
• Analysis
• Conclusion
• Reference
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3. Introduction
• Light emitting diodes (LEDs) are the most used
solid state light sources in almost all general
lightening applications.
• To drive this LEDs there are two types of drivers
which are generally used those are
i) Linear regulator
ii) Switched mode regulator
• Switched mode regulators are used widely due
to their high efficiency and high power density.
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4. Continued..
• LED drive has two power conversion stages:
i) power factor correction
ii) DC/DC conversion
• Flyback and forward converter are two
among several converters which use
transformer isolation between input and
output side.
• The proposed converter combines the
advantages of both of these.
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5. Conventional Flyback converter:
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 Derived from buck-boost converter.
Due to secondary winding on inductor electrical isolation is
achieved.

6. Continued…
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When switch is ON When switch is OFF

7. Conventional Forward converter:
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Practical forward converter has a magnetizing inductor and a
3rd winding called demagnetizing winding.
This converter is derived from buck converter.

8. Continued…
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When switch is ON When switch is OFF

9. Continued…
• During the time interval to, when i3 is flowing,
the voltage across the transformer primary as
well as Lm is
• Once the transformer demagnetizes, iLm = 0
and v1=0.
• The time interval t0 can be obtained by
recognizing that the time interval of voltage v1
across Lm must be zero over one period.
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10. Comparison of converters for PFC:
• Basic idea of doing PFC is to force the line
current to follow the waveform of the line
voltage which results in unity power factor.
• Incase of flyback converter, input current and
voltage have linear relationship which proves
that it has excellent self-PFC property.
• Due to the demagnetizing winding, forward
converter is not available for PFC.
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11. Continued….
Transformer magnetizing inductor currents of conventional flyback and forward
converters
• The magnetizing inductor offset current of flyback
converter is larger than that of forward converter.
• Due to less core loss and volume of transformer forward
converter is superior than flyback converter.
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12. Continued..
(a) PFC flyback converter (b) PFC forward converter
• The flyback converter can transfer the input energy to the
output side over an entire range of input voltage.
• The forward converter cannot transfer at the lower input
voltage than the reflected output voltage nVo to the
transformer primary side.
• The current dead zone near zero cross of AC input voltage
deteriorates the power factor in the forward converter.
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13. Proposed flyback-forward converter:
• Its primary side is exactly same as that of the conventional
flyback converter and its secondary side consists of one
output inductor (Lo) for forward operation.
• The balancing capacitor Cb makes the average current
through Cb during forward operation become exactly same as
that during flyback operation by the charge balance principle
of Cb
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14. Mode of operation:
Mode 1 (forward operating mode):
During this time interval the switch M1 is
closed. Diodes D2 and D3 are reverse biased
and D1 is forward biased.
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15. Continued…
Mode 2 (flyback operation):
The switch M1 is turned OFF and thus the input
source is disconnected from the rest of the
circuit. Thus the diode D1 will be reverse biased.
D2 and D3 are forward biased. 15

16. Continued…
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17. Analysis:
• Voltage conversion ratio:
– Voltage conversion ratio is defined as the ratio of dc output voltage to
the dc input voltage.
– The voltage across LM is equal to Vin during time interval DTs and
equal to n(Vo+Vcb) during time interval (1-D)Ts. Thus we can write.
DVin = n(Vo+Vcb)(1-D)
– Similarly voltage across Lo is equal to Vin/n +Vcb-Vo during the time
interval DTs and equal to Vo during time interval (1-D)Ts. Thus we can
write
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18. Continued…
– From the above two equations we can get the equation for Vcb as,
– From this we can write,
• Voltage stress of switch and diode:
– For a high turns ratio, the diode voltage stress is less but the the
switch voltage stress of the proposed converter is higher than that of
the conventional one due to the balanced capacitor voltage Vcb.
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19. Continued…
• Offset current of magnetizing inductor:
– Offset current of conventional flyback converter is,
– Offset current of proposed converter is,
– Thus the magnetizing offset current of the
proposed converter is lower than that of the
flyback converter with the aid of the balancing
capacitor Cb.
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20. Conclusion:
• Operation of flyback and forward converter is discussed and
performance of both are compared for PFC in terms of magnetizing
current and input output relationship.
• Proposed converter with balancing capacitor can operate as both
forward and flyback converter regardless of input voltage.
• Advantages of both flyback and forward converters are combined in
this converter.
• In this converter the power transfer is possible for entire switching
period and thus most favorable for LED drive operation.
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21. Reference:
[1] Yoon Choi, Moon-Hwan Keum,Sang-Kyoo Han, Jeong-il Kang,
“High efficiency and high power factor single-stage balanced
forward-flyback converter”, 39th Annual Conference of the IEEE
Industrial Electronics Society IECON 2013.
[2] Daniel W.Hart, Power Electronics, Pearson Education, Inc.,2010.
[3] N. Mohan, T. M. Undeland and W. P. Robbins, Power Electronics:
Converters, Application and Design, 2nd ed., John Wiley & Sons,
Inc., 1995.
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