Coupled-Inductor Boost Integrated Flyback Converter with High-Voltage Gain and Ripple-free Input Current

1. Coupled-Inductor Boost Integrated Flyback
Converter with High-Voltage Gain and Ripple-free
Input Current
Submitted by :-
Jeetendra Prasad

2. Contents
• Introduction
• Motivation and Objective
• Methodology
• Progress and Results
• Conclusions
• References

3. Introduction
• DC–DC converters with high-voltage gain and low-input current ripple
have attracted much attention in photovoltaic, fuel cells and other
renewable energy system applications.
• Conventional boost–flyback converter can achieve high-voltage set-
up ratio; however, its input current is pulsing and the voltage stress
across output diode of flyback-cell is high.
• By incorporating coupled-inductor into the boost-cell of boost–
flyback converter, the voltage stress across the output diode is
effectively reduced.

4. Coupled Inductor
• A coupled inductor is actually a pair of inductors.
• A normal inductor is coupled only to itself , that is, it has one input and one
output terminal, with voltage and current following the relationship.
• Coupled inductors are coupled together , that is, the input to one will
result in an output on both.
• Coupled inductors are often used to take advantage of the current-ripple
cancellation from magnetic coupling between the phases.
• Normally, current-ripple cancellation happens only at the output of the
multiphase buck converter when typical discrete inductors are used. When
these inductors are magnetically coupled, the current-ripple cancellation is
applied to all elements of the circuit: MOSFETs, inductor windings, PCB
traces.

5. • Thus, the switching from all the phases affects each single phase, so the
current ripple is reduced in amplitude and multiplied in frequency.
Reduction in the RMS of the waveforms can improve the efficiency of the
power converter or be traded for smaller magnetics, faster transient and,
therefore, smaller output capacitance.
Difference between coupled inductor and a normal inductor :
• A coupled inductor is generally a pair of inductors.
• A normal inductor is coupled only to itself , that is, it has one input and one
output terminal, with voltage and current following the relationship
• For a normal inductor :
V = L di/dt
• For coupled inductor :
V1 = L1 dI1/dt – M dI2/dt

6. Advantages of Coupled Inductor
• Coupled inductors are often used in multiphase topologies to take
advantage of the current-ripple cancellation from magnetic coupling
between the phases.
• When these inductors are magnetically coupled, the current-ripple
cancellation is applied to all elements of the circuit: MOSFETs, inductor
windings etc.

7. Flyback Converter
Circuit diagram :
Fig 1. Flyback converter
• The flyback converter is used in both AC/DC and DC/DC conversion
with galvanic isolation between the input and any outputs.
Vs
*
*
N1 N2
Lm
+
V1
_
_
V2
+
+ vD -
iC iR
iS
i2
i1
iLm
+
V0
_
RC
+
vSW
_
iD
Transformer

8. • The flyback converter is a buck-boost converter with the inductor split
to form a transformer, so that the voltage ratios are multiplied with
an additional advantage of isolation.
• When driving for example a plasma lamp or a voltage multiplier ,the
rectifying diode of the boost converter is left out and the device is
called a flyback transformer.

9. Motivation
• In recent years, high-voltage gain and input current ripple-free DC–DC
converters have attracted much attention in various industry applications.
• Example: Uninterruptible power supply, electric traction, distributed
photovoltaic (PV) generation systems, fuel cell energy conversion systems
and automobile high intensity discharge (HID) headlamps [1–3].
• Boost converter can provide high-voltage gain when operates at extremely
high duty cycle [4].
• However, high duty cycle operation increases input current ripple of boost
converter, as well as conduction losses and turned-off losses of power
switch.
• High duty cycle operation also affects transient response of boost converter
because of narrow turned-off time of power switches.

10. • To achieve high-voltage gain without operating at extremely high duty
cycle, many converters have been studied [5].
• By using switched capacitor or switched inductor cell, high-voltage
gain can be achieved [6].
• the power switch suffers from high transient current and large
conduction losses.
• Furthermore, a lot of switched capacitors or switched inductor cells
are indispensable to obtain extremely high-voltage step-up gain,
which increases the circuit complexity.

11. Objective
• Isolated or coupled-inductor-based converters [6] can be used to realize
high-voltage step-up gain, where the turns ratio of coupled-inductor can be
employed as a design freedom to increase the voltage gain.
• By incorporating coupled-inductor into boost-cell, high-voltage gain of the
proposed converter is achieved by adjusting turns ratio of coupled-
inductor.
• To reduce voltage spikes across the power switch induced by leakage
inductance of coupled-inductor, a passive lossless snubber circuit with a
diode and a capacitor is utilized and the leakage inductor energy is recycled
and voltage spike across the power switch is suppressed.
• Therefore a low-voltage stress, low on-resistance metal–oxide
semiconductor field effect transistor (MOSFET) can be used to reduce
conduction losses to improve power conversion efficiency of the converter.

12. Working of Flyback Converter
Switch closed :
• When switch is closed, then diode is reverse biased.
• Energy is stored in magnetizing inductor Lm .
Vs
*
*
N1 N2
Lm
+
V1=Vs
_
Is = iLm
0
0
iLm

13. Switch open :
• Diode is forward biased.
• Energy stored in inductor is transferred to load.
Vs
*
*
N1 N2
Lm
+
V1
_
_
V2
+
iLm
+
V0
_
RC
+
vSW
_
iD

14. Methodology
Fig 2. Coupled inductor Boost Integrated flyback-converter with high voltage gain
and ripple free input current
Vin
iLa
La
+ VLa -
- VC1 +
C1
np
ns2
Dc
ns1
iCc
Cc
+
VCc
-
Co1
+
Vo1
-
+
Vo
-
iDo2
Do2
Co2
+
Vo2
-
Io
Ro
S1
Is1
Cs1
Ds1
*
*
*
iDo1
iDc

15. Fig 3. Equivalent circuit of Coupled inductor Boost Integrated flyback-converter
with high voltage gain and ripple free input current
Vin
iLa
La
+ VLa -
- VC1 +
C1
np
ns2
Dc
ns1
iCc
Cc
+
VCc
-
Co1
+
Vo1
-
+
Vo
-
iDo2
Do2
Co2
+
Vo2
-
Io
Ro
S1
Is1
Cs1
Ds1
*
*
*
iDo1
iDc
ip
Ls
Lm
im
Do1
C1 : Intermediate storage capacitor
La : Input filter inductor
C0 : Output filter capacitor
R0 : Load resistor

16. Assumptions made ::
(a) Power switch S1 is ideal except its anti-paralleled diode and output
capacitor.
(b) Capacitors C1, Cc, Co1 and Co2 are so large that voltages Vc1, Vc2, Vo1
and Vo2 can be considered as constants in a switching cycle.
Circuit Description ::-
The coupled-inductor is modelled by an ideal transformer with turns
ratio of np:ns1:ns2 = 1:n1:n2, a magnetising inductor Lm and a leakage
inductor Ls.

17. Operating Mode Analysis
• In one switching cycle, there are three operation modes.
• At the beginning of each switching cycle, magnetising inductor
current im is > 0.
• Output diodes Do1 and Do2 are conducted for freewheeling current.

18. Mode 1:
Fig 4. Mode 1[t0 ∼ t1]
im(t)= 𝑖𝑚(𝑡0) +
𝑉𝑜1 −𝑉𝑐1
𝑙 𝑚
(𝑡 − 𝑡0)
Vin
iLa
La
+ VLa - - VC1 +
C1
np
ns2
Dc
ns1
iCc
Cc
+
VCc
-
Co1
+
Vo1
-
+
Vo
-
iDo2
Do2
Co2
+
Vo2
-
Io
Ro
S1
Is1
Cs1
Ds1
*
*
*
iDo1
iDc
ip
Ls
Lm
im
Do1

19. Mode 2:
Fig 5. Mode 2[t1 ∼ t2]
im(t)= 𝑖𝑚(𝑡0) -
𝑉𝑜2
𝑛2 𝑙 𝑚
(𝑡 − 𝑡1)
Vin
iLa
La
+ VLa -
- VC1 +
C1
np
ns2
Dc
ns1
iCc
Cc
+
VCc
-
Co1
+
Vo1
-
+
Vo
-
iDo2
Do2
Co2
+
Vo2
-
Io
Ro
S1
Is1
Cs1
Ds1
*
*
*
iDo1
iDc
ip
Ls
Lm
im
Do1

20. Mode 3:
• At time t2, current ip decreases to zero and diode Dc is turned off with
zero current, reverse-recovery loss of diode Dc is eliminated.
• The input inductor energy is transferred to the load and capacitor C1
is discharged to the load.
• Current im decreases linearly until the beginning of next switching
cycle.
Vin
iLa
La
+ VLa -
- VC1 +
C1
np
ns2
Dc
ns1
iCc
Cc
+
VCc
-
Co1
+
Vo1
-
+
Vo
-
iDo2
Do2
Co2
+
Vo2
-
Io
Ro
S1
Is1
Cs1
Ds1
*
*
*
iDo1
iDc
ip
Ls
Lm
im
Do1
Fig 6. Mode 3[t2 ∼ t3]

21. Fig 7. Typical waveforms
of the proposed
converter

22. Condition for ripple free input current :
• According to analysis of various modes, in one switching cycle, voltage
across the input filter inductor La can be written as :
VLa = La * diLa(t)/dt = Vin – Vo1 + VC1
• Since storage capacitor C1 and output filter capacitor Co1 are very large,
hence, their ripple voltages can be ignored, that is, the voltages across C1
and Co1 is constant in one switching cycle.
• Hence the voltage across inductor La is very close to constant.
• For acheiving ripple-free input current, inductor current iLa should be fixed,
that is, diLa /dt = 0, here, the voltage across intermediate capacitor C1 can
be expressed as:
VC1 = Vo1 – Vin
• As long as VC1 is satisfied, we can achieve ripple free input current.

23. Boundary condition for magnetising inductor Lm:
• For continuous conduction mode operation of inductor Lm:
2Im > ∆im
• From operation mode analysis:
Im = ILa + (n1+n2) *Io = (1+n1+n2) *Vo / ((1-D) *Ro)
∆im = Vin*DTs / Lm
• Therefore, for inductor Lm to operate in CCM:
2Im > ∆im
• From above condition:
K1 > Kcrit (n1, n2, D)
• Where, K1 = 2Lm / (RoTs)
Kcrit (n1, n2, D) = D*(1-D)2 / (1+n1+n2) *(1+n1D+n2D)

24. Performance comparison between proposed converter and flyback
converter:
• Through suitable design of the turns ratio of coupled inductor, high value of
voltage gain can be obtained to avoid operation at high duty cycle, this is
the reason why a low switch-on-resistance and a low voltage rated MOSFET
are used to avoid large conduction losses.
• When comparing with traditional active-clamp coupled-inductor boost
converter, voltage stress across the output diode Do1 in boost cell is been
reduced effectively by adjusting the turns ratio of the coupled inductor.
• By doing this we can reduce the reverse recovery losses and conduction
losses both of diode Do1 because we have already applied a low voltage
rated diode
• In the same regards, the voltage stress across the diode Do2 in flyback cell is
very low compared with boost flyback converter.

25. Results
• Main advantage of our simulated converter is that it increases the
voltage gain for the same input voltage over simple flyback converter.
• So, first we simulated flyback converter which has following
parameters:
• Input voltage, Vin = 30V
• Switching frequency, fs = 70KHz
• Output Voltage, Vo = 142.3V
• Gain = 4.74

26. Simulation diagram of flyback converter

27. Main parameters of the proposed converter :
• Input voltage, Vin = 30V
• Output Voltage, Vo =212.3V
• Switching frequency, fs = 70KHz
• Coupled inductor, Turns ratio = 15T : 23T : 60T
• Lm = 50uH
• Ls = 1uH
• Input inductor, La = 50uH
• Capacitor, Cc = 4.7uF
• Energy capacitor, C1 = 4.7uF
• Output capacitor, Co1 & Co2 = 200uF

28. Simulation diagram of Coupled-inductor
boost integrated flyback converter

29. Related Waveforms :
Switch voltage and current:
• Variation of voltage across the
switch and current through it is
shown in the figure. The voltage
across the switch is obtained
equal to 49V. In transient state
some voltage spikes were
present. The waveform shown is
of the steady state condition.
• This voltage spike present is due
to the resonance of leakage
inductor Ls and parasitic
capacitances of switch S1, diode
DC and capacitor CC when switch
is off.
Fig 8. Variation of voltage across the switch
and current through it

30. Current through diodes Do1 ,Do2
and Ls :
• In the figure leakage inductor
current iP, diode currents Do1
and Do2 has been shown. It can
be observed that when the
switch is turned off then the
leakage energy is transferred to
capacitor Cc. Value of leakage
inductor is chosen as 1 micro
henry. Leakage inductance of
the coupled inductor is
considered < 5% of magnetising
inductance of the transformer.
Fig 9 Current waveforms of diodes Do1, Do2 and Ls

31. Fig 10. voltage across capacitor Cc and
current through Ls
• In the figure current flowing through
the inductor La and voltage VCc is
shown. It can be observed easily that
the input current ripple of the
proposed converter is nearly zero. The
value of the input inductor La is taken
to be equal to 50 micro henry.

32. Output voltage :
• In the figure the output voltage
waveform of the proposed converter
and flyback converter has been
shown. For input voltage of 30V, we
have achieved the output voltage of
212.3V and 134V in case of
proposed converter and simple
flyback converter. Hence, the gain
obtained is 7.07 for our proposed
converter and 4.46 in case of flyback
converter, which is far better than
the gain obtained after the
simulation of simple flyback
converter. Furthermore, the ripple
content in the output voltage is
been reduced
Fig 11 Variation of voltage across the load
(flyback converter)
Fig 12 Variation of voltage across the load(project)

33. Current through magnetizing
Inductor, Lm :
• In the figure the current flowing
through the magnetising inductor
Lm is been shown. The current
through inductor Lm increases
linearly in mode 1. This same
current decreases linearly in mode
2, while in mode 3, this magnetising
current is freewheeled through the
secondary side of the coupled
inductor and diodes Do1 and Do2.
This current decreases linearly until
the beginning of next switching
cycle.
Fig 13 Magnetizing current, Im

34. Input and output power:
• In the figure the waveform for
the input and output power is
been shown. The value of input
power obtained is 112.6W and
the output power obtained is
197.5W with duty ratio of 0.47.
Fig 14. Input and output power

35. Conclusion :
• In this project a coupled-inductor boost integrated flyback converter with high-voltage
gain and ripple-free input current has been analyzed.
• The operating principle and operating characteristics of the proposed converter is
analyzed in detail.
• The turns ratio of coupled-inductor can be employed as a design freedom to extend the
voltage gain.
• Voltage stress of switch is far lower than output voltage.
• Output diode voltage stress is reduced.
• At the cost of increased count of the components, the efficiency of our proposed
converter is improved. Input current is also ripple free, which is the main reason for easy
designing of the input filter and it also reduces the size and weight of the converter.
• Based on above advantages, the proposed converter is suitable for PV, fuel cells and
other applications.

36. References :
1. Chen, S.M., Liang, T.J., Yang, L.S., et al.: ‘A safety enhanced, high step-up
DC–DC converter for AC photovoltaic module application’, IEEE Trans. Power
Electron., 2012, 27, (4), pp. 1809–1817
2. Lai, C.M., Pan, C.T., Cheng, M.C.: ‘High-efficiency modular high step-up
interleaved boost converter for DC-microgrid applications’, IEEE Trans. Ind.
Appl., 2012, 48, (1), pp. 161–171
3. Tseng, K.C., Tsai, M.H., Chan, C.Y.: ‘Design of high step-up conversion circuit
for fuel cell power supply system’. Proc. IEEE ISNE, 2013, pp. 506–509
4. Erickson, R., Maksimovic, D.: ‘Fundamentals of power electronics’ (Kluwer
Academic Publishers, 2001, 2nd edn.)

37. 5. Ismail, E.H., Al-Saffar, M.A., Sabzali, A.J., Fardoun, A.A.: ‘A family of single-
switch PWM converters with high step-up conversion ratio’
6. Axelrod, B., Berkovich, Y., Ioinovici, A.: ‘Switched capacitor/switched inductor
structures for getting transformerless hybrid DC–DC PWM converters’, IEEE Trans.
Circuits Syst. I, Regul. Pap., 2008, 55, (2),
7. Park, K.B., Moon, G.W., Youn, M.J.: ‘High step-up boost converter integrated with
a transformer-assisted auxiliary circuit employing quasi-resonant operation’, IEEE
Trans. Power Electron., 2012, 27, (4),

38. THANK YOU