Fault Tolerant Control Method for Interleaved DC-

DC Converters under Open and Short Circuit Switch

Faults
Elham Pazouki Jose Alexis De Abreu-Garcia Yilmaz Sozer
Department of Electrical and Computer Engineering,
The University of Akron,
Akron, OH, 44325-3904

Abstract— This paper proposes an adaptive democratic sharing of each module. In [5], a new current sharing method
current sharing control scheme for interleaved dc-dc which maximizes the global efficiency of N paralleled DC-DC
converters under faulty condition. The proposed fault boost converters is presented. The method proposed in [6] can
tolerant control approach can achieve the desired output
not only improve the current balance in each phase of
voltage generation, and equal current sharing among the
remaining healthy modules as more and more modules paralleled converters, but it also presents a framework for the
fail. The main idea is to adaptively change the outer stability analysis with active current sharing. In [7], in order to
voltage and inner current controller gains once the fault is avoid the current control loop in each phase of the interleaved
detected in the modules. The proposed control scheme is converter, a 16-phase interleaved power converter operating in
compared to the democratic current sharing control discontinuous conduction mode (DCM) is proposed for
scheme under the fault condition and step load variation. automotive applications [8]. The control method for the input
The results show that it achieves improved output
series output parallel (ISOP) connection proposed in [9], does
regulation and accurate current sharing among the
modules. The presented fault tolerant control method has not require any input-voltage of load current-share loop, and it
been analyzed, tested, and validated for the interleaved relies on the common duty ratio for all the converters. In
boost converter. general, all of the above approaches provide effective control
methods for modular converters. However, under abnormal
Index Terms— DC–DC power converters, fault tolerant conditions they lose the equal sharing of the voltage and
control, digital signal processors, power semiconductor
currents among the healthy modules.
switches, switched-mode power, and digital control.
Many fault tolerant control methods have been tested under
the fault condition of each of the modules in modular dc-dc
I. INTRODUCTION converters. In [10], a fault tolerant control and circuit topology
has been proposed for ISOP dc-dc converters. In [11], a new
Input parallel output parallel (IPOP) dc-dc converters, or
master-slave control scheme is proposed for ISOP dc-dc
interleaved dc–dc power converters, are used to achieve high-
converters, and it achieves fault tolerant operation through
power and high-current. The most important advantages of
changing the status of the master module with any other
interleaved dc-dc converters are their increased efficiency and
healthy module. In [12], a three-loop control scheme for ISOP
reduced current and voltage ripples, which result in reduced
dc-dc converters is proposed which consists of an output
inductor and capacitor size [1]. Interleaved dc-dc converters
voltage loop, individual inner current loops, and individual
also have improved fault tolerant capabilities because of
input voltage loops. In [13], the fault-tolerant strategy is
increased redundancy [2]-[3].
proposed for a cascade H-bridge multilevel converter (CHMC)
Many control methods have been proposed for modular based on a static synchronous compensator (STATCOM). It
power converters for equal sharing of voltages and current utilizes N H-bridge building blocks (HBBB) to achieve N-1
among modules. The droop method proposed in [4], controls redundancies in the system. A new structure of the multiphase
the reference voltage of each module in the parallel converter, and multilevel converter which achieves improved fault
and improves the output voltage regulation as well as current tolerance under the open and short circuit fault condition is

978-1-5090-2998-3/17/$31.00 ©2017 IEEE 1137

proposed in [14]. The proposed fault tolerant control method
L1 I L1
in this paper ensures continues power generation and equal
D1
sharing of current among the remaining healthy modules in the Iin L2 IL
2
IPOP dc-dc converter. In [15]-[16], the fault diagnosis method D2
can detect and identify the fault in less than one switching L3 IL
cycle for non-isolated dc-dc converters. The fault diagnosis + 3
+
D3 Ro Vo
Vin Co
module implemented on the DSP generates the fault detection S1 S2 S3
− −
and identification signals which identifies the switch fault type
and faulty switch. The proposed fault tolerant control method
Q1 Q2 Q3
in this digest utilizes the generated fault diagnosis signal to
adaptively change the gains of the voltage and current
controller under the fault condition. −
PWM Generator + I ref
The remainder of this paper is organized as follows. Section PI ¦
Phase Delay=0
II presents the basic operation of the interleaved dc-dc
converter. Section III presents the proposed fault tolerant PWM Generator
− −
− ¦ +
PI ¦− + PI
control method. Experimental results are provided in Section Phase Delay=2π / 3

IV. Finally, conclusions are given in Section V. Vref

PWM Generator
−
Phase Delay= 4π / 3
PI ¦ +

II. OPERATION OF INTERLEAVED CONVERTER

Fig. 1. Democratic current sharing scheme for interleaved dc-dc converter.
A. Basic Operation of Converter under Normal Condition
Interleaved dc–dc power converters are composed of n
phases connected in parallel; with the phase currents phase- loops. The current PI controllers regulate the current providing
shifted by 2ʌ/n. This configuration has been proposed to share the duty cycle command to the pulse width modulation (PWM)
the load current, and also to improve reliability with increased generator.
redundancy. In Fig. 1, a three-level interleaved dc-dc boost
converter is shown. The switch command pulses to the B. Boost Converter Operation under Faulty Condition
switches are phase-shifted by 2ʌ/3. In this section, the operation of the interleaved dc-dc
converter under the faulty condition of the modules is studied.
In an ideal situation, it is assumed that all the modules are
One of the main reasons for the failure of dc-dc power
identical. For the ideal operating condition, the voltage
converters is semiconductor switch faults. The major faults in
regulator is enough to ensure the desired output voltage
switches can be categorized as open circuit faults (OCF) and
generation and equal sharing of currents among each of the
short circuit faults (SCF) [17]. The SCF is the most severe
phases. However, in reality, the modules may be slightly
switch fault, and it causes a high current which exceeds the
different because of small mismatches in the component
induced rated current flowing in the faulty module and the
values. Thus, there should be a control scheme which ensures
entire converter. Therefore, failing to protect the converter fast
the stable operation of the converter under non-ideal conditions
enough, causes the failure of the entire converter. Although
as well.
OCF is not as severe as SCF, failing to diagnose it fast enough
In the closed loop control system, the outer voltage and protect the converter causes overstress and failures of
controller is employed to control the output voltage within the other switches and circuit components.
design specification. In a real operating condition, when there
The main goal for the converter operation under the fault
is a small mismatch in the components, after a short time, only
condition is that under the fault of one or two modules, the
one module is operational while the current in the other two
system can have the same power generation without any
modules drops to zero. Therefore, the inner current controllers
interruption. For the implementation of the fault tolerant
are required to share the current equally among the modules.
control method, the main assumption is that the dc-dc
The implementation of this democratic current sharing converters in each of the phases operates at less than rated
scheme is shown in Fig. 1. In this figure, the proportional power, and once the fault occurs, the remaining healthy
integral (PI) voltage controller regulates the output voltage, and modules can compensate for the power generation of the
it provides the current reference to all individual inner current faulty module.

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 III. LINEAR MODEL OF THE CONVERTER L1 I L
1
D1
The linear model of the converter is designed based on the I in IL
L2
small signal model, obtained by perturbing converter 2

D2
variables around the steady state operating point [18]. Then, IL
L3
the voltage and current controllers are designed based on the + +
3

F1 F2 F3 D3
linearized model of the converter. Vin Co Ro Vo
− S1 S2 S3 −
The method for deriving the small signal model, and
modeling the converter with the small signal transfer
functions have been extensively explained in the literature.
The input to output and control to output small transfer
Q1 Q2 Q3 IL
functions of the boost converter are given by [18], 1
Current PI
n-phase shifted PWM Controller
n: Number of Healthy
Modules IL Voltage PI
^ 2 Current PI Controller
Controller
vo ( s ) 1 1
Gvg ( s ) = = (1) IL Current PI
^
(1 − D) L LCo 3
vin ( s ) (1 + s 2
+ s2 ) Controller
(1 − D ) Ro (1 − D) 2

FI _ S1
FI _ S2
FI _ S3
FI _ S1
Fault Diagnosis Module, Identified Adaptive Democratic
FI _ S2
L Current Sharing
^ (1 − s ) Fault, Fault Type, and Faulty Switch FI _ S3
Controller
v (s) Vin (1 − D) 2 R o
Gvd ( s ) = o ^ = (2)
(1 − D ) L LCo
d (s) (1 + s 2
+ s2 )
(1 − D ) Ro (1 − D) 2 Fig. 2. Adaptive democratic current sharing scheme for interleaved dc-dc
converter for the fault condition.

^
where vo ( s ) is the small variation in the output voltage,
^ ^ isolation will be different depending on the fault type in the
vin ( s ) is the small variation in the input voltage, d ( s ) is the modules. In this study, the switch S1 fault condition is
small variation of the duty cycle, Vin is the input voltage, L is analyzed. Considering the symmetry in the interleaved dc-dc
the inductor, D is the duty cycle, Ro is the output resistor and convert, the results can be extended to the other two switches.
Co is the output capacitor. The transfer function shown in In the case of an OCF in S1 , since for boost operation the
Equation (2) is utilized to design the PI voltage controller. As output voltage is always higher than the input voltage, diode
it can been seen, the transfer function is a nonlinear function D1 is reversed biased (it is disconnected) and so, the faulty
of the duty cycle. module is isolated. Thus, the interleaved dc-dc converter is
For the closed loop control system, when a switch fault self-protected under OCF.
occurs in the converter, the current of the faulty module drops However, when there is an SCF in S 1 , the inductor
to zero. The duty cycle of the converter is changed in order to L1 starts charging, and since there is no path for the inductor
compensate for the current reduction. Therefore, the PI to discharge, the inductor current I L1 and the dc-link current
controllers need to be tuned for the new operating condition in
order to regulate the dc-link current and output voltages. I in increase rapidly. To prevent converter damage and for
fault tolerance purposes, three fuses ( F1 , F2 and F3 ) are
added in series with the switches, as shown in Fig. 2. Once
IV. PROPOSED FAULT TOLERANT CONTROL the current goes beyond the rated current of the fuse, the fuse
A. Isolation of Faulty Module breaks, and the current I L1 drops to zero. Similar to the OCF
In order to protect the converter and propose a suitable fault condition, the diode D1 is disconnected, and the faulty module
tolerant control, first, the fault, fault type, and faulty switch is isolated.
should be identified shortly after the fault occurs. The second In this paper, the fault in the input side is studied. When the
step for the fault tolerant operation is to isolate the faulty fault is in the output side, the ORing diode isolates the fault
module from the rest of the interleaved converter. The

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 3.0

I L1
I L1 1.5 1.5 Fuse action
0.0 0.0

I L2
1.5 1.5
I L2

0.0 0.0
S1 OCF S1 SCF
1.5
I L3

1.5

I L3
0.0 0.0
8.0
5.0
6.0

Iin
4.0
Iin

4.0
3.0 2.0
2.0 0.0
10.0 0.2 ms/div 10.0 0.2 ms/div
Time (ms) Time (ms)

(a) (a)

51.0
S1 OCF 51.0 S1 SCF
Vo (V)

50.0

Vo (V)
50.0
49.0 49.0
48.0 48.0
10.0 0.2 ms/div 10.0 0.2 ms/div
Time (ms) Time (ms)

(b) (b)

Fig. 3. Fault tolerant operation of interleaved boost converter under the Fig. 4. Fault tolerant operation of interleaved boost converter under the
OCF: (a) Inductor currents, (b) Output voltage. SCF: (a) Inductor currents, (b) Output voltage.

from the redundant bus, and it allows the safe operation of the I L3 increase. Following this transient condition, the converter
remaining healthy modules.
continues to operate with the same average current, as shown
in Fig. 3 (a). Fig. 3(b) shows the output voltage under the
B. Adaptive Democratic Current Sharing Scheme
fault condition for the studied closed loop configuration. As it
With the method proposed in [16] the fault can be detected
can be seen, the output voltage is generated at the desired
and the faulty switch can be identified within one switching
value with the two operational modules.
cycle after the fault occurs. The fault diagnosis module, shown
Fig. 4 shows the studied converter parameters with the
in Fig. 2, generates the fault detection (FD), and fault
proposed fault tolerant control scheme under the SCF in S1 .
identification (FI) signals ( FI _ S1 , FI _ S2 and FI _ S3 ). The FI
Clearly, once the SCF in switch S1 occurs, the current I L1
signals determine whether there is a fault in switches S 1 , S2
suddenly increases, and the current goes beyond the rated
or S3 . These pulses are sent to the current and voltage
current of the fuse. Therefore, fuse F1 breaks, the switch is
controller, as shown in Fig. 2, to shut down the PWM of the
removed, and I L1 drops to zero. Following this transient
faulty switch and to adaptively change the gains of the current
and voltage PI controller gains. condition, the converter operates similar to the condition when
Fig. 3 shows the studied converter parameters with the there is an OCF in switch S1 and the inductor current, I L2 and
proposed fault tolerant control scheme under the OCF in S 1 .
I L3 increase, as shown in Fig. 4(a). As it can be seen in Fig.
When the OCF is introduced in switch S 1 , I L1 drops to zero,
4(b), utilizing the proposed control method the same output
and the interleaved converter continues to operate with the two voltage is obtained under the SCF in switch S1 .
remaining phases. Through the provided fault tolerant control
scheme, the two healthy switches allow the converter to
continue to operate with a higher inductor current; that is, I L2
and

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 2.0 Ro = 15 Ω Ro = 20 Ω Ro = 15 Ω Ro = 15 Ω Ro = 20 Ω

I L1
1.0
0.0
S1 OCF
2.0

I L2
1.0
0.0
2.0
1.0

I L3
0.0

4.0
I in

3.0
2.0
1.0
20.0 0.2 ms/div
Time (ms)
(a)

Ro = 15Ω Ro = 20 Ω Ro = 15 Ω Ro = 15 Ω Ro = 20 Ω
50.0
Vo (V)

49.0
S1 OCF
48.0
47.0
20.0 0.2 ms/div
Time (ms)

(b)

Fig. 5. Democratic current sharing scheme for interleaved dc-dc converter.

Fig. 5 shows the comparative study of the democratic introduced a large ripple is observe on the output voltage
current sharing scheme and the proposed fault tolerant control when the democratic current sharing scheme is employed.
method for the step load variation under the healthy and faulty However, by changing the current and voltage controller gains
conditions. adaptively, the output voltage regulation is greatly improved.
As seen in Fig. 5(a), before the fault is introduced, the two
methods operate similarly, and the inductor currents are the I. CONCLUSION
same. Before the fault condition, under the load step up (from This paper has presented an adaptive democratic current
15 Ω to 20 Ω ), the current is shared equally among three sharing control scheme for interleaved dc-dc converters under
modules through either the democratic or adaptive democratic faulty condition. The approach utilizes the fault detection
current sharing scheme. The same result can be obtained under module output signals in order to define the operating
the load step up (from 20 Ω to 15 Ω ). Once the fault is condition of the converter. The proposed approach can
adaptively tune the voltage and current controller in the
introduced at 20 ms, utilizing the democratic current sharing
converter and achieve the desired output voltage generation,
scheme, the current highly decreases, and moreover, a large and equal current sharing among the remaining healthy
ripple appears on the current signals. However, utilizing the modules under the fault condition. The proposed control
proposed adaptive democratic current sharing scheme, no scheme is compared to the democratic current sharing control
change in the interleaved dc-dc converter operation is scheme under the fault condition and step load variation. The
observed under the faulty condition. results show that it achieves improved output regulation and
In Fig. 5 (b) the output voltage signal is studied under the desired current sharing accuracy. The method has been
analyzed and tested on the interleaved boost converter.
load variation for the faulty and healthy condition. Clearly, the
However, the result can be extended to other interleaved dc-dc
output voltage is regulated accurately to meet the design converters.
specification before the fault is introduced. After the fault is

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