217
Original scientific paper
 MIDEM Society
https://doi.org/10.33180/InfMIDEM2018.404
Reliability evaluation of buck converter based on 
thermal analysis
Mohammad Mojibi, Mahdi Radmehr
Electrical Engineering Department, Islamic Azad University, Sari branch, Iran
Abstract: The design, which is based on the concept of reliability, is impressive. In power electronic circuits, the reliability design has 
been shown to be useful over time. Moreover, power loss in switches and diodes plays a permanent role in reliability assessment. This 
paper presents a reliability evaluation for a buck converter based on thermal analysis of an insulated-gate bipolar transistor (IGBT) and 
a diode. The provided thermal analysis is used to determine the switch and diode junction temperature. In this study, the effects of 
switching frequency and duty cycle are considered as criteria for reliability. A limit of 150°C has been set for over-temperature issues. 
The simulation of a 12 kW buck converter (duty cycle = 42% and switching frequency = 10 kHz) illustrates that the switch and diode 
junction temperature are 117.29°C and 122.27°C, respectively. The results show that mean time to failure for the buck converter is 
32,973 hours.
Keywords: Reliability; Mean time to failure; Buck converter; Junction temperature.
Ocena zanesljivosti buck pretvornika na osnovi 
termične analize
Izvleček: V prispevku je predstavljena ocena zanesljivosti buck pretvornika na osnovi termične analize bipolarnega tranzostorja z 
izoliranimi vrati (IGBT) in diode. Termična analiza je uporabljena za določitev temperature spoja diode in stikala. Kriterij zanesljivosti je 
vpliv frekvence preklopa in obratovalnega cikla. Zgornja temperaturna meja je 150 °C. Simulacije 12 kW buck pretvornika (obratovalni 
cikel = 41%, frekvenca preklopa = 10 kHz) pokažejo temperaturo stikala 117.29 °C in spoja diode 122.27 °C. Povprečen pričakovani čas 
do okvare je 32,973 ur.
Ključne besede: zanesljivost; srednji čas do okvare; buck pretvornik; temperatura spoja.
* Corresponding Author’s e-mail: radmehr@iausari.ac.ir
Journal of Microelectronics, 
Electronic Components and Materials
Vol. 48, No. 4(2018), 217 – 227
1 Introduction
In recent years, the use of renewable energy has 
become more popular because of the negative impacts 
of fossil fuels and the environmental pollution they 
cause. Nowadays, various methods and topologies 
for extracting energy from different renewable 
sources are being introduced. Solar energy, which 
can be harnessed using photovoltaic panels, is one 
of the alternative sources of energy and offers many 
advantages (such as less negative environmental 
effects and affordability) in comparison with other 
sources. As renewable energy sources continue to be 
used more often, more attention is now being paid 
to power electronics. A converter frequently used for 
photovoltaic panels in power electronics, as well as in 
several wind turbine energy conversion systems, is the 
dc–dc converter. In the last few decades, there have 
been many dc-dc converter topologies introduced, 
which have been generally classified based on the 
ratio of voltage output to input (also known as gain) 
into three fundamental groups: buck, boost, and buck-
boost. This paper focuses on the buck converter type, 
often used in small or low power systems as a simple, 
remarkably efficient way to reduce the input voltage to 
a regulated dc voltage [1].
More efficient use of any device has always been a goal 
of manufacturers. In power electronics, the proper 
functioning of converters encompasses high output 
quality, a long lifespan, and less energy consumption. 
Due to the increase of power electronic converters in 
different devices, an especially important factor for 
optimizing converters is power quality, which can be 
described in terms of its thermal characteristics. Indeed, 

218
M. Mojibi et al; Informacije Midem, Vol. 48, No. 4(2018), 217 – 227
previous researches have clarified the relationship of 
converter performance and quality in terms of heat loss 
[2–4]. Furthermore, Usui and Ishiko presented a simple 
approach for the thermal design of an IGBT module 
practised only in steady state operation [5].
In recent decades, different approaches for thermal 
analysis have also been introduced, including the 
highly accurate method of computational fluid 
dynamics (CFD), based upon how airflow conditions 
determine heat transfer coefficients [6].
Converter lifespan is another significant factor with 
a direct relationship to reliability, which represents 
the probability of failure in a system at a specific time 
[7]. The reliability of a system depends on various 
parameters; for this reason, identifying the indicators 
and calculation of the reliability parameters of the 
system’s parts is required. Usually, two parameters 
are used to assess the reliability of the system. The 
first parameter is failure rate explained by failure 
distribution, and the next parameter is mean time to 
failure (MTTF) which presents the average operation 
time before the first failure of a component [8].
There are different researches related to the reliability 
assessment of various circuits and power converters. 
These circuits include multilevel inverters [9, 10], DC-
DC converters [11], and AC-AC converters [12].
Khosroshahi et al. [13] evaluated the reliability of two 
conventional and interleaved DC-DC boost converters 
based on the MIL-HDBK-217 procedure. They found 
that the interleaved boost converter performs better in 
terms of reliability in comparison with the conventional 
boost converter. Perhaps, the most crucial weakness of 
this article is using approximate relations for calculating 
power dissipation in the switch and diode, which are 
based on their internal resistances.
Rashidi-Rad et al. [14] performed a reliability analysis of 
modular multilevel converters (MMCs) with the presence 
of half and full-bridge cells. Their examination illustrated 
that the modular converters that used half-bridge cells 
have more reliable performance than other state.
Arifujjaman and Chang [12] compared the reliability 
of three ac-ac converter namely intermediate boost 
converter (IBC), intermediate buck-boost converter 
(IBBC), and back-to-back converter (BBC) with the 
well-known matrix converter. They concluded that the 
intermediate boost converter exhibits more reliable 
than other ones.
In [15], the reliability of a buck converter was assessed 
in the presence of N-channel and P-channel MOSFET 
drivers. They showed that the considered buck 
converter has more reliability when an N-channel 
MOSFET is used as switch. However, they ignored some 
portions of the power losses in switch and diode, thus 
the obtained results may not be referred.
Ranjbar et al. [16] carried out a reliability assessment 
of single/two stage power factor correction (PFC) 
converters. The MIL-HDBL-217 was considered as 
reliability estimation procedure in this analysis. The 
outcomes demonstrated that the lifespan of a single-
stage converter is about 1.6 times longer than the 
two-stage converter. In this study, for simplicity of 
calculations, the case temperature was intended to be 
a fix value of 35°C. This leads to an inaccuracy in the 
results.
The main purpose of this paper is to estimate the 
reliability of a buck converter based on the MIL-
HDBK-217 standard. Although this standard has been 
criticized for being obsolete, it is still extensively used 
in military and aerospace industries to provide a 
basis for comparison between two or more different 
circuits [17]. For this reason, several previous literatures 
have employed this standard for predicting the 
reliability of power electronic converters (e.g., [18-
22]). To investigate the reliability of semiconductor 
devices, there is a need for determining the junction 
temperature in these types of components, and in this 
study, the selected approach is based on information 
from manufacturer’s datasheet. A one-cell Cauer 
thermal model was utilized in order to provide a 
precise relationship between the power losses and the 
junction temperatures in the presence of a heatsink. 
This approach has an acceptable result as well as 
suitable speed in calculations. Additionally, this is the 
first time that the simultaneous impact of switching 
frequency and duty cycle on the power losses and the 
junction temperature has been analyzed.
The rest of this paper is structured as follows: Section 
2 describes the buck converter as a case study. The 
reliability principals employed for the analysis are 
discussed in Section 3. In Section 4, the accurate 
thermal analysis for the buck converter is discussed. 
In Section 5, the results and reliability evaluation are 
presented. Finally, conclusions are drawn in Section 6.
2 The buck converter
The buck converter circuit shown in Figure 1 is a highly 
efficient step-down dc-dc converter which is commonly 
used in switched-mode power supply circuits (SMPS). 
Generally, the dc input voltage of the buck converter 

219
is derived from the output of a rectifier through a dc-
link. In this paper, an IGBT is used as a switch for the 
converter. Also, the thermal analysis has been performed 
considering the effect of temperature on the voltage 
drop between the collector and emitter junctions of the 
diode and the transistor, because practically this voltage 
should be estimated by the means of both collector 
current and temperature, V
CE
(i
C
, T
j
). 
The voltage drop for an IGBT can be experimentally 
measured by sensing the current of switch. Typically, 
a low-ohmic resistor is placed between the ground 
and emitter, and by flowing the current through this 
sensing resistor, the occurred voltage drop can be 
identified by another monitoring circuit. An improved 
sensing method is based on four external connection 
nodes for finding R
DS
(on) of the switching power 
MOSFET, or V
CE
(on) of an IGBT. The drawback of these 
methods is need for protection circuits and expensive 
discrete components against high voltage [23]. But, the 
employed approach in this paper will provide us the 
opportunity to identify the voltage drop indirectly.
Figure 1: Topology of a buck DC-DC converter.
When the buck converter operates in continuous 
conduction mode (CCM), the current through the 
inductor (I
L
) will never fall to zero during the cycle. 
Assuming the steady state operation for this converter, 
it can be concluded that the energy stored in each of 
circuit components at the end of a cycle is equal to 
energy stored at the beginning of the cycle. Therefore, 
the input and output voltages in the buck converter 
have a direct relationship with the duty cycle of the 
pulses, which can be shown as follows:
 
out in
VD V =
     (1)
where V
out
, V
in
, and D are the output voltage, the input 
voltage, and the converter duty cycle, respectively. 
With regard to the value of 0<D<1, as a consequence, 
the output voltage is always lower than the input 
voltage. The basic characteristics of the converter are 
summarized in Table 1.
Table 1: Rated parameters for the desired buck 
converter.
Characteristic Value
Rated output active power Po 12 kW
Input voltage Vin 300 V DC
Output voltage Vout 125 V DC ± 1.2%
Switching frequency fs 10 kHz
Inductor L 3 mH
Capacitor C 1 µF
A buck converter with parameters based upon Table 
1 is simulated in MATLAB/Simulink. An open-loop 
controller is used for the simulation. Furthermore, a 
value of 42% is considered the duty cycle in this state. 
The results of the simulation are shown in Figure 2:
Figure 2: The simulation results of basic characteristics 
of the converter.
3 The reliability principle
Reliability means the ability of an item to perform a 
specific function under given conditions over a specific 
time period, which is expressed as a probability or 
failure frequency [24]. The importance of reliability in 
space and in the arms industry is more prominent than 
that of other industries because in these significant 
instruments, detecting or replacing a failed part is 
very difficult. Different methods have been introduced 
to improve the reliability of a system. One of these 
methods involves adding redundancy to parts of the 
converters, thereby increasing the global reliability of 
a system. Reliability is improved by adding more parts 
for redundancy, but cost is a deterrent to increasing the 
number of redundancy circuits [25].
One of the factors influencing reliability is failure rate. 
Failure rate can be expressed as the probability of 
failure per unit time occurring in the interval [t,t+∆t ], 
and there is no failure before time t. Usually, ∆t is a very 
small value, and it is close to zero [26].
M. Mojibi et al; Informacije Midem, Vol. 48, No. 4(2018), 217 – 227

220
If we present a failure rate with λ, the probability 
distribution function for failure can be expressed 
as a relationship in terms of failure rate, and can be 
obtained using the exponential distribution. Equation 
(2) presents the distribution function:
 
() ,
t
ft e
λ
λλ
−
=
    (2)
Also, the reliability function can be expressed as follows 
[8]:
 
() ,
t
Rt e
λ
λ
−
=
     (3)
where in the above equations, λ is the component’s 
failure rate. Another influential factor of reliability is 
mean time to failure (MTTF). MTTF is the average length 
of time before the first failure of a component or device 
occurs after it starts to work, after which the device is 
no longer able to continue with its normal operation. 
MTTF is expressed by the integral of reliability as 
follows:
 
()
0
MTTF Rtdt
+∞
=
∫
    (4)
A simple equation for the expression of MTTF is derived 
by substituting Equation (3) with Equation (4):
 1
MTTF
λ
=     (5)
In the last decades, various procedures have been 
introduced by different organizations to estimate the 
reliability. Some of the most popular procedures, such 
as RAC’s PRISM [27], Telcordia SR-332 [28], SAE’s PREL 
[29], CNET’s reliability prediction method [30], Siemens 
SN29500 standard [31] and British Telecom’s HRD-4 
[32], are described and discussed according to the 
organization’s strategies. A comprehensive comparison 
has been made among these procedures in [33]. Today, 
the MIL-HDBK-217F handbook is used as a suitable 
reference for estimating reliability. This paper also used 
a calculation based on the MIL-HDBK-217F procedure 
[34].
Two methods that include parts stress and parts count 
are discussed in the handbook. In the parts count 
method, less information is required, such as number 
of parts, quality level and environmental situation [35].
According to the series structure of the buck converter, 
the failure rate can be calculated using the summation 
of all failure rates of the circuit components, as shown 
in Equation (6) [36]:
 
System Components
λλ =
∑
    (6)
where λ
Components
 is the failure rate of each circuit 
component.
With the increasing complexity of the studied system, 
the overall system should be divided into subsystems 
so that the reliability evaluation becomes simpler and 
more concise [37]. 
3.1 The Reliability of Components
The buck converter consists of various components, 
including switch, diode, inductor and controller. 
In related studies on the reliability of electronic 
components (switches, diodes, capacitors and 
inductors), specific relationships for determining 
the failure rate for each component are expressed as 
follows [25, 34, 35, 38]:
 
()
pb CV QE
Capacitor λλ ππ π =   (7)
 
()
pb CQE
InductorT ransformer λλ πππ −= (8)
 
()
pb TARSQE
Switch λλ π πππππ =  (9)
 
()
pb TCSQE
Diode λλ πππππ =
                 (10)
In Equations (7)–(10), λ
b
 is the base failure rate, which is 
different for each component. Additionally, π
i 
is pi factor 
related to each component, and should be determined 
accurately. The controller failure rate can be considered 
0.88 (failures/10
6 
h) [35].
Due to the sudden progress of IGBTs, field data and 
reliability model related to this component are not 
available. Furthermore, one drawback of the employed 
standard is the lack of relationships for estimating 
the reliability of IGBT devices, because the latest 
update of the standard was before the comprehensive 
introduction of this kind of switch. In prior studies, 
some researchers preferred to use MOSFET equations 
for calculating IGBT failure rate [37], and alternatively, 
several papers claimed that using the reliability 
formulas of bipolar power transistors instead of IGBT 
would provide appropriate results [38]. In this paper, it 
is assumed that the IGBT modules have the same factors 
of high-power bipolar transistors. Another assumption 
is the fact that the value of base failure for an IGBT can 
be easily obtained by knowing the module’s field data, 
and according to [38], its value for a medium-power 
IGBT is equal to 100 FIT (λ
b (S)
 = 0.1 failures/10
6 
h).
M. Mojibi et al; Informacije Midem, Vol. 48, No. 4(2018), 217 – 227

221
The factors π
Q
 and π
E
, represent quality and 
environmental, respectively. The quality and environ-
mental factor values can be assumed to be equal to 
one, although the effects of these two factors were 
eliminated [25]. Another factor is the application factor, 
π
A
, and for switching application is equal to 0.7. The 
power rating factor, π
R
, is directly related to the rated 
power which is equal to 10 for a 500 W IGBT module. The 
voltage stress factor for IGBT (π
S (S)
) can be calculated by 
 
() ()  
0.045 exp(3.1)
SS SS
V π =× ×
                (11)
where V
S (S)
 is the ratio of applied collector-emitter 
voltage to rated voltage.
Diodes are usually used as power rectifiers in power 
electronic converters. In the industry, the diodes 
with a reverse recovery rate of 500 nanoseconds or 
less (approximately 0.1 of standard rectifiers) are 
categorized as fast rectifiers. If this period is reduced to 
100 nanoseconds or less, they will be named as super-
fast rectifiers [39]. Given that the diodes within the IGBT 
modules are classified as fast recovery power rectifier, 
they must be set to the appropriate level according to 
the standard, and the base failure rate for the diodes 
will be equal to 0.025 failures/10
6 
h.
In the following equation, π
S (D)
 is the stress factor for 
diodes:
 
()
()
2.43
 S SD
V π =
                  (12)
where V
S (D)
 is the ratio of operating voltage to nominal 
voltage.
π
C
 explains the contact construction. Considering it is 
metallurgically bonded, the contact construction leads 
to the value of 1 for π
C
 [35].
In the capacitor failure rate, π
CV
 is the capacitor factor 
which can be calculated as follows:
 
0.12
0.34
CV
C π =×                   (13)
where C is the capacitance in microfarad.
The inductor base failure rate can be expressed as 
follows:
 
()
15.6
 
273
0.000335 exp
329
HS
bL
T
λ
+ 
=×


             (14)
where T
HS
 is the hot spot temperature in degree Celsius, 
which can be determined using Equation (15):
 
1.1
HS A
TT T =+×Δ                 (15)
In Equation (15), T
A
 expresses the device ambient 
operating temperature in degree Celsius. Also, ∆T is 
the average temperature rise above the ambient [34, 
35]. The inductor failure rate is much lower than other 
circuit components, so it can be omitted from the 
analysis.
The capacitor failure rate can be described by the 
following equation:
 
()
5 3
 
273
0.00254 1e xp 5.09
0.5 378
A
bC
T S
λ
 
+  
=+ ×

  
 
 

  (16)
where S is the ratio of operating voltage to nominal 
voltage.
π
T
 is the temperature factor that, for the switch and 
diode, can be expressed as follows [35]:
 
()
11
exp 2114
273 298
TS
j
T
π
 
=−×−
 
+


        (17)
 
()
11
exp 3091
273 298
TD
j
T
π
 
=− ×−
 
+


         (18)
where T
j
 is the junction temperature.
One of the major concerns regarding reliable power 
electronics is the operating temperature. Thus, it 
seems that the precise determination of the junction 
temperature results in a more accurate analysis of 
the reliability. There are five different approaches 
introduced by Reliability Analysis Center (RAC) to 
predict the junction temperature for semiconductor 
devices. In this study, Method IV was used. This method 
is utilized when a heatsink is mounted on the device, 
and the exact value of the case temperature is also 
available [40].
According to the used approach, the junction 
temperature can be calculated from Equation (19):
 
jCjc loss
TT P θ =+×                  (19)
In Equation (19), T
C
 is the heat sink temperature, θ
jc
 is 
the thermal resistance of the diode or switch, and P
loss
 is 
the total power losses of switch or diode.
M. Mojibi et al; Informacije Midem, Vol. 48, No. 4(2018), 217 – 227

222
In fact, Equation (19) exhibits a scheme of the one-cell 
Cauer thermal network. Figure 3 shows this modeling.
Figure 3: One-cell Cauer thermal network model.
In Figure 3, R
th
 and C
th
 are the thermal resistance and 
capacitance from junction-to-case, respectively, and 
these indicators should be selected from the datasheet 
of the used IGBT module (both diode and IGBT). Also, by 
similarity of thermal modeling and electrical modeling, 
the junction temperature can be found easily from the 
total power losses.
Foster and Cauer thermal network models are 
commonly used for dynamic thermal modeling, 
and both models use thermal resistance (R) and 
thermal capacitance (C), which are joined together 
as ladders and form multiple layers. Although many 
manufacturers of power modules typically offer the 
Foster model limited to four layers [41], the others 
only provide the transient thermal impedance curve 
(Z
th
) within the datasheet; and the values for each layer 
should be obtained by curve fitting tools [42, 43] or 
algorithms [44]. Here, the particle swarm optimization 
(PSO) algorithm was used [45] to identify four RCs 
related to the four different layers.
Due to the need for a high number of simulations in 
this paper, a one-layer Cauer model has been employed 
for thermal analysis between junction and case. In 
addition to having the suitable precision, this model 
improves the simulation speed significantly. One issue 
for modeling thermal networks is the conversion of 
a Foster model to a Cauer model, and vice versa (for 
more details about these two thermal networks, refer 
to [46]). Considering that the four-layer Foster model 
is a four-order system, a circuit software is required to 
convert it to the first-order Cauer model. In this study, 
we used the LTspice software to find the values of R
th
 
and C
th
 for the one-cell thermal model. The resistance 
of the single-layer model is equal to the sum of the four 
resistances of the four-layer model, which is the same 
as the peak of the thermal impedance curve. In order 
to determine the value of capacitance, a curve fitting 
on the outputs of the LTspice software was performed. 
Thus, the obtained values of the thermal resistance and 
capacitance for the Cauer network are 0.25 K/W and 
0.18 J/K, respectively. Similarly, the extracted values for 
the diode are 0.46 K/W and 0.1 J/K, respectively.
To complete the thermal model, the heat transfer from 
the case to the ambient through the heatsink should 
also be added. Modeling of this part is shown in Figure 4.
Figure 4: Thermal modeling from case-to-ambient.
In order to implement this part, the thermal and physical 
elements of MATLAB/Simulink have been used. The 
case-heatsink thermal resistance value (R
th(c-h)
) is driven 
from the datasheet, and is equal to 0.05 K/W for the 
module under study. A 10-inch aluminum heatsink 
manufactured by the Wakefield-Vette under forced air 
cooling (500 feet per minute) is considered as the cooling 
system. According to the described circumstances, the 
value of R
th(h-a)
 will be equal to approximately 0.1 K/W. 
For simplicity, we consider the case-heatsink thermal 
capacitance to be 0.25 J/K. The heatsink thermal 
capacitor is also assumed to be 0.01 J/K, based on [47].
Finally, as mentioned earlier, the determination of 
semiconductors’ failure rate depends on their power 
losses. The utilized approach in this paper is based on 
calculating both conduction and switching losses for 
the diode and switch using lookup tables. Detailed 
explanation of this process is given in [48].
4 Thermal analysis of buck converter
In order to determine the thermal analysis of the 
converter, a Fuji 2MBI150U2A-060 600V/150A IGBT 
module is selected as the switch. The features of this 
module include high speed switching, voltage drive, 
and low inductance [49].
Figure 5 shows the IGBT on-state characteristics in 25°C 
and 125°C, based on Collector current versus Collector-
Emitter voltage.
The rated current distributions for the switch and 
diode are shown is Figure 6, which this figure clearly 
demonstrates the summation of switch and diode 
currents can produce the inductor current (when the 
switch is on, the diode is off). Conversely, when the 
diode is on, the switch is off. The inductor current will 
be a triangular waveform when its voltage analogue is 
pulsating in a rectangular form 
M. Mojibi et al; Informacije Midem, Vol. 48, No. 4(2018), 217 – 227

223
Figure 6: Current distributions of the switch and diode.
The most important factor in the evaluating converter 
reliability is temperature, which is directly related 
to power losses of the switch and diode. Thus, the 
calculation of the junction temperature is a sure way 
to assess reliability. Various elements can influence the 
junction temperature and its value will change with 
variations in component’s power losses; increasing the 
switching frequency can lead to more power losses 
in the switch and diode. Another important factor for 
power losses in the buck converter is the modulation 
index or duty cycle. By setting a different duty cycle 
for the converter, the gain of the output voltage will 
change. An analysis is undertaken to show the effects 
of the switching frequency and the duty cycle on the 
junction temperature and the heat sink temperature. 
Figures 7 and 8 represent the items that can affect 
temperatures in the form of two-dimensional and 
three-dimensional diagrams.
(a)
(b)
(c)
Figure 8: 3D plots for showing effects of duty cycle 
and switching frequency on (a) the switch junction 
temperature, (b) the diode junction temperature, (c) 
the heat sink temperature.
It is evident from Figures 7 and 8 that a lower duty 
cycle corresponds to a better performance in terms 
of temperature because of the decrease in the output 
voltage level. Therefore, it is possible to change the 
duty cycle to its desired value by changing the basic 
Figure 5: IGBT’s Collector current in terms of Collector-
Emitter voltage [49].
Figure 7: 2D plots for showing effects of duty cycle 
and switching frequency on (a) the switch junction 
temperature, (b) the diode junction temperature, (c) 
the heat sink temperature.
(a)
(b)
(c)
M. Mojibi et al; Informacije Midem, Vol. 48, No. 4(2018), 217 – 227

224
characteristics of the converter. Increasing switching 
frequency from 1 to 10 kHz has a negligible impact 
on the temperature, but switching frequencies higher 
than 10 kHz will increase the temperature. The over-
temperature is limited to 150°C, so the converter ceases 
to operate beyond this temperature. For duty cycles 
higher than 51%, the junction temperature of the 
switch rises beyond the over-temperature. This shows 
the weakness of heatsink for cooling the module under 
thermal pressure. Using a more efficient heatsink will 
result in a decrease in the junction temperature and the 
extension of authorized period for increasing the duty 
cycle. The calculated power losses for the switch and 
diode (based on the rated parameters) are 145.02 W 
and 89.69 W, respectively. Also, the results illustrate that 
the switch junction temperature for a duty cycle of 42% 
and f
s 
= 10 kHz is 117.29°C. The junction temperature of 
the diode is 122.27°C, and it has a higher value than the 
switch’s temperature.
Although the diode power losses are less than the 
switch, due to the greater diode thermal resistance in 
comparison with the switch (0.46 compared to 0.25 
K/W), the diode junction temperature would be higher. 
This issue can also be deduced from the physical 
structure of the IGBT module, because the cross-
section of the diode chip is smaller than the switch, 
and with the same heat flux applied to the both chips, 
the smaller chip will experience higher temperature 
increase. This shows that greater thermal resistance 
can produce higher junction temperatures. 
Typically, the heat sink temperature is much lower 
than that at the junction of other components, and in 
reliability designs, a temperature of 40°C is considered 
a stable value for the temperature of the heat sink [50]. 
However, the structure and design of the heat sink can 
affect its operating temperature. The simulation results 
showed that the heat sink temperature measured with 
the parameters rated was 69.32°C.
5 The reliability evaluation of buck 
converter
Estimated failure rates for each component under 
identical conditions are shown in Tables 2-5. Due to the 
application of the switch, a value of 0.7 is considered to 
be the application factor. Values of π
Q
 and π
E
 were set 
for the components according to [34].
A value of 0.88 was considered to be the failure rate 
of the controller, similar to [35], and the failure rate of 
the converter can be estimated by summing all of the 
failure rates (we only have one from each component). 
The failure rate of the entire system was calculated at 
30.328 (failures/10
6 
h) by the following equation:
     (20)
By reversing the failure rate, MTTF can be calculated as 
follows:
 
6
11 0
32,973 hours
30.328
System
MTTF
λ
== =
      (21)
Table 2: The estimated base failure rate for the switch.
P
Loss
 (W) T
j
 (°C) π
T
π
A
π
R
π
S
π
E
π
Q
λ
b
λ
P
 (failures/10
6
 h)
145.02 117.29 5.35 0.7 10 0.22 6 5.5 0.1 27.188
Table 3: The estimated base failure rate for the diode.
P
Loss
 (W) T
j
  (°C) π
T
π
C
π
S
π
E
π
Q
λ
b
λ
P
 (failures/10
6
 h)
89.69 122.3 12.85 1 0.19 6 5.5 0.025 2.014
Table 4: The estimated base failure rate for the capacitor.
Value T
A
 (°C) π
CV
π
E
π
Q
λ
b
λ
P
 (failures/10
6
 h)
1 µF 40 0.34 2 10 0.029 0.197
Table 5: The estimated base failure rate for the inductor.
T
A
 (°C) T
HS
 (°C) π
C
π
E
π
Q
λ
b
λ
P
 (failures/10
6
 h)
40 69.32 1 4 20 6.22×10
-4
0.049
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225
6 Conclusion
A new approach to reliability assessment based 
on thermal analysis of the switch and diode was 
presented. The thermal analysis of a buck converter 
with the basic characteristics shown in Table 1 was 
conducted by calculating the temperature at the 
switch and diode junction. The total failure rate of the 
converter was expressed by summing the failure rate 
of the components using the parts count method. The 
procedure employed for the reliability analysis was that 
given in the MIL-HDBK-217F handbook. The results of 
the simulation using MATLAB Simulink showed that 
the buck converter analyzed will operate reliably for 3.8 
years, which is an acceptable performance.
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Arrived: 27. 01. 2018
Accepted: 23. 11. 2018
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