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energies
Article
Over-Temperature-Protection Circuit for
LED-Battery Power-Conversion System Using
Metal-Insulator-Transition Sensor
Jin-Yong Bae
Department of Electric Vehicle Engineering, Dongshin University, Jeollanam-do 58245, Korea; bjy@dsu.ac.kr
Received: 11 April 2020; Accepted: 9 July 2020; Published: 13 July 2020
Abstract:
Extracting renewable energy from solar and wind energy systems, fuel cells, and tidal power
plants requires DC distribution and energy storage devices. In particular, a metal-insulator-transition
(MIT) sensor can be applied to the over-temperature-protection (OTP) circuit, to stop the LED-battery
power-conversion system when over-temperature occurs. Recently, there have been instances of
battery systems catching fire because of poor battery design, over-charging, over-voltage, cell balancing
failure, and an inadequate battery management system circuit. For continuous stabilization using an
LED-battery power-conversion system, a 450 Wh class battery system that can monitor the temperature
of battery packs with an MIT sensor was developed in this study. Furthermore, an OTP circuit
involving an MIT sensor to protect LED-battery power-conversion systems is proposed. According to
the results, this approach is required to continuously perform the stabilization of LED-battery systems.
Keywords:
metal-insulator-transition (MIT); over-temperature-protection (OTP); energy storage
system (ESS); LED-battery; power-conversion
1. Introduction
Recently, LEDs have attracted considerable attention because of their brightness and efficiency [
1
–
7
].
In particular, they are widely used for street and road-lighting, while LED lighting systems powered by
solar panels and lithium-ion batteries are actively used for outdoor-lighting. However, an LED-battery
power-conversion system involves photovoltaic power generation, and there is a risk of a fire
or an explosion occurring in the lithium-ion battery because of over-voltage, over-current, and/or
over-temperature. Therefore, battery stability is a very important factor in renewable energy systems [
8
].
In this paper, an over-temperature-protection (OTP) circuit based on a metal-insulator-transition
(MIT) sensor for LED-battery power-conversion systems with a lithium-ion battery involving
photovoltaic power generation is proposed. The most important feature of this study is the use
of an MIT sensor for temperature detection [9–18].
Thermo-couplers, which are widely used as conventional temperature sensors, have relatively
good characteristics, but are very expensive [
19
]. In the case of inexpensive negative temperature
coefficient thermistors, the resistance change is very moderate at fire-start temperatures, which is in
the range of 80 to 100
◦
C, and so there is the problem that an additional op-amp circuit is required [
20
].
In this study, an MIT sensor, which is cheaper and has better characteristics than conventional
thermo-couplers and negative temperature coefficient (NTC) thermistors, was used. According to
the results, the OTP circuit that protects the LED-battery power-conversion system was required to
continuously perform the stabilization of LED-battery systems.
Energies 2020,13, 3593; doi:10.3390/en13143593 www.mdpi.com/journal/energies
Energies 2020,13, 3593 2 of 11
2. Theory of MIT
Mott [
9
,
10
] was the first to propose a theory for MIT in 1949. His theory is that if the Coulomb
energy between metals and free-electrons becomes very large, transition to an insulator can occur
suddenly and discontinuously, without accompanying structural changes in the material.
Subsequent studies were conducted by Morin [
11
] and Imada [
12
], although they did not fabricate
MIT-based devices. In 2000, Kim [
13
] theoretically analyzed the possibility of MIT at the Electronics
and Telecommunications Research Institute (ETRI). Since 2004, Kim has developed MIT transistors
made of vanadium oxide (VO
2
) [
14
–
17
]. In 2007, Kim developed a critical temperature sensor (CTS),
which is an MIT sensor in which the resistance rapidly decreases when the insulator is heated to the
critical temperature [18].
Figure 1shows the theory of MIT. The theory implies that a Mott insulator with uniform Coulomb
energy can momentarily change its resistance to a value that corresponds to a metal when an electron
escapes [13].
Energies 2020, 01, x FOR PEER REVIEW 2 of 11
the results, the OTP circuit that protects the LED-battery power-conversion system was required to 42
continuously perform the stabilization of LED-battery systems. 43
2. Theory of MIT 44
Mott [9–10] was the first to propose a theory for MIT in 1949. His theory is that if the Coulomb 45
energy between metals and free-electrons becomes very large, transition to an insulator can occur 46
suddenly and discontinuously, without accompanying structural changes in the material. 47
Subsequent studies were conducted by Morin [11] and Imada [12], although they did not 48
fabricate MIT-based devices. In 2000, Kim [13] theoretically analyzed the possibility of MIT at the 49
Electronics and Telecommunications Research Institute (ETRI). Since 2004, Kim has developed MIT 50
transistors made of vanadium oxide (VO2) [14–17]. In 2007, Kim developed a critical temperature 51
sensor (CTS), which is an MIT sensor in which the resistance rapidly decreases when the insulator 52
is heated to the critical temperature [18]. 53
(a) (b)
Figure 1. The theory of metal-insulator-transition (MIT) [13]: (a) Mott insulator and (b) 54
Inhomogeneous. 55
(a) (b) (c)
Figure 2. Conventional and proposed temperature sensors: (a) thermo-coupler [19], (b) negative 56
temperature coefficient (NTC) thermistor [20], and (c) MIT sensor [18]. 57
Figure 1.
The theory of metal-insulator-transition (MIT) [
13
]: (
a
) Mott insulator and (
b
) Inhomogeneous.
Figure 2exhibits the conventional and proposed temperature sensors. Among the representative
conventional temperature sensors, thermo-couplers show relatively high accuracy for temperature
detection, but are expensive [
19
], while NTC thermistors are inexpensive but characterized by resistance
that changes slowly at temperatures above 70
◦
C [
20
]. In contrast, the MIT sensor used in this study is
inexpensive (made of VO2), and its resistance changes rapidly at temperatures above 70 ◦C [15,18].
Energies 2020, 01, x FOR PEER REVIEW 2 of 11
the results, the OTP circuit that protects the LED-battery power-conversion system was required to 42
continuously perform the stabilization of LED-battery systems. 43
2. Theory of MIT 44
Mott [9–10] was the first to propose a theory for MIT in 1949. His theory is that if the Coulomb 45
energy between metals and free-electrons becomes very large, transition to an insulator can occur 46
suddenly and discontinuously, without accompanying structural changes in the material. 47
Subsequent studies were conducted by Morin [11] and Imada [12], although they did not 48
fabricate MIT-based devices. In 2000, Kim [13] theoretically analyzed the possibility of MIT at the 49
Electronics and Telecommunications Research Institute (ETRI). Since 2004, Kim has developed MIT 50
transistors made of vanadium oxide (VO2) [14–17]. In 2007, Kim developed a critical temperature 51
sensor (CTS), which is an MIT sensor in which the resistance rapidly decreases when the insulator 52
is heated to the critical temperature [18]. 53
(a) (b)
Figure 1. The theory of metal-insulator-transition (MIT) [13]: (a) Mott insulator and (b) 54
Inhomogeneous. 55
(a) (b) (c)
Figure 2. Conventional and proposed temperature sensors: (a) thermo-coupler [19], (b) negative 56
temperature coefficient (NTC) thermistor [20], and (c) MIT sensor [18]. 57
Figure 2.
Conventional and proposed temperature sensors: (
a
) thermo-coupler [
19
], (
b
) negative
temperature coefficient (NTC) thermistor [20], and (c) MIT sensor [18].
Figure 3displays the resistance change in a NTC thermistor and a MIT sensor according to
temperature. Unlike the MIT sensor, the NTC thermistor does not exhibit a significant change in
Energies 2020,13, 3593 3 of 11
resistance at 25
◦
C room temperature, and is practical, as 70–80
◦
C is the first-start temperature range at
which a battery can catch fire. The problem with the NTC thermistor is that it is difficult to clearly detect
the change in its resistance value at the fire-start temperature [
20
–
22
]. By contrast, the proposed MIT
sensor’s resistance decreases abruptly at a temperature of around 70
◦
C. This facilitates the accurate
detection of a rise in temperature preceding the fire-start temperature and the prevention of fires and
explosions in the battery [15,18].
Energies 2020, 01, x FOR PEER REVIEW 3 of 11
(a) (b)
Figure 3. Resistance variation according to temperature: (a) NTC thermistor [20–22] and (b) MIT 58
sensor [15,18]. 59
60
Figure 4. Resistance measurement data for the MIT sensor. 61
Figure 1 shows the theory of MIT. The theory implies that a Mott insulator with uniform 62
Coulomb energy can momentarily change its resistance to a value that corresponds to a metal when 63
an electron escapes [13]. 64
Figure 2 exhibits the conventional and proposed temperature sensors. Among the 65
representative conventional temperature sensors, thermo-couplers show relatively high accuracy 66
for temperature detection, but are expensive [19], while NTC thermistors are inexpensive but 67
characterized by resistance that changes slowly at temperatures above 70 °C [20]. In contrast, the 68
MIT sensor used in this study is inexpensive (made of VO2), and its resistance changes rapidly at 69
temperatures above 70 °C [15,18]. 70
Figure 3 displays the resistance change in a NTC thermistor and a MIT sensor according to 71
temperature. Unlike the MIT sensor, the NTC thermistor does not exhibit a significant change in 72
resistance at 25 °C room temperature, and is practical, as 70–80 °C is the first-start temperature 73
range at which a battery can catch fire. The problem with the NTC thermistor is that it is difficult to 74
clearly detect the change in its resistance value at the fire-start temperature [20–22]. By contrast, the 75
proposed MIT sensor’s resistance decreases abruptly at a temperature of around 70 °C. This 76
facilitates the accurate detection of a rise in temperature preceding the fire-start temperature and 77
the prevention of fires and explosions in the battery [15,18]. 78
Figure 3.
Resistance variation according to temperature: (
a
) NTC thermistor [
20
–
22
] and (
b
) MIT
sensor [15,18].
Figure 4shows resistance measurement data at different temperatures for the MIT sensor invented
at the ETRI. The resistance was evidently 372.1 k
Ω
at 70
◦
C and rapidly decreased to 44.49
Ω
at 75
◦
C.
Energies 2020, 01, x FOR PEER REVIEW 3 of 11
(a) (b)
Figure 3. Resistance variation according to temperature: (a) NTC thermistor [20–22] and (b) MIT 58
sensor [15,18]. 59
60
Figure 4. Resistance measurement data for the MIT sensor. 61
Figure 1 shows the theory of MIT. The theory implies that a Mott insulator with uniform 62
Coulomb energy can momentarily change its resistance to a value that corresponds to a metal when 63
an electron escapes [13]. 64
Figure 2 exhibits the conventional and proposed temperature sensors. Among the 65
representative conventional temperature sensors, thermo-couplers show relatively high accuracy 66
for temperature detection, but are expensive [19], while NTC thermistors are inexpensive but 67
characterized by resistance that changes slowly at temperatures above 70 °C [20]. In contrast, the 68
MIT sensor used in this study is inexpensive (made of VO2), and its resistance changes rapidly at 69
temperatures above 70 °C [15,18]. 70
Figure 3 displays the resistance change in a NTC thermistor and a MIT sensor according to 71
temperature. Unlike the MIT sensor, the NTC thermistor does not exhibit a significant change in 72
resistance at 25 °C room temperature, and is practical, as 70–80 °C is the first-start temperature 73
range at which a battery can catch fire. The problem with the NTC thermistor is that it is difficult to 74
clearly detect the change in its resistance value at the fire-start temperature [20–22]. By contrast, the 75
proposed MIT sensor’s resistance decreases abruptly at a temperature of around 70 °C. This 76
facilitates the accurate detection of a rise in temperature preceding the fire-start temperature and 77
the prevention of fires and explosions in the battery [15,18]. 78
Figure 4. Resistance measurement data for the MIT sensor.
3. OTP Circuit Based on the MIT Sensor
Figure 5shows the proposed lithium-ion battery system with an MIT sensor, while Figure 6
presents the proposed LED-battery power conversion system with an OTP circuit.
Energies 2020,13, 3593 4 of 11
Energies 2020, 01, x FOR PEER REVIEW 4 of 11
Figure 4 shows resistance measurement data at different temperatures for the MIT sensor 79
invented at the ETRI. The resistance was evidently 372.1 kΩ at 70 °C and rapidly decreased to 44.49 80
Ω at 75 °C. 81
3. OTP Circuit Based on the MIT Sensor 82
Figure 5 shows the proposed lithium-ion battery system with an MIT sensor, while Figure 6 83
presents the proposed LED-battery power conversion system with an OTP circuit. 84
85
Figure 5. The proposed lithium-ion battery system with a MIT sensor. 86
87
Figure 6. The proposed LED-battery power-conversion system with an over-temperature-protection 88
(OTP) circuit. 89
Figure 5. The proposed lithium-ion battery system with a MIT sensor.
Energies 2020, 01, x FOR PEER REVIEW 4 of 11
Figure 4 shows resistance measurement data at different temperatures for the MIT sensor 79
invented at the ETRI. The resistance was evidently 372.1 kΩ at 70 °C and rapidly decreased to 44.49 80
Ω at 75 °C. 81
3. OTP Circuit Based on the MIT Sensor 82
Figure 5 shows the proposed lithium-ion battery system with an MIT sensor, while Figure 6 83
presents the proposed LED-battery power conversion system with an OTP circuit. 84
85
Figure 5. The proposed lithium-ion battery system with a MIT sensor. 86
87
Figure 6. The proposed LED-battery power-conversion system with an over-temperature-protection 88
(OTP) circuit. 89
Figure 6.
The proposed LED-battery power-conversion system with an over-temperature-protection
(OTP) circuit.
The MIT sensors can be placed at the desired position in the lithium-ion battery, and all the MIT
sensors are connected in parallel. The proposed LED-battery power-conversion system in Figure 6
was designed to prevent the occurrence of explosions and fires because of over-temperature in the
lithium-ion battery. It is based on the control of the flyback converter’s output. Power generated
from the solar cell charges the lithium-ion battery of the flyback converter, and the battery supplies
power to LED modules #1 to #8 and the load (LOAD). The MIT sensors (MIT1 to MIT4) detect the
temperature of a specific part of the lithium-ion battery, and the OTP circuit detects the reference
voltage (V
ref
) when over-temperature occurs. V
ref
of the flyback converter controller is controlled to
be 2.5 V. However, when over-temperature occurs, the voltage applied to the OTP circuit V
cc
rises to
12 V. At this voltage, the flyback converter controller does not generate a gate signal, resulting in the
operation of the converter being terminated.
Figure 7shows details of the proposed OTP circuit of the LED-battery power-conversion system.
The MIT sensors are connected in parallel, and the resistance of resistor a (R
a
) and the MIT sensors
divides the V
cc
voltage, after which the resistances of resistor b (R
b
) and resistor c (R
c
) divide the V
cc
Energies 2020,13, 3593 5 of 11
voltage again. The contact between R
a
and the MIT sensor is applied to the emitter terminal of the
NPN transistor, and the contact between R
b
and R
c
is applied to the base terminal of the NPN transistor.
The MIT sensors have resistance R
MIT
in the range of 1700 k
Ω
to 372.1 k
Ω
, at temperatures below
70
◦
C. Therefore, for the application of a voltage of 3 V to the base terminal of the NPN transistor,
the operating conditions of the OTP circuit can be expressed as
Rc
Rb+Rc
−RMIT
RMIT +Ra!×Vcc ≥3[V](1)
where,
Ra: resistance Ra[Ω];
Rb: resistance Rb[Ω];
Rc: resistance Rc[Ω];
RMIT: MIT sensor ’s resistance [Ω];
Vcc: voltage applied to the OTP circuit [V].
Energies 2020, 01, x FOR PEER REVIEW 5 of 11
90
Figure 7. The proposed OTP circuit. 91
The MIT sensors can be placed at the desired position in the lithium-ion battery, and all the 92
MIT sensors are connected in parallel. The proposed LED-battery power-conversion system in 93
Figure 6 was designed to prevent the occurrence of explosions and fires because of 94
over-temperature in the lithium-ion battery. It is based on the control of the flyback converter’s 95
output. Power generated from the solar cell charges the lithium-ion battery of the flyback converter, 96
and the battery supplies power to LED modules #1 to #8 and the load (LOAD). The MIT sensors 97
(MIT1 to MIT4) detect the temperature of a specific part of the lithium-ion battery, and the OTP 98
circuit detects the reference voltage (Vref) when over-temperature occurs. Vref of the flyback converter 99
controller is controlled to be 2.5 V. However, when over-temperature occurs, the voltage applied to 100
the OTP circuit Vcc rises to 12 V. At this voltage, the flyback converter controller does not generate a 101
gate signal, resulting in the operation of the converter being terminated. 102
Figure 7 shows details of the proposed OTP circuit of the LED-battery power-conversion 103
system. The MIT sensors are connected in parallel, and the resistance of resistor a (Ra) and the MIT 104
sensors divides the Vcc voltage, after which the resistances of resistor b (Rb) and resistor c (Rc) divide 105
the Vcc voltage again. The contact between Ra and the MIT sensor is applied to the emitter terminal 106
of the NPN transistor, and the contact between Rb and Rc is applied to the base terminal of the NPN 107
transistor. The MIT sensors have resistance RMIT in the range of 1700 kΩ to 372.1 kΩ, at 108
temperatures below 70 °C. Therefore, for the application of a voltage of 3 V to the base terminal of 109
the NPN transistor, the operating conditions of the OTP circuit can be expressed as 110
+−
+×
≥ 3[]
where,
111
: resistance [Ω]; 112
: resistance [Ω]; 113
: resistance [Ω]; 114
: MIT sensor’s resistance [Ω]; 115
: voltage applied to the OTP circuit [V]. 116
In this study, Ra, Rb, and Rc were set to 10 kΩ. Therefore, according to the voltage division law, 117
the contact between Ra and the MIT sensors was almost 12 V, which was applied to the emitter 118
terminal of the NPN transistor. The contact between Rb and Rc that was applied to the base terminal 119
of the NPN transistor was 6 V. 120
The NPN transistor cannot be turned on up to a temperature of 70 °C. Consequently, the PNP 121
transistor connected to the collector terminal of the NPN transistor can also not be turned on. 122
Therefore, since the OTP circuit does not generate any voltage with capacitor a (Ca) and diode a (Da), 123
Vref is 2.5 V, which is generated by the flyback converter controller. At temperatures above 70 °C, the 124
resistance of the MIT sensors decreases rapidly from 372.1 kΩ to 44.49 Ω. Therefore, the voltage at 125
any point between Ra and the MIT sensors is almost 0 V. Consequently, the voltage applied to the 126
Figure 7. The proposed OTP circuit.
In this study, R
a
,R
b
, and R
c
were set to 10 k
Ω
. Therefore, according to the voltage division law, the
contact between R
a
and the MIT sensors was almost 12 V, which was applied to the emitter terminal of
the NPN transistor. The contact between R
b
and R
c
that was applied to the base terminal of the NPN
transistor was 6 V.
The NPN transistor cannot be turned on up to a temperature of 70
◦
C. Consequently, the PNP
transistor connected to the collector terminal of the NPN transistor can also not be turned on. Therefore,
since the OTP circuit does not generate any voltage with capacitor a (C
a
) and diode a (D
a
), V
ref
is 2.5 V,
which is generated by the flyback converter controller. At temperatures above 70
◦
C, the resistance
of the MIT sensors decreases rapidly from 372.1 k
Ω
to 44.49
Ω
. Therefore, the voltage at any point
between Ra and the MIT sensors is almost 0 V. Consequently, the voltage applied to the emitter terminal
of the NPN transistor is 0 V, and a voltage of 6 V is applied to the base terminal. The condition of
Equation (1) is then satisfied, and the NPN transistor is turned on. This leads to V
cc
=12 V turning on
the PNP transistor and Cabeing charged at 6 V through resistor d (Rd).
The output through the cathode of D
a
. is around 6 V, resulting in the control circuit of the flyback
converter having a reference voltage of 2.5 V or more. V
ref
causes the gate voltage (V
gate
) to become 0 V,
and thereby terminates the charging of the lithium-ion battery.
4. Experimental Results
Table 1reports the parameters of the circuit elements, and Figure 8shows the experimental
apparatus.
Energies 2020,13, 3593 6 of 11
Table 1. Specifications and parameters of the LED-battery power conversion system.
Device Quantity Value
Solar Cell (LG Ltd., LG250SIC-23, Two
Series Connections)
Maximum Voltage 29.9 V
Maximum Current 8.27 A
Flyback Converter (Prototype)
Input Voltage 15–60 V DC
Output Voltage 27 V DC
Maximum Power 50 W
Main Switch IRF640, Fairchild
Diode DSSK28-01A, IXYS
Main Transformer PQ2625, TDK 20: 10, Llk =25 µH
Output Capacitor 2000 µF
18650 Li-ion Battery (Samsung SDI Ltd.)
Operating Voltage 21–28 V DC
Maximum Current 15 A
Maximum Capacity 450 Wh
Size 297 ×85 ×335 mm
LOAD
Non-inductive Bulk Resistance of 15
Ω
LED Module (LG Innotek Ltd., Eight
Parallel Connections)
Operating Voltage 25–30 V DC
Maximum Current 0.5 A
Over-temperature Sensor (ETRI, MIT Sensor)
120 nm thick VO2film on an AlN/Si
substrate
MIT temperature: 70 ◦C
Energies 2020, 01, x FOR PEER REVIEW 7 of 11
142
Figure 8. Experimental apparatus. 143
144
Figure 9. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/0.5 A). 145
146
Figure 10. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/1 A). 147
Figure 8. Experimental apparatus.
Figure 9shows the voltage and current waveforms of the main switch for an input voltage of 40 V
and an output of 27 V/0.5 A, while Figure 10 shows the voltage and current waveforms at an input
voltage of 40 V and an output of 27 V/1 A. The flyback converter was designed to operate in the input
voltage range 15 to 60 V to match the output voltage of the solar cell, and it is evident that it operates
in a discontinuous current mode.
Figures 11 and 12 show the waveforms of V
ref
and the output current of the flyback converter (I
o
)
(0.8 and 1.5 A, respectively) when the temperature of the MIT sensor changed from a normal temperature
to over-temperature, which is characterized by a sudden increase in V
ref
. The over-temperature of
70 ◦C increases the reference voltage rapidly and reduces the output of the flyback converter to zero.
The waveforms of V
ref
and Io are shown for the temperature change sequence normal temperature
→over-temperature →normal temperature.
Energies 2020,13, 3593 7 of 11
V
ref
is 2.5 V at normal temperature and 5 V at over-temperature. When the reference voltage
reaches 5 V at over-temperature, Io is cut off. Thus, the charging of the lithium-ion battery is terminated
when over-temperature occurs.
Energies 2020, 01, x FOR PEER REVIEW 7 of 11
142
Figure 8. Experimental apparatus. 143
144
Figure 9. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/0.5 A). 145
146
Figure 10. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/1 A). 147
Figure 9. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/0.5 A).
Energies 2020, 01, x FOR PEER REVIEW 7 of 11
142
Figure 8. Experimental apparatus. 143
144
Figure 9. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/0.5 A). 145
146
Figure 10. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/1 A). 147
Figure 10. Voltage (Vs) and current (Is) waveforms of the main switch (input 40 V, output 27 V/1 A).
Energies 2020, 01, x FOR PEER REVIEW 8 of 11
148
Figure 11. Reference voltage (Vref) and the flyback converter output current (Io) when the 149
temperature of the MIT sensor changed from normal to over-temperature (0.8 A). 150
151
Figure 12. Reference voltage (Vref) and the flyback converter output current (Io) when the 152
temperature of the MIT sensor changed from normal to over-temperature (1.5 A). 153
154
Figure 13. Gate voltage (Vgate) and flyback converter output current (Io) when the temperature of the 155
MIT sensor changed from normal to over-temperature (1.2 A). 156
Figure 11.
Reference voltage (V
ref
) and the flyback converter output current (I
o
) when the temperature
of the MIT sensor changed from normal to over-temperature (0.8 A).
Energies 2020,13, 3593 8 of 11
Energies 2020, 01, x FOR PEER REVIEW 8 of 11
148
Figure 11. Reference voltage (Vref) and the flyback converter output current (Io) when the 149
temperature of the MIT sensor changed from normal to over-temperature (0.8 A). 150
151
Figure 12. Reference voltage (Vref) and the flyback converter output current (Io) when the 152
temperature of the MIT sensor changed from normal to over-temperature (1.5 A). 153
154
Figure 13. Gate voltage (Vgate) and flyback converter output current (Io) when the temperature of the 155
MIT sensor changed from normal to over-temperature (1.2 A). 156
Figure 12.
Reference voltage (V
ref
) and the flyback converter output current (I
o
) when the temperature
of the MIT sensor changed from normal to over-temperature (1.5 A).
Figure 13 shows the V
gate
of Sand the Io waveform for an output current of 1.2 A when normal
temperature changes to over-temperature. When over-temperature occurs, the duty cycle of V
gate
decreases drastically, and Io decreases from 1.2 to 0 A.
Energies 2020, 01, x FOR PEER REVIEW 8 of 11
148
Figure 11. Reference voltage (Vref) and the flyback converter output current (Io) when the 149
temperature of the MIT sensor changed from normal to over-temperature (0.8 A). 150
151
Figure 12. Reference voltage (Vref) and the flyback converter output current (Io) when the 152
temperature of the MIT sensor changed from normal to over-temperature (1.5 A). 153
154
Figure 13. Gate voltage (Vgate) and flyback converter output current (Io) when the temperature of the 155
MIT sensor changed from normal to over-temperature (1.2 A). 156
Figure 13.
Gate voltage (V
gate
) and flyback converter output current (I
o
) when the temperature of the
MIT sensor changed from normal to over-temperature (1.2 A).
Therefore, the proposed OTP circuit changes the V
ref
of the flyback converter based on 70
◦
C,
and performs the function of stopping the battery charging.
Figure 14 shows the V
gate
of Sand the Io waveform when the over-temperature changes to normal
temperature for Io =1.2 A. After the change, V
gate
increases and Io increases from 0 to 1.2 A. In particular,
when the status changes back to normal temperature, Io transiently increases to 2.4 A instantaneously,
and then gradually decreases to 1.2 A.
Figure 15 shows the voltage (V
LOAD
) and current (I
LOAD
) waveforms for changes in the load
(2.5 A). Clearly, V
LOAD
(or battery voltage) is 27 V, even for a load of 15
Ω
. Thus, it was confirmed that
the proposed LED-battery power-conversion system stably follows load fluctuations.
Energies 2020,13, 3593 9 of 11
Energies 2020, 01, x FOR PEER REVIEW 9 of 11
157
Figure 14. Gate voltage (Vgate) and the flyback converter output current (Io) when the temperature of 158
the MIT sensor changed from normal to over-temperature (1.2 A). 159
160
Figure 15. Voltage (VLOAD) and current (ILOAD) waveforms for varying load (0–2.5 A). 161
Figures 11 and 12 show the waveforms of Vref and the output current of the flyback converter 162
(Io) (0.8 and 1.5 A, respectively) when the temperature of the MIT sensor changed from a normal 163
temperature to over-temperature, which is characterized by a sudden increase in Vref. The 164
over-temperature of 70 °C increases the reference voltage rapidly and reduces the output of the 165
flyback converter to zero. 166
The waveforms of Vref and Io are shown for the temperature change sequence normal 167
temperature → over-temperature → normal temperature. 168
Vref is 2.5 V at normal temperature and 5 V at over-temperature. When the reference voltage 169
reaches 5 V at over-temperature, Io is cut off. Thus, the charging of the lithium-ion battery is 170
terminated when over-temperature occurs. 171
Figure 13 shows the Vgate of S and the Io waveform for an output current of 1.2 A when normal 172
temperature changes to over-temperature. When over-temperature occurs, the duty cycle of Vgate 173
decreases drastically, and Io decreases from 1.2 to 0 A. 174
Therefore, the proposed OTP circuit changes the Vref of the flyback converter based on 70 °C, 175
and performs the function of stopping the battery charging. 176
Figure 14 shows the Vgate of S and the Io waveform when the over-temperature changes to 177
normal temperature for Io = 1.2 A. After the change, Vgate increases and Io increases from 0 to 1.2 A. 178
In particular, when the status changes back to normal temperature, Io transiently increases to 2.4 A 179
instantaneously, and then gradually decreases to 1.2 A. 180
Figure 14.
Gate voltage (V
gate
) and the flyback converter output current (I
o
) when the temperature of
the MIT sensor changed from normal to over-temperature (1.2 A).
Energies 2020, 01, x FOR PEER REVIEW 9 of 11
157
Figure 14. Gate voltage (Vgate) and the flyback converter output current (Io) when the temperature of 158
the MIT sensor changed from normal to over-temperature (1.2 A). 159
160
Figure 15. Voltage (VLOAD) and current (ILOAD) waveforms for varying load (0–2.5 A). 161
Figures 11 and 12 show the waveforms of Vref and the output current of the flyback converter 162
(Io) (0.8 and 1.5 A, respectively) when the temperature of the MIT sensor changed from a normal 163
temperature to over-temperature, which is characterized by a sudden increase in Vref. The 164
over-temperature of 70 °C increases the reference voltage rapidly and reduces the output of the 165
flyback converter to zero. 166
The waveforms of Vref and Io are shown for the temperature change sequence normal 167
temperature → over-temperature → normal temperature. 168
Vref is 2.5 V at normal temperature and 5 V at over-temperature. When the reference voltage 169
reaches 5 V at over-temperature, Io is cut off. Thus, the charging of the lithium-ion battery is 170
terminated when over-temperature occurs. 171
Figure 13 shows the Vgate of S and the Io waveform for an output current of 1.2 A when normal 172
temperature changes to over-temperature. When over-temperature occurs, the duty cycle of Vgate 173
decreases drastically, and Io decreases from 1.2 to 0 A. 174
Therefore, the proposed OTP circuit changes the Vref of the flyback converter based on 70 °C, 175
and performs the function of stopping the battery charging. 176
Figure 14 shows the Vgate of S and the Io waveform when the over-temperature changes to 177
normal temperature for Io = 1.2 A. After the change, Vgate increases and Io increases from 0 to 1.2 A. 178
In particular, when the status changes back to normal temperature, Io transiently increases to 2.4 A 179
instantaneously, and then gradually decreases to 1.2 A. 180
Figure 15. Voltage (VLOAD) and current (ILOAD) waveforms for varying load (0–2.5 A).
5. Conclusions
An MIT-sensor-based OTP circuit for an LED-battery power-conversion system was proposed.
The MIT sensor is cheaper than conventional thermo-couplers and NTC thermistors. An analysis of
the characteristics of the MIT sensor showed that it had a resistance of 372.1 k
Ω
at 70
◦
C, that rapidly
decreased to 44.49
Ω
at 75
◦
C. An OTP circuit was proposed for expanding the MIT sensor. If the MIT
sensor over-heats, the reference voltage of the OTP circuit is set to 5 V. Consequently, the main switch’s
gate signal in the flyback converter is turned off, resulting in the output of the converter being blocked.
This terminates the charging of the lithium-ion battery.
Furthermore, an LED-battery power-conversion system was proposed, in which the output
current of the flyback converter returns to normal when the temperature changes from
over-temperature to normal temperature. The proposed LED-battery power-conversion system with
OTP effectively suppresses the over-temperature generation of lithium-ion batteries for outdoor-lighting.
This power-conversion system is expected to help enhance the stability of lithium-ion batteries in
photovoltaic power and renewable-energy-based systems. The results of this study will lead to the
provision of OTP for lithium-ion batteries, thereby helping to secure battery stability for electric vehicles.
Energies 2020,13, 3593 10 of 11
Funding: This research received no external funding.
Acknowledgments:
The MIT sensor in this paper was provided by Kim, H.T. of ETRI. The authors also appreciate
Kim, H.T. for the constructive comments that helped improved the quality of this paper.
Conflicts of Interest: The authors declare no conflict of interest.
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