Patent Publication Number: US-10771055-B2

Title: Switching device and power conversion device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a switching device and a power conversion device. 
     BACKGROUND ART 
     Semiconductor switching devices (switches, keys), such as diodes, thyristors, bipolar transistors, field effect transistors (MOSFETs) and the like, are widely used in various electronic devices. Semiconductor switching devices are used, for example, in power conversion systems, communication devices, control and monitoring devices, amplification devices, and the like. The semiconductor switching devices and the switching elements may control electric current by an on/off operation. Further, the performance of electronic devices may depend on the performance of the semiconductor switching devices used therein. The characteristics required for the semiconductor switching devices include high power efficiency, small size, high operation speed, low loss, high reliability and the like. Particularly, since the operating speed of the semiconductor switching devices affects the power efficiency and the operating frequency, semiconductor switching devices having a high switching speed are required. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     Provided is a high-frequency switching device having a high switching speed. 
     Further, provided is a switching device having a high power capacity and a low power loss. 
     Solution to Problem 
     According to an aspect of an embodiment of the present disclosure, there is provided is a switching device including: a cascode switch including at least two transistors connected in series and receiving a switching control signal; and a third switch receiving the switching control signal, wherein the at least two transistors include a first transistor receiving the switching control signal through a control terminal and a second transistor having a control terminal connected to a first voltage source, and wherein the third switch is connected between the control terminal and the first terminal of the second transistor, is turned off when the first transistor is turned on, and is turned on when the first transistor is turned off. 
     The cascode switch may be connected between a ground node and an output node, a first terminal of the first transistor may be connected to the ground node, and a second terminal of the second transistor may be connected to the output node. 
     Each of the first transistor, the second transistor, and the third switch may be a Metal Oxide Silicon Field Effect Transistor (MOSFET), the control terminal of the first transistor, the second transistor, and the third switch may be a gate of the MOSFET, and the first terminal of the second transistor may be a source of the MOSFET. 
     Each of the first transistor and the second transistor may be an N-type MOSFET, and the third switch may be a P-type MOSFET. 
     Each of the first transistor, the second transistor, and the third switch may be a bipolar transistor, the control terminal of the first transistor and the second transistor may be a base, and the first terminal of the second transistor may be an emitter. 
     Each of the first transistor and the second transistor may be an npn transistor, and the third switch may be a pnp transistor. 
     The cascode switch may include at least one transistor connected between a second terminal of the first transistor and the first terminal of the second transistor. 
     A power capacity of the second transistor may be greater than that of the first transistor, and a power capacity of the third switch may be smaller than that of the second transistor. 
     The switching device may further include a first diode having a cathode connected to the control terminal of the second transistor and an anode connected to a second terminal of the second transistor, and being connected with the third switch in parallel between the control terminal and the second terminal of the second transistor. 
     The switching control signal may be an alternating current (AC) signal and the output node may output an AC current synchronized with a frequency of the switching control signal. 
     The first voltage source may be a direct current (DC) voltage source having a voltage level lower than a high level voltage of the output node. 
     The switching control signal may have a frequency between 50 MHz and 60 MHz. 
     The cascode switch may be connected between the ground node and the output node, and the output node may output an AC current that varies between a high voltage level in the range of 100V to 350V and a ground node potential. 
     According to another aspect of an embodiment of the present disclosure, there is provided a power conversion device including the switching device according to the disclosed embodiments. 
     According to an embodiment, the power conversion device may include: a first AC-DC converter configured to receive a first AC current and convert the first AC current into a first DC current having a first voltage level; an inverter configured to convert the first DC output from the first AC-DC converter into a second AC current of a first frequency; a transformer configured to convert the second AC current output from the inverter into a third AC current having a second voltage level and outputting the third AC current; and a second AC-DC converter configured to convert the third AC current output from the transformer into a second DC current having a rated output voltage level, wherein the inverter includes the switching device and the switching device may receive the switching control signal of the first frequency. 
     The first DC current output from the first AC-DC converter may have a voltage level higher than that of the first AC current. 
     Advantageous Effects of Disclosure 
     According to the disclosed embodiments, a high-frequency switching device having a high switching speed may be provided. 
     In addition, according to the disclosed embodiments, a switching device having a high power capacity and a low power loss may be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a structure of a switching device  100   a  according to an embodiment. 
         FIG. 2  is a diagram illustrating a structure of a switching device  100   b  according to an embodiment. 
         FIG. 3  is a diagram illustrating a structure of a switching device  100   c  according to an embodiment. 
         FIG. 4  is a diagram illustrating a structure of a switching device  100   d  according to an embodiment. 
         FIG. 5  is a diagram illustrating a structure of a power conversion device  500   a  according to an embodiment. 
         FIG. 6  is a diagram illustrating a structure of a power conversion device  500   a  according to an embodiment. 
         FIGS. 7A to 7C  are diagrams illustrating voltage levels of respective nodes of the power conversion device  500   a  according to an embodiment. 
         FIG. 8  is a diagram illustrating the structure of a switching device  810  according to a comparative example and a switching device  820  according to an embodiment of the present disclosure. 
         FIG. 9  is a diagram illustrating voltage levels at node D 1  NODE_D 1  and node D 2  NODE_D 2  in the comparative example shown in  FIG. 8  and in one embodiment of the present disclosure. 
         FIG. 10A  is a diagram illustrating response time and costs of a switching device according to a comparative example and an embodiment of the present disclosure.  FIGS. 10B, 10C, and 10D  are diagrams illustrating voltage level waveforms at control terminals of respective switching device structures. 
         FIG. 11  is a diagram illustrating an example of calculating the volume of a power conversion device according to an embodiment. 
     
    
    
     BEST MODE 
     According to an aspect of the present disclosure, provided is a switching device including: a cascode switch including at least two transistors connected in series and receiving a switching control signal; and a third switch receiving the switching control signal, wherein the at least two transistors include a first transistor having the switching control signal input to a control terminal and a second transistor having a control terminal connected to a first voltage source, and wherein the third switch is connected between the control terminal and the first terminal of the second transistor, is turned off when the first transistor is turned on, and is turned on when the first transistor is turned off. 
     Mode of Disclosure 
     The present specification explains principles of the present disclosure and discloses embodiments to clarify the scope of the present disclosure and allow one of ordinary skill in the art to perform the present disclosure. The disclosed embodiments may be implemented in various forms. 
     Like reference numerals refer to like elements throughout the specification. This specification does not describe all the elements of the embodiments, and general contents in the technical field of the present disclosure and duplicative contents between embodiments will be omitted. As used herein, the term “module” or “unit” may be embodied in software, hardware or firmware, or a combination of two or more, and may be embodied as a plurality of “modules.” Also, one “module” or “unit” may be configured to include a plurality of elements. Hereinafter, operation principles and embodiments of the present disclosure will be described with reference to the accompanying drawings. 
     Fast operating speed is one of the key requirements for switching devices. Since energy emitted from transistors of a switching device is lost as heat at the moment of switching, the high speed of the switching device is very important from the viewpoint of the efficiency of the device. Since the electric current flowing through the switching device in the turn-off state is close to 0 and the voltage applied to the switching device in the turn-on state is close to 0, the power emitted in the turn-off state and the turn-on state (the product of current and voltage) is close 0 and energy loss is small. However, since a state where both the current and the voltage of the switching device are not 0 may be generated at the moment of switching, considerable power may be consumed in the switching device. As long as the duration of the switching process is less than the time interval between switching operations, the average power consumption is relatively small. However, when the operating frequency of the switching device increases, the ratio of the switching process to the switching period may increase, the average power consumption may increase, and the power efficiency of the switching device may be reduced. To avoid such a situation, it may be appropriate to reduce the switching time to thereby increase the operating speed of the switching device. 
     One way to increase the operating speed of the switching device is to apply a cascode circuit which includes a main switching transistor which operates the gate at a fixed potential and is controlled from a source side and an auxiliary transistor which is controlled from the gate side and is connected in series with the main transistor. In this case, the main transistor may have a higher power capacity than the auxiliary transistor. For example, the main transistor may be a high power transistor and the auxiliary transistor may be a low power transistor. The state (for example, on or off) of such a switching device is determined by the state of the auxiliary transistor. Also, since the auxiliary transistor operates at a much lower drain-source voltage than the main transistor, the operating speed of the auxiliary transistor is much higher than the operating speed of the main transistor operating at relatively high power. Thus, the application of the cascode circuit enables the provision of a high speed switching device based on a general transistor. 
     A disadvantage of the cascode circuit, however, is a slow current turn-off process of the switching device, which is due to the passive turn-off of the main transistor. The main transistor is turned off by the interruption of the flow of the current passing through the auxiliary transistor, thereby delaying the turn-off process of the main transistor. In addition, the two transistors come to be in a non-conductive state due to the turn-off of the main transistor. Since the potential of the contacts of the two transistors remains undefined and floating, the voltage of the auxiliary transistor may exceed the maximum allowable voltage. To prevent this result, additional circuits such as diode Vz2 of U.S. Pat. No. 8,878,593 B2 (hereafter referred to as the “593 patent”) may be used. When the diode Vz2 is turned on, the potential difference between the gate and the source of the main transistor M1 decreases, and the turn-off of the main transistor M1 may be accelerated. However, the diode Vz2 will be turned on when the potential N1 of the contacts of the transistors exceeds the potential of the gate of transistor M1, i.e., the turn-on process has already begun and skips a first stage (the turn-off the auxiliary transistor M2). Therefore, 593 Patent involves a high switching loss caused by the switching delay of the high power N-channel MOS transistor during the operation of the switching device described above. 
     U.S. Pat. No. 8,779,841 B2 (hereinafter referred to as the “841 patent”) discloses a configuration of applying a more complicated supplementary circuit. The 841 patent discloses a cascode switch in which the first power transistor Q1 is located between the ground and the second transistor Q2, the second transistor Q2 is connected to the load, and the second transistor Q2 and the first power transistor Q1 are connected in series. The first power transistor Q1 is operable with an on/off switch responsive to a pulse source coupled to the gate of the first power transistor Q1. In addition, the 841 patent discloses a circuit further including a third transistor Q3. The additional bipolar transistor Q3 is opened when the control signal source voltage becomes zero and the auxiliary transistor Q1 is turned off. However, the method of accelerating the turn-off of the main transistor Q2 in the 841 patent has some disadvantages. 
     First, in order to open the additional transistor Q3, the potential of the base should be reduced after the voltage V1 of the control signal source is lowered. However, the base of the transistor Q3 may be connected to the control voltage source through the diode D 4 , and may be biased in the reverse direction at that moment to thereby allow the leakage current to be supplied as the control current to the base of the transistor Q3. 
     In addition, in the 841 patent, the potential of the gate of the main transistor Q2 may not be completely fixed but may be changed according to the applied control voltage V1. Moreover, the high charge capacity of the gate of the power transistor Q2 will require additional consumption of control signal power at the turn-on stage. 
     The speed reduction of the turn-off process of the main transistor Q2 of the 841 patent caused by the above-mentioned reasons causes additional power consumption and reduces the efficiency of the switching device. Moreover, this additional power consumption may be released as heat, causing a temperature rise of the device. 
     Thus, the solution of the 841 patent has disadvantages similar to those disclosed in the 593 patent. The switching loss caused by the switching delay of the high power N-channel MOS transistor Q2 is such that the high power N-channel MOS transistor Q2 starts the turn-off after the turn-off of the low power N-channel MOS transistor Q1 (the voltage of the Q1 drain should exceed a threshold) is finished. Furthermore, since the turn-off of the transistor Q1 is completed when the voltage of the drain of the transistor Q1 exceeds the threshold, it is difficult to avoid the delay of the turn-off operation of the transistor Q2. 
     Since the turn-on process of cascode circuits is generally much faster than the turn-off process and does not cause heat loss, the turn-on process has less impact on the switching performance than the turn-off process. 
     Also, high losses cause high heat emissions from the switching elements. Thus, there is a need to provide additional heat emitting elements, which increases the size of the device. 
     Further, improvement in the weight and size characteristics of the power supply (i.e., increase in specific power) reduces inductance and capacitance and reduces overall dimensions, which is closely related to increasing the operating frequency. 
     On the other hand, when the operating frequency of the switching device is increased, the limit speed of the switching elements becomes important because the switching device is accompanied by high energy emission (as heat) when the switching device is turned on from the turn-off state. When the switching occurs at a low frequency, the average emission power of the switching device is small, but when switching occurs frequently, the average emission power of the switching device may increase. In other words, the more frequent the switching, first, the less the efficiency of the switching device, because the more power is consumed, and second, as the heat released from the switching device increases, the more bulky heat sink is required, thereby preventing the reduction of the size of the power supply. 
     According to the disclosed embodiments, problems encountered in these comparative examples, such as high switching losses, performance degradation of the switching device due to switching delays of high power transistors, increase in the size of the switching device due to additional heat emitting elements, are resolved. That is, according to the disclosed embodiments, the power loss occurring during switching is significantly reduced and the performance degradation due to the switching delay of the high-power transistor is improved. In addition, the disclosed embodiments may reduce heat generation due to power loss, reduce the need for a heat sink, reduce the volume of the heat sink, and reduce the volume, inductance, and capacitance of the device. 
       FIG. 1  is a diagram illustrating a structure of a switching device  100   a  according to an embodiment. 
     The switching device  100   a  according to one embodiment includes at least two serially connected transistors VT 1  and VT 2 , and a third switch VT 3 . At least two serially connected transistors VT 1  and VT 2  may be connected in series between a ground node GND and an output node NODE_OUT, a control terminal of the first transistor VT 1  may be connected to a control signal source U 2  outputting a switching control signal, and a control terminal of the second transistor VT 2  may be connected to a first voltage source U 1 , which is a cascode switch structure. According to one embodiment, the first transistor VT 1  may be a low-power transistor and the second transistor VT 2  may be a high-power transistor having a higher power capacity than the first transistor VT 1 . Accordingly, the first transistor VT 1  may have a lower capacitance and a faster switching speed than the second transistor VT 2 . A second terminal of the second transistor VT 2  may be connected to the output node NODE_OUT. The first transistor VT 1  may be connected between a first terminal of the second transistor VT 2  and the ground node GND. According to one embodiment, at least one transistor may be further connected in series between the first transistor VT 1  and the second transistor VT 2 , or at least one transistor may be additionally connected in series between the first transistor VT 1  and the ground node. 
     The output node NODE_OUT may be a node connected to a predetermined load. The switching device  100   a  according to the disclosed embodiments may change the current applied from the load. For example, the switching device  100   a  may convert a direct current applied from the load to an alternating current of a predetermined frequency. As another example, the switching device  100   a  may output a control signal or a data signal of a predetermined waveform by modulating an input signal. The waveform of the output node NODE_OUT may be determined by the voltage level applied from the load LOAD and the frequency or waveform of the switching control signal. 
     The third switch VT 3  may be connected between the control terminal of the second transistor VT 2  and the first terminal of the second transistor VT 2 . The control terminal of the third switch VT 3  may be connected to the control signal source U 2  to thereby receive the switching control signal. That is, the control terminal of the third switch VT 3  and the control terminal of the first transistor VT 1  may receive the same switching control signal. Further, the second terminal of the third switch VT 3  is connected to the control terminal of the second transistor VT 2  to thereby be connected to the first voltage source U 1 . 
     According to another embodiment, the third switch VT 3  may receive a second switching control signal having the same frequency as the switching control signal and being synchronized at the same timing. 
     The third switch VT 3  is a switching element that is turned off when the first transistor VT 1  is turned on and is turned on when the first transistor VT 1  is turned off. According to one embodiment, the third switch VT 3  may be a transistor having a polarity opposite to that of the first transistor VT 1 . Also, according to one embodiment, the third switch VT 3  may be a low-capacity switch and may have a smaller power capacity than the second transistor VT 2 . 
     The first voltage source U 1  may be a direct current (DC) power source having a voltage level lower than the highest voltage level of the output node NODE_OUT and having a voltage level higher than a threshold voltage for turning on the second transistor VT 2 . 
     According to one embodiment, the control signal source U 2  is an alternating current (AC) signal source having a predetermined frequency and may output an AC signal having a voltage higher than the threshold voltage for turning on the first transistor VT 1 . 
     According to another embodiment, the control signal source U 2  is an AC signal source having a predetermined waveform for signal modulation and may output an AC signal having a voltage higher than the threshold voltage for turning on the first transistor VT 1 . 
     The present disclosure will be explained centering on an embodiment in which the first transistor VT 1  and the second transistor VT 2  are N-type MOSFETs, and the third switch VT 3  is a P-type MOSFET. However, this is an exemplary embodiment, and the first transistor VT 1 , the second transistor VT 2 , and the third switch VT 3  may be implemented by using various kinds of elements, and the scope of claims is not limited by the kinds of the elements. 
     The gate terminal of the third switch VT 3  and the gate terminal of the first transistor VT 1  may be connected to the control signal source U 2 . Therefore, since the first transistor VT 1  and the third switch VT 3  are controlled by the same voltage source, the third switch VT 3  may operate simultaneously with the first transistor VT 1 . That is, the switching operation of the first transistor VT 1  and the third switch VT 3  may be performed simultaneously. 
     The drain of the third switch VT 3  is connected to the gate terminal of the second transistor VT 2  and the source of the third switch VT 3  is connected to the source of the second transistor VT 2 . The third switch VT 3  is connected between the gate and the source of the second transistor VT 2  to thereby accelerate the turn-off operation of the second transistor VT 2  by providing a current path. Specifically, when the switching control signal is switched to a low level and a cascode switch  110   a  is turned off, the third switch VT 3  is turned on. As such, the third switch VT 3  generates a current path between the gate and the source of the second transistor VT 2  to thereby rapidly lower the Vgs of the second transistor VT 2  and accelerate the turn-off operation of the second transistor VT 2 . 
     The gate terminal of the second transistor VT 2  may be connected to the first voltage source U 1 , and the drain of the second transistor VT 2  may be connected to the load LOAD. The drain of the first transistor VT 1  may be connected to the source of the second transistor VT 2 , and the source of the first transistor VT 1  may be connected to the ground node GND. 
     Next, the switching-on operation of the switching device  100   a  will be described. 
     The state of the first transistor VT 1  is changed from the turn-off state to the turn-on state at the rising edge of the switching control signal. The gate of the third switch VT 3  is directly connected to the gate of the first transistor VT 1 , but since the third switch VT 3  has a polarity opposite to that of the first transistor VT 1 , the third switch VT 3  is turned off at the rising edge. That is, when the first transistor VT 1  is turned on, the third switch VT 3  is turned off. Since the source of the third switch VT 3  is connected to the gate of the second transistor VT 2  and the drain of the third switch VT 3  is connected to the source of the second transistor VT 2 , when the third switch VT 3  is at the turn-off state, the switching device  100   a  operates as a turn-on cascode switch that provides a low resistance. Since the node A NODE_A is connected to the ground node GND through the first transistor VT 1 , it has a ground potential or a potential close to the ground potential. Also, as the gate-source voltage of the second transistor VT 2  comes to have almost the same value as the voltage of the first voltage source U 1 , the second transistor VT 2  is turned on. 
     Next, the switching-off operation of the switching device  100   a  will be described. 
     The state of the first transistor VT 1  is changed from the turn-on state to the turn-off state at the falling edge of the switching control signal. At the same time, the third switch VT 3  is turned on since the gate thereof is directly connected to the gate of the first transistor VT 1  and has a polarity opposite to that of the first transistor VT 1 . When the third switch VT 3  is turned on, the node A NODE_A is connected to the first voltage source U 1  via the third switch VT 3  to thereby have the potential of the first voltage source U 1  or a potential close to the potential of the first voltage source U 1 . Further, since the gate and the source of the second transistor VT 2  have almost the same potential as the third switch VT 3  is turned on, the gate-source voltage of the second transistor VT 2  becomes nearly zero, and the second transistor VT 2  is turned off. 
     The second transistor VT 2  may be implemented as a high capacity transistor having a high power capacity. Particularly, when the cascode switch  110   a  is used in the power conversion device, a high power capacity is required for the second transistor VT 2 . However, transistors with high power capacities have a slow turn-off switching speed because they have high capacitance. Further, when the turn-off switching speed is slowed down, the power loss occurring at the time of turn-off is increased and the heat emission is increased. Therefore, according to the present embodiment, when the cascode switch  110   a  is switched to the turn-off state, a current path is generated by the third switch VT 3  between the gate and the source of the second transistor VT 2 , which increases the speed at which the gate-source voltage of the second transistor VT 2  drops, thereby improving the speed of the turn-off operation of the second transistor VT 2  and reducing the power loss and heat emission. 
     Further, in the cascode switch structure without the third switch VT 3 , when the cascode switch is turned off, the first transistor VT 1  and the second transistor VT 2  are turned off to thereby have high impedance, and the potential of the node A NODE_A remains in an undefined floating state. According to the disclosed embodiments, when the cascode switch is turned off, the node A NODE_A has the same potential as that of the first voltage source U 1  and is not left in the floating state. 
       FIG. 2  is a diagram illustrating a structure of a switching device  100   b  according to an embodiment. 
     The switching device  100   b  according to one embodiment may include at least two transistors VT 1  and VT 2  connected in series, a third switch VT 3 , and a first diode D 1 . 
     The first diode D 1  protects the third switch VT 3  when the switching speed of the third switch VT 3  becomes lower than the switching speed of the first transistor VT 1 . P-type MOSFETs generally have a slower switching speed than N-type MOSFETs. Therefore, when the third switch VT 3  is implemented by the P-type MOSFET, the switching speed of the third switch VT 3  may be slower than that of the first transistor VT 1 . This allows the potential of the node A NODE_A, which is the contact point of the first transistor VT 1  and the second transistor VT 2 , to be higher than the potential of a node B NODE_B, which is the source of the third switch VT 3 , and such a polarity may not be allowed when the third switch VT 3  is a P-type transistor. Therefore, the first diode D 1  prevents the erroneous polarity voltage from being applied to the third switch VT 3 . That is, when the potential of the node A NODE_A becomes higher than the potential of the node B NODE_B, the first diode D 1  generates a current path between the node A NODE_A and the node B NODE_B to thereby drop the potential of the node A NODE_A to the potential of the node B NODE_B. 
     According to another embodiment, the first transistor VT 1 , the second transistor VT 2 , and the third switch VT 3  may be implemented as bipolar transistors. Such bipolar transistors may have the same or similar structure as the circuit structure of the embodiment implemented as a MOSFET transistor. When a bipolar transistor is used, an n-p-n transistor is used instead of an n-type MOSFET, and a p-n-p transistor is used instead of a p-type MOSFET. 
       FIG. 3  is a diagram illustrating a structure of a switching device  100   c  according to an embodiment. 
     According to one embodiment, a cascode switch  110   c  may be implemented in such a manner that three or more transistors VT 1 , VT 2 , and VT 4  are connected in series between the ground node GND and the output node NODE_OUT. For example, as shown in  FIG. 3 , a first transistor VT 1 , a fourth transistor VT 4 , and a second transistor VT 2  may be connected in series between a ground node GND and an output node NODE_OUT. The fourth transistor VT 4  may be implemented as a transistor having the same polarity as that of the first transistor VT 1  and the second transistor VT 2 , for example, may be implemented as an N-type MOSFET. The control terminal of the fourth transistor VT 4  may be connected to the control signal source U 2 . The fourth transistor VT 4  may be a low-capacitance transistor having a lower power capacity than the second transistor VT 2 . 
     When a transistor is added to the cascode switch  110   c , a transistor may be added between the fourth transistor VT 4  and the first transistor VT 1 , and the control terminal of the added transistor may be connected to the control signal source U 2 . 
       FIG. 4  is a diagram illustrating a structure of a switching device  100   d  according to an embodiment. 
     According to one embodiment, a cascode switch  110   d  may be implemented in such a manner that three or more transistors VT 1 , VT 2 , and VT 5  are connected in series between the ground node GND and the output node NODE_OUT. For example, as shown in  FIG. 4 , a first transistor VT 1 , a second transistor VT 2 , and a fifth transistor VT 5  may be connected in series between a ground node GND and an output node NODE_OUT. The fifth transistor VT 5  may be implemented as a transistor having the same polarity as that of the first transistor VT 1  and the second transistor VT 2 , for example, may be implemented as an N-type MOSFET. The control terminal of the fifth transistor VT 5  may be connected to the first voltage source U 1 . The fifth transistor VT 5  may be a high capacity transistor having a higher power capacity than the first transistor VT 1 . 
     When a transistor is added to the cascode switch  110   d , a transistor may be added between the fifth transistor VT 5  and the second transistor VT 2 , and the control terminal of the added transistor may be connected to the first voltage source U 1 . 
     According to one embodiment, the cascode switch  110   d  may further include at least one transistor, which is connected in series between the second transistor VT 2  and the fifth transistor VT 5  and has a control terminal connected to the first voltage source U 1 , and at least one transistor, which is connected in series between the ground node GND and the first transistor VT 1  and has the control terminal connected to the control signal source U 2 . 
       FIG. 5  is a diagram illustrating a structure of a power conversion device  500   a  according to an embodiment. 
     A power conversion device  500   a  according to one embodiment includes a first AC-DC converter  510 , an inverter  520 , a transformer  530 , and a second AC-DC converter  540 . The inverter  520  may include the switching device  100  according to any of the previously disclosed embodiments. 
     The power conversion device  500   a  is provided in an adapter for an electronic device or a main body of the electronic device, and may convert power supplied from the outside into a voltage level and a current size corresponding to the rated input of the electronic device. The electronic device may be, for example, a communication terminal, a laptop computer, a tablet PC, a wearable device, or the like. 
     The power conversion device  500   a  receives a first AC current from a power source  560  and outputs a DC current corresponding to the voltage level of the rated output. The power source  560  may be a power grid. The power source  560  may supply, for example, AC power of 220V and 50 Hz AC power, or AC power of 110V and 50 Hz to 60 Hz. The power conversion device  500   a  according to the disclosed embodiments may support both 220V input power and 110V input power. 
     The first AC-DC converter  510  converts an input AC current, which is input from the power source  560 , into a DC current of a first voltage level and outputs the DC current to the inverter  520 . The first AC-DC converter  510  may have a rectifier structure. The first voltage level is a voltage higher than the input AC voltage, and may be, for example, a voltage level ranging from 300V to 400V. According to one embodiment, the first AC-DC converter  510  may be implemented by substitution of electrolytic capacitors by ceramic capacitors. 
     The inverter  520  converts the high voltage DC into a high voltage AC having a high frequency (0.01 to 50 MHz) and outputs the high voltage AC to the transformer  530 . The inverter  520  requires a switching device  100  that supports a fast switching operation to increase the AC frequency. The switching device  100  may have a switching structure, may include a gate driver, and may need isolation of a serial switch structure. The inverter  520  of the power conversion device  500   a  of the present disclosure may support high frequency and reduce the leakage current by employing one of the switching devices  100   a ,  100   b , and  100   c  according to the above-described embodiments. 
     The transformer  530  converts a high-voltage high-frequency AC output from the inverter  520  to a required low-voltage AC (for example, 5V) and outputs the low-voltage AC to the second AC-DC converter  540 . The mechanical and electrical structure of the transformer  530  may be varied and optimized for the efficiency of the transformer  530 . In addition, the size and weight of the transformer  530  decreases as the frequency increases. According to the disclosed embodiments, a high frequency AC may be output to the transformer  530  by using the switching device  100  that supports a high switching speed in the inverter  520 , thereby reducing the size and weight of the transformer  530 . 
     The second AC-DC converter  540  converts the high-frequency low-voltage AC output from the transformer  530  to a DC having a required voltage level and outputs the DC as an output of the power conversion device  500   a . The second AC-DC converter  540  may have a rectifier structure. For example, the second AC-DC converter  540  may output a DC converted to 5V, which is the rated output voltage level of the power conversion device  500   a.    
     An auxiliary circuit  550  is for improving the performance of the power conversion device  500   a  and may include at least one of, for example, a supply circuit, a feedback circuit, and a protection circuit or a combination thereof. 
       FIG. 6  is a diagram illustrating a structure of a power conversion device  500   a  according to an embodiment. 
     A first AC-DC converter  510   a  may be implemented in the form of a rectifier circuit implemented with four diodes D 2 , D 3 , D 4 , and D 5 , as shown in  FIG. 6 . According to the present embodiment, the second diode D 2  and the third diode D 3  are connected in series between the node C NODE_C and the ground node, and the fourth diode D 4  and the fifth diode D 5  are connected in series between the node C NODE_C and the ground node. A first circuit composed of the second diode D 2  and the third diode D 3  and the second circuit composed of the fourth diode D 4  and the fifth diode D 5  may be arranged in parallel between the node C NODE_C and the ground node. The first AC current supplied from the power source  560  may be input to the node between the second diode D 2  and the third diode D 3  and the node between the fourth diode D 4  and the fifth diode D 5 , respectively. The first AC current input to the first AC-DC converter  510   a  is stepped up to a voltage higher than the input voltage, is converted to a DC to thereby be output to an inverter  520   a  through the node C NODE_C. 
     The inverter  520   a  may have a circuit structure for filtering the input current. The inverter  520   a  according to the present embodiment may include a first capacitor C 1  connected between the ground node and the node C NODE_C, a first inductor L 1  connected between the node C NODE_C and a first output node NODE_OUT 1 , and a second inductor L 2  and a second capacitor C 2  connected in series between the first output node NODE_OUT 1  and the ground node. 
     The switching device  100   d  may be connected between the first output node NODE_OUT 1  and the ground node. A predetermined auxiliary circuit I_PROBE may be added between the first output node NODE_OUT 1  and the switching device  100   d.    
     The switching device  100   d  may include a first transistor VT 1 , a second transistor VT 2 , and a third switch VT 3 . According to one embodiment, a first resistor R 1  may be added between the first voltage source U 1  and the node B NODE_B, and a second resistor R 2  may be added between the control signal source U 2  and the node D. 
     According to another embodiment, the switching device  100   d  may be replaced by the switching devices  100   a ,  100   b , and  100   c  according to the above-described embodiments. 
     A transformer  530   a  may include a third capacitor C 3  connected between the first output node NODE_OUT 1  and the ground node, a third inductor L 3  and a fourth capacitor C 4  connected in series between the first output node NODE_OUT 1  and a node T NODE_T, a fourth inductor L 4  connected between the node T NODE_T and the ground node, and a fifth inductor L 5  and a sixth inductor L 6  arranged adjacent to the fourth inductor L 4 . The transformer  530   a  converts a high voltage AC current output from the first output node NODE_OUT 1  of inverter  520   a  to a low voltage AC. 
     A second AC-DC converter  540   a  may include a sixth diode D 6  connected in series between the fifth inductor L 5  and a second output node NODE_OUT 2 , a seventh diode D 7  connected in series between the sixth inductor L 6  and the second output node NODE_OUT 2 , and a fifth capacitor C 5  connected between the second output node NODE_OUT 2  and the ground node. The second AC-DC converter  540   a  converts the AC current output from the transformer  530   a  into a DC current having a voltage level corresponding to the rated output, and outputs the DC current to the second output node NODE_OUT 2 . 
     In the embodiment shown in  FIG. 6 , IRFBG 30 and IRFBG 2060 IR transistors may be provided in circuits, the rectifier may have a diode model with a forward voltage of 0.25 V and a zero capacitance and may have a frequency of 6.5 MHz and an output power of 160.8 W. 
       FIGS. 7A to 7C  are diagrams illustrating voltage levels of respective nodes of the power conversion device  500   a  according to an embodiment. 
       FIG. 7A  shows the voltage level of node D NODE_D in  FIG. 6 . The horizontal axis of the graph of  FIG. 7A  represents the time in nano second units, and the vertical axis represents the voltage in units of V (volt). As shown in  FIG. 7A , node D NODE_D may have an AC voltage level that swings between a ground voltage level (0V) and a predetermined high level voltage level (e.g., 10V). According to one embodiment, when the node D NODE_D has a high level, the first transistor VT 1  is turned on and the third switch VT 3  is turned off. Further, when the node D NODE_D has a low level, the first transistor VT 1  is turned off and the third switch VT 3  is turned on. 
       FIG. 7B  shows the voltage level of the first output node NODE_OUT 1  of  FIG. 6 . The horizontal axis of the graph of  FIG. 7B  represents the time in micro second units, and the vertical axis represents the voltage in kV. As shown in  FIG. 7B , the DC current output from the first AC-DC converter  510   a  may be converted to an AC current by the switching device  100  according to the disclosed embodiments to thereby be converted to the waveform shown in  FIG. 7B  in the first output node NODE_OUT 1 . The switching device  100  according to the disclosed embodiments may generate a high-frequency signal having a period of micro second units. 
       FIG. 7C  shows the voltage level of node T NODE_T of  FIG. 6 . The horizontal axis of the graph of  FIG. 7C  represents the time in micro second units, and the vertical axis represents the voltage in V units. As shown in  FIG. 7C , the AC current output from the inverter  520   a  may be converted to a low level voltage by the transformer to thereby be converted to the waveform shown in  FIG. 7C . 
       FIG. 8  is a diagram illustrating the structure of a switching device  810  according to a comparative example and a switching device  820  according to an embodiment of the present disclosure. 
     The switching device  810  according to the comparative example includes a sixth transistor VT 6 , a third resistor R 3 , and a control signal source U 2 . The sixth transistor VT 6  is connected between the first output node NODE_OUT 1  and the ground node, and the control terminal is connected to the control signal source U 2  through the third resistor R 3 . 
     The switching device  820  according to an embodiment of the present disclosure may have the same structure as that of the switching device  100   d  described above with reference to  FIG. 6 . 
       FIG. 9  is a diagram illustrating voltage levels at node D 1  NODE_D 1  and node D 2  NODE_D 2  in the comparative example shown in  FIG. 8  and in one embodiment of the present disclosure. 
     According to the comparative example, a waveform distortion  910  is observed at the falling edge of the voltage level of the node D 1  NODE_D 1  connected to the gate terminal of the sixth transistor VT 6 . Particularly, since the fall of the voltage level is delayed at the falling edge, the power consumption consumed in one period increases. As a result, according to the comparative example, the leakage current of the switching device increases, and the power efficiency is lowered. Leakage currents are dissipated in the form of heat, which increases the heat generation of the device. When the heat generation increases, an additional cooling system is required, which increases the size of the device and raises the unit price. In addition, such a waveform distortion  910  becomes an obstacle in increasing the frequency of the switching device, thereby making it difficult to implement the high-frequency switching device. 
     On the other hand, according to one embodiment of the present disclosure, little distortion occurs at the falling edge of the voltage level of node D 2  NODE_D 2  connected to the gate terminal of the first transistor VT 1  ( 920 ). In particular, the voltage level distortion  910  observed in the comparative example is rarely found in one embodiment of the present disclosure. According to the disclosed embodiments, the distortion is removed at the falling edge of the node D 2  NODE_D 2  as shown in  FIG. 9 , thereby reducing the leakage current and increasing the power efficiency. Further, according to the disclosed embodiments, as the switching response speed increases, a high switching speed may be possible and a high-frequency switching device may be provided. Further, the heat generation may be reduced and the cooling load of the cooling system may be reduced by reducing the leakage current, to thereby allowing the size and unit cost of the device to be reduced. 
     According to the measurement example shown in  FIGS. 8 and 9 , the efficiency of the switching device according to the embodiments of the present disclosure is equal to or greater than 93%, and the simulation accuracy has a +/−1% error range. According to one embodiment of the present disclosure, the efficiency may increase by about 9%, for example, efficiency may increase from 84% to 93%, when compared to a comparative example (for example, a MOSFET switch structure). 
       FIG. 10A  is a diagram illustrating response time and costs of a switching device according to a comparative example and an embodiment of the present disclosure.  FIGS. 10B, 10C, and 10D  are diagrams illustrating voltage level waveforms at control terminals of respective switching device structures. 
     The switching device having GaN EPC 2012  common source structure which uses GaN (Gallium Nitride) may have a very fast switching-on time by using GaN and may have a switching response time of about 3.2 nsec. Referring to  FIG. 10B , the switching device of the GaN EPC 2012  common source structure has a very fast switch reaction time, and little distortion is found in the falling edge interval T fall . However, since GaN is a very expensive material, it may increase the unit price of the product and may not be easily applied. 
     The IRF 820  common source structure has the structure of the switching device  810  of the comparative example of  FIG. 8 , and has a disadvantage that the switching speed is lowered because the switching-on time is the longest as 8.0 nsec as compared with the structures of other switching devices of  FIG. 10A . As shown in  FIG. 10D , the IRF  820  common source has a relatively long falling edge interval T fall . However, since the IRF  820  common source is cheaper than GaN switching device, it may be used to lower the product cost. 
     A silicon-based switching device (Si IRF  820  common gate+Si D2089UK common source (cascode structure)) according to embodiments of the present disclosure has a short switching-on time of about 4.5 nsec. As shown in  FIG. 10C , the silicon-based switching device according to the embodiments of the present disclosure has a relatively short falling edge interval T fall , and little distortion is found in the falling edge interval T fall . Further, the silicon-based switching device according to the embodiments of the present disclosure has the effect of providing a high-speed switching operation close to an expensive GaN switching device at a relatively low cost by using silicon of a low unit cost. 
       FIG. 11  is a diagram illustrating an example of calculating the volume of a power conversion device according to an embodiment. 
     The current source inductor indicates L 3  of  FIG. 6 , the input filter inductor indicates the first inductor L 1  of  FIG. 6 , the power transformer indicates the fourth inductor L 4 , the fifth inductor L 5 , and the sixth inductor L 6  of  FIG. 6 , the primary switching circuit indicates the switching device  100   d  of  FIG. 6 , and the rectifier circuit indicates the sixth diode D 6  and the seventh diode D 7  of  FIG. 6 . In general, since the inductors take up the largest volume in the circuit, the volume of the inductors makes a large contribution to the volume of the power conversion device. 
     The power conversion device according to an embodiment of the present disclosure may have a power density of about 25 or more, as shown in  FIG. 11 . According to the embodiments of the present disclosure, the power density of the power conversion device may be increased by reducing the volume of the transformer by the high-voltage high-frequency switching device. 
     The power conversion device based on the switching device according to the disclosed embodiments may be simulated using the software package Advanced Design System ADS 2011.6 for electrical simulation. Simulation results show that the operating frequency is significantly increased at sufficiently high efficiency (about 85%). According to one experimental example, it is possible to support up to 13.56 MHz by applying one embodiment whereas the switching device has previously supported up to 100-300 kHz. Further, according to the disclosed embodiments, a high direct current voltage (up to 400 V) generated by rectifying the main power voltage of 220 V, 50 or 60 Hz may be input to the inverter including the switching device. According to the simulation results, the output voltage of the inverter has a frequency of 13.56 MHz at a power of 231 W and an efficiency of 84.6% (input power 274 W). At these high operating frequencies, a reactive element (an inductor of about 1.0 μH) with the main reactance of the inverter may be implemented with a volume of only about 10 cm 3 , which allows a specific capacity of more than 10 W/cm 3 . 
     Therefore, according to the disclosed embodiments, a compact, low-loss, high-speed, high-power, high-frequency switching device may be provided. 
     It is also to be understood that the advantages of embodiments of the present disclosure include not only the general size reduction of the power supply device including the power conversion device using the switching devices according to the disclosed embodiments but also allowing production of such a power supply device in the form of a plane structure with a low height. These advantages have the effect of increasing consumer convenience. For example, the slim structure of the power supply device may reduce the thickness of the TV housing in a TV, thereby increasing consumer convenience. 
     The disclosed embodiments described have been described above with reference to the accompanying drawings. It will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims. The disclosed embodiments are illustrative and should not be construed as limiting.