Patent Publication Number: US-2019173385-A1

Title: Dc-dc converter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2017-0164333, filed on Dec. 1, 2017, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments relate to a DC-DC converter, and more particularly, to a DC-DC converter which may restrict an output current ripple. 
     2. Description of the Related Art 
     Power control units (PCUs) of geosynchronous earth orbit (GEO) satellites may include non-isolated DC-DC converters that receive power from a solar array and charge a battery. 
     An output current ripple may be generated in an output of the DC-DC converter. To reduce the output current ripple, a bus capacitor having a relatively large capacity and a relatively long life may be connected to the output of the DC-DC converter. However, a capacitor having a large size may be burdensome in terms of weight, size, and performance of a satellite. 
     SUMMARY 
     One or more embodiments include a non-isolated DC-DC converter which may restrict an output current ripple. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     According to one or more embodiments, a DC-DC converter includes a first transformer connected between a first node and a ground, the first node being located between an input terminal and an output terminal, and the first transformer comprising a first inductor and a second inductor magnetically coupled to each other, a first switch serially connected to the first inductor, between the first node and the ground, a second switch serially connected to the second inductor, between the first node and the ground, a third switch connected between a second node and the output terminal, the second node being located between the second inductor and the second switch, and the third switch being controlled in a same manner as the first switch, and a fourth switch connected between a third node and the output terminal, the third node being located between the first inductor and the first switch, and the fourth switch being controlled in a same manner as the second switch. 
     The DC-DC converter may further include a second transformer connected between the input terminal and the output terminal and including a third inductor and a fourth inductor magnetically coupled to each other, and a diode having an anode connected to the fourth inductor and a cathode connected to the output terminal. 
     The third inductor may be connected between the input terminal and the first node, and the fourth inductor may be connected between the first node and the diode. 
     When both of the first switch and the second switch are turned off, an output current may flow through the diode. 
     The first switch and the second switch may be alternately turned on, and when the first switch is turned on, the second switch may be turned off, and when the second switch is turned on, the first switch may be turned off. 
     The first switch and the second switch may be turned on at a same duty ratio, and the duty ratio may be less than about 50%. 
     A current may flow from the second node to the output terminal only when the third switch is turned on, and a current may flow from the third node to the output terminal only when the fourth switch is turned on. 
     The third switch may be a first thyristor having an anode connected to the second node and a cathode connected to the output terminal, and the fourth switch may be a second thyristor having an anode connected to the third node and a cathode connected to the output terminal. 
     The DC-DC converter may further include a controller outputting a first control signal to control the first thyristor and outputting a second control signal to control the second thyristor. 
     The first thyristor and the second switch may be controlled by the first control signal, and the second thyristor and the first switch may be controlled by the second control signal. 
     When the first switch is turned on, an induced current generated in the second inductor due to a current flowing into the first inductor may flow toward the output terminal via the third switch, and when the second switch is turned on, an induced current generated in the first inductor due to a current flowing into the second inductor may flow toward the output terminal via the fourth switch. 
     The third switch may be configured to be turned on when the first switch is turned on, and turned off when the first switch is turned off, and the fourth switch may be configured to be turned on when the second switch is turned on, and turned off when the second switch is turned off. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  schematically illustrates a configuration of a DC-DC converter according to an embodiment; 
         FIG. 2  is an exemplary circuit diagram of a DC-DC converter, according to an embodiment; 
         FIGS. 3A to 3D  exemplarily illustrate output current waveforms when a high output ripple current is generated, according to a comparative example; and 
         FIGS. 4A and 4B  exemplarily illustrate output current waveforms of a DC-DC converter, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. In the description of the present disclosure, certain detailed explanations of the related art are omitted when it is deemed that they may unnecessarily obscure the essence of the disclosure. 
     The terms used in the present specification are merely used to describe embodiments, and are not intended to limit the present disclosure. An expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. In the present specification, it is to be understood that the terms such as “including,” “having,” and “comprising” are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, steps, actions, components, parts, or combinations thereof may exist or may be added. Terms such as “first” and “second” used herein may modify various elements or components regardless of their order and/or importance. These terms may be used only to distinguish one element or component from another element or component, and these elements or components should not be limited by these terms. 
       FIG. 1  schematically illustrates a configuration of a DC-DC converter  10  according to an embodiment. 
     Referring to  FIG. 1 , the DC-DC converter  10  according to the present embodiment may include a first transformer T 1  including a first inductor L 1  and a second inductor L 2 , and first to fourth switches Q 1 , Q 2 , Q 3 , and Q 4 . 
     A first node N 1  is connected between an input terminal Tin and an output terminal Tout. The first transformer T 1  is connected between the first node N 1  and to a ground GND. The first inductor L 1  and the second inductor L 2  of the first transformer T 1  are magnetically coupled to each other. The first switch Q 1  is serially connected to the first inductor L 1  between the first node N 1  and the ground GND. The second switch Q 2  is serially connected to the second inductor L 2  between the first node N 1  and the ground GND. Diodes QD 1  and QD 2  may be respectively and anti-parallelly connected to the first switch Q 1  and the second switch Q 2 . 
     Input power Vin is connected between the input terminal Tin and the ground GND, and the input power Vin may output an input current Iin to the input terminal Tin. An output capacitor C for power stability is connected between the output terminal Tout and the ground GND. Furthermore, a load R is connected between the output terminal Tout and the ground GND, and an output current Iout is supplied to the load R via the output terminal Tout. When a relatively large output current ripple occurs in the output current Iout, the capacity of the output capacitor C needs to be large. 
     A second node N 2  is connected between the second inductor L 2  and the second switch Q 2 . The third switch Q 3  is connected between the second node N 2  and the output terminal Tout. A third node N 3  is connected between the first inductor L 1  and the first switch Q 1 . The fourth switch Q 4  is connected between the third node N 3  and the output terminal Tout. 
     The third switch Q 3  is controlled in the same manner as the first switch Q 1 , and the fourth switch Q 4  is controlled in the same manner as the second switch Q 2 . For example, the third switch Q 3  may be configured to be turned on when the first switch Q 1  is turned on, and to be tuned off when the first switch Q 1  is turned off. The fourth switch Q 4  may be configured to be turned on when the second switch Q 2  is turned on, and to be turned off when the second switch Q 2  is turned off. 
     When the third switch Q 3  is turned on, a current may flow only from the second node N 2  to the output terminal Tout. In other words, even when the third switch Q 3  is turned on, no current flows from the output terminal Tout to the second node N 2 . When the third switch Q 3  is turned off, no current flows between the second node N 2  and the output terminal Tout. 
     When the fourth switch Q 4  is turned on, a current may flow only from the third node N 3  to the output terminal Tout. In other words, even when the fourth switch Q 4  is turned on, no current flows from the output terminal Tout to the third node N 3 . When the fourth switch Q 4  is turned off, no current flows between the third node N 3  and the output terminal Tout. 
     The third switch Q 3  and the fourth switch Q 4  each may be provided as a thyristor. As illustrated in  FIG. 1 , the third switch Q 3  may be a first thyristor having an anode connected to the second node N 2  and a cathode connected to the output terminal Tout. The fourth switch Q 4  may be a second thyristor having an anode connected to the third node N 3  and a cathode connected to the output terminal Tout. In another example, the third switch Q 3  and the fourth switch Q 4  may be formed by a diode and a switching device that are serially connected to each other. 
     The first and second switches Q 1  and Q 2  each perform a switching operation according to a switch control signal, and may alternately repeat turning on and off. The first and second switches Q 1  and Q 2  are not turned on by the switch control signal at the same time. For example, when the second switch Q 2  is turned off by the switch control signal, the first switch Q 1  is turned on. Furthermore, when the first switch Q 1  is turned off, the second switch Q 2  is turned on. The first and second switches Q 1  and Q 2  may not be turned off at the same time. 
     The first switch Q 1  and the second switch Q 2  may be turned on at the same duty ratio. In this state, the duty ratio may be about less than 50%. In an example, the duty ratio may be about 40%. The duty ratio may vary according to a voltage level of the input power Vin and a voltage level of a desired output voltage. 
     When the first switch Q 1  is turned on by the switch control signal, an induced current is generated in the second inductor L 2  by a current flowing into the first inductor L 1 , and the induced current flows toward the output terminal Tout via the third switch Q 3 . Likewise, when the second switch Q 2  is turned on by the switch control signal, an induced current is generated in the first inductor L 1  by the current flowing into the second inductor L 2 , and the induced current flows toward the output terminal Tout via the fourth switch Q 4 . 
     The DC-DC converter  10  according to the present embodiment may further include a third inductor L 3  and a fourth inductor L 4 , which are serially connected between the input terminal Tin and the output terminal Tout, and a diode D 1  serially connected to the fourth inductor L 4 . The third and fourth inductor L 3  and L 4 , as illustrated in  FIG. 1 , are magnetically connected to each other, and may form a second transformer T 2 . Accordingly, an induced current is generated in the fourth inductor L 4  by the current flowing into the third inductor L 3 . 
     As illustrated in  FIG. 1 , the third inductor L 3  may be connected between the input terminal Tin and the first node N 1 , and the fourth inductor L 4  may be connected between the first node N 1  and the diode D 1 . The diode D 1  may have an anode connected to the fourth inductor L 4  and a cathode connected to the output terminal Tout. 
     The third and fourth inductor L 3  and L 4  and the diode D 1  form a current path between the input terminal Tin and the output terminal Tout. When both of the first and second switches Q 1  and Q 2  are turned off by the switch control signal, the current flows toward the output terminal Tout via the diode D 1 . In this state, since both of the third and fourth switches Q 3  and Q 4  are turned off, no current flows through the first and second inductors L 1  and L 2 . 
     Accordingly, the DC-DC converter  10  according to the present embodiment allows the current to flow toward the output terminal Tout via the third and fourth switches Q 3  and Q 4  when the first and second switches Q 1  and Q 2  are alternately turned on, and to flow toward the output terminal Tout via the diode D 1  when both of the first and second switches Q 1  and Q 2  are turned off, and thus the output terminal Tout may continuously output a current with a reduced output current ripple. 
     According to a comparative example, a diode, instead of the third switch Q 3 , is connected between the second node N 2  and the output terminal Tout, and a diode, instead of the fourth switch Q 4 , may be connected between the third node N 3  and the output terminal Tout. For convenience of explanation, according to the comparative example, the diode connected between the second node N 2  and the output terminal Tout is referred to as a third diode D 3  (not shown), and the diode connected between the third node N 3  and the output terminal Tout is referred to as a fourth diode D 4  (not shown). 
     When the first switch Q 1  is turned off by the switch control signal, the current flowing through the first inductor L 1  flows toward the output terminal Tout via the fourth diode D 4 . The first inductor L 1  and the second inductor L 2  are magnetically coupled to each other, forming the first transformer T 1 , and an induced current corresponding to the current flowing through the first inductor L 1  flows through the second inductor L 2 . The current flowing through the second inductor L 2  flows toward the output terminal Tout via the third diode D 3 . When the first switch Q 1  is turned off, the current flowing through the third diode D 3  and the current flowing through the fourth diode D 4  are summed at the output terminal Tout and a large output current ripple is generated. 
     As such, to prevent generation of a large output current ripple, in the DC-DC converter  10  according to the present embodiment, the third switch Q 3  is controlled in the same manner as the first switch Q 1 , and the fourth switch Q 4  is controlled in the same manner as the second switch Q 2 . In other words, when the first switch Q 1  is turned on, the third switch Q 3  is also turned on, and when the first switch Q 1  is turned off, the third switch Q 3  is also turned off. Furthermore, when the second switch Q 2  is turned on, the fourth switch Q 4  is also turned on, and when the second switch Q 2  is turned off, the fourth switch Q 4  is also turned off. 
     According to an embodiment, as illustrated in  FIG. 1 , the third and fourth switches Q 3  and Q 4  may be respectively first and second thyristors. In this state, the DC-DC converter  10  may further include a controller (not shown) for outputting a first control signal to control the first thyristor and a second control signal to control the second thyristor. 
     In an example, since the third switch Q 3  is controlled in the same manner as the first switch Q 1 , the first switch Q 1  may be controlled by the first control signal. Also, since the fourth switch Q 4  is controlled in the same manner as the second switch Q 2 , the second switch Q 2  may be controlled by the second control signal. In other words, the first and second control signals may be the above-described switch control signals. 
       FIG. 2  is an exemplary circuit diagram of a DC-DC converter according to an embodiment. 
     The circuit diagram of the DC-DC converter  10  of  FIG. 2  shows that additional devices are further included compared to  FIG. 1 . For example,  FIG. 2  illustrates that additional inductors are further connected to the first inductor L 1  of the first transformer T 1 . The additional inductors may be serially/parallelly connected to the first inductor L 1 . Furthermore,  FIG. 2  illustrates that additional inductors are further connected to the third inductor L 3  of the second transformer T 2 . The additional inductors may be serially/parallelly connected to the third inductor L 3 . 
       FIGS. 3A to 3D  exemplarily illustrate waveforms when an output current ripple is generated, according to a comparative example. 
     As described above, the comparative example of  FIG. 3  is an example in which, in the circuit diagram of  FIG. 1 , the third diode D 3 , instead of the third switch Q 3 , is connected between the second node N 2  and the output terminal Tout, and the fourth diode D 4 , instead of the fourth switch Q 4 , is connected between the third node N 3  and the output terminal Tout. 
       FIG. 3A  illustrates a control signal S 1  applied to the first switch Q 1  and a control signal S 2  applied to the second switch Q 2 . Both of the first and second switches Q 1  and Q 2  are turned on at a duty ratio of about less than 50%. The first and second switches Q 1  and Q 2  may have the same duty ratio. 
       FIG. 3B  illustrates a current IQ 1  flowing through the first switch Q 1  and a current IQ 2  flowing through the second switch Q 2 . 
       FIG. 3C  illustrates a current ID 1  flowing through first the diode D 1 , a current ID 3  flowing through the third diode D 3  between the second node N 2  and the output terminal Tout, and a current ID 4  flowing through the fourth diode D 4  between the third node N 3  and the output terminal Tout. 
     A portion “A” of  FIG. 3C  shows a moment directly after the second switch Q 2  is turned off. When the second switch Q 2  is turned on, the first switch Q 1  is turned off, and the current flowing through the second inductor L 2  flows to the ground GND via the second switch Q 2 . The first transformer T 1  allows an induced current to flow in the first inductor L 1  by the current flowing through the second inductor L 2 , and the induced current flows through the fourth diode D 4 . 
     When the second switch Q 2  is turned off, the induced current flowing through the fourth diode D 4  gradually decreases. However, since the current flowing through the second inductor L 2  does not flow through the second switch Q 2 , the current flows toward the output terminal Tout via the third diode D 3 . Accordingly, a sharp spike is generated in a current ID 3  flowing through the third diode D 3 , as indicated by the portion “A”. When the second switch Q 2  is turned off, a spike current having the same amount as the current ID 4  flowing through the fourth diode D 4  instantly flows through the third diode D 3 . 
       FIG. 3D  illustrates the input current Iin input through the input terminal Tin and the output current Iout output through the output terminal Tout. Due to the spike current instantly flowing through the fourth diode D 4  when the first switch Q 1  is turned off and the spike current flowing through the third diode D 3  when the second switch Q 2  is turned off, a large output current ripple is periodically generated in the output current Iout. 
       FIGS. 4A and 4B  exemplarily illustrate output current waveforms of a DC-DC converter according to an embodiment. 
       FIG. 4A  illustrates the input current Iin input through the input terminal Tin and the output current Iout output through the output terminal Tout.  FIG. 4B  illustrates a current IQ 3  flowing through the third switch Q 3 , a current IQ 4  flowing through the fourth switch Q 4 , and a current ID 1  flowing through the diode D 1 . 
     First, referring to  FIG. 4B , in a section in which both of the first switch Q 1  and the second switch Q 2  are turned off, a current flows through the diode D 1  only and the spike current as illustrated in  FIG. 3C  is not generated. Accordingly, as illustrated in  FIG. 4A , an output current ripple is hardly generated in the output current Iout. 
     As a result, according to the DC-DC converter  10  according to the present embodiment, since the output current ripple is much reduced, there is no need to use the output capacitor C having a large capacity in  FIG. 1 , and thus a problem that the life of the output capacitor C is abnormally decreased may not occur. 
     According to the DC-DC converter  10  according to the present embodiment, since the capacity of the output capacitor C may be reduced, the weight of an artificial satellite may be further reduced. Furthermore, when the output capacitor C is damaged, power may not be supplied to internal devices of the artificial satellite. Accordingly, by applying the DC-DC converter  10  according to the present embodiment, the expected life of the artificial satellite may be extended. 
     As described above, the present disclosure may provide a non-isolated DC-DC converter which may restrict an output current ripple. 
     It should be understood that embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. 
     While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.