Patent Publication Number: US-2009231772-A1

Title: Car charger and surge protection device thereof

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This non-provisional application claims priority under 35 U.S.C. § 119(a) of Patent Application No(s). 97108586 filed in Taiwan, R.O.C. on 2008 Mar. 11, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a charger and a surge protection device thereof, and more particularly to a car charger and a surge protection device thereof. 
     2. Related Art 
     A car charger provides power sockets in a car, so as to supply power to or charge mobile phones, GPS devices, or other portable electronic devices. Therefore, the car charger must comply with relevant safety specifications on cars. For example, the car charger must be capable of withstanding a certain degree of surge voltage, such that the surge voltage will not influence the normal operation of back end circuits. 
       FIG. 1  is a schematic view of a surge protection device in a car charger in the conventional art. Referring to  FIG. 1 , the surge protection device of the conventional art includes a diode A 10  and a transient voltage suppressor (TVS) A 20 . 
     According to the conventional art, the TVS A 20  is used to suppress the forward surge. In another aspect, a reverse voltage of the diode A 10  is used to solve the reverse surge. That is to say, to prevent the reverse surge from flowing back, the withstand voltage of the diode A 10  must be greater than the voltage of the reverse surge, so as to avoid the damage caused by the voltage of the reverse surge. However, though the problem caused by the reverse surge is solved, another negative problem appears; that is, when the reverse surge is greater, the withstand voltage of the diode A 10  must be higher, and accordingly, a forward voltage (Vf) of the diode A 10  must be higher as well. 
     The power loss of the diode A 10  will increase with the increase of the forward voltage of the diode A 10 , resulting in a temperature rise. The temperature rise caused by the diode A 10  is generated within the limited space of the car charger, so a heat sink must be added to solve this problem. Thus, the size of the car charger must be designed larger, and the additional heat sink increases the cost, which lowers the efficiency of the car charger. 
     For example, as specified in the JASO D001 B2 automotive standards, when the reverse surge is 260 V, to effectively block the voltage of the reverse surge, the reverse voltage of the diode A 10  must be 300 V, and the forward voltage must be 1.5 V. Assume that the current flowing through the diode A 10  is 1 A, the power loss on the diode A 10  is 1.5 w, and at this time, a heat sink must be used to solve the problem of temperature rise. 
     Therefore, the problem caused by the surge protection device of the car charger in the conventional art is an important problem for researchers to solve. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a car charger and a surge protection device thereof. The surge protection device of the present invention can use a diode having a low reverse voltage, so as to reduce a forward voltage of the diode. Thus, the diode will generate a low power loss, and will not cause the problem of temperature rise. Therefore, it is unnecessary to add a heat sink in a limited space of the car charger, thus saving the cost for the heat sink. 
     The present invention provides a surge protection device. The surge protection device is coupled between a car power source and a car charger, and includes a diode, a current-limiting resistor, and a first transient voltage suppressor (TVS). The diode has an anode coupled to the car power source and a cathode coupled to the car charger. The current-limiting resistor has one end coupled to the anode of the diode. The first TVS is connected to the current-limiting resistor in series, and is connected in parallel with the current-limiting resistor between the car power source and the car charger. A reverse breakdown voltage of the first TVS is smaller than a reverse breakdown voltage of the diode. 
     The present invention also provides another surge protection device. The surge protection device is coupled between a car power source and a car charger, and includes a diode, a current-limiting resistor, a first TVS, and a second TVS. The diode has an anode coupled to the car power source and a cathode coupled to the car charger. The current-limiting resistor has one end coupled to the anode of the diode. The first TVS is connected to the current-limiting resistor in series, and is connected in parallel with the current-limiting resistor between the car power source and the car charger. A reverse breakdown voltage of the first TVS is smaller than a reverse breakdown voltage of the diode. The second TVS has one end coupled to the cathode of the diode, and the second TVS is connected between the car power source and the car charger in parallel. 
     The present invention also provides a car charger. The car charger receives an input voltage from a car power source, and outputs an output voltage. The car charger includes a charging connector, a diode, a current-limiting diode, and a first TVS. The charging connector is provided to connect and charge external devices. The diode includes an anode receiving the input voltage and a cathode coupled to the charging connector. The current-limiting resistor has one end coupled to the anode of the diode. The first TVS is connected to the current-limiting resistor in series, and connected in parallel with the current-limiting resistor between the car power source and the charging connector. A reverse breakdown voltage of the first TVS is smaller than a reverse breakdown voltage of the diode. 
     Preferred embodiments of the present invention and their effects are described in the following with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic view of a surge protection device of a car charger in the conventional art; 
         FIG. 2  is a schematic view of a surge protection device of a car charger according to a first embodiment of the present invention; 
         FIG. 3  is a schematic view of a surge protection device of a car charger according to a second embodiment of the present invention; and 
         FIG. 4  is a schematic view of a car charger according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  is a schematic view of a surge protection device of a car charger according to a first embodiment of the present invention. Referring to  FIG. 2 , the surge protection device of a car charger of the present invention is coupled between a car power source and the car charger. The surge protection device according to the first embodiment includes a diode  10 , a current-limiting resistor  20 , and a first transient voltage suppressor (TVS)  30 . 
     The diode  10  includes an anode  12  and a cathode  14 . The anode  12  is coupled to the car power source  1 , and the cathode is coupled to the car charger  2 . The car charger  2  includes a DC-DC converter (not shown). 
     The diode is an electronic element having two electrodes, that is, two terminals including an anode and a cathode. In most cases, the diode is used for current rectification. The current rectification refers to that the electric current can flow in one direction. That is to say, the current is made to flow from the anode to the cathode (the forward bias), but cannot flow from the cathode to the anode (the reverse bias). The unidirectional electric current property is called current rectification. To put it simply, the diode allows the current to flow in one direction, but blocks the current in the reverse direction. Therefore, in the present invention, the diode  10  is coupled between the car power source  1  and the car charger  2 , such that the forward current from the car power source  1  is transmitted to the car charger  2  after passing through the diode  10 , and is used by an external electronic product to be charged or needing power. On the contrary, when a reverse surge appears on the car power source  1 , the diode  10  blocks the reverse surge generated by the car power source  1  from flowing back, so as to prevent the damage to the internal elements of the car. 
     To effectively reduce the reverse withstand voltage (also referred to as the maximum working peak reverse voltage (VRWM) or reverse breakdown voltage) and lower the power loss, the present invention further includes the current-limiting resistor  20  and the TVS  30 . 
     The current-limiting resistor  20  has one end coupled to the anode  12  of the diode  10 . The first TVS  30  is connected to the current-limiting resistor  20  in series, and then the serially connected first TVS  30  and current-limiting resistor  20  are connected in parallel between the car power source  1  and the car charger  2 . Here, the TVS is briefly introduced at first. The TVS is an effective protection element, which has characteristics similar to those of a Zener diode. Compared with the conventional Zener diode, the TVS has a P/N junction of a larger area, which, as a structural improvement, enables the TVS to have a stronger capability of withstanding high voltage, and reduces the voltage cut-off rate. Therefore, the TVS has a better effect in protecting the safety of the circuit through surge suppression. One of the most important features of the TVS is fast response, that is, a transient pulse can be effective suppressed before it causes damage to the circuit or elements. Moreover, the TVS has a lower leakage current, so the performance in processing the surge impact to high-rate transmission loops of the TVS is better. 
     In the present invention, the reverse breakdown voltage of the first TVS  30  is smaller than that of the diode  10 . Therefore, the diode  10  with a lower reverse withstand voltage can be used, which can still be protected and will not be broken down by the reverse surge. At this time, when the reverse surge appears on the car power source  1 , as the reverse breakdown voltage of the first TVS  30  is smaller than that of the diode  10 , the first TVS becomes conductive at first, and the reverse surge flows through the current-limiting resistor  20 , where the current of the reverse surge is lowered by the current-limiting resistor  20 . Then, the reverse surge returns to the car power source  1 . After the reverse surge flows through the first TVS  30  and the current-limiting resistor  20 , the current back into the car power source  1  is a safe current that will not damage the internal elements in the car. 
     Here, the reverse conduction voltage of the diode  10  only needs to be slightly greater than that of the first TVS  30 . The forward voltage of the diode  10  is proportional to the reverse breakdown voltage of the diode  10 , and the power loss of the diode  10  is also proportional to the forward voltage of the diode  10 . That is, the present invention uses the first TVS  30  and the current-limiting resistor  20 , so the diode  10  with a lower reverse breakdown voltage can be used. Further, as the reverse breakdown voltage of the diode  10  is lower, the forward voltage is also lower, such that the power loss is reduced as well. Thus, the temperature of the surge protection device is reduced naturally, and no additional heat sink is required to solve the problem of temperature rise, so the space and cost for the heat sink is saved, and the efficiency of the car charger is improved. 
     For example, as described in the Related Art, according to the JASO D001 B2 automotive standards, when the reverse surge is 260 V, if the surge protection device of the present invention is not used, the diode  10  must have a reverse voltage of 300 V and a forward voltage of 1.5 V. Assume that the current flowing through the diode  10  is 1 A, the power loss on the diode  10  is 1.5 w, and at this time, a heat sink must be used to solve the problem of temperature rise. However, in the surge protection device of the present invention, assume that the reverse breakdown voltage of the first TVS  30  (which can be, but is not limited to, model P6KE56CA) is 56 V. That is, when the voltage of the reverse surge exceeds 56 V, the first TVS  30  becomes conductive, such that the surge flows back to the power source through the current-limiting resistor  20 . Therefore, the reverse surge flows through the first TVS  30  and the current-limiting resistor  20 , and the current of the reverse surge is reduced, so as to realize the protection effect. In this embodiment, the diode  10  has the reverse breakdown voltage of 60 V and the forward voltage of 0.55 V. Assume that the current flowing through the diode  10  is also 1 A, the power loss on the diode  10  is 0.55 w, which is only one third of the power loss in the prior art. Therefore, the heat sink is not needed, and the cost is saved. 
       FIG. 3  is a schematic view of a surge protection device of a car charger according to a second embodiment of the present invention. The second embodiment further includes a second TVS  40 . 
     The second TVS  40  has one end coupled to the cathode  14  of the diode  10 , and the second TVS  40  is connected in parallel between the car power source  1  and the car charger  2 . Here, the second TVS  40  can be used to suppress the forward surge generated by the car power source  1 . The function of the second TVS  40  is similar to that of a Zener diode, so as to clamp the forward surge at a fixed value and output it to the car charger  2 . 
     For example, assume that the second TVS  40  is set to 40 V. As specified in the JASO D001A1 automotive standards, when the high-voltage forward surge generated by the car power source  1  is 70 V, without the help of the second TVS  40 , the forward surge of 70 V will flow to the car charger  2  directly, and then cause damage to the car charger  2  and the externally connected electronic products. The second TVS  40  can clamp the forward surge voltage of 70 V at 40 V, and then output it to the car charger  2 . Thus, the forward surge is suppressed, so as to protect the car charger  2  and the externally connected electronic products. 
     It is known from  FIG. 3  that the second TVS  40  can be unidirectional, and the first TVS  30  can be bidirectional. The unidirectional TVS  40  can suppress the forward surge, so as to prevent the forward surge from damaging the car charger  2  and the charged external electronic products. In another aspect, the bidirectional first TVS  30  not only can suppress the reverse surge as described earlier, but also can suppress the forward surge. That is, the first TVS  30  is designed to be bidirectional to suppress the reverse surge, and to help the second TVS  40  suppress the forward surge. 
       FIG. 4  is a schematic view of a car charger according to an embodiment of the present invention. In specific embodiments, the surge protection device can also be disposed in the physical space of the car charger to form a car charger having the surge protection function. 
     The car charger  3  has one end receiving an input voltage Vin provided by the car power source  1 , and the other end being a charging connector  50  connecting an external device to charge the device. The external device can be a mobile phone, PDA, GPS device, and the like. An output end  51  of the charging connector  50  outputs an output voltage Vout. 
     This embodiment is similar to the previous embodiments technically. Therefore, the same technology is not repeated here, and only the connection of the elements of this embodiment is described briefly. The car charger  3  includes a charging connector  50  for connecting and charging the external device. The diode  10  includes an anode  12  receiving the input voltage and a cathode  14  coupled to the charging connector  50 . The current-limiting resistor  20  has one end coupled to the anode  12  of the diode  10 . A first TVS  30  is connected to the current-limiting resistor  20  in series, and is connected in parallel with the current-limiting resistor  20  between the car power source  1  and the charging connector  50 . Here, the reverse breakdown voltage of the first TVS  30  is smaller than that of the diode  10 . The second TVS  40  has one end coupled to the cathode  14  of the diode  10 , and the second TVS  40  is connected in parallel between the car power source  1  and the charging connector  50 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.