Abstract:
A voltage converter includes: an input terminal receiving an input voltage; an output terminal providing an output voltage; a switching circuit having a main switch configured to regulate the output voltage, wherein a control end of the main switch is configured to receive a Pulse Width Modulation (PWM) signal, and the output voltage is controlled according to the duty cycle of the PWM signal; and a protection switch coupled between the input terminal and the switching circuit, and wherein when the output voltage is higher than a reference voltage, the protection switch is turned OFF.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of CN application No. 201210333505,0, filed on Sep. 11, 2012, and incorporated herein by reference. 
       TECHNICAL HELD 
       [0002]    The present invention generally relates to electrical circuit, more particularly but not exclusively relates to voltage converter and associated over-voltage protection method. 
       BACKGROUND 
       [0003]      FIG. 1  shows a prior art Direct Current to Direct Current (DC-DC) step-down converter or called Buck converter  100 . Buck converter  100  comprises a high-side switch M 1  and a low-side switch M 2 . The high-side switch M 1  functions as a main switch for regulating an output voltage Vout at an output terminal of the voltage converter  100 . The high-side switch M 1  is coupled to an input terminal of the voltage converter  100  to receive an input voltage Vin. An output capacitor Cout is coupled between the output terminal and a reference ground GND. Generally, the output capacitor Cout works under a limited voltage level. 
         [0004]    In some abnormal situations, for example, when the high-side switch M 1  is electrically shorted, the output voltage Vout will increase in short time, and the output capacitor Cout will be broken and the load will no longer work normally or will be broken too. 
         [0005]    In order to prevent the output voltage from increasing too high, a conventional over-voltage protection method couples a feedback signal FB indicating the output voltage to a control circuit  11 . When the feedback signal FB increases to a threshold voltage, the control circuit  11  turns ON the low-side switch M 2  for decreasing the output voltage Vout. However, in some situations, the over voltage situation happens because of load abnormity, and the control circuit  11  may not work normally. Accordingly, the low-side switch M 2  may not be turned ON and the over-voltage protection may fail. 
       SUMMARY 
       [0006]    In order to address one or some of the above deficiencies, the present invention discloses one type of voltage converter and associated over-voltage protection method. 
         [0007]    One embodiment of the present invention discloses a voltage converter, and the voltage converter has an input terminal configured to receive an input voltage and an output terminal configured to provide an output voltage for supplying a load. The voltage converter further comprises a switching circuit, having an input and an output, wherein the output of the switching circuit is coupled to the output terminal of the voltage converter, the switching circuit comprising a main switch configured to regulate the output voltage, wherein a control end of the main switch is configured to receive a Pulse Width Modulation (PWM) signal, and the output voltage is controlled according to the duty cycle of the PWM signal; and a protection switch having a first end, a second end and a control end, wherein the first end is coupled to the input terminal, the second end is coupled to the input of the switching circuit, and wherein when the output voltage is higher than a reference voltage, the protection switch is turned OFF via the voltage at the control end of the protection switch. 
         [0008]    Another embodiment of the present invention discloses an over-voltage protection circuit used in a voltage converter. The over-voltage protection circuit comprises: a switch having a first end, a second end and a control end, wherein the first end is coupled to an input terminal of the voltage converter, the second end is coupled to a switching circuit of the voltage converter; and a comparing circuit having a first input, a second input and an output, wherein the first input of the comparing circuit is coupled to an output terminal of the voltage converter, the second input of the comparing circuit is coupled to a threshold signal and the output of the comparing circuit is coupled to the control end of the switch. 
         [0009]    Yet another embodiment of the present invention discloses an over-voltage protection method in a voltage converter. The method comprises: coupling a switch between an input terminal of the voltage converter and a switching circuit of the voltage converter; sensing the output voltage of the voltage converter; comparing the output voltage to a reference voltage; and turning OFF the switch when the output voltage is higher than the reference voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Non-limiting and non-exhaustive embodiments are described with reference to the following drawings. The drawings are only for illustration purpose. Usually, the drawings only show part of the system or circuit of the embodiments. 
           [0011]      FIG. 1  shows a prior art DC-DC step-down converter. 
           [0012]      FIG. 2  illustrates a voltage converter comprising a protection switch at an input stage according to an embodiment of the present invention. 
           [0013]      FIG. 3  shows a voltage converter comprising a protection capacitor according to an embodiment of the present invention. 
           [0014]      FIG. 4  shows a voltage converter comprising a protection control circuit according to an embodiment of the present invention. 
           [0015]      FIG. 5  shows a voltage converter comprising a boost converter according to an embodiment of the present invention. 
           [0016]      FIG. 6  shows a voltage converter wherein the protection switch comprises a MOSFET according to an embodiment of the present invention. 
           [0017]      FIG. 7  illustrates a method of over-voltage protection in a voltage converter according to an embodiment of the present invention. 
       
    
    
       [0018]    The use of the same reference label in different drawings indicates the same or like components. 
       DETAILED DESCRIPTION 
       [0019]    Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
         [0020]    The term “couple” refers to direct connection or indirect connection through intermediate such as electrical conductor, diodes, resistors, capacitors, and/or other intermediaries. 
         [0021]    A “voltage converter” is a circuit/apparatus or system that converts an input voltage of a first voltage into an output voltage of a second voltage. In one embodiment, the first voltage or the second voltage each is at a predetermined value or in a predetermined range. However, the first voltage or the second voltage each may have any substantially constant value or variable value. 
         [0022]      FIG. 2  illustrates a voltage converter  200  comprising a protection switch  22  at input stage according to an embodiment of the present invention. Voltage converter  200  comprises an input terminal IN, an output terminal OUT, a switching circuit  21 , a protection switch  22  and a feedback circuit  23 . The input terminal IN receives an input voltage Vin. The output terminal OUT provides an output voltage Vout to a load  20 . Feedback circuit  23  having an input  231  and an output  232  wherein input  231  is coupled to the output terminal OUT to receive the output voltage Vout, and output  232  provides a feedback signal FB which indicates the output voltage Vout. Feedback signal FB may be a voltage signal, a current signal or any other type of signal. Switching circuit  21  comprises a main switch M 1  for regulating the output voltage Vout. The control end of the main switch M 1  receives a PWM signal, and the output voltage Vout is regulated by the duty cycle of the PWM signal. In one embodiment, the switching circuit  21  of voltage converter  200  comprises a Buck converter and converts an input voltage into an output voltage which is smaller than the input voltage, wherein the main switch is a high-side switch and the output voltage is proportional to the duty cycle of the PWM signal that is coupled to the control end of the high-side switch. In another embodiment, the voltage converter comprises a DC-DC step-up converter or called boost converter. In yet another embodiment, the voltage converter comprises an isolated DC-DC voltage converter. 
         [0023]    Continuing with  FIG. 2 , protection switch  22  has a first end  1 , a second end  2  and a control end  3 . The first end  1  is coupled to the input terminal IN, and the second end  2  is coupled to switching circuit  21 . When the voltage at the output terminal OUT is too high and feedback signal FB is higher than a threshold voltage Vth, protection switch  22  is turned OFF to disconnect switching circuit  21  from the input terminal IN. Thus, when main switch M 1  is electrically shorted, it is prevented that the output terminal OUT is coupled to the input terminal IN directly for a relatively long time period. In one embodiment, feedback signal FB is proportional to the output voltage Vout, and threshold signal Vth is also a voltage signal. In another embodiment, feedback signal and threshold signal are both current signal. In the shown embodiment, voltage converter  200  further comprises an output capacitor Gout for smoothing the output voltage Vout at the output terminal OUT. In one embodiment, the output capacitor can also be deemed as part of the switching circuit  21 . 
         [0024]      FIG. 3  shows a voltage converter  300  comprising a protection capacitor C 1  according to an embodiment of the present invention. Compared to voltage converter  200 , voltage converter  300  further comprises an input capacitor Cin and a protection capacitor C 1 . The input capacitor Cin is coupled between the input terminal IN and reference ground GND. And the protection capacitor C 1  is coupled between the second end  2  of protection switch  22  and reference ground GND. Protection capacitor C 1  prevents an abrupt voltage change at the second end  2  of protection switch  22  and smoothes the voltage supplied into an input  211  of switching circuit  21 . In another embodiment, protection capacitor C 1  is coupled between the input  211  and the output  212  of the switching circuit  21 . 
         [0025]      FIG. 4  shows a voltage converter  400  comprising a protection control circuit  43  according to an embodiment of the present invention. Switching circuit of voltage converter  400  comprises a Buck converter  21 . Buck converter  21  comprises a high-side switch M 1  functioning as a main switch, a low-side switch M 2  functioning as a rectifier and an inductor L. A first end of the high-side switch M 1  is coupled to the second end  2  of the protection switch  22 . A second end of the high-side switch M 1  is coupled to a first end of the rectifier M 2  and a first end of the inductor L. The second end of the rectifier M 2  is coupled to a reference ground GND. A second end of the inductor L is coupled to the output terminal OUT. And the control end of the high-side switch M 1  receives a PWM signal. The high-side switch M 1  and the low-side switch M 2  are turned ON and OFF in complementary pattern so as to provide a square waveform at a switching node SW between the high-side switch M 1  and the low-side switch M 2 . Inductor L and the output capacitor Cout filter the square waveform at node SW and provide the DC output voltage Vout at the output terminal OUT, wherein the output voltage Vout is lower than the input voltage Vin. The output voltage Vout equals the product of the input voltage Vin multiplying the duty cycle of the PWM signal which is supplied to the control end of the high-side switch M 1 . In the shown embodiment, the low-side switch M 2  comprises a non-synchronous rectifier of diode. In another embodiment, the low-side switch comprises a synchronous rectifier, for example, a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). In one embodiment, the Buck converter  21  comprises the output capacitor Cout. In one embodiment, an over-voltage protection circuit comprises protection switch  22  and protection control circuit  43 . 
         [0026]    In the embodiment shown in  FIG. 4 , the protection control circuit  43  comprises a comparing circuit COM. The comparing circuit COM has a first input, e.g. inverting input coupled to the output  42  of feedback circuit  23 , a second input, e.g. non-inverting input coupled to a threshold signal Vth, and an output coupled to control end  3  of protection switch  22 . When the feedback signal FB surpasses the threshold signal Vth, the comparing circuit COM outputs logic low signal and turns OFF the protection switch  22  via control end  3 . Protection control circuit  43  may further comprise other components or circuits, for example, a threshold signal generating circuit for generating the threshold signal Vth, and/or driving circuit coupled between the output of comparing circuit COM and control end  3  of protection switch  22 . It is noted that protection control circuit  43  may be used in any type of voltage converter, for example, a boost converter. In one embodiment, comparing circuit COM comprises a comparator. And the comparing circuit may comprise any other type of circuit or apparatus that can turn OFF a switch (such as switch  22 ) when a signal (such as signal FB) is higher than a threshold signal (such as signal Vth). 
         [0027]    In one embodiment, feedback circuit  23  comprises a first resistor R 1  and a second resistor R 2 . The first resistor R 1  has a first end coupled to the output terminal OUT of voltage converter  400  and a second end coupled to output  42  of feedback circuit  23 . The second resistor R 2  has a first end coupled to output  42  of feedback circuit  23 , and a second end coupled to reference ground CND. The feedback circuit may comprise any other suitable voltage sensing circuit or voltage sensing apparatus, for example, an RC voltage sensing circuit. In one embodiment, the feedback circuit comprises a conductor coupled between the protection control circuit and the output terminal. 
         [0028]      FIG. 5  shows a voltage converter  500  comprising a boost converter according to an embodiment of the present invention. Wherein, the switching circuit of voltage converter  500  comprises a boost converter  51 . In this embodiment, the protection switch  22  is coupled between the input terminal IN and the switching circuit  51 , and to be more particular, coupled between the input terminal IN and the inductor L of the boost converter  51 . 
         [0029]      FIG. 6  shows a voltage converter  600  according to an embodiment of the present invention, Switching circuit  21  of voltage converter  600  comprises a Buck converter an, a protection switch  62  of voltage converter  600  comprises a MOSFET. In one embodiment, the source of the MOSFET is coupled to the input terminal IN, the drain of the MOSFET is coupled to the high-side switch M 1  of switching circuit  21 , and the gate of the MOSFET is coupled to the output of comparing circuit COM of protection control circuit. In another embodiment, the protection switch at input stage may be in any other type, for example, a Junction Field Effect Transistor (JFET). 
         [0030]    Compared to voltage converter  500  in  FIG. 5 , voltage converter  600  further comprises a control circuit  61  for generating the PWM signal, Control circuit  61  has an input  611  and an output  612 . The input  611  receives feedback signal FB, and the output  612  provides the PWM signal. The PWM signal is generated at least based on feedback signal FB, and the duty cycle of the PWM signal is modulated at least according to the feedback signal FB. Control circuit  61  may further receive other signals such as output current signal for regulating the PWM signal. Control circuit  61  may further generates another signal to control a synchronous rectifier of the switching circuit. The control circuit may be in any conventional type used in a DC-DC voltage converter such as a constant on time control circuit. 
         [0031]      FIG. 7  illustrates a method of over-voltage protection in a voltage converter according to an embodiment of the present invention. The method  700  comprises in a first step  701  coupling a protection switch between an input terminal of the voltage converter and a switching circuit of the voltage converter. At the initial time, the protection switch is in ON state. In one embodiment, the switching circuit comprises a Buck converter. The protection switch is coupled between the input terminal of the voltage converter and the high-side switch of the Buck converter. In another embodiment, the switching circuit comprises a Boost converter, and the protection switch is coupled between the input terminal of the voltage converter and the inductor of the boost converter. The protection method  700  comprises in a second step  702  sensing the output voltage of the voltage converter. In one embodiment, sensing the output voltage of the voltage converter is performed by a voltage divider, for example the voltage divider of feedback circuit  23  shown in  FIG. 4 , and obtaining the feedback signal of the output voltage via the feedback circuit. The method  700  comprises in a third step  703  comparing the output voltage with a threshold voltage, for example, comparing the feedback signal FB with the threshold signal Vth as shown in  FIG. 3 . The over-voltage protection method  700  comprises in a fourth step  704  judging whether the output voltage is higher than the threshold voltage. If the output voltage is higher than the threshold voltage, turn off the protection switch in step  705 . And if the output voltage is not higher than the threshold voltage, turns to step  702  and continues to sense the output voltage. 
         [0032]    In one embodiment, the method  700  further comprises coupling a protection capacitor between the near-output end of the protection switch and the reference ground, where the near-output end of the protection switch is the end among two ends of the protection switch which is nearer to the output terminal, for example the end  2  of the protection switch  22  in  FIG. 2 . When the voltage converter comprises a Buck converter, the near-output end of the protection switch is the end which couples to the high-side switch. And when the voltage converter comprises a Boost converter, the near-output end of the protection switch is the end which couples to the inductor. The protection capacitor is configured to prevent the abrupt voltage change at the near-output end of the protection switch so as to make the system stable. 
         [0033]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.