Patent Publication Number: US-2005141158-A1

Title: Overvoltage projection circuit

Description:
FIELD OF THE INVENTION  
      The present invention is related to an overvoltage protection circuit, and more particularly to an overvoltage protection circuit for use in a power supply.  
     BACKGROUND OF THE INVENTION  
      With the incessant progress of technology, a computer has become a must-have appliance used prevalently in daily life. However, a computer needs electric power to start its operation as an ordinary electric appliance. To provide sufficient electric power for a computer, a power supply of which the main function is to convert an alternating current (AC) power supplied from an inlet into a direct current power (DC) for use by a computer, is contrived. A well-regulated power supply is required to comply with some industrial standards, for example, reliabilities, functionality specifications, safeguarding specifications, safety regulations, EMI compatibility, and other miscellaneous requirements.  
      When the feedback control circuit or other internal components of a power supply are impaired during operation and thus create an output voltage being higher than its maximum output voltage rating, the output voltage of power supply has to be restrained by a protection circuit in order to keep the circuit components of the load from being damaged. Such fail-safe utility that can restrain the output voltage of a power supply from increasing unlimitedly is referred to as overvoltage protection circuit, or OVP circuit.  
      The OVP function plays an extremely important role for a sensitive load, such as a central processing unit (CPU), memory, or a logic circuit. If a sensitive load is powered by a voltage being higher than its maximum tolerance, that would result in permanent damage and cause significant monetary losses.  
      Referring to  FIG. 1 , a schematic graph showing the output voltage waveform of a power supply provided with an OVP function is depicted. As can be understood from  FIG. 1 , the output voltage Vout will continue to rise up at time t 1  and will be restrained from outputting a voltage being higher than a maximum output voltage rating V 2  at time t 2 , and thereby protect the internal circuit components of the load to which the power supply connects.  
      In case that a power supply of a computer is out of order and cannot provide electric power for output, the computer is not possible to bring itself into action. In order to prevent the computer from becoming unworkable due to the failure of the internal power supply, a redundant power supply is invented to address this deficiency. A computer user can directly extract a redundant power supply from a computer host when the electric power is supplying to the computer host. In other words, the power supply of a computer host can be assured by a redundant power supply, even if the power supply of the computer cannot maintain a normal power supplying state. Besides, a portion of the backup power supplies or malfunctioned power supplies can be removed or replaced with new power supplies during the operation period of the computer. Generally, a computer system will be equipped with at least two redundant power supplies. Under normal condition, the required power supply amount of the computer system will be shared equally by the redundant power supplies. In the event that one of the redundant power supplies is impaired, the power supply can be changed to be provided by the other redundant power supply.  
      Referring to  FIG. 2   a,  a circuit block diagram of a redundant power supply system according to the prior art is shown. As indicated in  FIG. 2   a,  the redundant power supply  20  is configured to receive an input power from an input power source  22  and convert the input power into an output voltage tailored to power a load  24 . The redundant power supply  20  includes n power supplies  21  and is capable of transferring its output voltage having a DC characteristic to the load  24 , wherein each of the power supplies  21  includes a PWM (pulse-width modulation) controller  211 , a filter  212 , a diode  213 , and an OVP comparator  214 .  
      The PWM controller  211  is used to perform a PWM operation to the input power received from the input power source  22 . The filter  212  is a low-pass filter (LPF) that performs a filtering operation to the modulated pulse signals outputted from the PWM controller  211  and provides a filtered DC voltage for output. The filtered output DC voltage is transferred to the load  24  through the diode  213  and an output voltage bus  23 .  
      The OVP comparator  214  is used to detect the output voltage provided by the filter  212  and determines whether the output voltage exceeds a maximum output voltage rating. If the output voltage exceeds the maximum output voltage rating, the OVP function will be activated to stop the PWM controller  211  from providing an output voltage, such that the internal circuit components of the load  24  can be protected.  
      Referring to  FIG. 2   b,  a signal waveform diagram showing the output voltage being detected by an OVP comparator of a conventional power supply is illustrated. As shown, the bottom horizontal line indicated by a symbol V 1  represents the desired normal output voltage a load requires to sustain its operation. The horizontal line indicated by a symbol V 2  represents the noise margin of about ±0.6V that is generated due to the interference in the internal circuit components of the power supply. The horizontal line indicated by a symbol V 3  represents the overshoot margin that is generally rated at three percents of the output voltage V 1 . The horizontal line indicated by a symbol ΔVf represents the voltage drop caused by the forward-biased characteristics of the diode (about 0.2V). As shown in  FIG. 2   c,  the total margin equals to V 2 +V 3 +ΔVf, and the output voltage Vout detected by the OVP comparator equals to V 1 +V 2 +V 3 +ΔVf.  
      In the case that the load  24  requires an output voltage of 12 volts to sustain its operation, the maximum output voltage rating of a power supply generally ranges from 13.5V to 15V. Based on this rationale, as shown in  FIG. 2   b,  if the desired normal output voltage V 1  is 12.2 volts, the noise margin V 2  is 60 mV, the overshoot margin V 3  is 0.1 Volt, the voltage drop ΔVf of the forward-biased diode is 0.2V, and then the output voltage Vout of the power supply should be V 1 +V 2 +V 3 +ΔVf=12.2+0.06+0.1+0.2=12.56, which is limited within the maximum output voltage rating of 13.5V to 15V. Under this condition, there will not cause overvoltage problems.  
      However, if the load  24  is quite voltage-sensitive and belongs to a specifically-designed system, the gap between the desired normal output voltage and the maximum output voltage rating would be very small. For example, Unisys Corporation requires the desired normal output voltage V 1  requested by the load to be 12.2V, and requires the maximum output voltage rating of the power supply to be 12.4V, in which the gap between the desired normal output voltage and the maximum output voltage rating of the power supply is 0.2 only. In this way, the output voltage detected by an OVP comparator  214  of a conventional power supply  21  would be rated at as high as 12.56V, which is much higher than the maximum output voltage rating of 12.4V, and thereby the OVP function will be activated under this condition. Thus, the power supply cannot meet the requirements of allowing its output voltage to be 12.56V without activating the OVP function, which is set to react to the overvoltage problem at an output voltage of 12.4V. Moreover, the tolerance V of a conventional power supply that is equal to the gap between the output voltage detected by the OVP comparator  214  and the maximum output voltage rating is 0.36V, while a large percentage of the tolerance V is attributed to the voltage drop ΔVf across the forward-biased diode  213 .  
      Therefore, the present invention is dedicated to meet the requirements that an OVP circuit can satisfy the requirements of outputting a voltage being higher than the maximum output voltage rating of power supply when that the gap between the desired normal output voltage and the maximum output voltage rating is relatively small, without activating the OVP function.  
     SUMMARY OF THE INVENTION  
      A first object of the present invention is to develop an overvoltage protection circuit that enable a power supply to provide an output voltage being higher than its maximum output voltage rating by a relatively small gap without activating overvoltage protection function.  
      To attain the aforementioned object of the present invention, a first aspect of the present invention is focused on the provision of an overvoltage protection circuit for use in a power supply, wherein the power supply includes a voltage supply circuit and a diode having a first electrode and a second electrode. The overvoltage protection circuit is used to stop or reduce the output voltage of the voltage supply circuit when the output voltage of the voltage supply circuit is higher than a maximum output voltage rating of the voltage supply circuit. The overvoltage protection circuit includes: a first comparator electrically connected to the voltage supply circuit and the first electrode of the diode for detecting a first voltage at the first electrode of the diode and outputting a first detecting signal in response to a comparison between the first voltage and the maximum output voltage rating; a second comparator electrically connected to the second electrode of the diode for detecting a second voltage at the second electrode of the diode and outputting a second detecting signal in response to a comparison between the second voltage and the maximum output voltage rating; and a logic circuit electrically connected to the first comparator, the second comparator and the voltage supply circuit, and being used to receive the first detecting signal and the second detecting signal and output a control signal to control the voltage supply circuit to stop or reduce the output voltage when the first detecting signal and the second detecting signal indicate that both of the first voltage and the second voltage are higher than the maximum-output voltage rating.  
      In accordance with the present invention, the first electrode is an anode terminal of the diode and the second electrode is a cathode terminal of the diode.  
      In accordance with the present invention, the logic circuit is an AND gate which is used to control the voltage supply circuit to reduce its output voltage when both of the first voltage and the second voltage are higher than the maximum output voltage rating.  
      In accordance with the present invention, the first voltage is variable in accordance with a voltage drop as a result of the forward-biased characteristic of the diode, that is, the second voltage is obtained from subtracting the voltage drop across the forward-biased diode from the first voltage.  
      In accordance with the present invention, the overvoltage protection circuit further includes a latch electrically connected to the logic circuit and the voltage supply circuit, and being used to receive the control signal and turn off the voltage supply circuit by stopping or reducing the output voltage of the voltage supply circuit in response to the control signal.  
      In accordance with the present invention, the power supply is a redundant power supply.  
      In accordance with the present invention, the voltage supply circuit includes: a modulator for receiving a voltage and performs a modulation operation to a received voltage, and a filter electrically connected to the modulator for performing a filtering operation to a modulated voltage received from the modulator and outputting the first voltage.  
      In accordance with the present invention, the modulator is a pulse-width modulator (PWM) controller and the filter is a low-pass filter (LPF).  
      Another aspect of the present invention is associated with a power supply, comprising: a voltage supply circuit; a diode electrically connected to the voltage supply circuit and having a first electrode and a second electrode; and an overvoltage protection circuit electrically connected to the diode and the voltage supply circuit for stopping or reducing an output voltage of the voltage supply circuit when the output voltage of the voltage supply circuit is higher than a maximum output voltage rating of the voltage supply circuit. The overvoltage protection circuit includes: a first comparator electrically connected to the voltage supply circuit and the first electrode of the diode for detecting a first voltage at the first electrode of the diode and outputting a first detecting signal in response to a comparison between the first voltage and the maximum output voltage rating; a second comparator electrically connected to the second electrode of the diode for detecting a second voltage at the second electrode of the diode and outputting a second detecting signal in response to a comparison between the second voltage and the maximum output voltage rating; and a logic circuit electrically connected to the first comparator, the second comparator and the voltage supply circuit, and being used to receive the first detecting signal and the second detecting signal and output a control signal to control the voltage supply circuit to stop or reduce the output voltage when the first detecting signal and the second detecting signal indicate that both of the first voltage and the second voltage are higher than the maximum output voltage rating.  
      In accordance with the present invention, the power supply is a redundant power supply.  
      In accordance with the present invention, the voltage supply circuit includes: a modulator for receiving a voltage and performs a modulation operation to a received voltage, and a filter electrically connected to the modulator for performing a filtering operation to a modulated voltage received from the modulator and outputting the first voltage.  
      In accordance with the present invention, the modulator is a pulse-width modulator (PWM) controller and the filter is a low-pass filter (LPF).  
      In accordance with the present invention, the first electrode of the diode is an anode terminal of the diode, and the second electrode of the diode is a cathode terminal of the diode.  
      In accordance with the present invention, the logic circuit is carried out by an AND gate which is used to control the voltage supply circuit to stop or reduce the output voltage when both of the first voltage and the second voltage are higher than the maximum output voltage rating.  
      In accordance with the present invention, the first voltage is variable depending on the forward-biased characteristic of the diode, i.e. the second voltage is obtained by subtracting the voltage drop across the diode from the first voltage.  
      In accordance with the present invention, the overvoltage protection circuit includes a latch electrically connected to the logic circuit and the voltage supply circuit, wherein the latch is used to receive the control signal and turn off the voltage supply circuit by stopping or reducing the output voltage of the voltage supply circuit in response to the control signal.  
      Now the foregoing and other features and advantages of the present invention will be best understood through the following descriptions with reference to the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a timing diagram showing the output voltage waveform of a power supply under overvoltage protection;  
       FIG. 2   a  is a circuit block diagram of a redundant power supply system according to the prior art;  
       FIG. 2   b  shows the output voltage waveforms of a power supply being measured by an OVP comparator according to the prior art;  
       FIG. 2   c  is an I-V characteristic scheme of a forward-biased diode;  
       FIG. 3   a  is a circuit block diagram of a redundant power supply according to a preferred embodiment of the present invention;  
       FIG. 3   b  is a circuit block diagram showing a power supply according to a preferred embodiment of the present invention; and  
       FIG. 3   c  shows the output voltage waveform of the voltage supply circuit being measured at the output terminal of the second OVP comparator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 3   a,  a redundant power supply according to a preferred embodiment of the present invention is illustrated. As indicated in  FIG. 3   a,  a redundant power supply  30  is used to receive a predetermined input power from an input power source  32  and convert the input power into an output power tailored to power a load  34 . The redundant power supply  30  includes n power supplies  31  and is used to provide an output DC power by a series of power conversion processes to power the load through an output voltage bus  33 . The power supply  31  mainly includes a voltage supply circuit  310 , a diode  313 , and an OVP circuit  314 .  
      The voltage supply circuit  310  is configured to supply electric power in accordance with the requirement of the load  34 , wherein the voltage supply circuit  310  mainly includes a modulator and a filter. The modulator of the power supply circuit  310  is preferably made up of a PWM (pulse-width modulation) controller  311 , and is used to perform a PWM modulation operation to the power supplied by the input power source  32 . The filter of the voltage supply circuit  310  is a low-pass filter (LPF)  312 , and is made up of an inductor and a capacitor, as shown in  FIG. 3   a.  The filter is used to perform a filtering operation to the modulated pulse signals received from the PWM controller  311  and thereby generate an output DC voltage. The output DC voltage provided by the low-pass filter  312  is transferred to the load  34  via the diode  313  and the output voltage bus  33 .  
      The diode  313  is electrically connected between the low-pass filter  312  and the output voltage bus  33 , and includes a first electrode  3131  and a second electrode  3132 . When the first electrode  3131  is applied with a positive voltage, a forward-biased current is induced and thus prevents the voltage supplied by other power supplies to be inputted via the second electrode  3132 , and further protects other internal components of the power supply associated therewith. It should be noted that the first electrode  3131  is an anode terminal of the diode  313 , and the second electrode  3132  is a cathode terminal of the diode  313 .  
      The main function of the OVP circuit  314  is to instruct the voltage supply circuit  310  to stop or reduce the output voltage of the voltage supply circuit  310  when the output voltage exceeds a maximum output voltage rating. The core components of the OVP circuit  214  include a first comparator, a second comparator, and a logic circuit.  
      Turing to  FIG. 3   a  and  FIG. 3   b,  the first comparator is designated as a first OVP comparator  315 , which is connected between the low-pass filter  312  and the first electrode  3131  of the diode  313 . The first OVP comparator  315  is used to detect a first voltage being a fractional of the output voltage of the low-pass filter  312 , i.e. the first voltage is the voltage measured at the first electrode  3131  of the diode  313 . Also, the first OVP comparator  315  is configured to compare the first voltage with the maximum output voltage rating of the voltage supply circuit  310  and output a first detecting signal in response to the comparison between the first voltage and the maximum output voltage rating. Likewise, the second comparator is designated as a second OVP comparator  316 , which is connected between the second electrode  3132  of the diode  313  and the output voltage bus  33 , and is used to detect a second voltage being the voltage measured at the second electrode  3132  of the diode. Also, the second OVP comparator  316  is configured to compare the second voltage with the maximum output voltage rating and output a second detecting signal in response to the comparison between the second voltage and the maximum output voltage rating.  
      The logic circuit is electrically connected to the first OVP comparator  315 , the second OVP comparator  316  and the PWM controller  311 , and is preferably implemented by an AND gate  317 . The logic circuit is used to receive the first detecting signal from the first OVP comparator  315  and also the second detecting signal from the second OVP comparator  316 . The logic circuit is configured to output a control signal to regulate the PWM controller  311  to stop or reduce the output voltage of the voltage supply circuit  310  when the first detecting signal and the second detecting signal indicate that both of the first voltage and the second voltage exceed the maximum output voltage rating of the voltage supply circuit  310 , and thereby protect the internal circuit components of the load  34 .  
      Referring to  FIG. 3   b  again, a latch  318  is further provided and connected between the AND gate  317  and the PWM controller  311  for receiving the control signal from the AND gate  317  and regulating the PWM controller  311  in response to the control signal, and thereby stop or reduce the output voltage of the voltage circuit  310 .  
      When the first voltage is passed from the first electrode  3131  of the diode  313  to the second electrode  3132  of the diode  313 , it will transit to a second voltage by the voltage drop ΔVf of the diode  313  as a result of the forward-biased characteristics of the diode  313 , that is, the second voltage is obtained by subtracting the voltage drop ΔVf of the diode  313  from the first voltage. Because the OVP circuit  314  according to a preferred embodiment of the present invention is configured to measure the voltage at the first electrode  3131  of the diode  313  and the voltage at the second electrode  3132  of the diode  313 , respectively, and uses the AND gate  317  to determine the occurrence of overvoltage problem, the influence caused by the voltage drop ΔVf of the diode can be obviated.  
      For example, if the desired output voltage V 1  is 12.2V, and the maximum voltage rating is 12.4V, the first voltage measured by the first OVP comparator  315  is Vout 1 =V 1 +V 2 +V 3 +ΔVf=12.2+0.06+0.1+0.2=12.56, as shown in  FIG. 2   b.  Because the second OVP comparator  316  ignores the voltage drop ΔVf as a result of the forward-biased characteristics of the diode  313 , the second voltage is Vout 2 =V 1 +V 2 +V 3 +=12.2+0.06+0.1+=12.36, as shown in  FIG. 3   c.  The resulting first voltage is 12.56V and thus exceeds the maximum voltage rating 12.4V, while the resulting second voltage is 12.36V and thus does not exceed the maximum voltage rating. Because the first voltage and the second voltage do not both exceed the maximum voltage rating, the AND gate  317  will not activate the overvoltage protection function.  
      In conclusion, the OVP circuit according to the present invention takes advantage of two OVP comparators to detect the voltage at the anode terminal and the cathode terminal of the diode, respectively, and thereby ignore the effect caused by the forward-biased voltage drop of the diode. In this manner, the requirement of enabling the redundant power supply to provide an output voltage being higher than its maximum output voltage rating without activating the OVP function can be satisfied, even if the gap between the actual output voltage and the maximum output voltage rating is relatively small.  
      While the present invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention need not be restricted to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.