Abstract:
The present invention discloses a switching regulator, including: a power stage including at least one power transistor which switches according to a switch control signal to convert an input voltage to an output voltage; a pulse width modulation (PWM) signal generator generating a PWM signal according to the output voltage; an over current detection circuit comparing a current sensing signal with a reference signal to generate an over current signal indicating whether an over current is occurring; and a signal adjustment circuit adjusting the PWM signal or a clock signal to generate the switch control signal for controlling an ON time of the power transistor of the power stage.

Description:
CROSS REFERENCE 
     The present invention claims priority to TW 100214928, filed on Aug. 11, 2011. 
     BACKGROUND OF THE INVENTION 
     Field of Invention 
     The present invention relates to a switching regulator and a control circuit of a switching regulator, in particular to such switching regulator and control circuit that can avoid the occurrence of a large inrush current during the beginning of power start. 
     A flyback regulator is an isolation type regulator using a transformer to covert an input voltage to an output voltage. The energy is stored and transformed through charging and discharging of the windings of the transformer.  FIG. 1  shows a schematic diagram of a prior art AC-DC flyback regulator. The switching regulator  10  basically comprises a voltage conversion circuit  14  and a control circuit  15 . The voltage conversion circuit  14  converts the input voltage Vin at the primary side of a transformer  11  to the output voltage Vout at the secondary side of the transformer  11 . The control circuit  15  detects current and receives a feedback signal COMP to drive the voltage conversion circuit  14  for voltage conversion. The transformer  11  includes a primary winding and a secondary winding; the circuit connected to the primary winding is referred to as a primary side circuit, and the circuit connected to the secondary winding is referred to as a secondary side circuit. The secondary side circuit transmit signals to the primary side circuit through an opto-coupler  12 , for sending the feedback signal COMP to the control circuit  15 . The control circuit  15  generates a switch control signal Vgate to control a power transistor  13 , and the control circuit  15  detects the input current Iin flowing through the power transistor  13  by obtaining a voltage across a resistor Rcs connected in series with the power transistor  13 ; the voltage across the resistor Rcs is the primary side current sensing signal Vcs. 
     In the start-up stage of the regulator  10 , because the output voltage Vout is far lower than the target voltage, the feedback control mechanism will cause the power transistor  13  to operate under a maximum duty. This will incur a large inrush current at the primary side circuit, bringing a high risk of damaging the power transistor  13  and other components. The voltage Vcs at the node A will keep increasing due to the inrush current, which can be explained by the following formulas (1) and (2) and further in view of FIG.  2 .: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Iin 
                   
                   ∝ 
                   
                     Vin 
                     Lp 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Iout 
                   
                   ∝ 
                   
                     Vout 
                     Ls 
                   
                   ∝ 
                   
                     
                       n 
                       2 
                     
                     · 
                     
                       Vout 
                       Lp 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Wherein ΔIin is the variation in the input current; Lp is the inductance of the primary winding; ΔIout is the variation of the output current; Ls is the inductance of the secondary winding; Vout is the output voltage; n is the ratio of the turn number of the primary side to that of the secondary side of the transformer. Comparing formula (1) with formula (2), we can observe that ΔIin is larger than ΔIout during the start-up stage because the output voltage Vout is very low but the input Vin has already been provided at a normal level. Therefore, the voltage Vcs will gradually increase. The corresponding waveform is shown in  FIG. 2 . 
       FIG. 2  is a waveform diagram showing waveforms of the voltage Vcs at the node A and the switch control signal Vgate in  FIG. 1 . When the regulator  10  just starts-up, the switch control signal Vgate of the control circuit  15  periodically and continuously turns ON the power transistor  13 . The output voltage Vout is yet to be established and still at a low level, so the voltage Vcs keeps increasing because the input current Iin is increasing. In the figure, the increment Δ 1  of the voltage Vcs is in direct proportion to the increment ΔIin of the current variation of formula (1), that is, Δ1∝ΔIin. The decrement Δ2 of the voltage Vcs is in direct proportion to the variation ΔIout of the input current in formula (2), that is, Δ2∝ΔIout. Each time the power transistor  13  is turned ON, the voltage Vcs is increased even more, because Δ1&gt;Δ2, as shown in  FIG. 2 . 
     In order to suppress the impact by the inrush current, a typical solution is to provide a soft start circuit in the regulator. The conventional soft start circuit increases the start-up duration, so that power increases softly. However, such soft start circuit cannot effectively suppress the inrush current; it is found that the input current Iin and the voltage Vcs are still too large and there is high risk of damaging the current detection resistor and the power transistor of the regulator due to the over current. 
     In view of above, the present invention overcomes the foregoing drawbacks by providing a switching regulator and a control circuit that avoid the occurrence of a large inrush current during the beginning of the power start. The problems of damaging the current detection resistor, the power transistor and other components due to the over current can be solved, without requiring a circuit occupying a large area. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a switching regulator. 
     Another objective of the present invention is to provide a control circuit of a switching regulator. 
     To achieve the foregoing objectives, in one aspect, the present invention provides a switching regulator which comprises: a power stage including at least one power transistor which switches according to a switch control signal to convert an input voltage to an output voltage; a pulse width modulation (PWM) signal generator generating a PWM signal according to the output voltage; an over current detection circuit comparing a current sensing signal with a reference signal to generate an over current signal indicating whether an over current is occurring; and a signal adjustment circuit adjusting the PWM signal or a clock signal according to the over current signal to generate the switch control signal for controlling an ON time of the power transistor of the power stage. 
     In one embodiment, the over current detection circuit includes a comparator for comparing the current sensing signal with the reference signal. 
     In one embodiment, the signal adjustment circuit includes a logic gate which receives the over current signal and one of the PWM signal and the clock signal to generate the switch control signal. 
     In another embodiment, the signal adjustment circuit includes: a counter coupled to the over current detection circuit and operating according to the clock signal, the counter counting a number of occurrences of the over current signal indicating occurring of the over current and changing its output level when the number of occurrences reaches a predetermined count setting value; a flip flop coupled to the over current detection circuit and the counter, the flip flop generating a first output when the over current signal indicates that the over current is not occurring, the flip flop generating a second output when the over current signal indicates occurring of the over current, and the flip flop returning to the first output when a count of the counter reaches the predetermined count setting value; and a logic gate receiving an output of the flip flop and one of the PWM signal and the clock signal to generate the switch control signal. 
     The foregoing switching regulator is an AC-DC switching regulator or a DC-DC switching regulator. 
     In another embodiment, the present invention provides a control circuit of a switching regulator. The switching regulator includes at least one power transistor which switches to convert an input voltage to an output voltage. The control circuit controls the power transistor according to a current sensing signal. The control circuit comprising: an over current detection circuit comparing a current sensing signal with a reference signal to generate an over current signal indicating whether an over current is occurring; and a signal adjustment circuit adjusting the PWM signal or a clock signal according to the over current signal to generate a switch control signal for controlling an ON time of the power transistor of the power stage. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a prior art AC-DC switching regulator. 
         FIG. 2  shows the waveform diagram of the voltage Vcs at the node A and the switch control signal Vgate in  FIG. 1 . 
         FIG. 3  shows an embodiment of a switching regulator according to the present invention. 
         FIG. 4  shows an embodiment of the over current detection circuit and the signal adjustment circuit of a control circuit according to the present invention, illustrating. 
         FIG. 5  is a waveform diagram showing signal waveforms of a regulator according to the present invention. 
         FIGS. 6A and 6B  show schematic circuit diagrams of buck converters. 
         FIGS. 7A and 7B  show schematic circuit diagrams of boost converters. 
         FIGS. 8A and 8B  show schematic circuit diagrams of inverting converters. 
         FIGS. 9A and 9B  show schematic circuit diagrams of buck-boost converters. 
         FIGS. 10A and 10B  show schematic circuit diagrams of inverter-boost converters. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  shows an embodiment of a switching regulator according to the present invention. The switching regulator can be any type of AC-DC or DC-DC switching regulator, and is not limited to the AC-DC switching regulator shown in  FIG. 1 . In either the AC-DC or the DC-DC switching regulator, it is customary to generate current sensing signals. As shown in this figure, the current sensing signal Vcs generated by the voltage conversion circuit  34  of a switching regulator  30  is inputted to the over current detection circuit  351  of the control circuit  35 . The over current detection circuit  351  compares the current sensing signal Vcs with a reference signal Vh to generate an over current signal OC if Vcs is higher than Vh. The signal adjustment circuit  352  adjusts, according to the over current signal OC, a PWM signal or a clock signal CLK (one or both of these signals can be adjusted) generated from the clock generator  354 , to generate a switch control signal Vgate. The switch control signal Vgate controls an ON time (the ON and OFF timings, pulse width or frequency) of the power transistor (not shown in this figure, referring to  FIG. 1  and  FIGS. 6A-10B ) in a power stage of the voltage conversion circuit  34 . The PWM signal generator  356  generates the foregoing PWM signal according to the feedback signal COMP from the voltage conversion circuit  34 . To suppress the inrush current such that the current sensing signal Vcs does not keep increasing, for example, the switch control signal Vgate can temporarily reduce the turned-on times or the turned-on frequency of the power transistor. 
       FIG. 4  shows an embodiment of the over current detection circuit and the signal adjustment circuit of a control circuit according to the present invention. In this embodiment, the over current detection circuit  351  is a comparator. The signal adjustment circuit  352  includes a counter  3521 , an SR flip flop  3522  and a logic gate  3523 . When the current sensing signal Vcs is lower than the reference signal Vh, this means no over current and the over current signal OC is at low level. Thus, the setting input S of the SR flip flop  3522  is zero, and the counter  3521  has no count yet, so R=1 (initial value), Q=0, and QN=1. When QN is 1, the switch control signal Vgate is determined by the PWM signal. That is, if the PWM signal is at high level, the logic gate  3523  (such as an AND gate in the embodiment) outputs the switch control signal Vgate at high level. If the PWM signal is at low level, the AND gage  3523  outputs the switch control signal Vgate at low level, as shown in  FIG. 5 . 
     Referring to  FIGS. 4 and 5  and the previous description with respect to  FIG. 2 , if the switch control signal Vgate turns ON the power transistor during each cycle of the clock signal CLK, the current sensing signal Vcs will keep rising. When the current sensing signal Vcs is higher than the reference signal Vh, this means the occurring of the over current, and the over current signal OC changes to high level. Thus, the setting input S of the SR flip flop  3522  is 1, so the inverse output QN of the SR flip flop  3522  is zero and the switch control signal Vgate outputted by the AND gate  3523  is at low level and no longer determined by the PWM signal. Moreover, the counter  3521  starts to count, and its output keeps at 0 if the count of the counter  3521  has not reached the count setting value. When the count reaches the count setting value, the output of the counter  3521  switches to high level and the SR flip flop  3522  is reset, so R=1, Q=0, and QN=1. Thus, the switch control signal Vgate returns to be determined by the PWM signal. When the switch control signal Vgate keeps at low level, the power transistor of the voltage conversion circuit  34  will not be turned on. As shown in  FIG. 5 , the current sensing signal Vcs is suppressed and it does not rise any more. The count setting value for the counter  3521  to output high and the reference signal Vh determining the occurring of the over current can be set by a designer according to the requirements and specification of the power regulator  30 . The count setting value is set to 1 in the embodiment of  FIG. 5 , so the switch control signal Vgate keeps at low level for only one cycle. For the same purpose (to keep the switch control signal Vgate at low level for one cycle), an alternative way is to reduce the frequency of the clock signal. That is, if the frequency of the clock signal is reduced to half of the original frequency, it is equivalent to keeping the switch control signal Vgate keeps at low level for one cycle. If the frequency of the clock signal is reduced to one third of the original frequency, it is equivalent to keeping the switch control signal Vgate keeps at low level for two cycles. 
     In the foregoing embodiments, the switching regulator  30  is not limited to an AC-DC converter; it can be a DC-DC converter. For example, if the switching regulator  30  is an AC-DC converter, it may be the prior art switching regulator  10  as shown in  FIG. 1  (with a different controller). If the switching regulator  30  is a DC-DC converter, the DC-DC converter may be a buck converter ( 6   a ,  6   b ) shown in  FIG. 6A  or  6 B, boost converter ( 7   a ,  7   b ) shown in  FIG. 7A  or  7 B, inverting converter ( 8   a ,  8   b ) shown in  FIG. 8A  or  8 B, buck-boost converter ( 9   a ,  9   b ) shown in  FIG. 9A  or  9 B, or inverter-boost converter ( 10   a ,  10   b ) shown in  FIGS. 10A and 10B . 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, the flip flop is not limited to the SR flip flop  3521 , and may be other types of flip flops. Moreover, the meanings of the high level and low level of a digital signal are interchangeable, and the circuit can be correspondingly modified. In this regard, the positive and negative terminals of the comparators  351  are interchangeable, and the gate  3523  may be replaced by other proper logic gates and so on. Thus, the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.