Abstract:
A boost control module operates semiconductor switches of a boost converter circuit in an avalanche mode to precharge a boost output capacitor. The boost control module comprises a switching module that complementarily transitions a first semiconductor switch and a second semiconductor switch between ON and OFF states when a current does not exceed a maximum current threshold. The switching module transitions the first semiconductor switch and the second semiconductor switch to the OFF state when the current exceeds the maximum current threshold. The switching module maintains the first semiconductor switch and the second semiconductor switch in the OFF state until at least one of the inductor current is less than or equal to a minimum current threshold.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to boost converters, and more particularly to a control method that precharges a boost output capacitor of a boost converter.  
       BACKGROUND OF THE INVENTION  
       [0002]     A boost converter circuit is used to produce an output voltage that is greater than an input voltage from a voltage source. Boost converter circuits may also be used for other purposes, such as to provide noise isolation or regulate voltage levels. Referring now to  FIG. 1 , an exemplary boost converter circuit  10  includes a boost converter input module  12  and a boost converter control module  14 . The boost converter input module  12  receives an input voltage  16 . The boost converter control module  14  generates one or more control signals  18 . The boost converter input module  12  generates an output voltage  20  based on the input voltage  16  and the control signals  18 . The output voltage  20  is greater than the input voltage  16 .  
         [0003]     The boost converter circuit  10  as shown in  FIG. 1  may include one or more semiconductor devices. Semiconductor devices, such as MOSFETs, are rated for a maximum voltage. Operation above a maximum (drain-to-source) voltage causes current flow through the device. The current flow is referred to as an avalanche condition. In other words, operating a device above the maximum voltage is referred to as operating the device in an “avalanche mode.” Avalanche current flowing through the MOSFET device causes high power dissipation and temperature increase. This does not cause permanent damage to the MOSFET device as long as the energy does not exceed a maximum avalanche energy E A  of the MOSFET device.  
       SUMMARY OF THE INVENTION  
       [0004]     A boost control module for a boost converter circuit that includes a first semiconductor switch having an ON state and an OFF state, a second semiconductor switch having an ON state and an OFF state, and at least one inductor that affects output behavior of the boost converter circuit, comprises a comparing circuit that communicates with the boost converter circuit and that determines whether a current through the inductor exceeds a maximum current threshold. A switching module complementarily transitions the first semiconductor switch and the second semiconductor switch between the ON state and the OFF state when the current does not exceed the maximum current threshold. The switching module transitions the first semiconductor switch and the second semiconductor switch to the OFF state when the current exceeds the maximum current threshold. The switching module maintains the first semiconductor switch and the second semiconductor switch in the OFF state until at least one of the inductor current is less than or equal to a minimum current threshold.  
         [0005]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0007]      FIG. 1  is a functional block diagram of an exemplary boost converter circuit according to the prior art;  
         [0008]      FIG. 2  is a circuit schematic of an isolated boost converter circuit according to the prior art;  
         [0009]      FIG. 3  illustrates operating waveforms of an isolated boost converter according to the prior art and according to a normal operating mode of the present invention;  
         [0010]      FIG. 4  is a circuit schematic of an isolated boost converter circuit according to the present invention;  
         [0011]      FIG. 5  is a functional block diagram of a boost converter control module according to the present invention;  
         [0012]      FIG. 6  illustrates operating waveforms of an isolated boost converter according to a precharging mode of the present invention; and  
         [0013]      FIG. 7  is a flow diagram that illustrates steps of a precharging method for an isolated boost converter according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module and/or device refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.  
         [0015]     The present invention operates one or more MOSFET devices in an isolated boost converter circuit in the avalanche mode to precharge a boost output capacitor. Referring now to  FIG. 2 , an exemplary boost converter circuit  30  is shown. The boost converter circuit  30  includes a boost converter input portion  32 , a boost converter isolation portion  34 , and a boost converter control module  36 . The boost converter input portion  32  includes first and second transistors  38  and  40 , first and second inductors  42  and  44 , and an input capacitor  46 .  
         [0016]     The boost converter input portion  32  receives an input voltage Vin across input terminals  48  and  50 . The input voltage Vin causes a current through the first inductor  42  to rise and/or fall according to the input voltage Vin, the inductance characteristics of the first inductor  42 , and ON or OFF statuses of the first and second transistors  38  and  40 . The boost converter isolation portion  34  includes a transformer  52 , a rectifier module  54 , a precharge resistor  55 , and a boost output capacitor  56 . Current flows through the transformer  52  according to the ON or OFF statuses of the first and second transistors  38  and  40 .  
         [0017]     The rectifier module  54  rectifies an alternating current (AC) output of the transformer  52 . The rectifier module  54  is any suitable rectifier circuit as is known in the art. When current is flowing through the transformer  52 , an output of the rectifier module  54  is an output voltage Vout across output terminals  58  and  60 . Additionally, the output of the rectifier module  54  charges the boost output capacitor  56 . When current is not flowing through the transformer  52 , a charge stored in the boost output capacitor  56  provides the output voltage Vout. The transformer  52  isolates the rectifier module  54  and the boost output capacitor  56  from the boost converter input portion  32 .  
         [0018]     Conventionally, an initial output voltage Vout is required to be greater than a transformed input voltage (i.e. the output voltage is required to be greater than the transformed input voltage when the boost converter circuit  30  operates normally). When the initial output voltage Vout is not greater than the transformed input voltage, the boost converter circuit  30  is not able to control an initial current through a load (not shown). In other words, the current through the load will rise uncontrollably and may damage the circuit components. The precharge resistor  55  limits the initial current during a startup period of the circuit. A switch  61  is connected in parallel to the precharge resistor  55 . The switch  61  is open during the startup period. The switch  61  is closed and shorts out the precharge resistor  55  during normal operation. After a sufficient startup period, the boost converter circuit  30  provides the output voltage Vout.  
         [0019]     Referring now to  FIG. 2  and  FIG. 3 , the operation of the first and second transistors  38  and  40  determines the current flow through the first inductor  42  and the transformer  52 . The boost converter control module  36  outputs first and second switching signals  62  and  64  that are connected to gate nodes  66  and  68  of the first and second transistors  38  and  40 , respectively. The first and second transistors  38  and  40  turn ON and OFF according to the first and second switching signals  62  and  64 .  
         [0020]     The first transistor  38  is ON when the first switching signal  62  is high as indicated at  70 . Conversely, the first transistor  38  is OFF when the first switching signal  62  is low as indicated at  72 . The second transistor  40  is ON when the second switching signal  64  is high as indicated at  76 . The second transistor  40  is OFF when the second switching signal  64  is low as indicated at  78 . In this manner, the first and second switching signals  62  and  64  turn the first and second transistors  38  and  40  ON and OFF.  
         [0021]     An inductor current signal  80  indicates a current flowing through the first inductor  42 . A first transistor current signal  82  indicates a current flowing through the first transistor  38 . A transformer current signal  84  indicates a current flowing through the transformer  52 . When at least the first transistor  38  is ON, current through the first inductor  42  rises according to the inductor current signal  80 . When the first transistor is OFF, current through the first inductor  42  decreases. As shown, current flows through transformer  52  only when one of the first transistor  38  and the second transistor  40  is OFF. More specifically, current flows through the transformer  52  in a first direction when the first transistor  38  is ON and the second transistor  40  is OFF. Current flows through the transformer  52  in a second direction when the first transistor  38  is OFF and the second transistor  40  is ON.  
         [0022]     When both the first and second transistors  38  and  40  are OFF, there is no discharge path for current stored in the first inductor  42  and/or the second inductor  44 . In this situation, the current stored in the first and second inductors  42  and  44  will overcome maximum blocking voltages of the first and/or second transistors  38  and  40 , causing avalanche current flow. Conventionally, at least one of the transistors  38  and  40  is required to be ON at all times to prevent avalanche current through the transistors  38  and  40 .  
         [0023]     The present invention operates a boost converter circuit with both transistors OFF to precharge a boost output capacitor. In this manner, a precharge resistor is not required to limit current at startup. Referring now to  FIG. 4 , an exemplary isolated boost converter circuit  100  according to the present invention eliminates the precharge resistor and contactor as described in  FIG. 2 . Referring now to  FIG. 5 , a boost converter control module  110  according to the present invention includes a boost control circuit  112 , first and second gate drive modules  114  and  116 , a latch module  118 , and a comparator  120 . The first and second gate drive modules  114  and  116  generate switching control signals  122  and  124  according to boost control signals  126  and  128  and a switch enable signal  130 . The first and second gate drive modules  114  and  116  may be integrated within a single switching module  131 .  
         [0024]     The boost control circuit  112  generates the boost control signals  126  and  128  and a clock signal  132 . The latch module  118  generates the enable signal  130  according to the clock signal  132  and a comparator signal  134 . The comparator  120  generates the comparator signal  134  according to a current limit signal  136  and an inductor current signal  138 . The inductor current signal  138  is indicative of the current through the first inductor  42 . The current limit signal  136  is indicative of a maximum current desired current of the first inductor  42 . The comparator  120  compares the current limit signal  136  to the inductor current signal  138 . When the inductor current signal  138  is less than the current limit signal  136 , the comparator signal  134  is a first value. When the inductor current signal  138  is greater than or equal to the current limit signal  136 , the comparator signal  134  is a second value.  
         [0025]     The boost control circuit  112  controls the first and second gate drive modules  114  and  116  to turn the first and second transistors  38  and  40  ON and/or OFF according to conventional methods during normal operation. During a boost output capacitor precharge phase according to the present invention, the boost control circuit  112  turns the first and second transistors ON and OFF to precharge the boost output capacitor  56 . When the inductor current signal  138  is greater than the current limit signal  136 , the latch module  118  disables the first and second gate drive module  114  and  116  to turn both of the first and second transistors  38  and  40  OFF.  
         [0026]     When both the first and second transistors  38  and  40  are OFF, the current stored in the first and second inductors  42  and  44  causes avalanche current flow through the first and second transistors  38  and  40 . In other words, the first and second transistors  38  and  40  operate in the avalanche mode to allow the inductor current to decrease. When the inductor current decreases below a threshold, the boost control circuit  112  releases the reset signal of the latch module  118 . In this manner, the boost converter control module  110  operates the first and second transistors  38  and  40  in the avalanche mode for a short period of time to prevent the inductor current from increasing too high.  
         [0027]     Referring now to  FIG. 6 , the switching control signals  122  and  124  are controlled in a manner similar to the method described in  FIG. 3  when the inductor current signal  138  is less than the current limit signal  136 . Initially, when the first transistor  38  is ON, the inductor current ramps up linearly at a first rate. When the first transistor  38  is OFF, the inductor current ramps up linearly at a second rate that is less than the first rate. The output voltage  140  across the boost output capacitor  56  gradually ramps up (i.e. the boost output capacitor  56  is charged). When the inductor current is greater than or equal to the current limit signal  136 , the enable signal  130  becomes low, turning both the first transistor  38  and the second transistor  40  OFF.  
         [0028]     With the first and second transistors  38  and  40  are OFF, the inductor current decreases as indicated at  142 . When the inductor current decreases to a predetermined threshold (e.g. zero amps as shown in  FIG. 6 ) the enable signal  130  becomes high, completing a first phase  144 . Switching operation of the first and second transistors  38  and  40  resumes. In another implementation, the enable signal  130  may become high after a first period. In other words, the first and second transistors  38  and  40  may be OFF for the first period and then resume the switching operation.  
         [0029]     In a second phase  146 , the inductor current ramps up linearly when the first transistor  38  is ON at a rate similar to the first rate of the first phase  144 . However, when the first transistor  38  is OFF, the inductor current ramps up at a third rate that is less than the second rate of the first phase  144 . In subsequent phases, the inductor current ramps up at a decreasing rate and/or ramps down when the first transistor  38  is OFF as indicated at  148 . Therefore, more cycles of the first and second transistors  38  and  40  are required for the inductor current to reach the current limit signal  136 . In other words, the overall duration of each phase increases for subsequent phases. When the output voltage  140  is sufficient (e.g. reaches a predetermined output voltage threshold), the boost converter control module  110  terminates the precharging method and begins normal operation. For example, the boost converter control module  110  may control the isolated boost converter circuit  30  according to  FIG. 3 .  
         [0030]     Referring now to  FIG. 7 , an isolated boost converter precharge method  150  begins in step  152 . In step  154 , the boost converter circuit is powered on and enters a precharging mode. In step  156 , the method  150  determines whether the output voltage is greater than or equal to a threshold. If true, the method  150  continues to step  158 . If false, the method  150  continues to step  160 . In step  158 , the method  150  enters a normal mode. In step  162 , the method  150  terminates.  
         [0031]     In step  160 , the method  150  switches the transistors ON and OFF to increase the inductor current. In step  164 , the method  150  determines whether the inductor current is greater than or equal to a maximum inductor current threshold. If true, the method  150  continues to step  166 . If false, the method  150  continues to step  160 . In other words, the method  150  continues to operate the transistors until the inductor current is greater than or equal to the maximum inductor current threshold.  
         [0032]     In step  166 , the method  150  turns the transistors OFF (i.e. operates the transistors in the avalanche mode). In step  168 , the method  150  determines whether the inductor current is less than or equal to a minimum inductor current threshold. If true, the method  150  continues to step  156 . If false, the method continues to step  168 . In this manner, the method  150  continues to operate in the precharging mode until the output voltage is greater than or equal to a threshold.  
         [0033]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.