Abstract:
A power supply apparatus includes a power supply unit disposed within a casing. The power supply unit includes power semiconductor devices. A fan is provided in the casing for cooling the power semiconductor devices. The power supply unit is operated in a plurality of contiguous cycles each including a running and pausing modes of operation. Each cycle lasting a relatively short time period. The power supply unit operates in the running mode and the pausing mode, respectively in response to the closing and opening of a start switch. In association with the closing and opening of the start switch, a switch for the cooling fan causes the fan to rotate and stop, respectively, which results in minimizing the difference between the highest and lowest temperatures of the power semiconductor devices so that the lifetime of the power semiconductor devices can be prolonged.

Description:
This invention relates to a power supply apparatus for, for example, welders, cutters and projectors, which alternates between a running mode of operation and a pausing mode of operation in a plurality of cycles having a relatively short time period. 
     BACKGROUND OF THE INVENTION 
     An example of such power supply apparatus includes an input-side AC-to-DC converter which converts a commercial AC voltage to a DC voltage. The DC voltage is applied to an inverter where it is converted to a high-frequency voltage, which, in turn, is applied to a voltage transformer. The voltage transformer transforms the high-frequency voltage to a high-frequency voltage having a predetermined value. The voltage-transformed high-frequency voltage from the transformer is converted back to a DC voltage in an output-side high-frequency-to-DC converter. The resulting DC voltage is applied to a load. 
     The input-side AC-to-DC converter and the output-side high-frequency-to-DC converter each include at least one diode. The inverter includes at least one semiconductor switching device, e.g. an IGBT. 
     Since the described power supply apparatus includes an inverter, it can use small-sized reactors in the transformer and the output-side high-frequency-to-DC converter, which enables the downsizing of the power supply apparatus. 
     The power supply apparatus is placed in a casing, and, therefore, Joule heat generated by some components tends to accumulate in the casing. The diodes used in the input-side and output-side AC-to-DC converters and the power semiconductor device used in the inverter, in particular, generate a large amount of heat. A fan is disposed in the casing for the purpose of forcibly cooling such devices. 
     When this power supply apparatus is used with, for example, a welder or a cutter, of which the load includes a torch and a workpiece, it is used for a plurality of operation cycles. Each cycle consists of one running period in which the power supply apparatus operates in a running mode to generate an arc between the torch and the workpiece, and one pausing period in which the apparatus operates in a pausing mode. In the pausing mode, the apparatus is temporarily stopped. The time period of one cycle is predetermined for a particular apparatus. For example, it may be 10 minutes. Also, the proportion of the period of the running mode in one cycle (i.e. running ratio) is predetermined on the basis of the rating of the particular power supply apparatus. 
     Although the fan is driven to rotate in both the running and pausing periods, the power semiconductor devices including the above-mentioned diodes and semiconductor switching device, generate heat only during the running periods, causing the temperature in the casing to rise. During the pausing periods, the temperature within the casing rapidly decreases because the generation of heat by the power semiconductor devices is stopped and the fan is operating to cool the interior of the casing. 
     Experiments have revealed that the lifetime of power semiconductor devices decreases as the difference ΔTc between the highest temperature and the lowest temperature during their operation increases. Temperature difference frequently occurs in a power supply apparatus for an arc welder or an arc cutter, in particular, which has a predetermined running ratio, which results in decrease of the lifetime of power semiconductor devices used therein. In addition, the power consumption of such power supply apparatus is large because the fan continuously operates even during a time interval between one welding operation and another or between one cutting operation and another, during which the fan need not be operated. 
     There is a power supply apparatus in which the temperature within the casing is measured, and the rotation of the fan is controlled in accordance with the measured temperature, so that unnecessary power consumption can be reduced. However, this arrangement cannot provide a solution to the reduction of the lifetime of diodes and semiconductor switching devices used in the apparatus. 
     The present invention is based on a discovery that power semiconductor devices can be used for a longer time as the difference between the highest and lowest temperatures to which they are subjected during operation is smaller. An object of the present invention is to prolong the lifetime of power semiconductor devices used in a power supply apparatus and reduce unnecessary power consumption of the apparatus. 
     SUMMARY OF THE INVENTION 
     A power supply apparatus according to the present invention includes a casing within which a power supply unit is housed. The power supply unit includes heat generating components including power semiconductor devices. A fan is provided for the casing to cool the heat generating components. A start switch is closed and opened alternately and repeatedly to thereby cause the power supply apparatus to operate in a plurality of operating cycles each consisting of one running period and one pausing period following the running period. Each cycle lasts a relatively short time period. Fan control means responds to the closure and opening of the start switch by operating and stopping the fan, respectively. 
     Thus, according to the present invention, the fan is operated while the power semiconductor devices of the apparatus are generating heat so that excessive temperature rise of the power semiconductor devices is prevented. The fan is stopped when the power supply apparatus is placed in the pausing mode. Therefore, the temperature of the semiconductor devices at the instant when the power supply apparatus is brought into the pausing mode is highest. Since the fan is not operating during the pausing mode, the temperature of the power semiconductor devices does not fall much. Thus, the temperature at the instant when the start switch of the power supply apparatus is next closed is lowest. Since the temperature difference between the highest and lowest temperatures is small, the lifetime of the semiconductor devices can be prolonged. 
     The fan control means may be switching means for coupling and decoupling a power source to and from the cooling fan. The power source for the fan may be an AC power source or a DC power source. 
     The cooling fan may operate upon receiving an AC signal. In this case, the fan control means is provided by converter means which converts a DC signal from the power supply unit into an AC signal in response to the closure of the start switch and supplies the resulting AC signal to the cooling fan. The converter means stops supplying the AC signal when the start switch is opened. 
     The fan control means may be arranged such that it does not stop the fan immediately after the start switch is opened, but it may allow the fan to rotate for a predetermined time period after the opening of the start switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a power supply apparatus according to a first embodiment of the present invention. 
     FIG. 2 shows the lifetime of a power semiconductor device in relation to the difference between highest and lowest temperatures to which the semiconductor device is subjected. 
     FIGS.  3 ( a ) through  3 ( c ) show waveforms for use in explaining the operation of the power supply apparatus, in which 
     FIG.  3 ( a ) shows running and pausing periods of the power supply apparatus, 
     FIG.  3 ( b ) shows a change of the temperature in a casing of the power supply apparatus, and 
     FIG.  3 ( c ) shows the ON and OFF states of a fan motor. 
     FIG. 4 is a block diagram of a power supply apparatus according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A power supply apparatus according to a first embodiment of the present invention may be used with a welder and may include a power supply unit as shown in FIG.  1 . An input-side rectifier  2  rectifies an AC voltage, e.g. a single-phase commercial AC voltage, supplied to the power supply apparatus at input terminals  1  and coupled to the rectifier  2  via a power supply switch (not shown). The input-side rectifier  2  may be a full-wave rectifier circuit or a half-wave rectifier circuit. The output voltage from the input-side rectifier  2  is smoothed and converted into a DC voltage by smoothing means, e.g. a smoothing capacitor  3 . 
     The DC voltage is, then, applied to DC-to-AC converter means, e.g. an inverter  4 , which may include a full-bridge circuit including semiconductor switching devices  4   a,    4   b,    4   c  and  4   d.  The switching devices  4   a - 4   d  are PWM controlled by an inverter control circuit  5 , with one pair of semiconductor switching devices  4   a  and  4   d  being alternately turned and off when the other pair of switching devices  4   b  and  4   c  are alternately turned off and on, which results in the conversion of the inputted DC voltage into a high-frequency voltage. 
     The high-frequency voltage is then applied across a primary winding  6 P of a relatively small-sized high-frequency transformer  6 . As a result, a high-frequency voltage having a predetermined magnitude is induced across a secondary winding  6 S. The high-frequency voltage induced across the secondary winding  6 S is rectified by an output-side rectifier  7 , which may be a full-wave rectifier circuit or a half-wave rectifier circuit. The output of the output-side rectifier  7  is smoothed and converted to a DC voltage by smoothing means, e.g. a smoothing reactor  8 . 
     The DC voltage from the reactor  8  appears between output terminals  9   a  and  9   b  of the power supply apparatus and can be applied between a torch  10  and a workpiece  11  which form a load. 
     The input-side rectifier circuit  2  and the output-side rectifier circuit  7  each include a semiconductor device, e.g. a diode. Also, the inverter  4  may include, as the semiconductor switching devices  4   a,    4   b,    4   c  and  4   d,  power semiconductor devices, for example, thyristors, bipolar transistors or IGBTs. When operating, they generate heat. 
     For an AC load, the DC voltage developed between the output terminals  9   a  and  9   b  is converted to an AC voltage again in an inverter or the like (not shown) for application to the AC load. 
     The current applied to the load is detected by a load current detector  12 , which develops a load current representative signal representing the magnitude of the load current. The load current representative signal is applied to the inverter control circuit  5 , where it is compared with a reference signal. The inverter control circuit  5  PWM controls the inverter  4  in such a manner as to make the load current representative signal equal to the reference signal, i.e. to make the load current constant. 
     A start switch  13  is coupled to the inverter control circuit  15 . The power supply unit starts to run when the start switch  13  is closed, and pauses when the start switch  13  is opened. The start switch  13  may be disposed on, for example, the torch  10 . The power supply unit, except the start switch  13 , the torch  10  and the workpiece  11 , are disposed in a casing  20 . A fan  14  is disposed within the casing  20  for cooling the power semiconductor devices of the input-side rectifier circuit  2 , output-side rectifier circuit  7  and inverter  4 . Fan control means, e.g. a switch  15 , is connected between a fan driving motor  14   a  for the fan  14  and the AC power input terminals  1 . The switch  15  is ON-OFF controlled by a timer  16 . The timer  16  operates to close the switch  15  when the start switch  13  is closed and opens the switch  15  a predetermined time after the start switch  13  is opened. 
     When the start switch  13  is closed at a time t 1  as shown in FIG.  3 ( a ), the power supply unit is placed in a running mode of operation, in which the inverter control circuit  5  ON-OFF controls the semiconductor switching devices  4   a - 4   d  of the inverter  4 , and current flows through the load. In response to the closure of the start switch  13 , the timer  16  closes the switch  15  so as to supply an AC voltage e 1  to the motor  14   a  of the fan  14  as shown in FIG.  3 ( c ). Accordingly, the fan  14  rotates at a rate determined by the magnitude of the AC voltage e 1 . Due to the operation of the fan  14 , the temperature rise in the casing  20  is gradual as shown in FIG.  3 ( b ). If the fan  14  were not operated, the temperature within the casing  20  would rise more rapidly. 
     At a time t 2 , after a time period T 1  from the time t 1 , the start switch  13  is opened to terminate the running mode, and the inverter control circuit  5  turns off the semiconductor switching devices  4   a - 4   d  of the inverter  4 . Then, no current is supplied to the load. In other words, the power supply unit is placed in a pausing mode of operation. The timer  16  measures a predetermined time period T 3  from the time t 2  and opens the switch  15  at a time t 3  which is T 3  after the time t 2 . This causes the application of the AC voltage e 1  to the fan motor  14   a  to be interrupted as shown in FIG.  3 ( c ), and the fan  14  stops rotating. 
     Since the fan  14  continues to rotate for the time period T 3 , from the time t 2  to the time t 3 , the temperature within the casing  20  decreases. However, since the fan  14  stops rotating after that, the temperature within the casing  20  decreases by a smaller amount than when the fan  14  would be continuously rotated as indicated by a broken line slope in FIG.  3 ( b ). 
     At a time t 4 , when the time period T 2  starting at the time t 2  has lapsed, the start switch  13  is closed again to initiate the running mode, and the inverter  4  resumes operating, which results in a gradual rise of the temperature within the casing  20 . The operation is repeated after that. 
     The lengths of the time periods T 1  and T 2 , which constitute one cycle, are determined by the running ratio of the welder or cutter for which the power supply apparatus is used. The sum T 1 +T 2  may be, for example, 10 minutes. 
     As shown in FIG.  3 ( b ), there is a difference ΔTc between the highest temperature and the lowest temperature within the casing  20  when the power supply apparatus is being operated. The lowest temperature is the temperature when the start switch  13  is closed. Experiments conducted by the inventors have revealed that there is a relationship as shown in FIG. 2 between the number of the operating cycles of the power supply apparatus the power semiconductor devices of circuits, such as the inverter  4 , can be used (hereinafter referred to as useable cycles), i.e. the lifetime of the power semiconductor devices, and the temperature difference ΔTc. 
     It is seen that as the temperature difference ΔTc becomes larger, the lifetime of power semiconductor devices becomes shorter. For example, when ΔTc is about 35° C., the number of the useable cycles is 10 6 . If the fan  14  is driven continuously, resulting in ΔTc of, for example, about 75° C., the number of the useable cycles is 4×10 3 . It will be understood that the number of the useable cycles the power semiconductor devices can be used become larger when the fan  14  is stopped at intervals than when it is continuously driven. 
     Thus, according to the present invention, the operation of the fan  14  is stopped the predetermined time T 3  after the start switch  13  is opened, to thereby prevent increase of the temperature difference ΔTc. Since the fan is not operated continuously, but it is driven or stopped in relation to the operation of the start switch  13 , unnecessary power consumption can be avoided. 
     In order to minimize the temperature difference ΔTc, it is desirable to stop the fan  14  immediately after the start switch  13  is opened. According to the present invention, therefore, the fan  14  may be stopped at the same time the start switch  13  is opened. 
     On the other hand, if the fan  14  is stopped at the same time the start switch  13  is opened, the temperature of the junction of the semiconductor switching devices may exceed their allowable junction temperature. If the allowable junction temperature of a semiconductor switching device is exceeded, the semiconductor switching device can be used no longer. Therefore, as described previously, the fan  14  is kept operating for the time period of T 3  after the start switch  13  is opened, to thereby cool the semiconductor switching devices. 
     A power supply apparatus according to a second embodiment is shown in FIG.  4 . Components with the reference numerals or symbols same as the ones used in FIG. 1 are the same components as used in the first embodiment, and, therefore, no further description is made about them. 
     In the first embodiment, the switch  15  is connected between the fan  14  and the AC power input terminals  1  as shown in FIG.  1 . According to the second embodiment, AC power for the fan  14  is prepared by converting means including an inverter, e.g. a rotation rate control unit  17 , which converts the DC voltage across the smoothing capacitor  3  into an AC signal, e.g. an AC voltage. The rotation rate control unit  17  is controlled by a driving unit  18 . The driving unit  18  causes the rotation rate control unit  17  to generate an AC voltage of a predetermined value for rotating the fan  14  when the start switch  13  is closed. 
     When the start switch  13  is opened, the driving unit  18  causes the rotation rate control unit  17  to stop generating the voltage so that the fan  14  is stopped. Of course, as in the first embodiment, it may be so arranged that the fan  14  keeps rotating for a predetermined time period after the opening of the start switch  13  before it is stopped. 
     Alternatively, it may be so arranged that the fan  14  may be rotated at a lower rate for a predetermined time period after the opening of the switch  13 . 
     The present invention has been described as being embodied in a power supply apparatus which is used with a welder. However, it may be embodied in a power supply apparatus useable with other apparatuses, such as a cutter and a projector, which alternates between a running mode and a pausing mode in a plurality of contiguous cycles each having a relatively short time period. Also, the motor  14   a  for rotating the fan  14  may be a DC motor.