Load driving circuit

Abstract: This invention relates to a load driving circuit having a fail-safe breaking mechanism for breaking a primary power source when a failure occurs. This invention also relates to a load driving circuit capable of saving electricity in driving an inductive load and reducing a delay in stopping the load. The breaking mechanism for breaking the primary power source has no contact. The load driving circuit includes a power supply circuit involving a semiconductor switching element that turns ON and OFF the supply of power to the load. There is arranged a detector for detecting a failure in the semiconductor switching element. When detecting a failure, the detector provides an output signal to activate the breaking mechanism. To drive the inductive load, the power supply circuit may have two power supply sources. In response to a load driving instruction signal, the two power supply sources together apply a high voltage to the load. After a predetermined period, one of the power supply sources is stopped, and during a steady-state operation of the load, the remaining power source applies a low voltage to the load. The load driving instruction signal may be used to provide a pulse width modulated output, which is used to supply power to the load through a transformer. During a steady-state operation of the load, this arrangement supplies a voltage lower than an operation start voltage to the load, to thereby reduce power consumption and a delay in stopping the load.

Description: SPECIFICATION Technical Field The invention relates to a load driving circuit achieving excellent fail-safe performance with a non-contact breaking mechanism that breaks a primary power source if an abnormality occurs in the load driving circuit. Background Art Devices such as press controllers must provide a high degree of safety and must be fail-safe so that they are switched to a safety side when failures, short circuits, disconnections, etc., occur. Load driving circuits for driving loads such as motors and solenoids that are controlled must also be fail-safe. One of the conventional load driving circuits directly connects a semiconductor switch such as a thyristor, a solid-state relay (hereinafter referred to as SSR), or an electromagnetic relay having contacts to a load in series and provides a load driving instruction signal to turn ON and OFF the switch or the relay, to thereby control the operation of the load. If the semiconductor switch short-circuits or if the relay contact melts, a current will flow to the load even if there is no input signal (load driving instruction signal). Namely, the conventional circuit has a danger that it may erroneously provide an output to the load although there is no input. Such circuit is not fail-safe, and therefore, is unemployable for devices that require a high degree of safety. To be fail-safe, the load driving circuits may employ an electromagnetic relay having special contacts (for example, carbon contacts) that never melt. This sort of contacts, however, is short in service life. To secure fail-safe characteristics, another type of load driving circuits has been proposed (Japanese Unexamined Patent Publication Nos. 60-223445. and 60-227326 and U.S. Patent No. 4,661,880). These circuits directly control a load driving switch circuit with an input signal (load driving instruction signal) and monitor the ON/OFF status of the switch circuit through a fail-safe monitor circuit. Upon detecting electricity supplied to a load with no input signal, the monitor circuit forcibly breaks a primary power source, to surely prevent the most serious accident during the operation of the load. Another type of load driving circuits connects an input signal to a power supply circuit of a load via an electrically isolated signal receiving system involving a transformer. According to this type, an AC input signal (load driving instruction signal) is amplified by an amplifier, and the amplified signal is supplied to a primary winding of the transformer so that a secondary winding thereof may generate an alternating current. The alternating current is converted by a rectifier diode into a direct current, which is supplied to the power supply circuit of the load. This arrangement involves no semiconductor switches that may cause short-circuit failures nor has the problem of short service lives of electromagnetic relays, thereby ensuring fail-safe characteristics. Even of this type, load driving circuits of large capacity for, for example, presses usually employ contact breaking mechanisms having relays for breaking a primary power source that supplies electricity to a load. Since the contact breaking mechanisms always have the problem of melt and wear, they are unsatisfactory in reliability. According to the technique of indirectly driving a load through a transformer in response to an input signal, the load will generate a counter-electromotive force when the input signal is turned OFF, if the load is a DC electromagnetic valve or relay that is inductive. The counter-electromotive force produces a discharge current, which flows to a power supply circuit of the load through a rectifier diode. This results in causing a delay in stopping the load after the turning OFF of the input signal. Some loads such as electromagnetic valves and relays show hysteresis that an input level for starting the loads differs from an input level for stopping the loads. These hysteresis loads continuously operate if an input level sufficient for maintaining the operation is supplied thereto after the start thereof. In spite of this phenomenon, the prior art continuously supplies the starting input level as it is to the loads, thereby wasting electricity. Accordingly, an object of the invention is to provide a fail-safe load driving circuit employing a non-contact breaking mechanism for breaking a primary power source. Disclosure of Invention The invention provides a load driving circuit having a switching element that is connected to a load in series in a power supply circuit of a load and is directly turned ON and OFF by a load driving instruction signal, to control the supply of electricity to the load. The load driving circuit includes a switching power source having an input end electromagnetically coupled with a primary commercial AC power source through a first transformer and an output end electromagnetically coupled with the power supply circuit of the load through a second transformer, to supply a load driving current from the commercial AC power source to the power supply circuit of the load; a semiconductor switching element serving as the switching element connected in series with the load in the power supply circuit of the load, to close the power supply circuit in response to the load driving instruction signal and supply the current from the switching power source to the load; a semiconductor switching element status detector for detecting the ON/OFF status of the semiconductor switching element and providing a low level output of logical value 0 if the switching element is ON, a high-level output of logical value 1 if the switching element is OFF, and a low-level output of logical value 0 if the detector itself is out of order; and a power source stoppage decision unit for receiving an output of the semiconductor switching element status detector and the load driving instruction signal, determining that the switching element is abnormal if the output of the detector is at low level although there is no load driving instruction signal, and in this case, providing a low-level output to stop the power supplying operation of the switching power source. This arrangement employs no contacts in stopping the primary power source when the semiconductor switching element for controlling the supply of power to the load becomes abnormal, and therefore, is fail-safe to surely disconnect the load from the primary power source against any abnormality. The power source stoppage decision unit may employ fail-safe logical operation units to further improve fail-safe characteristics. Brief Description of the Drawings Fig. 1 is a circuit diagram showing a load driving circuit according to an embodiment of the invention; Best Mode for Carrying Out the Invention Embodiments of the present invention will be explained in detail with reference to the drawings. Figure 1 shows an embodiment of the invention. Fig. 1, a commercial primary AC power source 1 provides an AC input, which is passed through a first transformer 2 and supplied to a switching power source 3. The switching power source 3 includes a first rectifier 4 for rectifying an AC output generated by a secondary winding of the first transformer 2; a transistor 5 connected in series with a primary winding of a second transformer 6 to be explained later; and a first signal generator 24 operating in response to an output of a decision circuit 15 to be explained later. The switching power source 3 is excited by an AC output of the first signal generator 24. Namely, the AC output of the first signal generator 24 turns ON and OFF the transistor 5, which passes the AC output produced from the AC input provided by the primary AC power source 1. The AC output of the switching power source 3 causes a secondary winding of the second transformer 6 to generate an AC output, which is rectified by a rectifier 7. The rectified output is supplied to a load 8 such as a motor or a solenoid. A power supply circuit of the load 8 includes a switching element 9 (which may be a semiconductor switching element or a solid-state relay (SSR), the latter is used in this embodiment). The switching element 9 is closed in response to a load driving instruction AC signal I N rectified by a rectifier 19. In the figure, C is a capacitor. An impedance sensor 10 is a semiconductor switching element status detector for determining whether or not the SSR 9 is ON. The impedance sensor 10 includes a second signal generator 11; a magnetic core 12 having a primary winding N1 for receiving an AC signal from the second signal generator 11 through a resistor R, a secondary winding N2 for receiving an AC signal from the primary winding N1, and a power supply line of the power supply circuit of the load; a second amplifier 13 for amplifying the signal received by the secondary winding N2; and a level tester for receiving the amplified AC output from the amplifier 13 and providing a high-level output if the amplified AC output is greater than a predetermined level. When the SSR 9 is ON, the parallel-connected load 8 works to reduce a circuit impedance, thereby decreasing a voltage received by the secondary winding N2. As a result, an output I S of the level tester 14 falls to a logical value 0 to inform that the SSR 9 is ON. When the SSR 9 is OFF, the load 8 is disconnected to increase the circuit impedance, thereby increasing the voltage received by the secondary wining N2. As a result, the output I S of the level tester 14 rises to a logical value 1 to inform that the SSR 9 is OFF. The output I S of the level tester 14, i.e., the output of the impedance sensor 10 is sent to the decision circuit 15 serving as a power source stop decision unit. The decision circuit 15 receives the output I S of the impedance sensor 10 and the load driving instruction signal I N for controlling the SSR 9. If the load driving instruction signal I N is absent and the output I S of the impedance sensor 10 is at low level (indicating that a current is flowing to the power supply line to the load), the decision circuit 15 determines it is abnormal and provides an output of low level to stop the first signal generator 24 of the switching power source 3, thereby stopping the supply of power from the switching power source 3 to the load 8. The decision circuit 15 includes a fail-safe OR circuit 16 such as a wired-OR circuit for providing an OR of the load driving instruction signal I N and the output I S of the impedance sensor 10; a fail-safe first AND circuit 17 for providing an AND of the load driving instruction signal I N and the output I S of the impedance sensor 10; a fail-safe second AND circuit 18 for providing, as an abnormality decision output, an AND {(I N V I S ).(I N . I S ) } of the outputs of the OR circuit 16 and first AND circuit 17 and selfholding the torque of the first AND circuit 17 through a diode D; and rectifiers 20 to 23 for rectifying the load driving instruction signal I N and the output I S of the level tester 14. The first and second AND circuits 17 and 18 are known AND oscillators (disclosed in, for example, Japanese Unexamined Utility Model Publication NO. 57-4764). The diode D forms a rectifier for feeding an AC output of the second AND circuit 18 back to an input end of the second AND circuit 18. The decision circuit 15 provides an output of high level when the circuit 15 itself is normal, to drive the signal generator 24. The signal generator 24 generates an AC signal during operation, to turn ON and OFF the transistor 5, thereby activating the switching power source 3. The operation of the load driving circuit of this embodiment will be explained. The commercial primary AC power source 1 is set up to prepare for driving the load driving circuit. At this moment, the output I S of the impedance sensor 10 is at high level because the SSR 9 is open before receiving the load driving instruction signal I N . Due to the high-level output I S of the impedance sensor 10, the OR circuit 16 provides an output of high level. Accordingly, one input of the second AND circuit 18 is high. One input of the first AND circuit 17 is also high. As soon as the load driving instruction signal I N is provided, the first AND circuit 17 provides an output of high level to the other input terminal of the second AND circuit 18, which provides an output of high level accordingly. As a result, the first signal generator 24 provides an AC signal to the base of the transistor 5 of the switching power source 3, to turn ON and OFF the transistor 5 to drive the switching power source 3. A current from the commercial AC power source is passed through the first and second transformers 2 and 6 and supplied to the power supply circuit of the load 8. At this time, the SSR 9 is ON due to the load driving instruction signal I N , to close the power supply circuit of the load 8 and supply the current to the load 8, which is then driven. When the SSR 9 is turned ON to supply the current to the power supply line for the load 8, the output I S of the impedance sensor 10 falls to low level. As a result, the first AND circuit 17 provides an output of low level to one input terminal of the second AND circuit 18. Since the output of the second AND circuit 18 is connected through the diode D to the input terminal that is at low level, the output of the second AND circuit 18 maintains high level by itself. The output of the second AND circuit 18 is continuously supplied to the first signal generator 24, which causes the switching power source 3 to continuously operate. When the load driving instruction signal I N is stopped, the SSR 9 turns OFF to stop the supply of electricity to the load 8. Then, the output I S of the impedance sensor 10 rises to high level, and therefore, the output of the OR circuit 16 keeps high level even if the load driving instruction signal I N is stopped. Accordingly, the output of the second AND circuit 18 maintains high level to continuously activate the switching power source 3. In this way, the supply of power to the load 8 is controlled according to the ON and OFF statuses of the load driving instruction signal I N once the operation is started and if the load driving circuit is normal. To stop the operation of the circuit as a whole, the commercial primary AC power source 1 must be cut. An operation when the SSR 9 is short-circuited will be explained. When the SSR 9 is short-circuited, it is detectable because the output I S of the impedance sensor 10 falls to low level although the load driving instruction signal I N is absent. In this case, both inputs to the OR circuit 16 fall to low level, so that the second AND circuit 18 provides an output of low level to stop the signal generator 24. Accordingly, the ON/OFF operation of the transistor 5 of the switching power source 3 is stopped to deactivate the switching power source 3, so that no power is supplied to the load 8. Once the SSR 9 is short-circuited and the supply of a current to the power supply circuit of the load 8 is stopped, the output I S of the impedance sensor 10 maintains low level. The output of the first AND circuit 17, therefore, does not rise to high level, and even if the load driving instruction signal I N rises to raise the output of the OR circuit 16 to high level, the output of the first AND circuit 17 never rises to high level. Namely, one of the inputs to the second AND circuit 18 is kept at low level, to keep the switching power source 3 inactive. When the impedance sensor 10 becomes out of order, the output I S of thereof falls to low level to continuously stop the switching power source 3. The logical operation circuits 16 to 18 of the decision circuit 15 are fail-safe to provide an output of low level whenever any of them fails. Namely, if any one of them fails, the decision circuit 15 provides an output of low level to stop the switching power source 3. If the transistor 5 of the switching power source 3 is short-circuited or causes an open failure, the switching power source 3 will not produce an AC output. Accordingly, the second transformer 6 generates no AC output. If the transistor 5 and rectifier 4 are each short-circuited, the second transformer 6 will not provide an output because the frequency of the output signal of the first transformer 2 is low. In this way, this load driving circuit is safe against any failure because the circuit stops the supply of electricity to the load 8 and deactivates the load 8 if such failure occurs. When the SSR 9 causes an open failure, the output I S of the impedance sensor 10 will be continuously high. In this case, the output of the second AND circuit 18 rises to high level irrespective of the load driving instruction signal I N , to maintain the operation of the switching power source 3. The SSR 9, however is open to open the power supply circuit of the load 8 and supply no power to the load 8. The load 8, therefore, is never activated, to thereby secure the safety. As mentioned above, the load driving circuit of this embodiment is controlled to the safety side against any circuit failure. Namely, this circuit is fail-safe and has a high degree of safety. Electricity is supplied to the load 8 through the non contact switching power source 3. Unlike relays involving contacts, this arrangement is free from the problems of melt and wear. Compared with the conventional breaking mechanisms employing relays for breaking a primary power source, this embodiment of the present invention achieves improved safety and longer service life. As explained above, the invention provides a load driving circuit employing a non-contact breaking mechanism for breaking a primary power source, to eliminate the problems of melt and wear of contacts and improve the reliability and service life of the circuit. If the circuit fails, the supply of power to a load will be surely stopped and the load will never be erroneously driven. In this way, the circuit is highly fail-safe. Capability of Exploitation in Industry This invention safely and efficiently drives a load that is a final controlled object of industrial equipment that requires a high degree of safety. The present invention, therefore, has a great capability of exploitation in industry.

Claims: A load driving circuit for controlling the supply of power to a load (8) by directly turning ON and OFF a switching element (9), which is connected in series with a power supply circuit of the load, according to a load driving instruction signal (I N ), comprising a switching power source (3) having an input end electromagnetically coupled with a commercial primary AC power source (1) through a first transformer (2) and an output end electromagnetically coupled with the power supply circuit of the load (8) through a second transformer (6), to supply a load driving current from the commercial AC power source (1) to the power supply circuit of the load; a semiconductor switching element (9) serving as the switching element, connected in series with, the load in the power supply circuit of the load, to close the power supply circuit in response to the load driving instruction signal (I N ) so that the current from the switching power source (3) is supplied to the load (8); semiconductor switching element status detection means (10) for detecting an ON/OFF status of the semiconductor switching element (9) and generating a low-level output of logical value 0 if the switching element (9) is ON, a high-level output of logical value 1 if the switching element (9) is OFF, and a low-level output of logical value 0 if the detection means (10) itself is out of order; and power source stoppage decision means (15) for receiving an output (I S ) of the semiconductor switching element status detection means (10) and the load driving instruction signal (I N ), and if the load driving instruction signal (I N ) is absent and the output (I S ) of the detection means (15) is at low level, determining that the switching element (9) is abnormal and providing a low-level output to stop the power supplying operation of the switching power source (3). The load driving circuit according to claim 1, wherein the switching power source (3) includes a first rectifier (4) for rectifying an AC output that is produced by a secondary winding of the first transformer (2) according to the output of the commercial AC power source (1); a transistor (5) connected in series with a primary winding of the second transformer (6); and a first signal generator (24) for generating an AC signal to turn ON and OFF the transistor (5) when an output of the power source stoppage decision means (15) is at high level. The load driving circuit according to claim 1, wherein the semiconductor switching element status detection means (10) includes a second signal generator (11) for generating an AC signal; a magnetic core (12) having a primary winding for receiving the AC signal from the second signal generator (119 through a resistor (R), a secondary winding for receiving an AC signal from the primary winding, and a power supply line of the power supply circuit of the load (8); a second amplifier (13) for amplifying the signal received by the secondary winding; and a level tester (14) for providing a high-level output if the amplified AC output from the second amplifier is greater than a predetermined level. The load driving circuit according to claim 1, wherein the power source stoppage decision means (15) includes a fail-safe OR circuit (16) for providing an OR of the load driving instruction signal (I N ) and the rectified output of the semiconductor switching element status detection means (10); a fail-safe first AND circuit (17) for providing an AND of the load driving instruction signal (I N ) and the rectified output of the semiconductor switching element status detection means (10); and a fail-safe second AND circuit (18) for providing an AND of the outputs of the OR circuit (16) and first AND circuit (17) as a decision output to the switching power source (3), the second AND circuit (18) having a function of self holding the output of the first AND circuit (17).

Inventors: FUTSUHARA KOICHI, SAKAI MASAYOSHI, FUTSUHARA, KOICHI, SAKAI, MASAYOSHI, FUTSUHARA, KOICHI C/O YONO OFFICE, SAKAI, MASAYOSHI C/O YONO OFFICE
Applicants: NIPPON SIGNAL CO LTD, THE NIPPON SIGNAL CO. LTD.
Application year: 1993
Main classification: H01F7
IPC codes: H01F7, H01H47