JP2016046881A - Power source switching circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a power loss and to surely switch from a second power source to a first power source without using a step-down circuit that is large-scaled in area and cost, even if a voltage of the second power source of which the voltage precision is deteriorated unexpectedly exceeds a voltage of the first power source, in a power source switching circuit for switching the first power source and the second power source.SOLUTION: A power source switching circuit A for switching a first power source 11 of high voltage and a second power source 12 of low voltage includes: a transfer contact 15b for switching input terminals 21 and 22 from the power sources with respect to an output terminal 24; a voltage discrimination circuit 14 for driving a relay when the voltage of the first power source exceeds a specific value; and a mechanical relay 15 for switching the transfer contact 15b from the second power source side to the first power source side in accordance with an instruction from the voltage discrimination circuit 14. It is better to provide a series regulator 16 for bypass connection of the transfer contact 15b between an input terminal and an output terminal at the second power source side.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power supply switching circuit for selectively switching between a first power supply having a high voltage and a second power supply having a low voltage.

  FIG. 6 shows a circuit diagram of a conventional power supply switching circuit. In FIG. 6, 31 is a first power source (main power source) having a high voltage, and 32 is a second power source (sub power source) having a low voltage, such as a battery (storage battery) E31. As the first power supply 31, for example, a DC power supply obtained by converting power from an AC system power supply (commercial power supply) can be considered. The first power supply 31 includes a rectifying and smoothing circuit including a series circuit of a diode D31 and a smoothing capacitor (electrolytic capacitor) C31 connected between both ends of the secondary winding N32 of the transformer T31. Two diodes D41 and D42 are used as a power supply switching circuit 40 that selectively switches between the first power supply 31 and the second power supply 32. The anode of the diode D 41 on the high voltage supply side is connected to the input terminal 41 from the first power supply 31. The anode of the diode D42 on the low voltage supply side is connected to the input terminal 42 from the second power supply 32. Reference numeral 43 denotes an input terminal on the ground line. The cathode of the diode D41 on the high voltage supply side and the cathode of the diode D42 on the low voltage supply side are connected in parallel, and the connection point is connected to the output terminal 44 on the high side. Reference numeral 45 denotes an output terminal of the ground line, and the load circuit 33 is connected between the output terminals 44 and 45.

  When the first power source 31 that is the main power source is in a stopped state, power is supplied to the load circuit 33 from the second power source 32 that is the sub power source. At this time, the diode D42 on the low voltage supply side is in a conductive state, and the diode D41 on the high voltage supply side is in a nonconductive state.

  When the first power supply 31 is activated, charging of the smoothing capacitor C31 is started, and the voltage V31 of the first power supply 31 gradually increases. When the voltage V31 of the first power supply 31 exceeds the voltage V32 of the second power supply 32, the conduction cut-off of the diode D42 on the low voltage supply side and the conduction start of the diode D41 on the high voltage supply side occur simultaneously. That is, the power supply from the second power supply 32 having a low voltage is stopped, and the power supply from the first power supply 31 having a high voltage is started instead (automatic switching of the power supply).

  When the first power supply 31 is stopped again, the conduction interruption of the diode D41 on the high voltage supply side and the conduction start of the diode D42 on the low voltage supply side occur simultaneously. That is, the power supply from the first power supply 31 having a high voltage is stopped, and the power supply from the second power supply 32 having a low voltage is started instead (automatic switching of the power supply power supply).

  As similar to the conventional example shown in FIG. 6, there are technologies disclosed in Patent Documents 1 and 2.

JP 2012-151942 A JP 2012-177621 A

  In the power supply switching circuit 40 of the conventional example described above, in the power supply state from the second power supply 32, there is power consumption due to current flowing through the diode D42 on the low voltage supply side, and the first power supply Even in the power supply state from the power supply 31, there is power consumption due to current flowing through the diode D41 on the high voltage supply side. Assuming that the forward voltages of the diodes D42 and D41 are Vf, a loss of [output current × Vf] occurs in any case, resulting in a total loss.

  In particular, when the secondary second power source 32 with a low voltage is the battery E31, the following problem occurs. The battery has a relatively low voltage accuracy, and in some cases, the battery may be in a voltage output state higher than the voltage V31 of the main first power supply 31 having a high voltage. However, if the first power supply 31 needs to be supplied from the first power supply 31 to the load circuit 33 and the first power supply 31 is activated, the second power supply 32 is unexpectedly more than the first power supply 31. If the voltage is high, there is a situation in which power supply from the first power supply 31 cannot be performed. This is because the two diodes D41 and D42 are connected in parallel to the output terminal 44. This is because when the voltage V32 of the second power supply 32 is unexpectedly higher than the voltage V31 of the first power supply 31, the diode D42 does not switch to the cut-off state and the diode D41 cannot conduct. For example, this is a problem when the load circuit 33 is a DC-DC converter and it is necessary to supply power from the first power supply 31 instead of from the second power supply 32 that is a battery.

  In order to cope with this problem, it is conceivable to add a step-down circuit to the second power source, which is a sub power source. However, the equipment is large for the effect, and there is a problem in terms of downsizing and cost. Remains.

  The present invention has been created in view of such circumstances, and aims to reduce loss in a power supply switching circuit that selectively switches between a first power supply having a high voltage and a second power supply having a low voltage. In particular, even if the second power source, which is a sub power source, is a battery with relatively poor voltage accuracy and a higher voltage is output than the first power source, which is the main power source, a large step-down voltage is generated. The object is to be able to cope without using a circuit.

  The present invention solves the above problems by taking the following measures.

The power supply switching circuit according to the present invention includes:
A power supply switching circuit for selectively switching a first power supply having a high voltage and a second power supply having a low voltage to output a selected voltage from an output terminal;
Switch means for switching connection of the input terminal from each power supply to the output terminal;
A voltage determination circuit connected to the input terminal on the first power supply side for detecting the voltage of the first power supply and instructing switching of the switch means when exceeding a specified value;
Switch driving means for switching the switch means from the second power supply side to the first power supply side in response to the switch switching instruction from the voltage determination circuit.

  In the power supply switching circuit of the present invention configured as described above, when the first power supply having a high voltage is in a stopped state, the voltage determination circuit does not instruct switch switching, and the switch means is inoperative. The switch means connects the second power source having a low voltage to the output terminal. That is, power is supplied to the load circuit from the second power source having a low voltage.

  Next, when the first power supply is activated, the voltage of the first power supply gradually increases. The voltage determination circuit monitors the rising process of the voltage of the first power supply, and instructs switching when the voltage exceeds a specified value. As a result, the switch driving means is activated to operate the switch means, and the switch means that has been connected to the second power supply side so far is switched to the first power supply side.

  With regard to the switching operation of the switch means, the switching is surely executed as long as it is determined by the determination operation in the voltage determination circuit that the voltage of the first power source has exceeded the specified value. Even if the voltage of the second power supply is unexpectedly higher than that of the first power supply due to poor voltage accuracy because the second power supply is a battery or the like, the switch means Is switched. That is, unlike the conventional example in which the cathodes of the two diodes are connected in parallel to the output terminal, the power supply state of the first power supply is changed from the power supply state of the second power supply regardless of the voltage state of the second power supply. It is possible to switch to steadily.

  In the above, what is inserted into the line connecting the first power supply or the second power supply and the output terminal is the switch means, which has a resistance value substantially zero compared to the diode in the conventional example. Since it is small, the power loss in the power supply switching circuit is greatly reduced. In addition, as a condition for switching the power supply main body from the second power source to the first power source, the voltage of the first power source by the voltage determination circuit exceeds a specified value, so that it is not affected by the voltage state of the second power source. Not receive. That is, even if a situation occurs in which the second power source is a battery with relatively poor voltage accuracy and unexpectedly outputs a voltage higher than the first power source, Therefore, it is possible to cope with a reliable switching to a power source without using a step-down circuit which is large and disadvantageous in terms of area and cost.

  According to the present invention, it is possible to reduce power loss for a power supply switching circuit that selectively switches between a first power supply having a high voltage and a second power supply having a low voltage. Furthermore, since the second power source is a battery with relatively poor voltage accuracy, even if a situation in which a voltage higher than the first power source is unexpectedly generated, the step-down circuit has a large scale and is disadvantageous in terms of cost. Without using, it is possible to realize reliable switching from the second power source to the first power source.

The circuit diagram which shows the structure of the power supply switching circuit of the Example of this invention Timing chart showing the operation of the power supply switching circuit of the embodiment of the present invention The circuit diagram which shows the structure of the power supply switching circuit of another Example of this invention. Circuit diagram of photo MOS relay in another embodiment of the present invention (light-off state) Circuit diagram of photo MOS relay in another embodiment of the present invention (lighting state) Circuit diagram showing the configuration of a conventional power supply switching circuit

  The power supply switching circuit of the present invention having the above configuration has several preferred modes as follows.

  The switch means includes a transfer contact for switching the input terminal from each power supply to the output terminal, and the transfer contact from the second power supply side to the first power supply according to the switch switching instruction from the voltage determination circuit. And a series regulator that bypass-connects a transfer contact between the input terminal and the output terminal on the second power supply side.

  Here, the transfer contact is a switch means having two contacts, a contact (normally open contact) and b contact (normally closed contact), and its operation mode is that the a contact is normally off and the b contact is on. The contact b is turned off and the contact a is turned on by a relay drive instruction (switch switching instruction). In this case, a neutral state in which the two contacts are both non-conductive occurs at the contact switching timing, but the connection state is switched through the neutral period.

  A series regulator is a constant current DC power supply circuit with a continuous current that can only be stepped down, in which a voltage control element such as a transistor is connected in series to a line connecting a power supply and an output terminal (load circuit). The series regulator has a function of adjusting the load circuit so that a constant voltage is applied by causing a voltage drop by consuming electric power in the series regulator by an amount that is too high.

  In the neutral state in which the transfer contact is being switched from the second power supply side to the first power supply side, the transfer contact is temporarily connected to both the first and second power supplies, although this is a short time. There is a new problem in that there is a possibility that power supply interruption may occur because the connection is not established. However, the series regulator outputs to the output terminal in a constant voltage state in which the voltage of the second power supply having a low voltage is further lowered. In other words, a neutral state occurs, but it is avoided that the voltage suddenly drops and power supply stops or power supply deterioration occurs, and although the voltage level slightly decreases, the power supply state to the output terminal is continued. Become.

  On the other hand, when the first power supply is stopped again, the voltage of the first power supply gradually decreases and falls below the specified value, and the voltage determination circuit stops the relay driving instruction (switch switching instruction). Then, the relay reverses and performs an operation of switching the transfer contact that has been connected to the first power supply side to the second power supply side. Even during this switching operation, a neutral state similar to the above occurs, but due to the action of the series regulator, although a slight decrease in the voltage level is observed, the state of power supply to the output terminal is continued.

  In order to reduce loss, the present invention employs switch means that is switched and driven instead of the two diodes connected in parallel in the conventional example. However, there arises a new problem that power supply interruption may occur due to a neutral state during switching between the two contact points of the transfer contact point. Therefore, since power supply is continued by adding a series regulator, the problem of power supply interruption in the neutral state does not occur.

  The switch means may be a contactless semiconductor relay in addition to a mechanical relay. In the case of a non-contact type semiconductor relay, the reliability is higher than that of a mechanical relay and high-speed switching operation can be expected.

  The voltage determination circuit preferably includes a constant voltage element, a biasing resistance element, and a relay driving switching element as follows. That is, a constant voltage element (zener diode) is connected between the input terminal on the first power supply side and the ground line via a biasing resistance element. The switching element (transistor) for driving the relay has its control terminal connected to the connection point between the constant voltage element (zener diode) and the biasing resistance element, the high side terminal connected to the relay drive coil, and the low side terminal To the ground line.

  The series regulator is preferably composed of a step-down switching element, a biasing resistance element, and a constant voltage element as follows. That is, the high-side terminal and the low-side terminal of the step-down switching element (transistor) are connected in a bypass manner between the input terminal and the output terminal on the second power supply side, and the bias resistance element is the step-down switching element. The constant voltage element (zener diode) is connected between the control terminal of the step-down switching element (transistor) and the ground line.

  Hereinafter, the embodiment of the power supply switching circuit of the present invention having the above configuration will be described in detail at the level of specific examples. FIG. 1 is a circuit diagram showing a configuration of a power supply switching circuit according to an embodiment of the present invention, and FIG. 2 is a timing chart showing an operation of the power supply switching circuit according to the embodiment of the present invention.

  In FIG. 1, A is a power source switching circuit, 11 is a main first power source having a high voltage, 12 is a sub second power source having a low voltage, 13 is a load circuit, 14 is a voltage determination circuit, and 15 is a mechanical relay. , 16 is a series regulator. Reference numeral 21 denotes an input terminal of the power supply switching circuit A in the high-side power supply line LH, to which the output terminal of the first power supply 11 (the positive terminal of the smoothing capacitor C11) is connected. Reference numeral 22 denotes an input terminal in the low-side power supply line LL, to which the output terminal of the second power supply 12 is connected. Reference numeral 23 denotes an input terminal of the ground line (common line) LG, 24 denotes a high-side output terminal, and 25 denotes an output terminal of the ground line.

  The first power supply 11 is constituted by a rectifying / smoothing circuit including a series circuit of a diode D11 and a smoothing capacitor (electrolytic capacitor) C11 connected between both ends of the secondary winding N12 of the transformer T11. For example, a power conversion circuit (not shown) is connected to the previous stage of the primary winding N11 of the transformer T11. The second power source 12 is typically configured by a battery (storage battery) E11. The load circuit 13 is, for example, a DC / DC converter. The power supply switching circuit A selectively switches between the first power supply 11 and the second power supply 12, and includes a voltage determination circuit 14, a mechanical relay 15, and a series regulator 16.

  The mechanical relay 15 is an example of a switch unit that switches connection of the input terminals 21 and 22 from the power supplies 11 and 12 to the output terminal 24. The mechanical relay 15 includes a drive coil 15a and a transfer contact 15b. The drive coil 15a is an example of a switch drive unit. In the transfer contact 15b, the b contact that is normally active is connected to the low-side power supply line LL, the a contact that is active during driving is connected to the high-side power supply line LH, and the c contact is the high-side output terminal. 24. The transfer contact 15b in the mechanical relay 15 includes an a contact connected to the input terminal 21 on the first power supply 11 side, a b contact connected to the input terminal 22 on the second power supply 12 side, and a high side output. It has a c contact connected to the terminal 24 and a switching operation actuator d provided at the c contact. The actuator d is always in contact with the contact b to form a state in which the second power source 12 is connected to the high-side output terminal 24. When the drive coil 15a is energized and energized, it is switched to the contact a. The state in which the first power supply 11 is connected to the high-side output terminal 24 is formed. In this embodiment, the number of c contacts is one, but this type of transfer contact is also referred to as “1c contact”.

  The voltage determination circuit 14 includes bias resistance elements R11 and R12, a Zener diode ZD11 as a constant voltage element, and an NPN transistor Q11 as a relay driving switching element. The cathode (high side terminal) of the Zener diode ZD11 is connected to the input terminal 21 on the first power supply 11 side via a biasing resistance element R11. The anode (low side terminal) of the Zener diode ZD11 is connected to the ground line LG via a biasing resistance element R12. The transistor Q11 for driving the relay has its base (control terminal) connected to the connection point between the anode of the Zener diode ZD11 and the biasing resistance element R12, its emitter (low side terminal) connected to the ground line LG, A collector (high side terminal) is connected to one end of the drive coil 15 a in the mechanical relay 15. The other end of the drive coil 15a is connected to the input terminal 21 on the first power supply 11 side.

  The series regulator 16 includes an NPN transistor Q12 as a step-down switching element, a biasing resistance element R13, and a Zener diode ZD12 as a constant voltage element. The step-down transistor Q12 has its collector (high side terminal) connected to the low side power supply line LL (b contact) and its emitter (low side terminal) connected to the high side output terminal 24 (c contact). Yes. The biasing resistance element R13 is connected between the base (control terminal) and the collector (high side terminal) of the step-down transistor Q12. The Zener diode ZD12 has its cathode (high side terminal) connected to the base (control terminal) of the transistor Q12 and its anode (low side terminal) connected to the ground line LG. In the series regulator 16, a step-down transistor Q12 is connected in a bypass manner between the b contact and the c contact of the transfer contact 15b.

  Next, the operation of the power supply switching circuit A according to the embodiment of the present invention configured as described above will be described with reference to the timing chart of FIG. 2A compares the change in the voltage V11 of the first power supply 11 with the voltage V12 of the second power supply 12, the constant voltage V12 ′ by the series regulator 16, and the specified value Vth of the determination criterion of the voltage determination circuit 4. FIG. 2B is a timing chart showing a change in the output voltage Vout appearing at the high-side output terminal 24. The timings of FIG. 2A and FIG. 2B are displayed in synchronization.

  Now, it is assumed that the first power supply 11 having a high voltage is in a stopped state in a time zone before t0 in terms of timing, and the voltage V11 is at a zero level. At this time, the voltage determination circuit 14 does not operate, and the mechanical relay 15 does not operate. The transfer contact 15b is in contact with the b contact, and power is supplied to the load circuit 13 from the second power source 12 having a low voltage.

  Next, it is assumed that the first power supply 11 is activated at the timing t0. At this time, in the transformer T11, the electric power induced in the secondary winding N12 induced from the primary winding N11 is half-wave rectified by the diode D11, and charging of the smoothing capacitor C11 is started. The voltage V11 appearing at the positive terminal of the smoothing capacitor C11 is applied to the voltage determination circuit 14 and gradually increases (period t0 to t3). The voltage determination circuit 14 outputs a relay drive signal to the mechanical relay 15 at a timing t1 when the voltage V11 exceeds the specified value Vth. That is, when the voltage V11 appearing in the smoothing capacitor C11 gradually increases and the voltage V11 rises before the cathode applied voltage of the Zener diode ZD11 exceeds the Zener voltage, the Zener diode ZD11 becomes conductive. As a result, a voltage due to a voltage drop in the biasing resistance element R12 is applied to the base of the relay driving transistor Q11, and the relay driving transistor Q11 becomes conductive. Then, a current flows through the relay drive coil 15a, the drive coil 15a is excited, and the switching operation of the transfer contact 15b starts. That is, the actuator d that has been in contact with the b-contact until then leaves the b-contact at the timing t1, and then transitions to a state of contacting the a-contact at the timing t2 when a minute time has passed. For driving the mechanical relay 15, the voltage V11 of the first power supply 11 is used.

  What is important in this case is that the second power supply 12 is the battery E11 and the voltage accuracy is poor, so that the voltage V12 is higher than the voltage V11 of the first power supply 11 or not. This means that the switching of the contact 15b is performed reliably. That is, when the voltage V11 of the first power supply 11 is monitored and exceeds the specified value Vth, the function of the voltage determination circuit 14 that instructs relay driving is activated. This is because the operating condition of the voltage determination circuit 14 is exclusively at the level of the voltage V11 of the first power supply 11 and is not related to the state of the voltage V12 of the second power supply 12. Therefore, as long as the first power supply 11 is activated, the voltage V11 reaches the specified value Vth, and the voltage determination circuit 14 only detects this, the first power supply 11 from the power supply state of the second power supply 12 is detected. The power supply state can be reliably switched to. That is, unlike the conventional example in which the cathodes of the two diodes are connected in parallel to the output terminal, the second power supply 12 is switched to the first power supply 11 regardless of the state of the voltage V12 of the second power supply 12. The power supply switching can be ensured.

  In particular, when the load circuit 13 is a DC-DC converter, avoid a situation where an excessively high voltage (a voltage higher than the voltage V11 of the first power supply 11) is applied from the battery in an abnormal output state. There must be. However, according to the embodiment of the present invention, it is not necessary to use a large step-down circuit for reducing the excessive high voltage of the battery.

  During the switching of the transfer contact 15b from the second power supply 12 side to the first power supply 11 side (transition period T1 between timings t1 and t2), the transfer contact 15b is temporarily connected. It is in a neutral state that is not connected to any of the first and second power supplies 11 and 12. Then, a state occurs in which the series regulator 16 outputs to the output terminal 24 in the state of the voltage V12 ′ obtained by further reducing the voltage V12 of the second power supply 12 having a low voltage. That is, even if the neutral state occurs, it is possible to avoid the sudden drop of the output voltage Vout and the occurrence of power supply stoppage or power supply deterioration, and although the voltage level is slightly decreased, the power supply state to the load circuit 13 continues. Will be. This is the effect of the series regulator 16.

  At timing t2, the transfer contact 15b comes into contact with the b contact, and the voltage V11 of the first power source 11 having a high voltage is applied to the c contact of the transfer contact 15b, and the timing t3 slightly delayed from this. The voltage V11 is stabilized at the rated voltage V11max when the smoothing capacitor C11 is fully charged (stationary period T2 from timing t3 to t4). The magnitude of the rated voltage V11max is represented by an upward arrow in FIGS. 2 (a) and 2 (b).

  Next, when the first power supply 11 is stopped again at the timing t4, the voltage V11 of the first power supply 11 gradually decreases and becomes lower than the specified value Vth at the timing t5, and the Zener diode ZD11 becomes conductive. As a result of the transition from the state to the non-conducting state and the voltage across the biasing resistance element R12 disappearing, the relay driving transistor Q11 is turned off. Then, the drive coil 15a is not energized, the mechanical relay 15 reverses, and the transfer contact 15b that has been connected to the contact a on the first power supply 11 side becomes the contact b on the second power supply 12 side. Start the switching action. Even during this switching operation, the neutral state similar to the above occurs, but the power supply to the output terminal 24 is continued (timing t5) although the voltage level is slightly lowered by the action of the series regulator 16. ˜t6 transition period T3). At timing t6, the transfer contact 15b contacts the b contact, and the voltage V12 of the second power supply 12 is output instead of the voltage V12 ′ of the series regulator 16.

  The series regulator 16 operates only during the transition period T1 from timing t1 to t2 and the transition period T3 from timing t5 to t6. That is, it is only a period during which the transfer contact 15b is in the neutral state during switching.

  In the above, the transfer contact 15b is inserted into the line connecting the first power supply 11 or the second power supply 12 and the output terminal 24, which has a substantial resistance value as compared with the diode in the conventional example. Therefore, the power loss in the power supply switching circuit A is greatly reduced. In addition, as a condition for switching the power supply main body from the second power source 12 to the first power source 11, the voltage V11 of the first power source 11 by the voltage determination circuit 14 exceeds the specified value Vth. There is no need to be affected by the state of the 12 voltages V12. That is, even when the second power source 12 is a battery having relatively poor voltage accuracy and outputs a voltage higher than that of the first power source 11, the second power source 12 changes to the first power source 11. Therefore, it is not necessary to use a step-down circuit that is disadvantageous in terms of downsizing and cost, and can be easily handled.

  Further, since the series regulator 16 is provided, even when the mechanical relay 15 in which the neutral state is generated as the switch means is employed, it is possible to prevent the output voltage Vout from dropping suddenly and causing the power supply stop or power supply deterioration to occur. 13 can be stably supplied with power.

  It should be noted that a contactless semiconductor relay having no moving parts may be used as the switch means instead of the mechanical relay 15, and high-speed switching can be expected with higher reliability than the mechanical relay. One example is a semiconductor relay (photo MOS relay).

  FIG. 3 is a circuit diagram showing a configuration of a power supply switching circuit according to another embodiment in which a photo MOS relay is used as the switch means. In FIG. 3, the same reference numerals as those used in FIG. 1 of the previous embodiment denote the same components, and detailed description thereof will be omitted. A characteristic part in this embodiment is a photo MOS relay 26. The photo MOS relay 26 includes a normally closed type photo MOS relay 26a and a normally open type photo MOS relay 26b. In the normally closed type photo MOS relay 26a, the photo MOS switch Q21 is turned on when the light emitting diode LED1 is turned off, and is cut off when the light emitting diode LED1 is turned on. In the normally open type photo MOS relay 26b, the photo MOS switch Q22 is cut off when the light emitting diode LED2 is turned off and is turned on. The two light emitting diodes LED1 and LED2 are connected in series, the anode of the light emitting diode LED1 is connected to the resistance element R11 in the voltage determination circuit 14 (node N1), and the cathode of the light emitting diode LED2 is the collector of the transistor Q11 in the voltage determination circuit 14 (Node N2). The high potential side terminal of the photo MOS switch Q21 is connected to the input terminal 22 on the second power supply 12 side (node N3), and the low potential side terminal is connected to the high side output terminal 24 (node N4). ). The high potential side terminal of the photo MOS switch Q22 is connected to the input terminal 21 on the first power supply 11 side (node N1), and the low potential side terminal is connected to the high side output terminal 24 (node N4). ). The low potential side terminal of the photo MOS switch Q21 and the low potential side terminal of the photo MOS switch Q22 are connected in parallel to the high side output terminal 24.

  4 and 5 show detailed circuit configurations and operation modes of the normally closed type photo MOS relay 26a and the normally open type photo MOS relay 26b. FIG. 4 shows a light-off state, and FIG. 5 shows a light-on state. The nodes N1 to N4 correspond in FIG. 3, FIG. 4, and FIG. Each of the photo MOS switches Q21 and Q22 includes two MOS-FETs Q21a and Q21b (Q22a and Q22b) whose gates are connected in common, and body diodes D21a and D21b (D22a, D22b) between the source and drain of each MOS-FET. D22b), photovoltaic cells SC21 and SC22 as photoelectric conversion elements, and diodes D21c and D22c and thyristors TH21 and TH22 that form a discharge path.

  Next, the operation of the power supply switching circuit of the embodiment of FIG. 3 will be described.

  When the first power supply 11 having a high voltage is in a stopped state before the timing t0 in FIG. 2, the voltage determination circuit 14 does not operate, and both the light emitting diodes LED1 and LED2 are turned off in the photoMOS relay 26. The normally closed type photo MOS-FET Q21 is in a conductive state, and the normally open type MOS-FET Q22 is in a non-conductive state. Therefore, power is supplied to the load circuit 13 from the second power supply 12 having a low voltage. In FIG. 4, the photocells SC21 and SC22 are inactive, the thyristors TH21 and TH22 are in a conductive state, and the gates of the normally closed MOS-FET Q21 and the normally open MOS-FET Q22 are both biased. There is no state.

  Next, when the first power supply 11 is activated at the timing t0, the voltage V11 appearing in the smoothing capacitor C11 gradually increases, and the voltage V11 increases until the cathode applied voltage of the Zener diode ZD11 exceeds the Zener voltage. ZD11 becomes conductive. As a result, the relay driving transistor Q11 becomes conductive. Then, as shown in FIG. 5, both the light emitting diodes LED1 and LED2 are lit, the normally closed photo MOS-FET Q21 is inverted to the non-conductive state, and the normally open photo MOS-FET Q22 is in the conductive state. Invert. In FIG. 5, the photocells SC21 and SC22 operate, the thyristors TH21 and TH22 are commutated off, the gate of the normally closed MOS-FET Q21 is set to “L” level by reverse bias, and the normally open MOS− The gate of the FET Q22 becomes “H” level with forward bias.

  In the middle of this switching (transition period T1 between timings t1 and t2), although it is a short time, the two photoMOS-FETs Q21 and Q22 in the photoMOS relay 26 are temporarily either the first or second. A neutral state is established in which the power supplies 11 and 12 are not connected. Then, as in the previous embodiment, a state occurs in which the series regulator 16 outputs to the output terminal 24 in the state of the voltage V12 ′ obtained by further reducing the voltage V12 of the second power supply 12 having a low voltage. That is, a neutral state occurs, and although the voltage level is slightly decreased, the power supply state to the load circuit 13 is continued.

  At the timing t2, the normally open MOS-FET Q22 becomes conductive (see FIG. 5), and the voltage V11 of the first power supply 11 having a high voltage is applied to the output terminal 24, which is slightly delayed. At the timing t3, the voltage V11 is stabilized at the rated voltage V11max when the smoothing capacitor C11 is fully charged (steady period T2 from timing t3 to t4).

  Next, when the first power supply 11 is stopped again at the timing t4, the voltage V11 of the first power supply 11 gradually decreases and becomes lower than the specified value Vth at the timing t5, and the Zener diode ZD11 becomes conductive. From the state to the non-conductive state, the relay driving transistor Q11 is turned off. As a result, the light-emitting diodes LED1 and LED2 are no longer energized, the normally closed MOS-FET Q21 is inverted to the conductive state, and the normally open MOS-FET Q22 is inverted to the non-conductive state (see FIG. 4). ). Even during this switching operation, the neutral state similar to the above occurs, but the power supply to the output terminal 24 is continued (timing t5) although the voltage level is slightly lowered by the action of the series regulator 16. ˜t6 transition period T3). At timing t6, the normally-closed MOS-FET Q21 is turned on, and the voltage V12 of the second power supply 12 is output instead of the voltage V12 ′ of the series regulator 16.

  As described above, even in the case of a semiconductor relay (photo MOS relay), a neutral state in which the normally closed MOS-FET Q21 and the normally open MOS-FET Q22 are both open can occur. The presence of the series regulator 16 is significant.

  The present invention provides a power supply switching circuit for selectively switching between a first power supply having a high voltage and a second power supply having a low voltage, while reducing the power loss and reducing the voltage accuracy of the second power supply. Even if the voltage is unexpectedly higher than the voltage of the first power supply, it is possible to reliably switch from the second power supply to the first power supply without using a step-down circuit that is disadvantageous in terms of downsizing and cost. It is useful as a technology to do.

DESCRIPTION OF SYMBOLS 11 1st power supply 12 2nd power supply 14 Voltage determination circuit 15 Mechanical relay (switch means)
15a Driving coil (switch driving means)
15b Transfer contact 16 Series regulator 21 Input terminal from first power supply 22 Input terminal from second power supply 24 Output terminal A Power supply switching circuit LH High side line LL Low side line LG Ground line Q11 Relay switching element (transistor )
Q12 Step-down switching element (transistor)
R12 Bias resistance element in voltage judgment circuit R13 Bias resistance element in series regulator ZD11 Constant voltage element (zener diode) in voltage judgment circuit
Constant voltage element (Zener diode) in ZD12 series regulator

Claims (5)

A power supply switching circuit for selectively switching a first power supply having a high voltage and a second power supply having a low voltage to output a selected voltage from an output terminal;
Switch means for switching connection of the input terminal from each power supply to the output terminal;
A voltage determination circuit connected to the input terminal on the first power supply side for detecting the voltage of the first power supply and instructing switching of the switch means when exceeding a specified value;
A power supply switching circuit comprising switch drive means for switching the switch means from the second power supply side to the first power supply side according to the switch switching instruction from the voltage determination circuit. The switch means includes a transfer contact for switching connection of an input terminal from each power supply to the output terminal, and the transfer contact from the side of the second power supply according to the switch switching instruction from the voltage determination circuit. A mechanical relay comprising a drive coil for switching to the first power source side;
The power supply switching circuit according to claim 1, further comprising a series regulator that bypass connects the transfer contact between the input terminal on the second power supply side and the output terminal.   The power supply switching circuit according to claim 1, wherein the switch means is a contactless semiconductor relay. The voltage determination circuit includes:
A constant voltage element connected via a biasing resistance element between the input terminal on the first power supply side and the ground line;
A control terminal is connected to a connection point between the constant voltage element and the bias resistance element, a high side terminal is connected to the drive side of the switch means, and a low side terminal is connected to the ground line. The power supply switching circuit according to any one of claims 1 to 3, comprising a switching element. The series regulator is
A step-down switching element in which a high-side terminal and a low-side terminal are connected in a bypass manner between an input terminal and an output terminal on the second power supply side;
A biasing resistance element connected between a high-side terminal and a control terminal of the step-down switching element;
The power supply switching circuit according to claim 2, further comprising a constant voltage element connected between a control terminal of the step-down switching element and a ground line.

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