Limiting circuit used in explosion-proof environment

Abstract

A limiting circuit for use in an explosion-proof environment, in particular for a power supply, comprising: a first reverse polarized Zener diode connected and configured to limit a voltage between two output terminals depending on a first breakdown voltage; a second reverse polarized Zener diode connected and configured to limit a voltage between the two output terminals depending on a second breakdown voltage; a semiconductor switch coupled in series with the second reverse polarized Zener diode to open or close depending on a switching signal; a first shunt resistor coupled in a load path and coupled with the first reverse polarized Zener diode and the second reverse polarized Zener diode to carry a load current and a current flowing through the first reverse polarized Zener diode and the second reverse polarized Zener diode; and a comparison device configured to generate the switching signal depending on a voltage drop across the first shunt resistor.

Description

Technical Field

[0001] The present invention relates to a technology for preventing overvoltage and overcurrent, in particular to a protection circuit for explosion-proof applications. The invention further relates to measures for reducing the maximum power consumption of this limiting circuit. Background Art

[0002] According to the standards IEC 60079-0 and IEC 60079-11, explosion proof applications particularly require limiting circuits. Such limiting circuits must be rated to maintain high power dissipation even in the event of a fault. Typically, common circuits utilize high power Zener diodes and high power resistors, which have a large volume and therefore occupy most of the available space in the device. In addition, the cost of these components is relatively high. In particular, Zener diodes have recently shown low availability and increased cost.

[0003] Current solutions to reduce the power consumption of limiting circuits are based on the use of transient open circuit voltage limiters, which apply SCRs, TRIACs and / or thyristors with avalanche behavior.

[0004] It is an object of the present invention to provide a limiting circuit complying with the standards required for explosion proof applications having a substantially reduced maximum power consumption. Summary of the invention

[0005] The object of the present invention is to provide a limiting circuit used in an explosion-proof environment to solve the above problems.

[0006] The present invention provides a limiting circuit for use in an explosion-proof environment, particularly for a power supply, comprising:

[0007] a first reverse polarized Zener diode connected and configured to limit a voltage between the two output terminals depending on a first breakdown voltage;

[0008] a second reverse polarized Zener diode connected and configured to limit the voltage between the two output terminals depending on a second breakdown voltage;

[0009] a semiconductor switch coupled in series with the second reverse polarized Zener diode and configured to be opened or closed depending on a switching signal;

[0010] a first shunt resistor coupled in a load path and coupled with the first and second reverse polarized Zener diodes to carry a load current and a current flowing through the first and second reverse polarized Zener diodes;

[0011] A comparison device is configured to generate the switching signal in dependence on a voltage drop across the first shunt resistor.

[0012] According to standard IEC 60079-11, Zener diodes are used that guarantee voltage limiting, the maximum value of which is defined by the nominal reverse voltage of the Zener diode and its tolerance. The maximum value of the voltage limiting of each Zener diode must be rated to dissipate 1.5 times the maximum power that can occur in the event of a fault. The maximum power dissipation of the Zener diode is defined by the Zener voltage x 1.7 x the breaking current of the circuit breaker, which results in the requirement for high-power Zener diodes with a power dissipation of typically 3 to 5 W.

[0013] Furthermore, according to the above standards, the output resistor connected in series to the circuit breaker must be rated to dissipate 1.5 times the maximum power that can occur in the event of a fault. Typically, it is necessary to use a high-power resistor with a power rating of 3 to 5 W. In order to reduce the power ratings of the Zener diode and the output resistor, it is found that the maximum power dissipation depends directly on the maximum value of the voltage limitation. By reducing the maximum value, the current flowing into the voltage limiting circuit part and / or the current limiting circuit part can be reduced.

[0014] It may be provided that the limiting circuit is connected to the input terminals of the power supply via a circuit breaker.Furthermore, a first reverse polarized Zener diode may be coupled in series with the first shunt resistor, and a series connection of a semiconductor switch and a second reverse polarized Zener diode is coupled in parallel with the first reverse polarized Zener diode.

[0015] Therefore, the above-mentioned limiting circuit is basically connected to the input terminal of the power supply through a circuit breaker and includes a first reverse polarized Zener diode and a second reverse polarized Zener diode. The first reverse polarized Zener diode is connected in series with the shunt resistor and is connected between the first input terminal and the second input terminal via the circuit breaker. The series connection of the second reverse polarized Zener diode and the semiconductor switch is provided in parallel with the first reverse polarized Zener diode. Depending on the switching signal (e.g., the switching voltage), the semiconductor switch is normally open (high conductivity) and normally closed (low conductivity). The switching signal is generated depending on the node voltage on the node between the first reverse polarized Zener diode and the shunt resistor.

[0016] The Zener diodes are connected with reverse polarity so that they can substantially limit the voltage to an inherent breakdown voltage. The switching of the switch is made dependent on a comparison result in a comparison device that compares a node voltage of a node between the first reverse polarized Zener diode and the shunt resistor with a trip voltage. If the node voltage exceeds or experiences a given threshold voltage given as the trip voltage, the comparison device closes (becomes conductive) the semiconductor switch so that a second reverse polarized Zener diode is connected in parallel with the first reverse polarized Zener diode.

[0017] The breakdown voltage of the second reverse polarized Zener diode is set lower than the breakdown voltage of the first reverse polarized Zener diode so that the voltage across the Zener diode decreases once the node voltage changes due to an excess input voltage or decreases if an excess load current occurs.

[0018] Overvoltage protection is established by a Zener diode. Overcurrent protection is provided so that when the current flowing through the shunt resistor causes a voltage drop across the shunt resistor so that the node voltage exceeds or experiences a trip voltage of the comparison device, a switching signal is generated to close the semiconductor switch. Typically, the comparison device switches, i.e. changes its output controlling the switch, when the voltage at the node exceeds or experiences a trip voltage. The current trip value corresponds to the trip voltage divided by the shunt resistance.

[0019] A general characteristic of the circuit system for voltage-current relationship described above is that the output voltage is the breakdown voltage corresponding to the first reverse polarized Zener diode for an output current from 0 to the current trip value defined by the shunt resistor and the comparison device, and is the breakdown voltage corresponding to the second reverse polarized Zener diode for a load current above the current trip value. This results in the maximum power dissipation of the first reverse polarized Zener diode being the breakdown voltage multiplied by the current trip value, and the maximum power dissipation of the second reverse polarized Zener diode being the breakdown voltage of the second reverse polarized Zener diode multiplied by 1.7 multiplied by the trip current of the circuit breaker. The maximum power dissipation of the output resistor is the output resistance multiplied by the square of the current trip value or the square of the breakdown voltage of the second reverse polarized Zener diode divided by the output resistance, whichever is greater.

[0020] The current trip value can be set between the maximum functional operating current required by the load and the nominal value of the circuit breaker. This allows to substantially reduce the maximum power consumption of the limiting circuit.

[0021] The comparison means may be provided as a comparator with a set trip voltage, wherein the switching signal is provided as a digital output signal of the comparator.

[0022] In an alternative embodiment, the comparison means may be provided by a bipolar transistor, with the trip voltage being defined by the forward bias necessary for conduction of the current base-emitter path, which is typically 0.6 to 0.7 V at ambient temperature. Since the base-emitter voltage is highly dependent on the junction temperature, temperature compensation using an NTC resistor with a negative temperature coefficient in parallel with the base-emitter path may be provided.

[0023] Another embodiment of a comparative device is to use a shunt voltage regulator, which has the advantage of being more accurate and stable over temperature, but typically has a higher trip voltage of greater than 1.2V and is more expensive.

[0024] According to an embodiment, a first reverse polarized Zener diode may be connected in series with a second shunt resistor and the first shunt resistor such that the load current flows only through the first shunt resistor and not through the second shunt resistor, wherein the comparison device is connected to a node between the first reverse polarized Zener diode and the second shunt resistor. Here, the voltage is limited to a value defined by the breakdown voltage of the first reverse polarized Zener diode and the voltage drop across the second shunt resistor.

[0025] According to another embodiment, the first reverse polarized Zener diode can be connected in series with a sixth resistor, wherein the node between the first reverse polarized Zener diode and the sixth resistor controls a transistor, which is configured to drive a further semiconductor switch, in particular a MOSFET, wherein the drain-source path of the further semiconductor switch is coupled between the input terminals.

[0026] Specifically, a series connection of a second reverse polarized Zener diode and a semiconductor switch is coupled to a node between the first reverse polarized Zener diode and the sixth resistor.

[0027] Furthermore, the further semiconductor switch is connected in series with the second shunt resistor and the first shunt resistor such that the load current flows only through the first shunt resistor and not through the second shunt resistor, wherein the comparison device is connected to a node between the first reverse polarized Zener diode and the second shunt resistor.

[0028] The proposed solution involves the use of a high-power Zener diode. Due to the low spatial volume, another adaptation of the above circuit system can be made to imitate the behavior of the Zener diode by simply replacing the high-power Zener diode as previously described. The Zener diode can be replaced by a diode replacement circuit, which is connected to the terminals of the high-power Zener diode. The diode replacement circuit includes another semiconductor switch, such as a MOSFET, a bipolar transistor, a first low-power Zener diode, and a fifth and sixth resistor. The further semiconductor switch simulates the current path through the replaced first reverse polarized Zener diode and is normally open. The further semiconductor switch will be closed once the threshold voltage at the output terminal corresponding to the breakdown voltage of the simulated Zener diode is exceeded. This is determined by the series connection of the sixth resistor and the first low-power Zener diode, wherein the node between the sixth resistor and the first low-power Zener diode is coupled to the control input of the bipolar transistor, and the base-emitter path of the bipolar transistor can be connected in parallel with the sixth resistor. The emitter-collector path of the bipolar transistor is connected in series with the fifth resistor, wherein the node between the path and the fifth resistor is coupled to the control terminal of the further semiconductor switch.

[0029] The breakdown voltage of the high power Zener diode is thus replaced by a threshold voltage defined by the breakdown voltage of the first low power Zener diode plus the base emitter voltage of the transistor. Once the output voltage exceeds the threshold voltage, the further semiconductor switch is driven to form a conductive path, as is the case with an overvoltage at the first high power Zener diode.

[0030] A second low-power Zener diode is connected in series with the semiconductor switch and a third resistor, which already holds some voltage when the semiconductor switch is turned on, so that the breakdown voltage of the second low-power Zener diode can be selected to be significantly lower than the breakdown voltage of the second high-power Zener diode. The power consumption of this diode replacement circuit is mainly absorbed by the MOSFET.

[0031] In order to reduce the value of the output voltage for the two different levels of the current trip value, a second low impedance shunt resistor can be added in series with the source of the further semiconductor switch (MOSFET) of the diode replacement circuit, which shunt resistor is also connected to the input of the comparison device. The comparison device obtains a voltage equal to the sum of the product of the resistance value of the first shunt resistor multiplied by the sum of the load current and the current through the further semiconductor switch and the shunt resistance of the second shunt resistor multiplied by the current through the further semiconductor switch.

[0032] As is evident from the equation, the current from the voltage emitter has a greater influence than the current from the current limiter (IO). This condition permits reducing the maximum power dissipation of the voltage limiter when the current from the voltage limiter is less than the current trip value. Basically, the resistance value of the second shunt resistor must be very low, for example below 10Ω, to avoid reducing the capacity of the voltage limiter to sustain the high surge currents mainly discharged by the high power MOSFET. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The embodiments are described in more detail with reference to the accompanying drawings, in which:

[0034] Figure 1 To schematically illustrate the basic circuit system of the limiting circuit;

[0035] Figure 2 The characteristic of the output voltage varying with the sum of the voltage limit current and the load current;

[0036] Figure 3 is a schematic diagram of another embodiment of a limiting circuit;

[0037] Figure 4 is a schematic diagram of another embodiment of a limiting circuit;

[0038] Figure 5 is a schematic diagram of another embodiment of a limiting circuit; and

[0039] Figure 6 FIG. 4 is a schematic diagram of another embodiment of a limiting circuit. DETAILED DESCRIPTION

[0040] Figure 1 Schematic diagram showing the basic principle of the limiting circuit. Figure 1 A power supply 1 is shown, which has a transformer T1 and an AC / DC converter 2 to provide an input voltage V between two input terminals E1, E2. 输入 . Input voltage V 输入 Used to drive the load R between output terminals A1 and A2 负载 One input terminal E1 is provided with a circuit breaker F1 , for example a fuse.

[0041] In addition, the limiting circuit 3 is basically provided when the input voltage V is applied 输入 The circuit breaker F1 is connected between one input terminal E1 and the limiting circuit 3.

[0042] The limiting circuit 3 is described in more detail. A first high power Zener diode DZH is provided in series with a first shunt resistor R1shunt between the high potential input terminal E1 and the low potential input terminal E2. The first high power Zener diode DZH is reverse polarized and connected so that the cathode is connected to the high potential input terminal E1. Connected in parallel with the first high power Zener diode DZH is a series connection of a semiconductor switch S and a second high power Zener diode DZL.

[0043] The switch S may be formed as a semiconductor switch, for example, by a MOSFET or the like, which is set to be normally open and controllable by a switching signal C of a comparator 31 as a comparison device. The switching signal C may be coupled to the switch S via a second resistor R2. The inverting input of the comparator 31 may be coupled to a node between a first high-power Zener diode DZH and a first shunt resistor R1shunt via a high-impedance first resistor R1. A fixedly set trip voltage may be applied at the non-inverting input of the comparator 31. The trip voltage V 跳脱 Can be set to I 跳脱 x R1shunt, where I 跳脱 can be set between the maximum functional operating current required to operate the load and the nominal value I of the circuit breaker F1 熔断器 Between. 熔断器 is the breaking current of circuit breaker F1.

[0044] The reverse breakdown voltage of the second high power Zener diode DZL is selected to be lower than the reverse breakdown voltage V of the first high power Zener diode DZH. DZHBasically, the output voltage of the power supply 1 is first limited by the first high power Zener diode DZH. For current limiting, the first shunt resistor R1shunt conducts the load current plus the voltage limiting current through the first high power Zener diode DZH and the second high power Zener diode DZL. The current flowing through the first resistor R1 and the second resistor R2 can be regarded as negligible due to the high impedance of these resistors compared to the low impedance of the shunt resistors.

[0045] Figure 2 Show Figure 1 The behavior of the circuit. Figure 2 The output voltage U is shown as a function of the current through the first shunt resistor R1shunt. o The voltage limit, the current is the output current I o and the voltage limiting current I through the first high power Zener diode DZH Z It can be seen that the maximum power consumption of the first high-power Zener diode DZH is the breakdown voltage V DZH Multiply by the trip current I 跳脱 =V 跳脱 / R1shunt(V DZH x I 跳脱 ), and the maximum power consumption of the second high power Zener diode DZL corresponds to the breakdown voltage V DZL Multiply by 1.7 times the breaking current I of circuit breaker F1 熔断器 (V DZL x1.7 x I 熔断器 ).

[0046] In addition, the current limiting resistor R O can be provided in series with circuit breaker F1, where the limiting circuit is connected to the output resistor R O The maximum power dissipation in the output resistor is R o x I 跳脱 x I 跳脱 or V DZL x V DZL / R O , whichever is greater. Trip current I 跳脱 The value of can be set to the threshold of the comparator 31 divided by the resistance value of the first shunt resistor R1shunt. 跳脱 The value of can be set between the maximum functional operating current required by the load Rload and the nominal value of the circuit breaker F1.

[0047] exist Figure 2 As can be seen, the limiting circuit 3 and the current limiting resistor R OThe maximum power consumption can be greatly reduced compared with existing technology solutions.

[0048] Figure 1 The circuit provides voltage and current limiting. The voltage limiting is provided by the first high power Zener diode DZH, which limits the voltage to the breakdown voltage of the first high power Zener diode DZH. The current limiting is provided by the first shunt resistor R1shunt, which is in the current path of the load Rload and in the current path of the first high power Zener diode DZH, so that the combined current (I O +I Z ) exceeds the current trip value of the output switching state of the comparator 31, thereby closing the switch S to connect the second high power Zener diode DZL in parallel with the first high power Zener diode DZH.

[0049] like Figure 3 As shown in , the switch S can be realized by a P-MOSFET transistor TR1, which is connected to the comparator 31 via a second resistor R2. Therefore, the drain-source path of the P-MOSFET is connected in series with the second high-power Zener diode DZL. The source terminal and the gate terminal of the P-MOSFET are interconnected with a third resistor R3 having a high impedance of several kΩ.

[0050] Alternatively or additionally, Figure 3 As shown in FIG. 1 , the comparator 31 can be implemented by a bipolar NPN transistor TR2, wherein the trip voltage V 跳脱 is defined by the forward bias necessary for conduction of the base-emitter path, which is typically between 0.6 and 0.7 volts at ambient temperature. Since the base-emitter voltage is highly dependent on the junction temperature, it is convenient to connect a temperature compensating NTC resistor (NTC: Negative Temperature Coefficient) between the base and emitter terminals of TR2. Therefore, the value of the first shunt resistor R1shunt must be chosen as R1shunt=0.6V / (I 跳脱 ).

[0051] like Figure 4 As shown in FIG. , the comparator 31 may alternatively be implemented as a shunt voltage regulator 31 ′, which has the advantage of being more accurate and stable over temperature, but provides a trip voltage V 跳脱 A higher value, such as 1.2 volts.

[0052] Due to the low availability of high power Zener diodes, Figure 5 Shown based on Figure 3A method of an embodiment of the present invention, wherein the function of a high-power Zener diode DZH is obtained by a sub-circuit 4 providing the same electrical voltage limiting characteristics. The sub-circuit 4 mimics the behavior of the high-power Zener diode DZH and can be evaluated as a single component connected by cathode and anode terminals. The sub-circuit 4 may include the following components: a first low-power Zener diode DZ1H, a high-power N-MOSFET TR4, a fifth resistor R5, a sixth resistor R6, a semiconductor switch S (e.g., MOSFET), and a bipolar PNP transistor TR3. The breakdown voltage of the replaced high-power Zener diode is defined by the breakdown voltage of the low-power Zener diode DZ1H and the base-emitter voltage of the bipolar PNP transistor TR3. The forward voltage of the high-power Zener diode DZ1H is defined by the inherent part of the body diode of the high-power N-MOSFET TR4. The high-power N-MOSFET TR4 defines high power dissipation according to its situation, its mounting conditions and its maximum junction temperature. The functionality is defined by the same components as described for the first high power Zener diode DZH, so the components of the second low power Zener diode DZ2L and the P-MOSFET TR1 operate as a switch S.

[0053] exist Figure 5 In the circuit, the drain terminal of the high power MOSFET TR4 is connected to the input terminal E1 (via the circuit breaker F1), and the source terminal of the high power MOSFET TR4 is connected to the trip voltage V 跳脱 The emitter terminal of the bipolar PNP transistor TR3 is connected to the drain terminal of the N-MOSFET TR4, and the collector terminal is connected to the gate terminal of the N-MOSFET TR4. The gate terminal of the N-MOSFET is also connected to the second output terminal with the fifth resistor. The base terminal of the bipolar transistor TR3 is connected to the cathode of the first low-power Zener diode DZ1H. The anode of the first low-power Zener diode DZ1H is connected to the second output terminal A2. The base terminal of the bipolar transistor TR3 is further connected to the source terminal of the P-MOSFET forming the switch S, and the drain terminal of the P-MOSFET is connected to the cathode of the second low-power Zener diode DZ2L. The anode of the second low-power Zener diode is also connected to the second output terminal A2. The source terminal of the P-MOSFET is connected to the base terminal of the transistor together with the sixth resistor.

[0054] When the sum of the voltage limit current and the load current is higher than the trip current value I 跳脱 When the comparator 31, TR2, 31' drives the switch S, TR1 to start the second low power Zener diode DZ2L to reduce the output voltage U O Output voltage U OIt is now defined by the breakdown voltage of the second low power Zener diode DZ2L and the base emitter voltage of the bipolar PNP transistor TR3.

[0055] In addition, if Figure 6 As shown in O The value of can be additionally reduced by two different levels for tripping current value. Here, a low impedance second shunt resistor R2shunt can be connected in series to the source of the N-MOSFET TR4 so that the source terminal remains directly connected to the input of the comparison device 31, TR2, 31', which is the high resistance first resistor R1. The comparison device 31, TR2, 31' receives an input voltage, namely V comp =R1shunt x(I o +I z )+R2 x I z From this equation, it is clear that the current from the voltage limiting circuit is proportional to the current from the load R 负载 This allows the voltage to limit the current to less than the trip current value I 跳脱 This further reduces the maximum power dissipation of the voltage limiting circuit.

Claims

1. A limiting circuit for use in explosion-proof environments, particularly for power supplies, characterized in that: include: a first reverse polarized Zener diode (DZH, DZ1H) connected and configured to limit the voltage between the two output terminals (A1, A2) according to a first breakdown voltage; a second reverse polarized Zener diode (DZL, DZ2L) connected and configured to limit the voltage between the two output terminals (A1, A2) according to a second breakdown voltage; a semiconductor switch (S) coupled in series with the second reverse polarized Zener diode (DZL, DZ2L) and configured to be opened or closed according to a switching signal; a first shunt resistor (R1shunt) coupled in a load path and coupled with the first reverse polarized Zener diode (DZH, DZ1H) and the second reverse polarized Zener diode (DZL, DZ2L) to carry the load current and a current flowing through the first reverse polarized Zener diode and the second reverse polarized Zener diode (DZH, DZ1H); A comparison device (31, TR2, 31') is configured to generate the switching signal according to the voltage drop across the first shunt resistor (R1shunt).

2. The limiting circuit according to claim 1, characterized in that: Connect to the input terminals of the power supply through a circuit breaker.

3. The limiting circuit according to claim 1 or 2, characterized in that: A breakdown voltage of the second reverse polarization Zener diode (DZL, DZ2L) is set lower than a breakdown voltage of the first reverse polarization Zener diode (DZH, DZ1H).

4. The limiting circuit according to claim 3, characterized in that: The comparison device (31, TR2, 31') comprises a comparator (31), a bipolar transistor (TR2) or a shunt voltage regulator (31') combined with an NTC resistor for temperature compensation.

5. The limiting circuit according to claim 4, characterized in that: The first reverse polarized Zener diode (DZH, DZ1H) is coupled in series with the first shunt resistor (R1shunt), and the series connection of the semiconductor switch (S) and the second reverse polarized Zener diode is coupled in parallel with the first reverse polarized Zener diode (DZH, DZ1H).

6. The limiting circuit according to claim 5, characterized in that: The first reverse polarized Zener diode (DZH, DZ1H) is connected in series with a second shunt resistor (R2shunt) and the first shunt resistor (R1shunt), so that the load current flows only through the first shunt resistor (R1shunt) and not through the second shunt resistor (R2shunt), wherein the comparison device (31, TR2, 31') is connected to a node between the first reverse polarized Zener diode (DZH, DZ1H) and the second shunt resistor (R2shunt).

7. The limiting circuit according to claim 4, characterized in that: The first reverse polarized Zener diode is connected in series with a sixth resistor (R6), wherein a node between the first reverse polarized Zener diode (DZH, DZ1H) and the sixth resistor (R6) controls a transistor (TR3), which is configured to drive a further semiconductor switch (TR4), in particular a MOSFET, the drain-source path of the further semiconductor switch being coupled between the input terminals (E1, E2).

8. The limiting circuit according to claim 7, characterized in that: A series connection of the second reverse polarized Zener diode (DZL, DZ2L) and the semiconductor switch (S) is coupled to the node between the first reverse polarized Zener diode (DZH, DZ1H) and the sixth resistor (R6).

9. The limiting circuit according to any one of claims 7 or 8, characterized in that: The other semiconductor switch is connected in series with a second shunt resistor (R2shunt) and the first shunt resistor (R1shunt) so that the load current flows only through the first shunt resistor (R1shunt) and not through the second shunt resistor (R2shunt), wherein the comparison device (31, TR2, 31') is connected to the node between the first reverse polarized Zener diode (DZH, DZ1H) and the second shunt resistor (R2shunt).

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