FI107848B - Coupling arrangement for reducing transients caused by an electromechanical coupling with overcurrent protection - Google Patents

Description

107848

Switching arrangement for reducing transients caused by electromechanical switch with overcurrent protection

FIELD OF THE INVENTION This invention relates to attenuation of transients caused by an electromechanical switch with overcurrent protection and to overcurrent protection of the switch.

BACKGROUND OF THE INVENTION Many electrical systems utilize a circuit board which supplies external operating voltages and has filters for filtering the operating voltages. In addition to the various electrical circuits and connectors available, the card can be fitted with a manual switch that switches the supply voltage on and off to an external load. The load may be located 15 away from the circuit board. When you place this type of card in a frame or case, the circuit breakers will remain outside the case so that they are easy to operate. Cards can be placed in a rack or case in several • φ: ·. Cain. The usual circuit board use is in power supplies.

Figure 1 is a rough illustration of the above card scheme. 20 heifers. A positive and negative operating voltage + V and -V from an external voltage source is applied to the circuit board 1 delimited by the dashed line. Voltages can be supplied via the; · *; * connectors, which connects the connectors on the card to the connectors on the case when the card is installed. The circuit board may further include various electronic circuits for performing various functions, generally shown in the figure by reference numeral 6.

Filters 2 and 3 are placed on the circuit board for DC operating voltages: to filter out any exchange components and transients • # ·; * ··. In practice, the situation is almost always such that the filters 2 and 3 .. * · are specifically dimensioned not only for the characteristics of the DC supply but also for the electrical circuits 6, with the potential s · of their inductances.

'··· * the effect on the filter parameters is taken into account. Instead of filters:. · *; the design could not take due account of the switch 4 provided with an internal overcurrent protection j \ f, which is also mounted on the circuit board. This • · switch switches the voltage to the external load 5 and switches 35 off the load accordingly. The load may be, for example, an electric motor or some substantially resistive load controlled by a manual switch 2 107848 4 on the card. Depending on the nature of the load, a suitable commercially available switch shall be selected and mounted on a circuit board. The selection criteria are mainly rated load current and maximum overcurrent. The switch must thus withstand a certain continuous rated current and must have an internal overcurrent breaker which will open the load current of the 5 switches at the maximum allowable overcurrent. The circuit breaker uses a magnetic overcurrent protector with a "current sensing coil", ie a relatively large inductance through which the load current passes. When the current exceeds a certain value higher than the rated current, the coil-induced magnetomotor force forces the switch to open.

10 The problem with using the switch is the high transient voltages caused by its opening. They interfere with the operation of the electronic circuits on the circuit board, propagate to the DC network fed through the filters, and, in addition, cause interference with the operation of nearby electronic circuits as radio frequency signals. Since the inductances of different types of switches cannot be taken into account in the design of the filters, it happens that the inductance of the switch to some extent reduces the inductances of the filters ... effect, resulting in an increase in transients. Electromagnetic Compatibility (EMC) Regulations, to which European countries are committed, oblige equipment designers to calculate '20 product interference levels so that products comply with EMC regulations. Another problem with · *. '··' electromechanical switches is that the overcurrent trip current limit · *. ·. · Is not accurate due to tolerances.

•• A: However, a reduction in the level of interference in a structure like Figure 1 is very difficult to accomplish. Of course, careful dimensioning of the filters can greatly reduce the transients caused by a particular switch, but this would require the use of a specific switch for each switch. . ·. This is an uneconomic solution. One might also think. moving the switch away from the circuit board, but doing so will increase the inductance, which will further increase the transients. It would be desirable to move the switch • ·: * · 30 remotely, since it will be easier to position the switch with good ergonomics and layout design of the circuit board: '·' would be freer. So far, however, no * · * · * 'solution to the above problem has been found.

It is therefore an object of the present invention to provide a switch which, when the mechanical switch 35 is closed, significantly reduces switching transients and can set a precise overcurrent limit at which the switch opens to isolate the load from the circuit.

The set object is achieved by a switching arrangement as defined in the independent claim.

5

BRIEF SUMMARY OF THE INVENTION A controllable semiconductor switch is placed adjacent to the electromechanical switch, whereby the total load current is distributed to each switch. Its control voltage is formed by reference circuits.

10 The first comparator circuit monitors the current flowing through the electromechanical switch and compares it to a reference value. As soon as the current flowing through the switch has reached the reference value, the first comparator circuit provides a control which opens the semiconductor switch, i.e. sets it to the conductive state. As a result, the rate of rise of current through the switch is substantially reduced because a large portion of the total load current is passed through the semiconductor switch. The function of the first comparator circuit ... is to detect as quickly as possible the electromechanical switch «, *. 'After closing, the rising current flowing therethrough and transferring the bulk of this • e \ l current and hence the total load current to the semiconductor switch. Therefore, the * · · * · '' 20 reference value must be as small as possible.

The second reference circuit monitors the total load current and compares it to another reference value, which is set to the maximum load current value at which the load must be disconnected from the voltage source. When the total load current exceeds the reference value, the second comparator circuit provides control which immediately closes the semiconductor switch, i.e. sets it to a non-conductive state. The portion of the total load current that has passed the semiconductor switch. ·. will now pass through the electromechanical switch, whereby the current through its * · /•'.I will increase step by step so that the internal overcurrent protection of this switch operates immediately and • opens the switch.

• ·; * · 30 The control voltages of the reference circuits cause the semiconductor switch to · · *. '* Not conduct when the electromechanical switch is in the open position or; * · 'The maximum load current set value is exceeded but results almost immediately • · «* Γ *! after closing the electromechanical switch. In this way, when the switch is normally closed, the transient 35 caused by the high current rise rate is greatly reduced because the current cannot rise to a high value at all.

4, 107848

The coupling according to the invention has other advantages besides reducing transients. Since the current limit for the overcurrent opening is determined by the first reference value instead of the internal layout of the electromechanical switch, the current limit can be precisely set as desired. When the semiconductor switch draws a large portion of the load current, an electromechanical switch with a low rated current can be used, however, the overcurrent limit of the overcurrent protection can be set higher than the internal overcurrent limit of the minimum rated current switch. Thus, only one size switch can be used for loads of different sizes. A further advantage is that the positioning of the electromechanical switch on the circuit board 10 is no longer critical and can be located at any desired position even away from the circuit board.

List of figures

The invention will now be described in more detail with reference to the accompanying schematic drawings, in which Fig. 1 shows a prior art load control ... Fig. 2 illustrates a switching arrangement according to the invention, Fig. 3 illustrates the input and output states of the reference circuits, and Fig. 4 illustrates a practical embodiment of the circuit of Fig. 2.

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Detailed Description of the Invention

The circuit shown in Fig. 2, where applicable, uses the same reference numeral as Fig. 1, in which the essential parts of the invention are a load 5 and an electromechanical switch 4 which switches the load to + V - -V. In Figure 2, the switch is located outside the circuit board 1.

. ... An on / off controlled semiconductor switch ♦ · .. *. *. * 21 is located on the circuit board and is electrically connected to the electromechanical switch 4.

• «*. . The current flowing through the electromechanical switch 4 is measured by lswitCh (or • ♦: '30 proportional magnitudes) measured by the switch current measurement circuit 24 and measured.' *** The current value is applied to the first comparator circuit 25 which compares this current and: the first reference current (or • ·! proportional magnitude) lref, min to each other. If the current through the electromechanical switch 4 exceeds the reference value, i.e. I switch> lref, min>> 35, the output of the first comparator circuit in the "on" state controls the semiconductor switch 5 to 107848 21 to the conductive state. Otherwise, the output of the first comparator circuit 25 is in a state where the semiconductor switch 21 receives control "off".

The second comparator circuit 23 operates in a similar manner. The load current measurement circuit 22 measures the total current passing through load 5, ITot, or 5 proportional to it. The total current measured ITot is applied to a second comparator circuit 23, which compares this current and the reference current (or its magnitude) lref, max acting at the other input of the comparator circuit. If the total current exceeds this reference value, i.e. I> i> lref, max, the output of the second comparator is in the state ofT, which controls the semiconductor switch 21 to a non-conducting state. Otherwise, the output of the second comparator circuit 23 is in a state where the semiconductor switch 21 receives control "on".

The operation of the connection according to Fig. 2 will be explained with reference to Figs. Before the voltage is applied to the load 5 by the electromechanical switch 4, the lswitCh is zero so that the first comparator circuit 25 keeps the semiconductor switch 21 non-conducting. The total current ITOs is also zero, with the output of the comparator circuit 23 being in the "on" state. The outputs of the reference circuits are coupled to the control input of the semiconductor switch such that the output j of either circuit. · When off switch in non-conductive mode.

«»: 'At t = 0, when the electromechanical switch 4 is turned on, * ·: The current of 20 switches lSwitch starts to rise rapidly from 0 at the rate of rise of the circuit inductances • V ·. After a short time ΔΤ, Fig. 3c, it is: .j. has reached the reference value lref, min of the first comparator circuit 25, which is thus: Y small value as possible. In practice, it is easier to compare voltages instead of currents, so that voltage referents Uref, min and Uref, max · are plotted instead of currents in the graphs. When the current reaches the reference value ΔΤ, the output state of the first comparator circuit changes from state to off. · "On", Figure 3d, as a result of which the semiconductor switch 21 is guided to a conductive * · state. Most of the load current is now channeled to the semiconductor switch • * *. . branch because the branch inductances are much smaller than the branch inductances of the electromechanical switch 4. The directing of the load current to the semiconductor switch 21 almost immediately upon closure of the electromechanical switch: Y 4 effectively prevents the transients otherwise caused by the • ♦ switch.

• * * * * Thereafter, a steady state is achieved in which the major part of the load current 35 Iit passes through the semiconductor switch and a smaller part through the electromechanical switch 6 107848. In this way, a switch with a lower rated current can be used than what should be used without the parallel semiconductor switch.

Next, suppose that the current ITot taken by load 5 at the moment ti starts to rise for some reason. The cause may be a partial short circuit in the load or, in the case of a motor, an overload condition. The second comparator circuit 23, which monitors the total current Ito, detects that at t2 it reaches a reference value Uref, max (= Iref. Max), Fig. 3a. The reference value is set to correspond to the maximum allowable current value for load 5. At the same time t2, the output state of the second comparator circuit changes from an "on" to an "off", Figure 3b. The load current Itot value is currently greater than the current limit value of the internal overcurrent protection of the switch, so the overcurrent protection operates almost immediately by disconnecting the load from circuit 5. The lSWitch goes to zero so the output of reference circuit 25 In this way, the current limit value of the overcurrent protector can be set with the reference value of the second reference circuit, whereby a smaller,,,. switch with internal overcurrent protection than what should be used without the parallel * m ^ · semiconductor switch.

* 4 'Finally, Figure 4 shows a practical connection that implements ·· «

20 according to the principle of the invention shown in Figure 2. From the terminals + V and -V

«• V · provides the filtered positive and negative voltages between which the load (not shown) is connected by an electromechanical switch 4. The paths of the load current: V are represented by dashed lines. The gate voltage of the FET transistor 41 adjacent to the switch is controlled by a comparator C1 of the second comparator circuit and a comparator C2 of the first comparator circuit whose outputs are directly connected to the gate via resistor R9. The gate receives a positive pull-up voltage via resistor R10 from transformer T, which converts the supply voltage to + V (e.g.

* · 48 V) suitable for operating comparators U, eg 15 V.

'', The measurement of load current I is converted to a voltage measurement which measures the voltage caused by the load current • · f '· 30 across resistor R6. The second comparator circuit: ** 'comprises a comparator C1 which is positively feedbacked by resistor R11 to provide hysteresis. Circuit Positive Input • *. effective constant reference voltage Uref, max formed by • * * · '· voltage divider R3-R4 from operating voltage U. Approximately: p 77 «c rcf, max

U

7 107848 where it is observed that the reference voltage is determined by the ratio of the resistors. The resistor R6 is then dimensioned after knowing the magnitude of the overcurrent I as follows: r, ^ ref .max * '= - r ~ 5 The value of resistor R6 is chosen to be as small as possible, for example 25 mQ. The reference voltage is then calculated and, with it, the voltage divider resistance values.

As the load current increases to the overcurrent side, the voltage at point P1 rises above the reference voltage, whereby the output voltage of comparator C1 oscillates from positive to negative operating voltage and the gate voltage of FET 41 decreases to close the FET.

The measurement of the current of the electromechanical switch is also in practice a voltage measurement, which measures the voltage caused by the very small current flowing through the switch over resistor R5 which is intended to conduct the FET of the first comparator circuit comparator C2. Let us denote this starting current lsw, mjn. The reference voltage of comparator C2 is then calculated from the formula: 8 107848

A switching arrangement for reducing switching transients caused by an electromechanical switch with overcurrent protection, characterized in that it comprises: 5 controllable semiconductor switches (21) disposed parallel to the electromechanical switch (4), whereby the load current (Itot) can be distributed to both switches having an input proportional to the current (lSwitch) passing through the electromechanical switch (4) and a minimum reference value (lref, mini Uref, min), the output of which is operatively coupled to the control input of the semiconductor switch (21) the value proportional to the current flowing through is greater than the minimum reference value.

A switching arrangement according to claim 1, characterized in that it further comprises a second comparator circuit (23) having inputs a value proportional to the load current (Itot) and a maximum reference value (lref, maX; Uref, max) and operatively connected to the semiconductor switch (21). ). to the control input by controlling the current of the semiconductor switch to a nonconductive state when the value proportional to the load current exceeds the maximum reference value.

Switching arrangement according to Claim 1, characterized in that the minimum reference value is the smallest reliably detectable value (1SWitch) of the current flowing through the electromechanical switch (4), whereupon, after closing of said switch (4), part of the load current is passed through · 25 (4) current transient (lSWitch) transients.

A switching arrangement according to claim 2, characterized in that the maximum reference value corresponds to the overcurrent value of the load current at which. . ·. the electromechanical switch must isolate the load from the supply voltage.

A switching arrangement according to claim 1, characterized in that the controllable semiconductor switch is an FET transistor.

A switching arrangement according to claim 1, characterized in that the first reference circuit is a comparator (C2) having a reference: X as a product of a voltage divider (R1-R2) and a second: · · · [ | 35 is the voltage drop caused by the 9 107848 current (Iswitch) passing through the electromechanical switch (4) in the measuring resistor (R5) in series with the switch (4).

A switching arrangement according to claim 1, characterized in that the second comparator is a comparator (C1) having a reference input 5 formed by a voltage divider (R3-R4) and a second input a voltage drop caused by an electromechanical switch (4) and FET in a measuring resistor (R6) in series with the transistor (41).

A switching arrangement according to claim 7, characterized in that the measuring resistor (R6) is connected to the input of the comparator (C1) via an input resistor (R7) and by changing the resistance value of the input resistor (R7) to set the overcurrent value .

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Claims (8)

107848 1. Coupling arrangement for reducing transients caused by an electromechanical coupling with overcurrent protection, characterized in that it includes: a controlled semiconductor coupler (21) positioned parallel to an electrical mechanical coupling (4), whereby the load current (Ιγογ) can be distributed on the coupler, a first reference circuit (25) whose inputs comprise a current (lswjtch) proportional to the current passing through the electromechanical coupling (4) and a minimum reference value (lref msn; Uref msn) and whose output is functionally coupled to the control input of the semiconductor coupler (21) and controlling the semiconductor coupler to a current-conducting state when the minimum reference value after closing the electromechanical coupling is exceeded by the value proportional to the current flowing. Coupling arrangement according to claim 1, characterized in that it also comprises a second reference circuit (23) whose inputs are a value proportional to the load current (1T0T) and a maximum reference value (lref max; Uref max) and whose output is functionally coupled to the control input of the semiconductor coupler (21) and control the semiconductor coupler to a non-current conducting state as the value proportional to the load current exceeds the maximum reference value. · j. ' 3. Coupling arrangement according to claim 1, characterized in that the minimum reference value is the least reliably expressable value of the current (lswitch) passing through the electromechanical coupling (4), whereby a portion of the load current after closing the coupling (4) starts pass through the semiconductor coupler and prevent a rise that causes transients in »· · v: the current passing through the coupling (4) (lswitch). Coupling arrangement according to claim 2, characterized in that the maximum reference value corresponds to the overflow value of the load current with which the electromechanical coupling is to be separated. the load from the supply voltage. • ♦ · '* * | 5. A coupling arrangement according to claim 1, characterized in that the controlled semiconductor coupler is an FET transistor. Coupling arrangement according to claim 1, characterized in that the first reference circuit is a comparator (C2) whose reference input is a reference voltage formed with a voltage divider (R1-R2) and the second input is the voltage drop which the The current lswltch passing through the electromechanical coupling (4) causes a measurement resistance (R5) located in series with the coupling (4). Coupling arrangement according to claim 1, characterized in that the second reference circuit is a comparator (C1) whose reference input is a reference voltage formed with a voltage divider (R3-R4) and whose second input is the voltage drop caused by the load current (1X0T). a measurement resistor (R6) located in series with the electromechanical coupling (4) and the FET transistor (41) parallel connection. Coupling arrangement according to claim 7, characterized in that the measurement resistor (R6) is coupled to the input of the comparator (C1) via an input resistor (R7) and that by changing the resistance value of the input resistor (R7), an overflow value is set on the voltage current with the electrical current. the supply voltage 15 from the load. • I · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · »· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·