EP0914664A1

EP0914664A1 - Circuit arrangement for reducing transients caused by an electromechanical switch with overcurrent protection

Info

Publication number: EP0914664A1
Application number: EP97929330A
Authority: EP
Inventors: Toivo Ridal
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1996-07-25
Filing date: 1997-07-03
Publication date: 1999-05-12
Also published as: CN1230284A; AU3346897A; FI962973A0; FI107848B; CN1064476C; WO1998005050A1; FI962973A

Abstract

When voltage is connected to load by using an electromechanical switch, high switching transients are generated. They can be suppressed significantly by placing a controllable semiconductor switch (21) in parallel with the switch (4), in which case the total load current (ITOT) is distributed among both switches. The first comparison circuit (25) monitors the current (Iswitch) which passes through the electromechanical switch and compares it to a very small reference value. As soon as the current which passes through the switch reaches the reference value, the first comparison circuit gives a control voltage which forces the semiconductor switch into the conductive state. Because of this, the rate of increase of the current which passes through the switch (4) decreases essentially because a large part of the total load current (ITOT) passes through the semiconductor switch (21). The second comparison circuit monitors the total load current (ITOT) and compares it to a maximum current value at which the load must be disconnected from the voltage source. When the total load current exceeds the reference value, the second comparison circuit gives a control voltage which immediately forces the semiconductor switch to the nonconductive state. At this point the current which passes through the electromechanical switch increases suddenly so that the internal overcurrent protection of the switch reacts immediately and opens the switch.

Description

Circuit arrangement for reducing transients caused by an electromechanical switch with overcurrent protection

Field of the Invention This invention relates to the suppression of the transients caused by an electromechanical switch and the overcurrent protection of the switch.

Background for the Invention

Many electric systems utilize a circuit board to which the external operating voltages are brought and on which are located the filters for filtering the supply voltages. It is possible to install on the board, in addition to different electronic circuits and connectors, a manually operated switch which is used to switch the supply voltage on and off to an external load. The load can be located far from the circuit board. When a board of this type is placed in a frame or housing, the switches are left outside the casing so that they are easy to operate. It is possible to place several boards next to one another in the frame or housing. Usually the circuit boards are used for power sources.

Figure 1 is a rough illustration of the board arrangement described above. The supply voltage +V and -V is brought in to the circuit board 1 which is outlined by a dashed line from an external voltage source. The voltages can be brought in through connectors, in which case the connectors located on the board are connected to the connectors located in the housing when the board is installed. Additionally, there can be different electric circuits which perform different tasks on the circuit board and which are indicated generally by the reference number 6 in the figure.

Filters 2 and 3 have been placed on the circuit board to filter any alternate components and transients included in DC supply voltages. In practice the situation is almost always such that the properties of the filters 2 and 3 are selected according to the characteristics of the DC feed and also the charac- teristics of the electric circuits 6, in which case the possible effects caused by their inductances have been taken into account in the filter parameters. On the other hand, it has not been possible in the filter properties to sufficiently account for switch 4 which is equipped with an internal overcurrent protection and which is also placed on the circuit board. This switch is used to switch on the voltage to an external load 5 and, correspondingly, to switch off the voltage from the load. The load can be, for example, an electric motor or some mainly resistive load which is controlled by the manually operated switch 4 located on the board. A suitable commercially available switch is selected according to the nature of the load and installed on the circuit board. The selection criteria are mainly the nominal current of the load and the maximum value of the overcurrent. The switch should thereby tolerate a certain continuous nominal current and it must contain an internal overcurrent protection which opens the switch when the load current exceeds the maximum current value allowed. A magnetic overcurrent protection which includes a "current sensing coil", in other words, a relatively large inductance through which the load current passes, is used as the switch. When the current exceeds a certain value which is higher than the nominal current, the magnetomotoric force inducted by the coil forces the switch to open.

The problem in the use of the switch is the high transient voltages caused by its opening. They cause interference in the operation of the electric circuits located on the circuit board, they progress via the filters into the feeding DC power net with the result that additionally, they cause interference in the operation of nearby electronic circuits as radio frequency signals. Because it is not possible to account for the inductances of switches of different types in the filter design, the final result is that the inductance of the switch de- creases the effect of the inductances of the filters, which results in increased transients. The regulations which relate to interference caused by electric devices EMC (Electromagnetic Compatibility) which the European countries are committed to, force the device designers to decrease the interference level of the products so that they meet the EMC regulations. Another problem related to electromechanical switches is that the current limit for triggering the overcurrent protection is not exact because of tolerances.

The decrease of the interference level in a structure of the type shown in Figure 1 is, however, very difficult to implement. Of course it is possible to decrease the transients caused by a certain switch by designing the properties of the filters carefully, but in this case each switch would require a separate, specially designed circuit board which is an uneconomic solution. It is also possible to locate the switch far from the circuit board, but in this case the increased conduit lengths only increase the inductance and thereby further increase the transients. It is, however, desirable to locate the switch as far away as possible, because then it is easier to take into account good ergonomics in its positioning and the lay-out design of the circuit board would have less restraints. So far a solution has not been found to the problem described above.

The objective of this invention is, therefore, a circuit which significantly reduces the switching transients when the switch is closed and which can be used to set an exact overcurrent limit at which the switch opens and thus separates the load from the circuit.

The set objective is achieved by using a switch arrangement specified in an independent claim.

Summary of the Invention

A controllable semiconductor switch is placed parallel to the electromechanical switch so that the total load current is distributed on both switches. The control voltage is formed by using comparison circuits.

The first comparison circuit monitors the current passing through the electromechanical switch and compares it to a reference value. As soon as the current passing through the switch reaches the reference value, the first comparison circuit generates a control voltage which opens the semiconductor switch, in other words, sets it in the conductive state. Because of this the rate of increase of the current passing through the switch decreases signifi- cantly, because most of the total load current starts passing through the semiconductor switch. The task of the first comparator circuit is therefore to detect any rising current passing through the electromechanical switch as soon as possible after it has been closed and to switch most of this current and thus most of the total load current to the semiconductor switch. Because of this the reference value must be as small as possible.

The second comparison circuit monitors the total load current and compares it to another reference value which is set as the maximum current value of the load at which the load must be disconnected from the voltage source. When the total load current exceeds the reference value, the second comparison circuit generates a control voltage which immediately closes the semiconductor switch, in other words, sets it to the nonconductive state. The part of the total load current which was passing through the semiconductor is switched to pass through the electromechanical switch, in which case the current passing through this switch increases suddenly so that the internal overcurrent protection of this switch operates immediately and opens the switch. The control voltages of the comparison circuits cause the following: the semiconductor switch does not conduct when the electromechanical switch is in the open position or the set maximum load current value is exceeded, but it starts to conduct almost immediately after the electromechani- cal switch has been closed. In this manner the transient normally caused by the high rate of increase when the switch is closed decreases significantly, because the current does not get an opportunity to rise to a large value at all.

The circuit according to the invention has other advantages in addition to decreased transients. Because the current limit for the overcurrent triggering is determined by the first reference value instead of the internal setting of the electromechanical switch, the current limit can be set to exactly the desired value. When the semiconductor switch takes on a large part of the load current it is possible to use an electromechanical switch with low nominal current and the overcurrent protection limit can still be set higher than the internal overcurrent protection limit of a switch with low nominal current. In this manner it is possible to use a switch of one size for different loads. Another remarkable advantage is that the positioning of the electromechanical switch on the circuit board is no longer a critical concern and it can be placed as desired, and even located in a place separate from the circuit board.

Brief description of drawings

A detailed description of the invention will now be made with reference to the attached drawings, in which

Figure 1 shows the implementation of the load control according to prior art by using an electromechanical switch, Figure 2 shows a switching arrangement according to the invention, Figure 3 illustrates the states of the inputs and outputs of the comparison circuits, and Figure 4 shows a practical application of the circuit shown in Figure 2.

Detailed Description of the Invention

The circuit shown in Figure 2 uses, as applicable, the same reference numbers as Figure 1 in which the essential parts for the invention are the load 5 and the electromechanical switch 4 which connects the load to the voltage +V - -V. In Figure 2 the switch is located outside the circuit board 1. An on/off controlled semiconductor 21 is located on the circuit board and it is connected electrically in parallel to the electromechanical switch 4. The current l_swιtch which passes through the electromechanical switch 4 (or a quantity proportional to it) is measured by the measurement connection of current 24 of the switch and the measured current value is conducted to the first comparison circuit 25 which compares against this current the first reference current l_{ref mιn} (or a quantity proportional to it) which acts as the second input of the reference circuit. If the current which passes through the electromechanical switch 4 exceeds the reference value, or, in other words, l_swιtch > _{f, mm}- ^tne output of the first comparison circuit is in the "on" state which forces the semiconductor switch 21 to the conductive state. Otherwise the output of the first comparison circuit 25 is in the state in which the semiconductor switch 21 is given the "off signal.

The second comparison circuit 23 operates in a corresponding man- ner. The measurement circuit of load current 22 measures the total current l_τoτ which passes through the load 5 or a quantity proportional to it. The measured total current l_τoτ is conducted to the second comparison circuit 23 which compares this current against the reference current l_{ref rnax} (or a quantity proportional to it) which acts as the second input. If the total current exceeds this reference value, or, in other words, l_τoτ > l_{ref maχι}, the output of the second comparison circuit is in the "off state which forces the semiconductor switch to the nonconducting state. Otherwise the output of the second comparison circuit 23 is in the state in which the semiconductor switch 21 is given the "on" signal. Figures 3a to 3d describe the operation of a switch according to Figure 2. Before the electromechanical switch 4 is used to connect the voltage to load 5 the current l_swltch equals zero, so the first comparison circuit 25 keeps the semiconductor switch 21 in the nonconducting state. The total current l_τoτ is also zero, in which case the output of the comparison circuit 23 is in the "on" state. The outputs of the comparison circuits are connected to the control input of the semiconductor switch so that when either of the circuit outputs are in the "off state, the switch is in the nonconducting state.

When the electromechanical switch 4 is switched on at the moment t=0, the current l_swltch of the switch starts to increase rapidly from the value 0 at a rate determined by the inductances of the circuit. After a short time ΔT, in Figure 3c, the current has reached the reference value l_{ref m},_n of the first com- parison circuit 25 which is as small a value as possible. In practice, it is easier to measure voltages than currents, so the figures show, instead of current references, voltage references U_{ref mn} and U_{ref max}. When the current has reached the reference value after the time ΔT the output of the first compari- son circuit 25 switches from the "off state to the "on" state, as seen in Figure 3d, because of which the semiconductor switch 21 is forced to the conducting state. Most of the load current now passes through the branch which includes the semiconductor switch, because the inductances in this branch are much smaller than those of the branch which includes the electromechanical switch 4. As the load current starts passing through the semiconductor switch 21 almost immediately after the electromechanical switch 4 has closed, the transients that the latter switch would otherwise cause are thereby efficiently prevented.

After this the system reaches a stable state in which greater part of the load current l_τoτ passes through the semiconductor switch and a smaller part passes through the electromechanical switch. Thus it is possible to use a switch with a smaller nominal current than would have to be used without the parallel semiconductor switch.

Let it be assumed next that at moment t the current l_τoτ taken by the load 5 starts to increase for some reason. The reason may be a partial short- circuit in the load or, in the case of a motor, an overload condition. The second comparison circuit 23 which monitors the total current l_τoτ detects at moment t₂ that the current reaches the reference value U_{ref max} (= l_{ref max} ), as shown in Figure 3a. The reference value has been set to correspond to the highest current value allowed for the load 5. At the same moment t₂ the state of the output of the second comparison circuit switches from the "on" state to the "off state as shown in Figure 3b. At this point the semiconductor switch is forced into the nonconductive state, in which case the entire load current is suddenly switched to pass through the electromechanical switch. The value of the load current l_τoτ at this moment exceeds the current limit of the internal overcurrent protection of the switch, so the overcurrent protection acts almost immediately by disconnecting the circuit from the load 5. The current l_switch drops to zero so the output of the comparison circuit 25 also switches to the "off state, as shown in Figure 3d. In this manner the current limit of the over- current protection can be set by the reference value of the second comparison circuit, because of which it is possible to use a switch with a lower internal overcurrent limit than would have to be used without the parallel semiconductor switch.

Finally, Figure 4 shows a practical circuit which implements the principle according to the invention shown in Figure 2. Connectors +V and -V provide filtered positive and negative voltage, between which the load (not shown) is connected by the electromechanical switch 4. The routes of the load current are indicated by dashed lines. The grid voltage of the FET transistor 41 connected parallel to the switch is controlled by the comparator C1 of the second comparison circuit and comparator C2 of the first comparison circuit whose outputs are connected directly to the grid via the resistor R9. The grid receives the positive windup voltage via the resistor R10 from the transformer T which transforms the input voltage +V (for example, 48 V) to the operating voltage U which is suitable for comparators, for example, 15 V. The measurement of the load current I has been converted to voltage measure- ment, in which the voltage caused by the load current over the resistor R6 is measured. The second comparator circuit consists of the comparator C1 which features a positive feedback via resistor R11 to generate hysteresis. A constant reference voltage U_{ref max} acts in the positive input of the circuit. This reference voltage is formed from the operating voltage U by using the voltage splitter R3-R4. In this case, the following equation is approximately true:

D 77

"4 ref ,max

R, U from which it can be seen that the reference voltage is determined by the ratio of the resistances. The value of the resistor R₆ is defined after this, when the value of the load overcurrent I is determined as follows

Λ 6 = ^{re max}

As small a value as possible is selected for the resistor R₆, a suitable value is, for example, 25 mΩ. After this the reference voltage is calculated and then the resistance values of the voltage splitter are calculated from this. When the load current increases to overcurrent value, the voltage in point P1 exceeds the reference voltage, at which point the output voltage of the comparator C1 swings from positive to negative operating voltage and the grid voltage of the FET 41 decreases and closes the FET. The measurement of the current of the electromechanical switch is, in practise, achieved by the measurement of voltage caused by the very small current over the resistor R₅, at which current it is desirable that the comparator C2 of the first comparison circuit switches the FET to the conductive state. Let this activation value of the current be l_{sw mιn}. In this case the reference voltage of the comparator C2 is calculated from the equation:

Let R₆, for example, be four times the resistance of the first measurement resistor R₆ and let the value of the activation current l_{sw mm} > 100 mA. When the reference voltage has been calculated, the resistances of the voltage splitter R R₂ which forms the reference voltage can be calculated from the equation:

Λ, ^" U

The circuit according to the invention can be used to effectively de- crease the transients caused by an electromechanical switch equipped with an overcurrent protection and to exactly determine the overcurrent value at which the switch is closed. Additionally, it is possible use one switch for load currents and overcurrents of different sizes.

According to the specifications of the following claims the component level circuit can be implemented in several ways other than the one described above.

Claims

1. A circuit arrangement for reducing transients caused by an electromechanical switch with an overcurrent protection w h e r e i n the arrangement consists of: a controllable semiconductor switch (21) placed in parallel with the electromechanical switch (4) so that the load current (l_τoτ) can be distributed to both switches, a first comparison circuit (25) whose inputs are a value proportional to the current (l_swltch) which passes through the electromechanical switch (4) and a minimum reference value (l_{ref mιn}; U_{ref mιn}) and whose output is functionally connected to the control input of the semiconductor switch (21 ) so that it forces the semiconductor switch to the conductive state when the value proportional to the current which passes through the electromechanical switch exceeds the minimum reference value after the electromechanical switch has been closed.

2. A circuit arrangement according to claim 1 , w h e r e i n the said arrangement includes a second comparison circuit (23) whose inputs are a value proportional to the load current (l_τoτ) and a maximum reference value C_{ref, ma} : U_{ref max}) and whose output is functionally connected to the control input of the semiconductor switch (21) so that it forces the semiconductor switch to the nonconductive state when the value proportional to the load current exceeds the maximum reference value.

3. A circuit arrangement according to claim 1 , w h e r e i n the minimum reference value is the smallest value of the current (l_SWItcr,) which passes through the electromechanical switch which can be detected reliably, in which case, when the aforementioned switch (4) closes, part of the current starts passing through the semiconductor switch thus preventing the increase in the current (l_swιtch) which passes through the electromechanical switch (4) which causes transients.

4. A circuit arrangement according to claim 2, w h e r e i n the maximum reference value corresponds to the overcurrent value of the load current at which the electromechanical switch must disconnect the load from the supply voltage.

5. A circuit arrangement according to claim 1 , w h e r e i n the con- trollable semiconductor switch is a FET transistor.

6. A circuit arrangement according to claim 1 , w h e r e i n the first comparison circuit is a comparator (C2) whose reference input is the reference voltage generated by a voltage divider (R1-R2) and the other input is the voltage loss which is caused by the current (l_swtch) which passes through the electromechanical switch (4) in the measurement resistor which is connected in series with the switch (4).

7. A circuit arrangement according to claim 1 , w h e r e i n the second comparison circuit is a comparator (C1) whose reference input is the reference voltage generated by the voltage divider (R3-R4) and the second input is the voltage loss caused by the load current (l_τoτ) in the measurement resistor (R6) which is connected in series with the electromechanical switch (4) and the FET transistor (41) which are connected in parallel with one another.

8. A circuit arrangement according to claim 7, w h e r e i n the measurement resistor (R6) is connected in the input of the comparator (C1) via an input resistor (R7) and the overcurrent value at which the electromechanical switch is disconnected the load from the supply voltage is set by changing the resistance value of the input resistor (R7).