Circuit for detecting an overvoltage on an electric load

Abstract: The invention relates to a circuit for detecting an overvoltage in an electrical load (Z1) inserted with a first and a second terminal between a feed line (AL) and a control switch (S), the circuit having an output voltage (Vout) at an output terminal (OUT) and comprising: at least one first threshold comparator (C1) having a first input terminal held at a first reference voltage (E1), a second input terminal connected to the feed line (AL) and an output terminal, at least one second threshold comparator (C2) having a first input terminal held at a second reference voltage (E2), a second input terminal connected to a second terminal of the electrical load (Z1), and an output terminal, at least one output transistor (T1) inserted with a first and a second terminal between the feed line (AL) and an output terminal (OUT) of the circuit (1), the transistor (T1) being controlled by at least one logic block (D) having inputs connected to the outputs of the threshold comparators (C1) and (C2), at least one feedback block (R) inserted with an input terminal and an output terminal respectively between the output terminal (OUT) of circuit (1) and a further input terminal to the logic block (D).

Description: This invention relates to a circuit for detecting overvoltage on an electrical load inserted between a feed line and a controlled switch. The field of application of the invention relates in particular to the motor vehicle industry and the description which follows is made with reference to a circuit to detect an overvoltage in a coil inserted between a feed line and a controlled switch, solely for the purpose of simplifying the description. It is known that in an electronic ignition device for an internal combustion engine it is necessary to detect whether a spark in the secondary circuit of an ignition coil included in the device is present or absent during the exhaust stage. If a spark is in fact absent it is necessary to perform subsequent actions in order to avoid deterioration of the catalytic converter. A known technical arrangement for meeting this need is described in European Patent Application EP-A-0 344 349, which suggests that the present or absence of a spark in the secondary circuit of the engine's ignition coil should be detected by examining the different behaviour of the voltage present in the primary circuit of the coil. For a better understanding of this phenomenon we refer to Figure 1. When a control switch S included in the electronic ignition device of an internal combustion engine closes, a current which increases linearly with time passes through the primary circuit L' of the ignition coil L. If on the other hand control switch S opens, voltage Va present in the primary circuit L' of ignition coil L increases rapidly and then stops, after a certain time interval, at a value equal to that of the voltage Vs provided by the vehicle's battery. In the latter case it is possible to detect that the behaviour of voltage Va is approximately that shown in Figure 2 if there is no spark in the secondary circuit L'' of ignition coil L. In this first situation the time for which the overvoltage lasts is approximately 100 µs. If on the other hand there is a spark in the secondary circuit L'' of ignition coil L, the behaviour of voltage Va is that shown in Figure 3. In this latter situation the overvoltage lasts approximately 2 ms. The technical problem underlying this invention is that of providing a circuit for correct and accurate determination of a time duration of an overvoltage on an electrical load between a feed line and a controlled switch. The technical problem is solved by a circuit of the abovementioned type as defined in the claims appended hereto. Features and advantages of the circuit according to the invention will be apparent from the description below of an embodiment provided not restrictively by way of illustration, with reference to the appended drawings. In these drawings: Figure 1 shows a diagram of an electronic ignition device for an internal combustion engine, Figures 2 and 3 show respective diagrams of the electrical signals present in the device in Figure 1 on the same time base, Figure 4 shows a block diagram of a circuit constructed according to the invention, Figure 5 shows a possible application of the circuit constructed according to the invention, Figure 6 shows an embodiment of the circuit in the block diagram illustrated in Figure 4, Figures 7 and 8 show the change in an output voltage of the circuit according to the invention. With reference to these figures, 1 indicates as a whole a circuit for detecting an overvoltage in an electrical load Z1 inserted with a first and a second terminal between a feed line AL and a controlled switch S. With particular reference to Figure 4, it will be noted that circuit 1 comprises a first threshold comparator C1 which has a first input terminal held at a first reference voltage E1, a second input terminal connected to feed line AL and an output terminal. Again with reference to Figure 4, it will be noted that circuit 1 also includes a second threshold comparator C2 which has a first input terminal held at a second reference voltage E2. This second comparator C2 also includes a second input terminal connected to the second terminal of electrical load Z1 through a one-way block ZN which is connected in series to a first resistance element R1, and an output terminal. The second input terminal of comparator C2 is also connected to a fourth reference voltage GND, for example a ground, through a third resistance element R2. Circuit 1 also includes an output transistor T1, for example of the NPN type, inserted with a first and a second terminal between feed line AL and an output terminal OUT from circuit 1. This transistor T1 is controlled by means of a logic block D which has input connected to the output terminals of threshold comparators C1 and C2. Circuit 1 also includes a feedback block R inserted with an input terminal and an output terminal respectively between the output terminal OUT of circuit 1 and a further input to logic block D. More specifically, this feedback block R includes a third threshold comparator C3 which has a first input terminal held at a third reference voltage E3, a second input terminal connected to the output terminal OUT of circuit 1 and an output terminal. In turn logic block D includes a first logic gate P1, for example of the OR type, which has a first input terminal connected to the output terminal of the second threshold comparator C2 and a second input terminal connected to the output terminal of the third threshold comparator C3. In addition to this, logic block D also includes a second logic gate P2, for example of the AND type, having a first input terminal connected to the output terminal of first comparator C1 and a second input terminal connected to the output terminal of first logic gate P1. This second logic gate P2 also includes an output terminal by means of which output transistor T1 is controlled. Finally, Figure 4 shows that a condenser C1, which is connected in parallel to a second resistance element R1, is inserted between the output terminal OUT of circuit 1 and the fourth reference voltage GND. For convenience of description, the operation of circuit 1 according to the invention will now be described with reference to an application in which electrical load Z1 is of the inductive type, for example an engine's electronic ignition coil L comprising a primary circuit L' and a secondary circuit L'', as shown in Figure 5. An initial condition in which switch S is open will also be considered. In this case it is found that a voltage Va, which changes as shown in Figures 2 and 3 according to whether a spark in the secondary circuit L'' of the coil is absent or present, is present at the ends of primary circuit L' of ignition coil L. The peak value of voltage Va is approximately 400 V if a spark is absent and approximately 250 V if a spark is present. The case in which there is no spark in secondary circuit L'' of coil L will first be considered assuming that a first condition obtains in which: Va>Vzn + [R1 + R2] x E2/R2 = Vth1 where Vzn is the voltage present at the terminals of one-way block ZN, which in this application is approximately 40V, with E2 of the order of 1 volt and Vth1 a first upper threshold value. In addition to this, with the voltage at the terminals of the third resistance element R2 being V2, it follows from condition (1) that: V2 > E2 It is also assumed that a second condition obtains in which: V1 > Vs + E1 = Vth2 where V1 = V2 + Vzn , Vs is the supply voltage, which in this application is the same as the vehicle's battery voltage, and lies between approximately 6V and approximately 24V, E1 is of the order of a few volts and Vth2 is a second lower threshold value. The existence of both conditions (1) and (2) implies that the first threshold comparator C1 and the second threshold comparator C2 perform a switching operation, providing their output terminals with a voltage Vc1 and Vc2, which are at a high logic level, respectively. Voltage Vc2 activates logic gate P1 to provide an output voltage Vp1 which is itself at a high logic level. As a consequence, because logic gate P2 has a high logic level on both its inputs it is activated to provide an output voltage Vp2 which is also at a high logic level. This voltage Vp2 causes output transistor T1 to become active. When this transistor becomes active, the OUT output terminal of circuit 1 has a voltage VOUT = Vp2-VbeT1 and condenser C1 begins to charge up. It is also assumed that a third condition obtains in which: Vout > E3 with E3 of the order of 1 volt. The existence of this third condition implies that third threshold comparator C3 performs a switching operation, providing a voltage Vc3 at a high logic level as an output. As a consequence, because logic gate P1 has both inputs at a high logic level it continues to maintain voltage Vp1 at a high logic level. As a result of what has been described above, voltage Vout remains at the value Vp2-VbeT1 and condenser C1 remains charged. If the current flowing through one-way block ZN is I, then: Va = Vzn + (R1 + R2) x I With reference to Figure 5, when Va ≅ Vzn, I is cancelled out and as a consequence Va becomes equal to V1. In this situation then also V2 = R2 x I = 0 However, if condition (2) is still fulfilled, and therefore: V1 = Va > Vth2, first threshold comparator C1 continues to maintain voltage Vc1 at a high logic level while second threshold comparator C2 performs a switching operation, reducing voltage Vc2 to zero. As condenser C1 is charged to a voltage Vout > E3, third threshold comparator C3 maintains voltage Vc3 at a high logic level and there is still a voltage Vp1 at a high logic level at the output from logic gate P1. In turn, as logic gate P2 still has inputs at a high logic level, it maintains voltage Vp2 at a high logic level, and as a consequence output transistor T1 remains active. When it is then found that: Va = V1 < Vth2 first threshold comparator C1 switches, reducing voltage Vc1 to zero. As a consequence, logic gate P2 also switches, reducing voltage Vp2 to zero and deactivating output transistor T1. In these circumstances the course of voltage Vout follows that of the discharge of condenser C1 to second resistance member R1. More specifically, the course of voltage Vout is that shown in Figure 7. It should be pointed out that once voltage Vout has fallen below the value of reference voltage E3 it does not become positive again unless condition (1) is reestablished. An identical method of functioning to that just described occurs when there is a spark in secondary circuit L'' of ignition coil L. In this case however there is a problem associated with the course of voltage Va. As shown in Figure 3, after a first short-lived overvoltage of approximately 10 ms, voltage Va in fact undergoes a sequence of alternations from zero to approximately 70-80 V in rapid succession lasting a few tenths of a microsecond. From what has been said above it follows that when Va < Vth2, output transistor T1 becomes deactivated and condenser C1 begins to discharge. In order that this should not happen condenser C1 and second resistance element R1 have to be dimensioned so that condition Vout > E3 still obtains throughout the sequence of alternations to which voltage Va is subjected. In this way, third comparator C3 in fact maintains voltage Vc3 at a high logic level. It is then sufficient for voltage Va to rise above lower threshold value Vth2 in order for voltage Vc2 to be again at a high logic level. In this way logic gate P2 is activated again to provide an output voltage Vp2 at a high logic level, which reactivates output transistor T1. If there is a spark in secondary circuit L'' of ignition coil L, the course of voltage Vout is therefore that shown in Figure 8. Also, as shown in Figures 2 and 3, once the spark in secondary circuit L'' of coil L has been exhausted, voltage Va shows a transient to zero and then stops at the supply voltage Vs, which is less than lower threshold voltage Vth2. As a consequence, voltage Vout tends to zero over the discharge time of condenser C1 to second resistance element R1, as shown in Figures 7 and 8. It should be pointed out that circuit 1 and controlled switch S, which is constructed for example using a power transistor, are incorporated into the same silicon chip using technology of the VIPower type. This results in the filtering of voltage Va through condenser C1 being performed only at the output from circuit 1. In fact, because of the alternation which voltage Va undergoes when a spark is present in secondary circuit L'' of coil L, which causes it to adopt negative values with respect to ground, any filtering performed at an intermediate point in circuit 1 would be negated through the activation of spurious active components which would deactivate the circuit itself. Figure 6 shows a preferred embodiment of circuit 1 according to the invention. With reference to this figure, one-way block ZN comprises a first diode, Z1, a second Z2, a third Z3 and a fourth Z4, of the Zener, type which are connected together in series. First threshold comparator C1 comprises a second transistor T2, of the PNP type, while second threshold comparator C2 and first logic gate P1 are provided by a third transistor T3, of the NPN type. A fifth diode D1 is connected to an emitter terminal of second transistor T2 to provide first reference voltage E1. Figure 6 also shows that second logic gate P2 is provided by a fourth transistor T4, of the NPN type, while a fifth transistor T5 and a sixth T6, of the PNP type, in a mirror-current connection, provide third threshold comparator C3. In addition to this, circuit 1 as shown in Figure 6 also comprises a sixth diode Z5 of the Zener type, inserted with a first and a second terminal between a drive terminal first output transistor T1 and a ground. As far as the functioning of the embodiment of the circuit just described is concerned, when voltage Va is less than voltage Vzn present at the ends of the chain of Zener diodes Z1, Z2, Z3, Z4, third transistor T3 is deactivated while fourth transistor T4 is activated and keeps first output transistor T1 deactivated. As a consequence voltage Vout is equal to zero. When on the other hand voltage Va rises above voltage Vzn, current I which flows in the chain of Zener diodes Z1, Z2, Z3, Z4 activates third transistor T3 which causes fourth transistor T4 to be deactivated. At the same time, second transistor T2 also becomes active providing an output current which causes first transistor T1 to become active. When this transistor becomes active, voltage Vout rises to a value which is greater than zero. It should be pointed out that the existence of diode Z5 causes voltage Vout not to exceed a maximum value of Vz5-VbeT1. In addition to this, if the condition: Vout > E3 obtains, the comparator formed by fifth transistor T5 and sixth T6 switches, causing T6 to be deactivated and T5 to be activated, keeping third transistor T3 active. The latter continues to keep fourth transistor T4 inactive. When voltage Va again falls below voltage Vzn, current I flowing in the chain of Zener diodes Z1, Z2, Z3, Z4 becomes equal to zero. However, if the condition: Va > Vs + VbeT2 + VD1 obtains, second transistor T2 continues to remain active, keeping first transistor T1 activated. If on the other hand: Va < Vs + VbeT2 + VD1 obtains, second transistor T2 is deactivated, and no longer provides any current to first transistor T1, which is in turn deactivated. As a consequence voltage Vout begins to fall in accordance with the discharge function for condenser C1 to second resistance member R1. However, as long as: Vout > E3 fifth transistor T5 continues to remain active, keeping third transistor T3 active. In turn, third transistor T3 keeps fourth transistor T4 inactive. In these circumstances it is then sufficient for voltage Va to rise above the threshold value Vs + Vd1 + VbeT2 in order to again activate second transistor T2, which causes first transistor T1 to become activated. In this way voltage Vout again adopts a value which is greater than zero. When however voltage Va falls below the threshold value for a sufficiently long time to allow voltage Vout to fall below reference voltage E3, then sixth transistor T6 becomes active and fifth transistor T5 becomes inactive. As a consequence, third transistor T3 also becomes inactive, while fourth transistor T4 becomes active, rendering first transistor T1 definitively inactive. The latter can only be made active again when voltage Va again exceeds voltage Vzn, reactivating the chain of Zener diodes Z1, Z2, Z3, Z4. At this point the cycle which has just been described is repeated.

Claims: A circuit for detecting a time duration of an overvoltage signal (Va) on an electrical load (Z1) inserted with first and second terminals between a feed line (AL) and a control switch (S), said circuit having an output voltage signal (Vout) at an output terminal (OUT) and comprising: at least one first threshold comparator (C1) having a first input terminal receiving a voltage indicative of the overvoltage (Va), said voltage including a first reference voltage (E1) applied to said first input terminal, a second input terminal coupled to the feed line (AL) and an output terminals for producing a first voltage signal (Vc1) at a first logic level when the overvoltage signal (Va) exceeds a second threshold voltage (Vth2) and at a second logic level when the overvoltage signal (Va) falls below a second threshold voltage (Vth2), at least one second threshold comparator (C2) having a first input terminal held at a second reference voltage (E2), a second input terminal receiving a voltage indicative of the overvoltage (Va), and an output terminal for producing a second voltage signal (Vc2) at a first logic level when the overvoltage signal (Va) exceeds a first threshold voltage (Vth1) and at a second logic level when the overvoltage signal (Va) falls below the first threshold voltage (Vth1), at least one output transistor (T1) inserted with first and second terminals between the feed line (AL) and the output terminal (OUT)for producing the output voltage signal (Vout), said transistor (T1) being connected to at least one logic block (D) having inputs connected to the outputs of threshold comparators (C1) and (C2), said logic block (D) being for controlling the output transistor (T1) so as to have the output voltage signal (Vout) at the first logic level when the first (Vc1) and the second (Vc2) voltage signal are at the first logic level, at least one feedback block (R) inserted with an input terminal and an output terminal respectively between the output terminal (OUT) and a further input terminal to the logic block (D), said feedback block (R) being for maintaining the output voltage signal (Vout) at the first logic level for a time interval sufficient to suppress changes in said logic level due to rapid changes of the overvoltage (Va), said time duration being detected by the condition Va<Vth2 persinsting for a sufficient long time in order to distinguish between rapid changes and regular overvoltage duration during the time duration of the overvoltage signal (Va). A circuit according to claim 1, characterised in that the feedback block (R) includes at least a third threshold comparator (C3) having a first input terminal held at a third reference voltage (E3), a second input terminal connected to the output (OUT) terminal from circuit (1) and an output terminal. A circuit according to claim 1, characterised in that the logic block (D) comprises: at least one first logic gate (P1) having a first input terminal connected to the output terminal of the second threshold comparator (C2), a second input terminal connected to the output terminal from the third threshold comparator (C3) and an output terminal, at least one second logic gate (P2) having a first input terminal connected to the output terminal from the first comparator (C1), a second input terminal connected to the output terminal from the first logic gate (P1) and an output terminal capable of controlling the output transistor (T1). A circuit according to claim 1, characterised in that the second input terminal to the second comparator (C2) is connected to the second terminal of the electric load (Z1) through at least one one-way block (ZN) connected in series to a first resistance element (R1). A circuit according to claim 1, characterised in that a condenser (C1) connected in parallel to a second resistance element (R1) is inserted between its output terminal (OUT) and a fourth reference voltage (GND). A circuit according to claim 3, characterised in that the first logic gate (P1) is of the OR type. A circuit according to claim 3, characterised in that the second logic gate (P2) is of the AND type. A circuit according to claim 1, characterised in that the output transistor (T1) is of the NPN type. A circuit according to claim 1, characterised in that it is integrated using technology of the VIPower type.

Inventors: PALARA SERGIO, SCHIACCIANOCE SALVATORE, SUERI STEFANO, PALARA, SERGIO, SCHIACCIANOCE, SALVATORE, SUERI, STEFANO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CO.RI.M.ME.CONSORZIO PER LA RICERCA SULLAMICROELETTRONICA NEL MEZZOGIORNO, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: F02P17
IPC codes: H02H3, G01R31, F02P17, G01R19, G01R29