EP0427801A1 - Semiconductor switch, especially a high-tension ignition switch for an internal combustion engine. - Google Patents

Abstract

The invention relates to a semiconductor switch, used in particular as a high voltage contactor for applying an ignition voltage to a spark plug of an internal combustion engine, comprising a cascade circuit formed by semiconductor constituent elements arranged in series, for connecting an operating voltage to a user, the semiconductor elements each having a barrier layer capacitance, and the junction between two semiconductor elements forming a parasitic capacitance relative to the ground, determined by the electrical field distribution. For a symmetrical voltage distribution without additional coupling members, it is expected that a stall current (ik) flowing through the semiconductor elements (T1 to Tn) before reaching the conductive state, is located, with respect to a displacement current (iver), in the interval defined below: iver <ik <a.iver the displacement current (iver) being produced by a voltage increase (duo / dt) of the operating voltage uo aux capacitances with stop (C1) and ground (C2) layer of the cascade assembly, varying successively with the connection of said semiconductor elements, and the factor (a) having a value between 5 and 10.

Description

Semiconductor switch, in particular as a high-voltage ignition switch for internal combustion engines

State of the art

The invention relates to a semiconductor switch, in particular as an ignition voltage switch for applying an ignition voltage to a spark plug of an internal combustion engine, with a cascade circuit formed by semiconductor components for switching an operating voltage to a consumer, according to the preamble of the main claim.

In the case of semiconductor high-voltage switches, it is known to carry out a cascade connection (series connection) of semiconductor components, so that the dielectric strength is ensured. Here is a precise dimensioning of the connection elements necessary for preferably uniform voltage distribution of the semiconductor components is important, since exceeding the reverse voltage limit values leads to the destruction of the semiconductors. The known circuit elements are implemented by relatively complex RC networks. An uneven voltage distribution that occurs as a result of specimen controls and due to the inevitable stray capacities is thus avoided. The circuit elements result in a relatively complex and expensive circuit structure.

From DE-OS 37 31 412 a high-voltage switch equipped with phototransistors is known, in which each transistor has a resistor connected in parallel. The voltage divider thus formed is used to evenly distribute the operating voltage to be switched. In addition to the disadvantages of such wiring elements already mentioned, the current flowing through the voltage divider also leads to undesirable losses.

Advantages of the invention

The device according to the invention with the features mentioned in the main claim has the advantage that no circuit elements have to be used and nevertheless a largely symmetrical voltage distribution is realized. This significantly reduces the circuitry and there are no additional voltage control losses. The cascaded semiconductor components connected in series each have a junction capacitance and the between Any connection made by two semiconductor components forms a corresponding (parasitic) earth capacitance due to the electrical field distribution present there. These capacitances known per se are unavoidable and therefore have nothing in common with the wiring elements known from the prior art. They are used for the symmetrical voltage distribution of the semiconductor switch according to the invention, since they cause a displacement current due to a voltage increase in the operating voltage. According to the invention it is provided that a breakdown current flowing through the semiconductor components before the conductive state is reached, relative to the displacement current, is within the range i _ver <i _k <a · i _ver , where i _{ver is} the displacement current, i _{K is} the breakover current and a is a factor , whose value is between 5 and 10. When the individual semiconductor components lying in series are switched through in succession, the capacities of the cascade connection mentioned change successively in a corresponding manner. There are several options for realizing the teaching according to the invention, which can be used individually or in combination. The most important possibility is to specify the size of the breakdown current by choice or in the manufacture of the semiconductor components in such a way that the condition according to the invention is fulfilled. An influence is also given by the setting of the junction capacity of the semiconductors used. Furthermore, the construction of the cascade allows the respective Erdka within certain limits capacity vary. Finally, the displacement current is also determined by the rate of voltage rise of the operating voltage, so that it can be influenced. The above-mentioned variables are therefore to be coordinated with one another in such a way that the condition according to the invention is observed. In practice it can be assumed that both the junction and the earth capacitance are fixed within relatively narrow limits. Also, the rate of voltage rise of the operating voltage is usually determined within narrow limits by external circumstances not relevant to semiconductor switches. The rate of voltage rise is often determined by the requirements of the consumer. If the voltage change rate is predetermined by the external circumstances, there can be no influence on the implementation of the teaching according to the invention.

As already mentioned above, the component-dependent specification of the tipping current remains as a significant influence. “Breakdown current” is to be understood as the current of the semiconductor component which flows shortly before its conductive state is reached. The breakover voltage assigned to the breakover current, which corresponds to the ignition voltage which leads to the semiconductor being switched on, is present at the semiconductor component which is still in the blocked state. Accordingly, if there is an increase in voltage up to the ignition voltage, the semiconductor assumes its conductive state. Here, the previously flowing, small breakdown current changes into the forward current (operating current). According to the teaching according to the invention, the limits of the tipping current result from the need that no semiconductor component of the cascade may switch through before the breakover voltage and thus the breakover current is reached on the output-side semiconductor component leading to the consumer. On the other hand, it is important to avoid excessive overcoupling from the input of the cascade to the consumer shortly before switching through, that is, excessive voltage build-up and / or excessive power loss is undesirable at the consumer.

It is preferably provided that the displacement current consists of the junction capacitance C ₁ of the cascade component on the output side leading to the consumer and the earth capacitance C ₂ according to the relationship i _ver = (C ₁ + C ₂ )

results.

The voltage increases up to the ignition voltage of the semiconductor components at which switching takes place. This is called "overhead ignition" if the ignition process takes place without additional control, triggering or the like. If, for example, the semiconductor component is a thyristor, there is an overhead ignition if, without driving the gate, the anode-cathode voltage is increased to the zero breakover voltage at which the semiconductor changes into its conductive state . In the semiconductor switch according to the invention, however, components with control connections can also be used, so that switching can be carried out by driving these control connections.

Each semiconductor component is preferably designed as a thyristor, photo thyristor or breakover diode.

drawing

The invention is explained in more detail below with reference to the figure. Show it:

FIG. 1 shows a schematically illustrated cascade of the high-voltage switch provided with semiconductor components with a connected consumer,

Figure 2 is a diagram of the operating voltage and

FIG. 3 shows a current / voltage diagram of a semiconductor component.

FIG. 1 shows a series connection of a plurality of thyristors T ₁ to T _n . These form a cascade 1 of the high-voltage switch 2 according to the invention. One end of the series circuit forms an input 3 and the other end forms an output 4 of the high-voltage switch 2. A junction capacitance C ₁ lies parallel to each thyristor T ₁ to T _n . The size of the junction capacitance C ₁ can be influenced within certain limits in semiconductor production. In practice it can be assumed that the junction capacitances C _{1 of} the thyristors T ₁ to T _n due to Variations in copies do not all have the same value.

The connections between two calf conductor components T ₁ to T _n are connected to a parasitic earth capacitance C ₂ determined by the electrical field distribution. Depending on the design of the cascade 1, it is possible to influence the size of this earth capacitance C ₂ within certain limits. In this respect, the size of the individual earth's capacities C _{2 can} vary depending on the location within the cascade; with a symmetrical structure of the cascade 1, however, it is possible that all earth capacitances C _{2 have} approximately the same value.

Both the junction capacitances C ₁ and the earth capacitances C ₂ are inevitable, parasitic capacitances, and therefore not additional circuit elements, as are known from the prior art. To illustrate this difference, a dashed representation has been chosen in FIG.

The operating voltage u _{O is} applied to the input 3 of the cascade 1 and a consumer 5 is connected to the output 4. The high-voltage switch 2 is preferably used as an ignition voltage switch for applying an ignition voltage to a spark plug of an internal combustion engine. In this respect, the operating voltage u _o is the secondary voltage of an ignition coil and the consumer 5 is a spark plug Z _K. The respective gate 6 of the thyristors T ₁ to T _n is not wired in the illustrated embodiment. This means that the Thy Ristor T ₁ to T _n assume their conductive state when the anode-cathode voltage exceeds a certain limit value (zero breakover voltage) U _K0 . If, according to an exemplary embodiment (not shown), the gates 6 are activated, the ignition voltage of the thyristors T ₁ to T _n is dependent on the control current flowing in each case. In the following, however, the activation-free embodiment shown in FIG. 1 will be discussed.

FIG. 2 shows the voltage profile of the secondary voltage (operating voltage u _o ) of an ignition coil (not shown). The negative half wave has an edge with the voltage rise speed du _o / dt.

FIG. 3 shows the current-voltage diagram of one of the thyristors T ₁ to T _n . The pass quadrant of the diagram is shown. With increasing anode-cathode voltage U _D , the current initially hardly increases, but is limited to the blocking current I _AK . If the breakover voltage u _{K is} reached, the current suddenly rises to the breakover current i _K and then jumps into the forward current I _T. Since the gates 6 of the thyristor T ₁ to T _{n are} not activated (FIG. 1), the breakover voltage u _K is the zero breakover voltage U _K0 .

According to the invention, it is now provided that for uniform distribution of the secuncar voltage (operating voltage u ₀ ) over the thyristors T ₁ to T _n before the conductive state of the thyristors is reached T ₁ to T _n flowing breakover current i _κ relative to a displacement current i _ver lies within the range i _ver <i _k <a · i _ver , the factor a assuming a value approximately between 0 and 10. The teaching according to the invention ensures that, even without additional wiring elements and despite the semiconductor being scattered, the voltage distribution for the individual cascade elements is substantially uniform, so that the dielectric strength of the individual thyristors T ₁ to T _{n is} not exceeded.

The relationship applies to the displacement current

i _ver = (C ₁ + C ₂ )

such that the relationship (C ₁ + C ₂ ) <i _k <a (C ₁ + C ₂ )

results.

The high-voltage switch 2 according to the invention works as follows:

The voltage curve indicated in FIG. 2 is applied to input 3 of cascade 1 by appropriate activation of the ignition coil (not shown). Accordingly, the negative half-wave runs into the circuit at a voltage change rate du _o / dt, so that the thyristor T ₁ appropriate junction capacitance C ₁ charges and the current through the thyristor T ₁ assumes the value of the breakover current i _k . The breakover current i _k then flows to the thyristor T ₂ and charges the existing junction capacitance C ₁ and earth capacitance C ₂ there. The breakover current i _k also occurs in the case of the thyristor T ₂ . This process is repeated in the subsequent thyristors T3 to T _n , the current coming from the thyristor T _n stage (T _n-1 ) of the cascade 1 receiving a charge of the capacitors in parallel on the output side stage (thyristor T _n ). The total capacitance of the last stage is thus composed of the sum of the junction capacitance C ₁ associated with the thyristor T _n and the ground capacitance C ₂ . This total capacity is the largest capacity compared to the other stages of cascade 1, since there is no series connection of capacities in the last stage. The voltage change rate du _o / dt is applied to it, which leads to the formation of the displacement current i _ver .

Overall, the sum of the breakover voltage u _{k of} the individual semiconductors of the cascade 1 is set exactly as the sum breakover voltage, so that sample variations are not negatively noticeable. The overall symmetrical voltage distribution without additional wiring elements leads to an almost simultaneous control of all tyristors T _k to T _n when the ignition voltage is reached.

The invention will be illustrated in more detail using a numerical example:

It is assumed that the sum of junction capacitance C ₁ and earth capacitance C _{2 is} 1 pF. The The voltage rise rate du _o / dt is given as 1000 V / μs. For a factor a = 5, i _k > 1 mA i _k <5 mA

If the breakover current i _k is in the range between 1 and 5 mA, an essentially symmetrical distribution of the cascade input voltage (operating voltage u _o ) over the individual stages of the cascade 1 can be assumed. When developing the semiconductor components for the circuit arrangement dealt with in the example, measures familiar to the semiconductor expert must be taken that the breakover current i _{k is} in the range between 1 and 5 mA.

Claims

Expectations

1.Semiconductor switch, in particular as a ignition voltage switch for applying an ignition voltage to a spark plug of an internal combustion engine, with a cascade circuit formed by series semiconductor components for switching through an operating voltage to a consumer, the semiconductor components each having a junction capacitance and between two each Semiconductor components existing connection forms a parasitic earth capacitance determined by the electrical field distribution, characterized in that a breakdown current (i _k ) flowing through the semiconductor components (T ₁ to T _n ) before reaching the conductive state relative to a displacement current (i _ver ) within the range i _ver <i _k <ai _ver The displacement current (i _ver ) is caused by a voltage rise (du _o / dt) of the operating voltage u _o at the cascade circuit (C ₁ ) and earth capacitances (C ₂ ) of the cascade circuit, which change successively with the switching of the semiconductor components and factor (a) has a value between 5 and 10.

2. Semiconductor switch according to claim 1, characterized in that the displacement current (i _ver ) from the junction capacitance (C ₁ ) of the output side of the cascade (1) to the consumer (5) leading semiconductor component (T _n ) and the earth capacitance (C ₂ ) is determined according to the relationship i _ver = (C ₁ + C ₂ ) du _o / dt.

3. Semiconductor switch according to one of the preceding claims, characterized in that the voltage rise (du _o / dt) up to the switching-on ignition voltage (U _κ0 ) of the semiconductor components (T ₁ to T _n ).

4. Semiconductor switch according to one of the preceding claims, characterized in that the semiconductor components (T ₁ to T _n ) have control terminals (gates 6) and that the control of the semiconductor components (T ₁ to T _n ) by driving the control connections (gates 6) is done.

5. Semiconductor switch according to one of the preceding claims, characterized net that the breakover current (i _k ) is determined in the manufacture of each _{¬ of} the semiconductor device (T ₁ to T _n ).

6. A semiconductor switch according to any one of the preceding claims, dadurchgekennzeich _¬ net that each semiconductor component (T ₁ to T ₆₎ as a thyristor, photo-thyristor, the photo IC or Kipodiode is formed.