WO1994021020A1

WO1994021020A1 - Power load improvement of semiconductor valves

Info

Publication number: WO1994021020A1
Application number: PCT/EP1994/000729
Authority: WO
Inventors: Georg Preleuthner; Grammenos Nicoltsios
Original assignee: SIEMENS AKTIENGESELLSCHAFT öSTERREICH
Priority date: 1993-03-09
Filing date: 1994-03-09
Publication date: 1994-09-15

Abstract

A process and device are disclosed for improving the power load of one or several semiconductor switches with at least one semiconductor valve. Each semiconductor valve is loaded with a switching current during a conduction period and the depletion layer temperature of at least one semiconductor valve is calculated from an n-degree thermal model consisting of n RC members. For that purpose, a switching current substitute value is at first determined and the power loss is calculated therefrom. In addition, a substitute depletion layer temperature for the depletion layer of the semiconductor valve is derived and recursively calculated by a digital computer from above-said model. If required, the device applies protective measures when the value of said substitute depletion layer temperature exceeds a selected reaction limit.

Description

Improvement of the current utilization of semiconductor valves

The invention relates to a method and a device for improving the current utilization of semiconductor switches which can be switched on and off, each having at least one semiconductor valve, in particular for use in an at least two-pulse converter, with each semiconductor valve periodically during a conductive period with electrical Current is loaded and the barrier layer heating of at least one semiconductor valve is monitored at each switching period with the introduction of a replacement valve which, based on the characteristics of the semiconductor valve, is formed on the basis of a n-degree thermal model consisting of n RC elements, each with a thermal resistance and a time constant is, and a switching current flowing through the semiconductor valve is used for determining the junction heating.

To take advantage of the thermal possibilities of semiconductor valves in a controlled manner, the manufacturer or user takes various measures. For example, the European patent application EP-A2-451 324 describes a device for determining the junction temperature of a semiconductor component using at least one RC element, the thermal behavior of the junction being electrically simulated by an RC network. With such a network which simulates the thermal behavior, only a rough approximation of the complicated thermal behavior of the barrier layer can be achieved with reasonable hardware and cost expenditure. However, the advantage of extensive freedom from delays is offset by the large amount of hardware described, which relates to the use of this device for monitoring overcurrents in semiconductor valves, in particular its use on a whole range of converter devices with different converter valves per individual device, in comparison with a more generous dimensioning of the valves which without Surveillance can be operated, seems uneconomical.

In "Digital control of electric drives" by Rolf Schönfeld, published by Hüthig Verlag, 2nd edition, 1990, on page 133 the use of a thermal model for electric motors or power converters, which is implemented in the computer controller, is addressed, including a flow chart for monitoring the critical temperature is given. Since the thermal behavior of power converters and motors is complicated, their sufficiently precise modeling is not considered possible and is therefore proposed as an alternative to storing a maximum permissible load characteristic in the computer and the current loads actually measured at the respective sampling times with this permissible characteristic to compare.

The object of the invention is to exploit the thermal limits of the semiconductor switches or their semiconductor valves for the different switching current profiles that occur in practice in the sense of the greatest possible economy when using the semiconductor valves as flexibly and largely as possible. This objective implies the approval of a time-limited, monitored overload operation, in which the semiconductor valves are loaded over a switching period for a certain time with a higher current than the continuous limit current specified by the manufacturer, none of the usual overcurrent protection devices, such as fuses or circuit breaker.

In practice, there is a desire for a mixed load of semiconductor valves (ie any sequence of normal and overload phases) in particular when the semiconductor valves have a current average of currents less than the continuous limit current and during most of their operating time are only subjected to overload currents during comparatively short periods of time.

While there is no difficulty in checking compliance with normal load currents, a monitoring device is required to monitor temporal mixed load current curves, which on the one hand ensures that the maximum permissible junction temperature of the semiconductor valves is never exceeded during the overload phases and on the other hand also ensures this carries that none of the usual overcurrent protection devices respond. In any case, the cost of the technical monitoring effort must be less than the cost difference of a system of semiconductor switches of a higher performance class, in which the same currents can be classified as normal load currents.

This is achieved according to the invention by means of the method mentioned at the outset in that a digital computer provided for system control, among other things, also takes on the task of monitoring the junction temperature of the semiconductor valves and the method used further comprises the following steps:

- Determining a switching current substitute value for the considered leading phase of the semiconductor device, e.g. by averaging the current flowing through the semiconductor valve during the conduction period,

Calculating a power loss converted by the switching current in the replacement valve during a switching period,

- Calculation of the replacement junction temperature for the replacement valve with the aid of the calculated value for the power loss and the parameters for the thermal resistances and the time constants of the individual RC elements of the thermal model, and also taking into account the ambient temperature, A particularly advantageous embodiment of the method additionally has the following steps:

Provision of a set of n previous values for replacement partial barrier layer heating of the individual RC members of the replacement valve,

- Closed and recursive calculation of the replacement barrier layer temperature for the replacement valve using the set of n previous values,

- Closed and recursive calculation of the replacement partial barrier layer heating for use as previous values in the subsequent switching period.

In many practical applications, the method is characterized in that protective measures are taken if the calculated equivalent junction temperature exceeds a selected response threshold.

The method features listed below, which are additionally used for the calculation of the equivalent junction temperature, enable the above-mentioned method to be carried out particularly easily, quickly and economically in each embodiment.

In the method described above, it is assumed as a model assumption that the semiconductor valve is loaded by a rectangular current pulse, the pulse width of which is equal to the conduction duration and the amplitude is equal to the switching current replacement value.

Furthermore, the forward loss power is used for the power loss, whereby characteristic data for the calculation of the

Forward power loss can be obtained from a polygon approximation that is a function of the lock voltage and the Equivalent resistance at a certain reference temperature, for example the maximum permissible junction temperature.

The peak value of the junction temperature of the semiconductor device that prevails at the end of the conduction period is advantageously calculated.

Optionally, a fixed value can be specified for the ambient temperature, or a currently measured value can be used.

The response threshold for protective measures is set in an advantageous embodiment of the invention in such a way that no overlapping with conventional overcurrent protection measures, such as fuses or circuit breakers, is possible, the maximum permissible junction temperature being able to be selected as the highest possible value for this response threshold.

If the selected response threshold is exceeded, the switching current is reduced on average over the switching period to at least 100% of the continuous limit current.

If the power supply fails, the current set of previous values is at least partially stored for later reuse, initial values which have not been stored being replaced by permanently stored worst-case values.

Furthermore, a binary heating detector is expediently activated during such a supply failure, which, when operation is resumed, indicates whether the barrier layer of the semiconductor device has already cooled to ambient temperature. In order to carry out the method mentioned at the beginning for improving the current utilization of semiconductor switches, a device of the type mentioned at the outset is disclosed which has the following features:

Means for determining a switching current substitute value, for example by averaging the switching current measured over a switching period,

- Means for calculating a power loss converted by the switching current mean value in the semiconductor valve and the replacement junction temperature of the replacement valve.

A particularly advantageous embodiment of the invention also has the following features:

Means for providing a set of n predecessor values for replacement partial junction heating of the individual RC elements of the n-degree thermal model,

Means for the closed and recursive calculation of the

Replacement junction temperature of the replacement valve and

Update the replacement partial junction heats for each RC element of the nth degree thermal model.

In a device according to the invention, additional means for setting protective measures are advantageously provided, which become active when the calculated substitute barrier layer temperature exceeds a defined response wave.

For practical applications, thyristors, diodes or power transistors are used as semiconductor valves for the semiconductor switch.

The means for determining the switching current mean value are implemented by a current converter and an analog / digital converter. in order to supply the switching current or load current and, in a further embodiment, the ambient temperature in digital form to the calculation means.

Furthermore, an advantageous embodiment of the device has an input / output module coupled to a control panel for the selection of the protective measures by the user.

In addition, a device for the early detection of a supply failure can be installed, which initiates the securing of current replacement partial barrier layer heating.

In cooperation with this early detection, a binary heating detector can also be provided, which is activated in the event of a supply failure and, when the power supply is resumed, supplies a binary signal which indicates whether the barrier layer of the semiconductor device has cooled to ambient temperature.

In one embodiment of the invention, means for measuring the ambient temperature are provided, which deliver a measured value to the calculation means in defined time intervals.

The device can be provided with a timer which outputs a signal for starting the temperature calculation for each switching period.

In many practical applications, the monitoring device is provided with a display for issuing warnings and error messages.

Further advantages and details of the present invention, as well as the basics of the theoretical model used and an exemplary embodiment are explained in more detail with reference to the attached FIGS. 1 to 7. In these show: FIG. 1 shows an equivalent circuit diagram of the thermal model used,

Figure 2 shows the time course of a rectangular

Switching currents, the associated forward loss and junction temperature of a semiconductor valve,

FIG. 3 shows a polygon approximation of a real pass characteristic,

FIG. 4 shows the time course of transient thermal resistances for direct current,

FIG. 5 shows a structogram of the main routine of the program code processed in the microcomputer unit for determining the equivalent junction temperature of the semiconductor device,

FIG. 6 shows the thermal monitoring of a converter for speed control of a DC machine as an application example for the invention

Figure 7 is a schematic diagram of the microcomputer unit used in Figure 6 for thermal monitoring.

For a better understanding of the n-degree thermal model shown in FIG. 1, which is used for the calculation according to the invention of the equivalent junction temperature of a semiconductor device, the values in each switching period 1, 2, 3, ... , q-1, q, q + 1, ... calculation steps to be carried out, provided that the ambient temperature is measured and all power loss components with the exception of the forward power loss are neglected, based on the following theoretical consideration:

For the time being, the transmission power loss Pτ, q converted during the conducting period t _{p on the basis of} the switching current substitute value I, qi ^m semiconductor valve has ^to be determined. A polygon approximation of the pass characteristic of the semiconductor device V shown in FIG. 3 is used for this, the calculation of the pass loss conduction, as shown in FIG. 2, based on a rectangular current pulse with the width tp and the amplitude Iτ, q. The forward power loss ^p τ, q determined on the basis of the polygon approximation of the forward characteristic according to FIG. 3 is calculated according to the following relationship, which is known from the prevailing teaching:

(1) ^p T, q = ^U T0 ^I T, q ^{+ r} T ^τ τ, q "

where U- o is the lock voltage and r * _j * is the equivalent resistance of the semiconductor valve V. The index q indicates the number of computation cycles (switching periods started).

The peak value of the equivalent junction temperature Θg _{/ Inax} for the clock cycle q shown in FIG. 1 results from the following relationship:

n ■ ^' 2) Θ _qftnax = Θ _{a +} P _{T; q} R _th + Σ e-TΛi AΘ _{± ι q} . _lt i = l

where Θ _{a is} the ambient temperature, T is the duration of the switching period and t is the conducting time. ΔΘ-j ^ qi are the previous values of the replacement partial junction heating of the respective RC elements i of the n-th order thermal model and the quantity R _t h with the dimension of a thermal resistance is according to equation (3) below with the partial thermal resistance - stand between r _t ^ _j _, the time constants τ * j_ of the individual RC elements of the thermal model and the lead time tp.

n (3) R _th = ∑ r _{thf i} (1-e-tpAi) i = ι

If the partial heat capacity C ^ - i in addition to the partial heat resistance ^r th i is known ^{as a} model parameter for the i-th RC element of the thermal model, the associated time constant τ * j_ is calculated according to the known relationship * c "th, i ^{= τ} i- ^c th, i. Two time profiles of the transient heat resistance of the Zth for direct current, one for the heat transfer between the barrier layer environment (JA = junction to ambient) and one time for the transition between the barrier layer housing (JC = junction to case), are shown in Fig. ready to 4, wherein R ^ YES ^{un <^} ^R th JC ^ ^e supplied ^¬ impaired final values for large time, the so-called stationä¬ ren thermal resistances call. at the time specified in relationship (2) calculation of the absolute replacement Sperrschichttem The temperature of a switching period q generally requires the complete set of the replacement partial barrier layer heaters Θi, π- _l calculated in the previous switching cycle q-1, which are updated for the next cycle q + 1 takes place according to

(4) ΔΘ _{i / q} = ΔΘ ^ q.- _{L +} P _{T / q} r _{th ±} (l- e ^ p / H); i = 1.2 n.

The derivation of the relationships (2) to (4) has linearity and time invariance of the thermal model under consideration and was carried out using the convolution theorem of the Laplace transformation.

After their appropriate standardization for overflow-proof and time-saving processing in fractional fixed-point arithmetic, equations (1), (2) and (4) take the following form, with the dimensionless quantities occurring here being written down in small letters: ; 5. PT, q = ^c 2 i ^ q (c- ^X T, ₍

n

< ⁶ - ^Θ q, max = ^Θ a + ^k PT, q ^{+ Σ e} i ^ΔΘ i, q- l < ^{mi te} i = e ^{"τ τ} i, i = l

(7) ΔΘ _{i / q} = ΔΘi _^ g. ! + k _± p _{T /} g;

^c _l * ^c 2 ^unc ^ - ^ i are dimensionless quantities, where k = Σ k ^.

The method designed to improve the current utilization for a semiconductor valve to be monitored is now extended to a system of semiconductor switches to be monitored, each consisting of at least one semiconductor valve, and using the equation systems (1) to (4) or (5) described above ) to (7), that is also described in detail on the assumption of the above-mentioned requirements on which these equations are based.

The real semiconductor valves of the system under consideration, for which it is assumed that the same characteristic curve data apply throughout, are replaced by a single fictitious replacement valve V. This substitute valve V is assumed that during the switching period q it is loaded with a switching current substitute value Iτ, q for the conductance tp of the real semiconductor valves, which is obtained, for example, by appropriately averaging the real switching current curve Is, over the conductance tp . With different loads on the individual semiconductor valves, the maximum of these mean values is expediently taken. In the embodiment of a line-guided converter explained later, for example, the mean value of the load current IL () is selected. Such a temporal valve current curve is shown in simplified form in FIG. 2 below. Using equation (1) or (5), the amplitude of the in the replacement valve implemented forward power loss Pp "or pp _Q are calculated, a polygon approximation of the forward characteristic curve shown in FIG. 3 being used for this calculation. This polygon approximation is carried out with those values for the lock voltage U-pn and the equivalent resistance rp that are specified by the manufacturer for the maximum permissible barrier layer temperature θ. The associated time profile of the forward power loss used for calculating the junction temperature is shown in FIG. 2 above.

Based on the relationship (2) and (3) or (6), the current peak junction temperature Θ _{g max} or Θq, max of the replacement valve, which according to FIG. 2 is assigned to the negative edge of the current or forward loss power pulse is closed and recursively calculated as the sum of the ambient temperature and the n partial junction heating for the switching cycle q, namely as a folding integral depending on suitable previous values of the replacement partial junction heating for the switching cycle q-1. If necessary, the value of the junction temperature Θg can also be calculated at any other time.

This calculation is based on the n-degree thermal model seen in FIG. 1, which consists of a series connection of n RC elements, the heat resistances r _t h and capacitances C * j_ and time constants τ ^ of this equivalent circuit being specified by the manufacturer are or are determined from the time course of the transient thermal resistance of the heat transfer in the junction environment shown in FIG.

Suitable predecessor values for the replacement partial barrier layer heating can be provided in different ways. When the junction temperature of the semiconductor valves in operation (switching cycle q) is continuously monitored, they become during the previous one Switching period q-1 values calculated according to equation (4) or (7) are used. These values are updated in each switching period after calculation of the equivalent barrier layer temperature for use in the next switching cycle. At the beginning of the thermal monitoring (1st computing cycle), stored values representative of the cold semiconductor valve are used for the respective n replacement partial barrier layer heating. When the semiconductor switch is switched off or the power supply fails, the predecessor values are saved with relevantly large time constants τ * j_. The amount of data stored in a resistant manner depends primarily on the time available after the detection of a supply failure, the available memory space and the memory access time of the storage medium provided for this purpose, for example an EEPROM. If these conditions are so unfavorable that updated replacement partial junction heating is lost, they are replaced by worst-case values permanently stored (for example in an EPROM) when operation is resumed.

In this context, it can be assumed that a known binary heating detector in the form of a time measuring element is provided in the system to be monitored, which is started after detection of a supply failure or after the system has been switched off and starts immediately, a reference time, for example, for cooling Count the necessary cooling time of the barrier layer of the replacement valve to the ambient temperature. When operation is resumed, this time measuring element uses a control bit to indicate whether this reference time has already passed or not, after which the suitable previous values are selected in accordance with the criteria mentioned above. If the reference time has not yet elapsed (warming state "warm"), the values saved before switching off or the so-called worst-case values are used as previous values. Otherwise, the predecessor values are replaced by the predecessor values associated with the "cold" heating state, which are generally permanently stored. Is not a binary warming provided that the previous values are used for the replacement high-temperature layer heating, which can be assigned to the warming state "warm" (either the values saved before switching off or the worst-case values), regardless of whether the semiconductor valve already exists cooled down or not.

As long as the selected response threshold is not exceeded, the program-controlled thermal monitoring permits any conceivable form of overloading of the current-carrying semiconductor valves. If the calculated temperature exceeds the response threshold, the protective measures specified by the user are taken. At least the switching current is reduced on average over the switching period to the value of the continuous limit current. When the power supply is completely switched off, in many cases, for example in the case of a crane or elevator, mechanical fuses are activated at the same time in order to avoid accidents.

FIG. 5 shows a structogram of the main routine in which the above-mentioned standardized equations (5), (6) and (7) are processed. Because of the time-saving and overflow-proof processing, these equations are implemented in fixed-point fractional arithmetic. In the algorithm shown in FIG. 5, in addition to the prerequisites mentioned at the outset, constancy of the values for the lead time t _p and switching period T is required, as is the case, for example, for line-guided converters. The steps of this algorithm denoted by A to F are explained in more detail below.

In the first calculation step A, the amplitude of the forward loss power implemented in the replacement valve is calculated according to relationship (5).

In step B, the first summand is calculated from equation (6) and added to the ambient temperature. In the next step C, the routine is informed using a bit INIT whether the current computing cycle was preceded by an arbitrary shutdown or a supply failure (INIT = 0) or not (INIT = 1), after which the routine branches accordingly.

If INIT = 0, the information provided by the heating detector about the heating state of the semiconductor valves also flows into the calculation step D via a further control bit ANF_ERW. In this step D of the main routine, the third summand from equation (6) is processed while accessing the stored model parameters (heat resistances ^ - ^ -j_, heat capacities C ^ - ^ or time constants t-j_ of the RC elements), wherein with INIT = 0 and ANF_ERW = 0 the permanently stored replacement partial junction heating for the heating state "cold" and with INIT = 1 the replacement partial junction heating updated in the course of the previous computing cycle are processed. With INIT = 0 and ANF_ERW = 1, the h first (h <n) stored replacement layer heaters, which were updated in the course of the previous computing cycle, are first processed as predecessor values. Permanently stored worst-case values are then called for the remaining (n-h) RC elements of the thermal model. After step D has ended, the INIT bit is set to INIT: = 1 in any case.

In the next program step E, if the response threshold is exceeded, the protective measures defined by the user are triggered by the value calculated for the replacement barrier layer heating.

In a final calculation step F, the processor updates the replacement partial barrier layer heating for each element i of the thermal model in accordance with relationship (7) and makes it available for use in the next calculation cycle. Then the processor can turn to other tasks. until a signal supplied by a clocked timer orders the execution of the next computing cycle (steps A to F).

FIG. 6 shows an application example of thermal monitoring using a speed-controlled direct current drive. As can be seen in FIG. 6, a three-phase converter SR3 takes over the function of the load converter. On the DC side, it feeds the armature circuit of a GM DC machine.

For control at a fixed speed and load value by a fixed mechanischeε moment of resistance at the Maschinen¬ wave arises in connection with the control of the speed, a fixed Laststromistwert Ii _s t _<ie a stationary operating state. If the six-pulse three-phase bridge circuit that is normally used is assumed for the load converter, then a different pair of converter valves of the converter leads this current I-j_ _Sf I course of each clock cycle during each 60-degree phase interval a participation of all converter valves in the load current supply.

Not only in the steady state, but also in every dynamic operating case or when regulating to an arbitrary temporal speed curve, ie in the case of an almost arbitrary temporal load current curve, all real converter valves become one for them using the fiction which has already been explained in detail representative replacement valve, which is assumed to be loaded with the load current mean value over a switching period during its lead time, continuously thermally monitored by the microcomputer unit in real time. If the computing power, computer load and memory data allow it, the program-controlled thermal monitoring can be decisively tightened, refined and accelerated in the reaction by the replacement junction temperature of the respective in each sixth period immediately preceding sixth period of the commutated thyristor is calculated taking into account the respectively valid predecessor values for the replacement partial barrier layer heating.

6 also shows, among other things, the input or output signal bundle which the microcomputer has to process or deliver in connection with the thermal monitoring and the speed and current control. The load current value is measured with a current transformer SW and converted into the analog signal I act, wt. list.gew ^ ^{st e} ^ - ⁿ Key signal both for thermal monitoring and for the current control subordinate to the speed control. In addition, in one embodiment of the invention, a measuring unit for measuring the ambient temperature MU can be installed on the heat sinks of the load converter. This then supplies the analog signal Θ _a to the microcomputer unit as a measure of the ambient temperature. The control panel enables the user, inter alia, to switch the thermal monitoring on and off and to select the protective measures which are to take effect when the monitoring is activated. If the protective measures are supplemented by warnings and error messages, these, but also, for example, the measured ambient temperature, can be taken from a display.

The microcomputer unit is supplied via the power supply SV, which among other things also contains a VN module for implementing an early warning in the event of a power failure. In the event of a critical drop in the supply voltage, this module sends a digital early warning signal to the microcomputer unit for processing. Another task of the microcomputer unit is, among other things, the control of the phase angle α via the unit for generating the ignition pulses for the thyristors.

In the following some details of FIG. 6 are discussed which are not directly related to the invention in Are related and only serve to explain the function of the DC drive shown.

The speed controller implemented digitally in the microcomputer unit requires the signals n _so n and n-j_ _st as information about the setpoint and actual value of the speed. The target speed is specified by the user, the actual speed is measured by the tachometer machine TM on the shaft.

The direct current machine GM is externally excited, the excitation winding of the direct current machine GM with the inductor Le is thus externally supplied, as is generally the case by a two-phase converter SR2 (field converter). In the armature circuit, the ripple of the actual load current value is reduced to the desired level by a smoothing choke Lg. On the three-phase side, the two converters SR2 and SR3 are equipped with commutation chokes to support commutation, SR3 is protected by string fuses e.g. protected against short-circuit currents. The dash-dotted line in FIG. 6 frames all those assemblies which are combined in the converter device SG.

The microcomputer unit shown in FIG. 7, clocked by a clock generator 14, consists of a microcomputer core with a processor unit 10, an analog / digital converter 12 and a timer 8. Furthermore, there are three digital memories 16, 18 and 20, a time measuring element 22 in this microcomputer unit with associated voltage buffer 24, digital input / output units 26, 28 and 30 and the required bus connections, such as address, data and control bus are provided, only the control bus being shown in FIG. 7 for the sake of simplicity. In the present case, a microcontroller of the type SIEMENS 80C166 clocked at 36 MHz is used as the processor. However, any other commercially available processor with the required computing power can also be used. The RAM 18 is generally used for storing changeable data and the EPROM 16 for storing unchangeable data, such as tables and, inter alia, the programs indicated in FIG. 7. The input and output blocks 26, 28 and 30 provide and pick up the digital information to be exchanged with modules external to the computer. For example, information about the activation or deactivation of the thermal monitoring and the choice of measures for protecting the power output are obtained via the component 26 and the information about the phase angle α is provided for the unit for ignition pulse injection by the component 28. The module 30 takes on the one hand the output of error messages, warnings and possibly values of the ambient temperature, but on the other hand also in cooperation with the other modules of the microcomputer unit and the early warning device VN the function already described in detail in the event of an arbitrary shutdown, failure of the power supply SV or when the short-term protection trips.

The application of the present invention is not limited only to the example of use of a line-guided converter explained in FIG. 6. In principle, all clocked semiconductor switches can be provided with such a device and thermally monitored using the method according to the invention. Further possible applications are, for example, power supplies clocked with a certain duty cycle, such as inverse converters, step-down or step-up converters.

In addition to the advantages of the invention mentioned at the outset, the use of the thermal model for calculating a replacement junction temperature of a semiconductor device by means of a digital computer has the further great advantage that, on the basis of this model, project planning documents can be calculated precisely for many practical applications, and thus the customer for his Select an inexpensive, yet overheat-proof system for a specific application can. The method proves to be particularly cost-saving when applied to a whole type series of semiconductor circuits if all circuit types are operated with one and the same software, which accesses type-specific information when processing type-specific information.

Claims

1. A method for improving the current utilization of one or more semiconductor switches that can be switched on and off, each with at least one semiconductor valve (V), in particular for use in a power converter (SR3), each semiconductor valve (V) periodically during a conduction period (t _p ) is loaded with current and the barrier layer heating of at least one semiconductor valve (V) is monitored at each switching period (q) by introducing a replacement valve (V) which, based on the characteristics of the semiconductor valve (V), uses a thermal model of nth degree from n RC Gates, each with a thermal resistance (--h) and a time constant (τ), and a switching current (Is, q) flowing in the conducting state through the semiconductor valve (V) is used to monitor the junction heating , characterized in that the method comprises the following steps:

a) Determining a switching current substitute value dτ, - 'for example by averaging the switching current (Ig ₍ g) flowing through the semiconductor valve (V) during the conduction period (tp).

b) Calculating a power loss (P * p, q) / converted by the switching current substitute value Üτ,) in the substitute valve (V) /

c) Calculation of a replacement junction temperature (Θ _g ) for the replacement valve (V) with the aid of the calculated value for the power loss (P <p, q) ^and the parameters for the thermal resistances (r _j - ^ -j_) and the time constants (j_) of the individual RC elements of the thermal model, and taking into account the ambient temperature (Θ _a ),

2. The method according to claim 1, dadurchge ¬ indicates that it further comprises the following steps: Provision of a set of n process values (ΔΘ - * _ _g ) for replacement barrier layer heating of the individual RC elements of the replacement valve (V),

f) Closed and recursive calculation of the replacement junction temperature (Θg) of the replacement valve (V) with the aid of the set of n previous values (ΔΘ _{1 (} g).

g) Closed and recursive calculation of the replacement partial barrier layer heating (AΘ _lι „) for use as previous values in the subsequent switching period (q + 1).

3. The method according to any one of claims 1 or 2, that the protective measures are taken if the calculated substitute barrier layer temperature (Θg) exceeds a selected response threshold.

4. The method according to claim 1, 2 or 3, characterized in that the equivalent valve (V) for calculating the equivalent junction temperature (Θ _g ) is subjected to a load by a rectangular current pulse, the amplitude of which is equal to the switching current value d , q- and its pulse width equal to the lead time (t _p )

5. The method according to any one of claims 1 to 4, since ¬ characterized in that the forward power loss is used for the power loss (P * p _; q), characteristic data for calculating the forward power loss being obtained from a polygon approximation, which is a function the lock voltage (U-po) and the equivalent resistance (r * p) at a specific reference temperature, for example the maximum permissible junction temperature ( ^■ &).

6. The method according to any one of claims 1 to 5, since ^{¬ characterized in} that when executing step c) according to claim 1 of the time associated with the end of the lead time (t _p ) peak value of the equivalent junction temperature (Θ _g , max _' of Replacement valve (V) is calculated.

7. The method according to any one of claims 1 to 6, since ^{¬ characterized in} that a currently measured value (Θ _a ) is used for the ambient temperature.

8. The method according to any one of claims 3 to 7, so that the response threshold according to claim 1d) is set such that overlaps with conventional overcurrent protection measures, such as fuses or circuit breakers, are avoided.

9. The method according to any one of claims 1 to 8, so that the maximum permissible barrier layer temperature (τ_>) is selected as the response threshold.

10. The method according to any one of claims 3 to 9, since ¬ characterized in that when exceeding the response threshold of the semiconductor valve (V) the switching current ds, q) i ^m time average over a Schaltperi¬ ode (q) at least to 100% of Continuous limit current is reduced.

11. The method according to any one of claims 1 to 10, since ¬ characterized in that in the event of a power failure, the current set of n replacement Teü- barrier layer heating (ΔΘ * j_ _# q) is at least partially stored for later reuse and 2α

that unsaved values are replaced by permanently stored worst ^ case values.

12. The method according to claim 11, so that a binary heating detector is activated during the supply failure, which, when operation resumes, indicates whether the barrier layer of the semiconductor valve (V) is already cooled to ambient temperature.

13. Device for improving the current utilization of one or more semiconductor switches that can be switched on and off, each with at least one semiconductor valve (V), in particular for use on a converter (SR3), each semiconductor valve (V) periodically during the lead time (t _p ) is loaded with current and the barrier layer heating of at least one semiconductor valve (V) is monitored at each switching period (q) using a replacement valve (V), which consists of an n-th degree thermal model based on the characteristic data of the semiconductor valve (V) n RC elements, each with a thermal resistance (r ^ h) and a time constant (τ-j), and a means (SW) for monitoring the junction heating for detecting the switching current flowing through the semiconductor valve (V) (Is , q) is provided, characterized in that the device has the following:

a) means (SW, 12, 10) for determining a switching current replacement value (Iτ, q) _<for example by averaging the measured switching current (Is, q) over the conductance (tp),

b) means (10) for calculating a power loss (Pτ, q *) converted by the switching current replacement value dτ, q) ^{in the} replacement valve ^{and the} replacement junction temperature (Θg) of the replacement valve (V).

14. Device according to claim 13, so that it also has the following:

c) means (16, 18, 20) for providing a set of n predecessor values (ΔΘj _/ qi) for the replacement partial blocking layer heating of the individual RC elements of the thermal model of the nth degree for the replacement valve (V).

d) Means (10) for the closed recursive calculation of the replacement junction temperature (Θ _g ) of the replacement valve (V) and the current replacement partial junction heating (Δθ _{i # q} ).

15. Device according to claims 13 and 14, since ¬ by g- indicates that it has means (10, 16, 28, 30) for setting protective measures which become active when the calculated replacement junction temperature (Θg) exceeds a selected response threshold.

16. Device according to claim 13, 14 or 15, that the semiconductor valves (V) are thyristors.

17. Device according to one of claims 13 to 16, characterized in that it further comprises: with the calculation means (10) connected analog / digital converter means (12) for digitizing input variables, such as a load current or a prevailing ambient temperature, one with a Control panel coupled input / output module (26) for the selection of the protective measures by the user, a device (VN) for early detection of a supply failure, which initiates the execution of the steps according to claim 11 and means (30) for Output of error messages and warnings.

18. Device according to one of claims 13 to 17, so that it has a means (MU) for measuring the ambient temperature which is connected to the analog / digital converter means (12).

19. Device according to one of claims 13 to 18, so that it has a timer (8) coupled to the calculation means (10), which outputs a signal for starting the method steps according to claim 1 per switching period.