EP1275203A1

EP1275203A1 - Circuit for reducing switching losses in electronic valves

Info

Publication number: EP1275203A1
Application number: EP01923373A
Authority: EP
Inventors: Wolfgang Croce; Günther Danhofer
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-04-12
Filing date: 2001-04-12
Publication date: 2003-01-15
Also published as: AU2001250144A1; WO2001080425A1

Abstract

The invention relates to a circuit for reducing switching losses in electronic valves, wherein a saturation coil (2) for reducing switching losses is mounted in series relative to the electronic valve (1). In order to reduce losses as effectively as possible both during connection and disconnection, an electronic auxiliary valve (3) is connected in parallel to the electronic valve (1) by means of a capacitor (4) and a serial circuit consisting of at least one inductive resistor (6) and a diode (7) is mounted in parallel to the electronic auxiliary valve (3) valve for discharging the capacitor (4), wherein the electronic auxiliary valve (3) is triggered before the electronic valve (1) is disconnected so that the auxiliary valve (3) absorbs the current flowing through the electronic valve (1), thereby minimizing power loss of the electronic valve (1) during disconnection. A saturation coil (5) for reducing the connection losses of the auxiliary valve is also advantageously mounted in series relative to said auxiliary valve (3).

Description

Circuit for reducing the switching losses of electronic valves

The application relates to a circuit for reducing the switching losses of electronic valves, a saturation coil for reducing the switching losses being arranged in series with the electronic valve.

Controlled and uncontrolled semiconductor components are subsumed under the term electronic valves. The controlled semiconductor components are controlled either by the electrical current supplied to an electrode or by the electrical potential applied to such an electrode. These include, for example, bipolar and unipolar transistors, thyristors or comparable electronic components. Diodes in which the current is a function of the voltage are, for example, uncontrolled electronic valves.

Various losses occur when switching or controlling such semiconductor components. A distinction is made between transmission losses on the one hand and switching losses on the other. The transmission losses dominate at low switching frequencies and long switching times, while at higher switching frequencies the switching losses make up a significant part of the total losses. To dissipate the not inconsiderable power losses, heat sinks or the like are often used. necessary, which often take up the largest part of the volume of the electronic circuit.

To reduce the switch-on losses, for example of transistors, coils are connected in series with the transistor. The inrush current can be delayed by appropriate dimensioning of the coil, as a result of which the power loss, characterized by the integral of the product of current and voltage, can be reduced. Similarly, a capacitor can be used in parallel with the transistor to reduce the voltage steepness. However, the maximum operating frequency of the electronic valve is greatly reduced by the inductance or capacitance. The power loss that normally occurs in the transistor is also destroyed in another component, for example in the coil or the capacitor, with damping resistors connected in series, as a result of which there is in fact only a displacement, but not a real reduction in the power loss. Such saturation coils or step chokes for reducing the switch-on losses of electronic valves are known for example from DE 35 42 751 AI, DE 33 34 794 AI or DE 28 29 840 AI.

To reduce the switch-off losses, it is known to use uasi resonance circuits, which, however, do not allow the valve to be completely switched off.

When using thyristors as electronic valves, it is known, for example from the document "Power Electronics: Fundamentals and Applications" by Rainer Jäger, VDE Verlag Berlin 1980, pages 165-181, to arrange a further thyristor in parallel via a capacitor in order to quench the thyristor. A swinging circuit, which is arranged parallel to the auxiliary thyristor, serves to ensure a periodic function of the thyristor. The capacitor is recharged via this oscillating circuit, which consists of an inductor and diode in series, for example. However, the arrangement does not reduce the switching losses of the thyristor.

In order to reduce both the switch-on and switch-off losses, compromises are usually made between valve relief and the maximum operating frequency, since it has so far not been possible to combine the circuits for reducing the switch-on losses and the switch-off losses with a reasonably high operating frequency.

Known methods for limiting the switching losses of electronic valves have the disadvantage that, in turn, power in the externally connected components is converted into heat, and effectively only the power loss is shifted from the electronic valve to other components. The reduction of the power loss peaks is often only achieved by temporarily storing the power, which loads the electronic valve with a time delay.

For example, US Pat. No. 5,341,004 A discloses a semiconductor circuit with reduced switching losses, in which a second semiconductor element, which is of the same type as the first semiconductor element, is applied in parallel to the semiconductor element, for example the transistor. The second semiconductor element has a higher saturation voltage and a shorter fall time than the first semiconductor element. Because the faster semiconductor element during the switch-off phase the current takes over, the turn-off losses of the first semiconductor element can be reduced in accordance with the shortened switching time.

The same circuit arrangement is also shown, for example, in JP 6290863 A.

The object of the invention is to achieve the most effective possible reduction of both the switch-on and switch-off losses in electronic valves. The disadvantages of known systems are to be avoided or at least reduced. The maximum frequency of the electronic valve should not be significantly reduced by the circuit according to the invention and the circuit according to the invention should be as simple as possible and therefore inexpensive.

The object of the invention is achieved in that an electronic auxiliary valve is arranged parallel to the electronic valve and a series circuit of at least one inductor and a diode for discharging the capacitor is arranged parallel to the electronic valve, the electronic auxiliary valve being activated before the electronic valve is switched off , so that the auxiliary valve takes over the current flowing through the electronic valve and thus the power loss of the electronic valve is minimized during the switch-off process. The known saturation coil connected in series with the electronic valve has an inductance which is variable as a function of time or current, which is as large as possible when the electronic valve is switched on and as small as possible after the switch-on process, so that the current is opposite the voltage of the electronic valve is delayed during the switch-on process and the power loss of the electronic valve is minimized during the switch-on process. This inductance ensures that the current is delayed so much during a switch-on process that the power loss occurring in the electronic valve, characterized by the integral of the product of the voltage and the current, is almost negligible, while the inductance decreases so strongly after the switch-on process , so that there is no significant limitation of the maximum operating frequency of the electronic valve. The coil can be arranged anywhere in the current path of the electronic valve. The electronic auxiliary valve, which is arranged parallel to the electronic valve via a capacitor, becomes controlled before the electronic valve is switched off, so that the circuit branch takes over the current flowing through the electronic valve. This also minimizes the power loss of the electronic valve during the switch-off process. A series connection of at least one inductor and one diode for discharging the capacitor is provided in parallel with the auxiliary valve. As a result of both the reduced Einais and the reduced turn-off losses, circuits with such electronic valves can be made significantly smaller, since the heat sinks do not have to be as large or the electronic valves themselves can be dimensioned smaller. The efficiency of circuits with such electronic valves can also be increased.

If a saturation coil is arranged in series with the electronic auxiliary valve to reduce the switch-on losses of the auxiliary valve, the total switching losses can be reduced further.

According to a further feature of the invention, the non-linearity of the inductance of the coil is achieved in that it is formed by a choke with a magnetic core, and in that the choke is dimensioned such that it saturates immediately after the electronic valve or the electronic auxiliary valve is switched on goes. The choke loses its inductive resistance in saturation, since all elementary magnets of the core material are magnetized. Thus, the choke does not limit the effective operating frequency. Since the choke hardly stores any more energy in the magnetic field in saturation, the losses and overvoltages generated and their energy content are very low and can be fed back into the supply by suitable circuits, for example. By choosing the material of the magnetic core, the number of turns, the core volume and the voltage, the time and the type of transition of the choke into saturation can be precisely determined. As a result, the circuit according to the invention for reducing the switch-on losses can be adapted exactly to the respective application.

In order to achieve as little hardware expenditure as possible, a microcontroller is provided for controlling the electronic valves and electronic auxiliary valves. This allows the circuit to be sentences are adjusted.

The features of the present invention are described in more detail with reference to the accompanying figures. In it show:

1 is a schematic circuit consisting of a transistor as an electronic valve,

2a-2d the time profiles of some characteristic quantities according to FIG. 1,

3 shows the circuit according to FIG. 1 with a saturation choke to reduce the switch-on losses,

4a-4d the time profiles of some characteristic quantities according to FIG. 3, and

5a - 5c the time profiles of the switching current, the current through the saturation inductor and the inductance of the saturation inductor during a switch-on process,

6 shows an embodiment variant of the circuit according to the invention for the combined reduction of the switch-on and switch-off losses of transistors, and

7a-7h the time profiles of some characteristic quantities of the circuit according to FIG. 6.

Fig. 1 shows a transistor T as an electronic valve. The transistor T switches a voltage U ₀ to a load, represented by a series connection of a load resistor R _L and a load inductance L _L. To control the circuit, a corresponding base current I _{B is} applied to the base of the transistor T.

The time profiles of the base current I _B , the collector-emitter voltage U _CE , the collector current I _c and the resulting power loss P _v during a switching on and off process of the transistor are shown in FIGS. 2a to 2d. The courses are only schematic representations. After switching on, the collector current I _c gradually increases to its maximum value after a certain switch-on delay time t _d . The collector-emitter voltage U _CE gradually drops to a minimum value which depends on the transistor type and the collector current I _c . During the switch-off process, the collector-emitter voltage U _CE gradually rises again during the so-called storage time t _s and the collector current I _c then drops to a negligible residual current. The power loss P _v during a period of time Δt = t _B -t _A is determined by the following relationship:

It is clearly evident from the course of the power loss P _v according to FIG. 2d that relatively high power loss peaks occur during the switch-on phase and during the switch-off phase. Between the on and off phase, the total power loss P _{v is} only determined by the forward power loss. In order to reduce the power loss P _v during the switching processes, it is necessary to keep the voltage or the current as low as possible during the switch-on or switch-off process according to the above formula, so that the integral of the product of voltage and current is as low as possible.

Fig. 3 shows a simplified circuit of an electronic valve in the form of a transistor _x with a saturation inductor L _x in series. It is irrelevant at which point the inductance L _{x is} interposed. With suitable dimensioning of the saturation choke L _x , the switch-on losses can be almost completely eliminated, so that only the passage losses and the switch-off losses are decisive for the heat balance of the transistor T _x or the electronic valve, and the switching frequency is neither due to thermal losses nor due to insufficient current rise and waste time is limited.

4a to 4d, which show the time profiles of the base current I _B , the collector-emitter voltage U _CE , the collector current I _c and the resulting power loss P _v during an on and off operation of the transistor T _x according to the circuit in FIG. 3 show, the advantage is clear in comparison to FIGS. 2a to 2d. After the switch-on process by increasing the base current I _B , the steepness of the current I _c is reduced as much as possible by the saturation choke h ₁ . If a normal inductance were used, however, the increase in the collector current would be reduced so much by this measure that the maximum achievable switching frequency of the transistor T _x would become impermissibly small. For this reason, it is sought that after the collector-emitter voltage U _{CE drops,} the collector current I _{c increases} as quickly as possible. This is achieved through use a saturation choke h _x with a magnetic core, which is dimensioned so that it saturates immediately after the collector-emitter voltage U _CE drops and thus has a very low inductance. At this point in time, the product of the collector-emitter voltage U _CE and collector current I _c no longer makes any significant contribution to the switch-on power loss. With this measure, the start-up losses can be reduced in such a way that they are negligible compared to the forward losses.

5a shows the base current I _{B of} the transistor T _lt, which represents the switch-on process of this controlled semiconductor. 5b shows the corresponding time profile of the current I (t) through the saturation inductor L _x and in FIG. 5c the inductance L (t) of the saturation inductor L _x as a function of the time t during the switch-on process. After switching on, the current rises very slowly due to the relatively high inductance of the saturation choke _x . By appropriate dimensioning of the saturation choke h _{x it} can be achieved that at a precisely defined current I _s , given by the operating voltage and the already elapsed switch-on time, the saturation choke L _{j ^} goes into saturation. The area of the core saturation is characterized in that the magnetic flux Φ or the induction B cannot be significantly increased despite the increase in the current in the saturation choke L _x . In the area of saturation, almost all elementary magnets of the core material are aligned in the preferred direction. In the area of saturation, the inductive resistance of the winding drops, as a result of which only the undesirable ohmic component of the resistance limits the current in the winding. The inductance of the saturation inductor L _x therefore drops to a minimum value L _min . This is mainly determined by the number of turns and the core material of the saturation choke L _x . The current I (t), on the other hand, now increases more rapidly to its maximum value I _max limited by the load. The dimensioning of the choke L _x is preferably carried out by suitable selection of the magnetic core material, the number of turns and the core volume. These parameters influence not only the point in time t _s at which the inductance L _x goes into saturation, but also the behavior of how the transition to saturation takes place, that is to say, for example, the steepness of the current rise in the area of saturation of the saturation choke h ₁ .

6 shows a circuit according to the invention for combining th reduction in turn-on and turn-off losses of a transistor T _x . Compared to the circuit according to FIG. 3, an auxiliary transistor T ₂ with a further saturation inductor L _{2 is} connected via a capacitor C in parallel to the transistor 1 _X and the saturation inductor L _x connected in series. The saturation inductor L ₂ minimizes the turn-on losses of the auxiliary transistor T ₂ in the same way as the saturation inductor L _: minimizes the turn-on losses of the main transistor T ₁ . The parallel to the auxiliary transistor T ₂ and the saturable reactor L ₂ connected diode D _x and inductance L ₃ is used to reverse the polarity of the capacitor C according to any one switching cycle, so that the starting conditions for a renewed switching cycle to be created. Often, a diode D _{2 is} used parallel to the output (load R _L and L _L ), which enables the demagnetization of the inductance L _L , so that there are no impermissibly high voltage peaks when switching off.

The mode of operation of the circuit according to FIG. 6 is explained in more detail on the basis of the time profiles according to FIGS. 7a to 7h. In it are the time profiles of the base current I _B1 , the collector-emitter voltage U _CE1 , the collector current I _cl and the resulting power loss P _{vl of} the transistor T ₁ and the base current I _B2 , the collector-emitter voltage U _CE2 , the collector current I _c2 and the resulting power loss P ^ of the auxiliary transistor T _{2 shown} during a switching on and off of the transistor T _x . The switch-on process of the main transistor T _x corresponds to that which has already been shown and explained in FIGS. 4a to 4d. Before the switch-off process, the auxiliary transistor T _{2 is switched} through by a corresponding base current I _B2 . The collector current I _{c2 also} increases due to the inductor L _{2 with a} delay. The secondary branch formed by transistor T ₂ finally takes over the current, so that after reaching the maximum of collector current I _C2, main transistor T _x no longer carries collector current I _C1 and can be switched off by base current I _B1 . Since the collector current I _{cl of} the transistor _{x has} already dropped to its minimum at this point in time, the turn-off power loss of the transistor T _{x is} reduced to a minimum. The current I _c2 drops automatically due to the charge on the capacitor C. At the time the transistor T ₂ is switched off, it carries only a minimal residual current I _c2 , so that the power loss when the T _{2 is switched off is} also reduced. The negative collector-emitter chip _voltage U _CE2 after the switching on of the transistor T _x stems from the swinging over of the capacitor C via the diode D _x and the inductance L ₃ . The switching losses according to the invention are reduced by combating the cause, namely the simultaneous occurrence of a current and a voltage.

The wiring according to the invention is characterized by particular simplicity and particular effectiveness. As a result, all circuits which contain electronic valves or switches can be made significantly smaller or deliver a higher output power with the same volume.

The circuits according to the invention offer particular advantages when used in devices in which very high currents and / or voltages and therefore also very high power losses occur. These include, for example, welding devices in which very high currents usually occur or ballasts for gas discharge lamps.

Claims

Claims:

1. Circuit for reducing the switching losses of electronic valves, a saturation coil (2) for reducing the switching losses being arranged in series with the electronic valve (1), characterized in that an electronic one is connected via a capacitor (4) parallel to the electronic valve (1) Auxiliary valve (3) and a series connection of at least one inductor (6) and one diode (7) for discharging the capacitor (4) is arranged in parallel to the electronic auxiliary valve (3), the electronic auxiliary valve (3) before the electronic valve ( 1) is controlled so that the auxiliary valve (3) takes over the current flowing through the electronic valve (1) and thus the power loss of the electronic valve (1) is minimized during the switch-off process.

2. Circuit according to claim 1, characterized in that a saturation coil (5) for reducing the switch-on losses of the auxiliary valve (3) is arranged in series with the electronic auxiliary valve (3).

3. Circuit according to claim 1 or 2, characterized in that the saturation coil (2, 5) is formed by a choke with a magnetic core, and that the choke is dimensioned such that it is immediately after the switching on of the electronic valve

(1) or the electronic auxiliary valve (3) goes into saturation.

4. Circuit according to one of claims 1 to 3, characterized in that a microcontroller is provided for controlling the electronic valves (1) and electronic auxiliary valves (3).