EP1484947A2 - Switch circuit arrangement for driving lamps - Google Patents

Abstract

The circuit has two switches in a half-bridge arrangement. Each switch has a bipolar transistor (B2) and a field effect transistor (F2) with control, working and reference electrodes in a cascode circuit. The half bridge center point is coupled to the at least one load circuit. Each cascode circuit has an input stage (E2) with the series circuit of a diode (D21) and transformer secondary winding (L2) arranged parallel to the bipolar transistor's control electrode and the FET's reference electrode. The transformer primary (L 0) is in the load circuit to carry the load current (IL) when the lamp(s) is operating.

Description

Technical field

The present invention relates to a control circuit for the operation of at least one Lamp in an associated load circuit, in which the connections for the at least one Lamp are arranged with two switches in a half-bridge arrangement.

State of the art

Such a control circuit known from the prior art is shown schematically in FIG. 1. The so-called intermediate _circuit voltage U _{zw is present} on the input side. This is a DC voltage that is usually generated from the mains voltage using circuits familiar to those skilled in the art. Two switches S1, S2 are arranged in series in a half-bridge arrangement and are controlled via a respective input circuit E1, E2, not shown. The connection point of the two switches is connected via a choke L to the lamp La, through which the lamp current I _L flows during operation. On the output side, the two coupling capacitors C _K1 , C _{K2 complete} the circuit. Alternative circuit structures are familiar to the person skilled in the art, but are not described in more detail below, since they are irrelevant for the implementation of the invention. When used in the so-called medium voltage range, the switches S1, S2 must be designed to switch voltages between 400 and 1000 volts. The switching frequency is in the order of 40 to 50 kHz. The duty cycle of the circuit shown in Figure 1 is 50 percent. The network power to be switched is more than 100 watts. In order to enable a relatively simple controllability from microcontrollers or integrated control modules, MOSFETs (metal oxide semiconductor field effect transistors) and IGBTs (insulated gate bipolar transistors) are currently used as switches. Since the forward losses increase with the square of the current in the field effect transistor, and the chip area has to be correlated with the forward losses, MOSFETs become relatively expensive for currents above one ampere and average voltages of approximately 600 volts. The IGBTs, on the other hand, have large transmission losses. In the case of pure bipolar transistors, in which the conduction losses are directly proportional to the current, components designed for such boundary conditions are cheaper because they require less chip area, but their poor dynamic switching behavior has a negative effect. Since the collector current cannot be switched off quickly enough, the switching overlaps with the collector-emitter voltage result in high switching losses.

Presentation of the invention

The present invention is therefore based on the object of a generic control circuit to develop in such a way that they operate at medium currents of approx. 1 to 10 amperes has low transmission losses, at the same time low costs and given controllability from microcontrollers or integrated control modules.

This object is achieved by a control circuit with the features of claim 1.

On the one hand, the present invention is based on the knowledge that a fast Switching can be achieved when combined with a low voltage MOSFET a bipolar transistor is used. The MOSFET therefore only needs the small control voltage for the bipolar transistor and can therefore be designed small and cheap become. The bipolar transistor, whose power loss is only linear with the one flowing through it When electricity is linked, it can be dimensioned at low cost for large currents. This is the advantage of the MOSFET - high dynamics and controllability from an integrated Circuit and that of the bipolar transistor - large, processable at an affordable price Performance - optimally linked.

The second finding on which the invention is based is that such Control circuit can be started easily if part of the in the load circuit flowing energy is transferred to the input circuit of the respective switch. Since it is a bipolar transistor is essentially a current-controlled component, A corresponding control current must be made available to him at the base. To do this a primary winding of a transformer is formed in the load circuit, its secondary windings are arranged in the input circuit of each bipolar transistor and thus the base of the bipolar transistor supply with electricity. To reduce the conduction losses of the bipolar transistors it is preferred to dimension the transformer so that the base current is approx Makes up a fifth of the collector current. With a practical current gain of 20 the bipolar transistor is overdriven by a factor of 4. This results in low transmission losses. While driving the bipolar transistor as a result of the large required Control currents from an integrated circuit would not be possible, this is the case with the MOSFET an essentially voltage-controlled component is very possible.

Cascode circuits with a bipolar transistor and a are from the prior art MOSFET transistor known, but which are used for completely different purposes: So is it is known from EP 0 753 987 D1, such a cascode circuit in which the bipolar transistors controlled by MOSFETS arranged in the emitter, for switching off to use a half-bridge arrangement when the lamp to be operated has aged. In US 5,998,942, see FIG. 4 there, is also a cascode circuit of this type used, but here lies a constant due to the different purpose Voltage at the base of the bipolar transistor 20 on. In US 4,894,587, Figure 6, is also such a cascode circuit is shown, but in contrast to the present Invention, no defined magnetic reversal of the transformer takes place. To one To prevent saturation, it should only be switched on for a short time, then you would have to wait at least twice as long until the magnetic field is reduced again Has. Therefore, such a circuit structure would not be in the present invention used. In addition, only such a switch is used to implement a dim device. In the present invention, the duty cycle of the two switches is Half bridge is essentially 50 percent, ensuring that the transformer is not goes into saturation because it is remagnetized by the other transistor current.

A preferred embodiment of the present invention is characterized in that that a diode is arranged such that it turns on in the case of an npn bipolar transistor Outflow of a positive base current through the secondary winding, in the case of a pnp bipolar transistor an outflow of a negative base current through the secondary winding prevented. This is important because the base current flows through the secondary winding the formation of a voltage between the control electrode of the bipolar transistor and would prevent the reference electrode of the field effect transistor and thus the Formation of a sufficiently high base-emitter voltage. Parallel to the control electrode of the Bipolar transistor and the reference electrode of the field effect transistor can be at least one Diode or a zener diode between the potential of the control electrode of the bipolar transistor and the potential of the reference electrode of the field effect transistor. In order to is at least the voltage at the pn junction of the diode as the base-emitter voltage at the pn junction of the bipolar transistor. An opening of the bipolar transistor can thus be ensured. The same applies when using a Zener diode.

Parallel to the control electrode of the bipolar transistor and the reference electrode of the field effect transistor is also preferably a series connection of an ohmic resistor and a capacitor arranged. This is a simple and inexpensive way to start implement the control circuit according to the invention. Detailed information on this follow below. The control electrode of the field effect transistor is preferred with a integrated driver circuit connected. As already mentioned, it is one Field effect transistor around a voltage-controlled element, which is due to the low requirement can be controlled at control current from an integrated circuit.

The diode or the zener diode, which is parallel to the control electrode of the bipolar transistor and the reference electrode of the field effect transistor between the potential of the control electrode of the bipolar transistor and the potential of the reference electrode of the field effect transistor is preferably dimensioned such that a voltage of at least 1 volt, preferably, is applied to it approx. 2 volts, drops.

The reference electrode of the field effect transistor of each switch is preferably a first Reference potential connected while the control electrode of the bipolar transistor of each switch is connected to a second reference potential via a high-resistance resistor. This resistor is used to supply charge carriers to the base of the bipolar transistor, as long as the secondary winding of the transformer has no charge carriers in the Introduces input circle, especially at the start.

It is also preferred between the control and the reference electrode of the bipolar transistor an ohmic resistor is arranged on each switch. This ensures that the transistor is not switched on at the wrong time by interference pulses when switched off. In a Cascode circuit as here, it can also be used to charge or discharge parasitic Capacities of the field effect transistor serve. Finally, it also increases the dielectric strength of bipolar transistors.

The switches are preferably designed such that they operate at a frequency between 100 Hz and 300 kHz and a voltage of 100 to 1000 volts can be operated. Further advantageous embodiments result from the subclaims.

Brief description of the drawings

Figur 1

in schematischer Darstellung ein Prinzipschaltbild mit einer aus einer Halbbrückenschaltung angesteuerten Lampe;

Figur 2

ein erstes Ausführungsbeispiel eines Eingangskreises einer erfindungsgemäßen Ansteuerschaltung;

Figur 3

ein zweites Ausführungsbeispiel eines Eingangskreises einer erfindungsgemäßen Ansteuerschaltung; und

Figur 4

den zeitlichen Verlauf des Basisstroms, des Kollektorstroms und der Kollektor-Emitter-Spannung bei einem Abschaltvorgang des Schalters der Halbbrückenanordnung in einer erfindungsgemäßen Ansteuerschaltung.

Exemplary embodiments of the invention are described in more detail below with reference to the accompanying drawings. They represent:

Figure 1

a schematic representation of a basic circuit diagram with a lamp controlled from a half-bridge circuit;

Figure 2

a first embodiment of an input circuit of a drive circuit according to the invention;

Figure 3

a second embodiment of an input circuit of a drive circuit according to the invention; and

Figure 4

the time course of the base current, the collector current and the collector-emitter voltage when the switch of the half-bridge arrangement is switched off in a drive circuit according to the invention.

Preferred embodiment of the invention

Figures 2 and 3 show embodiments of the input circuit E2 of Figure 1 in a drive circuit according to the invention. Identical components are provided with the same reference symbols and are only explained once. A bipolar transistor B2 and a field effect transistor F2 in a cascode arrangement form the switch S2. The gate of the field effect transistor F2 is connected via its connection 10 to the output of an integrated driver circuit. A transformer, preferably designed as a toroid, is located in the load circuit with its primary winding L ₀ . Secondary windings are arranged in the respective input circuit, in the present case the secondary winding L2 in the input circuit E2. A diode D21 prevents charge carriers from flowing out of the base via the secondary winding L2. Using a high-resistance resistor R21, which is connected on the one hand to the base of the bipolar transistor B2, and on the other hand with the intermediate circuit _voltage U _zw , charge carriers can be provided on the base. The base of the bipolar transistor, on the other hand, is connected to the reference potential via a parallel connection of a diode D22 and, on the other hand, via a parallel connection of a diode D22 and a resistor R22, on which the reference electrode of the field effect transistor F2 is located. This allows a sufficiently large base-emitter voltage to be generated with which the circuit arrangement can be started. A resistor R23 is used to withstand the voltage of the associated bipolar transistor.

A typical value for R21 is 1 MΩ, a typical value for R22 is 100 Ω. Instead of Diode D22 can also provide a Zener diode, of course in reverse order his.

In FIG. 3, the series circuit comprising secondary winding L2 and diode D21 is connected in parallel with a Zener diode Z2 on the one hand, and the series circuit with a resistor R22 and a capacitor C2 on the other hand. The base of the transistor is in turn connected to the intermediate circuit voltage U _zw via a high-resistance resistor R21 and to the working electrode of the field effect transistor F2 via a resistor R23.

If for example the Zener diode Z2 dimensioned to two volts, upon application, the intermediate circuit voltage U _ZW of the capacitor C2 via resistors R22 and R21 charged to about 2 volts. When the field effect transistor F2 is switched on via a suitable signal at the connection 10, which opens the bipolar transistor B2, the capacitor C2 discharges and leads to a base current I _B of 100 mA when the resistor R22 is dimensioned to 10 Ω. As a result, the switch S2 is switched on for one to two microseconds, a load current I _L begins to flow and a signal is coupled into the input circuit E2 by linking primary winding L ₀ and secondary winding L2, as a result of which the circuit arrangement is started.

In a particularly advantageous manner, the switch-off behavior of the circuit is also improved by this solution. The problem is that when the field effect transistor F2 is switched off, the emitter current I _{E of} the bipolar transistor suddenly goes to zero. However, since the collector current I _C wants to continue to flow, the base is flooded with charge carriers, which leads to long switch-off times. Long switch-off times are associated with the problem that the collector current I _C and collector-emitter voltage U _CE have positive values over a certain period of time. Since the product of these two sizes dominates the transmission loss, this results in undesirably high power losses. Through the parallel connection of diode D22 and ohmic resistor R22 in FIG. 2 and the Zener diode Z2 and the series connection of resistor R22 and capacitor C2 in the embodiment according to FIG. 3, a downsider C2 in the embodiment according to FIG. 3 becomes a on the base side low-resistance branch provided to ground. The collector current I _C can therefore continue to flow to the ground almost unhindered after the field effect transistor F2 has been switched off as a negative base current -I _B. This results in fast switch-off times. The resistor R23 is dimensioned, for example, with 100 Ω and serves to ensure that no current can flow out as long as the field effect transistor is high-resistance.

This is confirmed in FIG. 4 by a graphically represented example measurement on a laboratory sample, the resolution of the base current I _{B being} approximately one hundred times the resolution of the collector current I _C. After the field-effect transistor is switched off, the base current I _B drops to very large negative values, namely to -I _C, and rises again to its zero value after a relatively short time. The collector current also goes to zero after a few oscillations. The collector voltage U _CE increases, but only at a point in time when the collector current I _{C has} already dropped very far. The power loss, see for example the point marked P, which defines the maximum, is very low. Leading line A denotes the zero line for the collector current I _C , reference line D the zero line for the base current I _B.

Figures 2 and 3 show an example of the input circuit E2. It is obvious to the person skilled in the art that the input circuit E1 should be designed symmetrically for this in a corresponding manner is.

Claims (10)

Control circuit for the operation of at least one lamp (La) in an associated load circuit, in which the connections for the at least one lamp are arranged, with two switches (S1, S2) in a half-bridge arrangement, each switch (S1, S2) in cascode circuit having a bipolar transistor (B2) with a control, a working and a reference electrode and a field effect transistor (F2) with a control, a working and a reference electrode, the center of the half-bridge arrangement being coupled to the at least one load circuit, and each such cascode circuit has an input circuit (E1, E2) in which the series circuit of a diode (D21) and a secondary winding (L2) of a transformer is arranged in parallel to the control electrode of the bipolar transistor (B2) and the reference electrode of the field effect transistor (F2), the primary winding (L ₀ ) is arranged in the load circuit in such a way that when the at least one lamp (La) is in operation from the load circuit current m (I _L ) is flowed through. Control circuit according to claim 1,
characterized in that the diode (D21) is arranged in such a way that, in the case of an npn bipolar transistor (B2), a positive base current (I _B ) flows out via the secondary winding (L2), in the case of a pnp bipolar transistor, a negative one flows out Base current prevented through the secondary winding. Control circuit according to one of claims 1 or 2,
characterized in that parallel to the control electrode of the bipolar transistor (B2) and the reference electrode of the field effect transistor (F2) at least one diode (D22) or a Zener diode (Z2) between the potential of the control electrode of the bipolar transistor (B2) and the potential of the reference electrode of the field effect transistor ( F2) is arranged. Control circuit according to one of the preceding claims,
characterized in that a series circuit of an ohmic resistor (R22) and a capacitor (C2) is arranged parallel to the control electrode of the bipolar transistor (B2) and the reference electrode of the field effect transistor (F2). Control circuit according to one of the preceding claims,
characterized in that the control electrode of the field effect transistor (F2) is connected to an integrated driver circuit. Control circuit according to one of the preceding claims,
characterized in that the duty cycle of the two switches (S1, S2) of the half-bridge is essentially 50 percent. Control circuit according to one of Claims 3 to 6,
characterized in that the diode (D22) or the zener diode (Z2) is dimensioned such that a voltage of at least 1 V, preferably approximately 2 V, drops across it. Control circuit according to one of the preceding claims,
characterized in that the reference electrode of the field effect transistor (F2) of each switch (S1, S2) is connected to a first reference potential and the control electrode of the bipolar transistor of at least one switch via a high-resistance resistor (R21) to a second reference potential (U _ZW ). Control circuit according to one of the preceding claims,
characterized in that an ohmic resistor (R23) is arranged between the control and the reference electrode of the bipolar transistor (B2) of each switch (S1, S2). Control circuit according to one of the preceding claims,
characterized in that the switches (S1, S2) are designed to be operated in operation with a frequency between 100 Hz and 300 kHz and a voltage of 100 to 1000 V.

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