Combined Bipolar and MOSFET Switch

The present invention relates to an electronic switch with a MOSFET and a bipolar transistor, in which the collector-emitter section of the bipolar transistor and the drain-source section of the MOSFET are connected in series.

Such a switch has been described, for example, in the periodical "Elektronik," 23/1981, pages 93 to 96. This circuit takes into account the fact that MOSFETs which handle large voltages, for example, those of over 300 V have a relatively high on-state DC resistance R.sub.on. The combination of bipolar transistor and a MOSFET makes it possible to use a MOSFET designed to handle low voltages with a low on-state DC resistance and a high voltage bipolar transistor, with an on-state DC resistance which is lower than that of a MOSFET with a corresponding voltage handling capability. The disadvantage of the series circuit described above is due to the fact that a separate source of DC voltage is required for the bipolar transistor, or else a capacitor that is dynamically charged through a transformer and whose capacity must be attuned to the clock frequency of the switch must be used, in order to prevent saturation of the bipolar transistor.

The object of the invention is to provide an electronic switch of the type referred to but with the additional features that its operation is independent of the clock frequency, and that a separate voltage source for the bipolar transistor is unnecessary.

This object and others are achieved by providing a switch with the following characteristics:

(a) A series circuit comprising a bipolar transistor and a MOSFET is arranged in parallel with a series circuit comprising another MOSFET and a threshold element.

(b) The threshold element is placed between the base terminal of the bipolar transistor and the terminal of the MOSFET that is not connected to the bipolar transistor.

(c) A control signal is fed to the gate terminals of the MOSFETs.

In the case where the switches must drive an inductive load with revovery operation, the signal is routed through a delay element that becomes active when the circuit is turned on. The delay element may comprise a resistor and a diode connected in parallel.

In a preferred embodiment the bipolar transistor, referred to above, comprises a Darlington amplifier to provide greater load handling capabilities.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments, and from the claims.

For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention and to the accompanying drawings.

FIG. 1 is a schematic of a first switch embodiment.

FIG. 2 is a schematic of a second switch embodiment.

FIGS. 3A, 3B, 3C and 3D show characteristic voltage/current waveforms for the circuit of FIG. 2.

FIG. 4 provides a further embodiment of an electronic switch.

The switch shown in FIG. 1 has a series circuit consisting of a high voltage bipolar transistor 1 and a relatively low voltage MOSFET 2. It passes through a load 5 to a supply voltage U.sub.B. To the above-mentioned series circuit consisting of a bipolar transistor and a MOSFET is added, in parallel, a series circuit consisting of another MOSFET 3 and a threshold element 4. The threshold element 4, which may be a Zener diode, a diode or several diodes connected in series, is placed between the base terminal of the bipolar transistor 1 and the terminal of the MOSFET 2 that is not connected to the bipolar transistor, in this case the source terminal. The other MOSFET 3 is connected to the supply voltage and therefore has a high voltage cut-off capability. The gate terminals of MOSFETs 2 and 3 are connected to an input terminal, to which a control voltage u.sub.1 can be applied.

In explaining the operation, it will be assumed that no control voltage is being applied, so that the electronic switch is closed. At the output of the series circuit consisting of bipolar transistor 1 and MOSFET 2, there is present the voltage u.sub.3, which corresponds to the supply voltage U.sub.B. When a control voltage u.sub.1 is applied, MOSFETs 2 and 3 are conductively driven. This drive has no output, other than the dielectric losses in the MOSFETs. Through MOSFET 3 there then flows a current i.sub.1, while MOSFET 2 initially remains without any current, since the bipolar transistor 1 is still cut off. However, the emitter potential of the transistor 1 is approaching zero. As a result, the current i.sub.1 flows at least in part as a base current into the bipolar transistor 1, which it opens. If the threshold voltage of the threshold element 4 is higher than the sum of the base-emitter resistance of the bipolar transistor 1 and the drain-source resistance of MOSFET 2, then the flow of current through the threshold element 4 stops. The load current now flows almost entirely as current i.sub.2 through the bipolar transistor 1 and MOSFET 2. Through MOSFET 3 flows only the control current i.sub.1 for the bipolar transistor 1. Since this current is usually small, the losses in MOSFET 3, which has a relatively high resistance, are negligible. The electronic switch closes, when the input voltage u.sub.1 reaches zero.

The switch shown in FIG. 1 can also be used, with minor changes, for inductive loads with recovery operation, as occurs for example in the case of motor-regulating units. A switch of this kind is shown in FIG. 2. The load here consists of an inductance 6 and a recovery diode 7. Such an arrangement is driven at a frequency of, for example, about 10 kHZ. In this case a mediumsized load current I flows through the necessary diode 7 in the timing gaps. If the electronic switch is then turned on, it will operate in a short circuit mode until the diode 7 recovers, and is thus exposed to the keenest stresses.

The switch itself is distinguished from the one in FIG. 1 mainly in that the control signal u.sub.1 is routed to the MOSFET 2 through a delay element 8. This delay element is effective for voltages of the polarity that cause MOSFET 2 to become conductive. This is shown through a diode 9 which is wired in parallel with the delay element 8. When MOSFET 2 is turned off its gate-source capacitance can be rapidly discharged, and the transistor is quickly cut off. The delay element 8 may consist, for example, of a resistive element.

When a control voltage u.sub.1 is applied, only MOSFET 3 is initially turned on, due to the delay element 8. The current then flows first only through the threshold element 4, which, as before, may be a Zener diode, a diode or several diodes 14 in series. Due to the differential resistances of the diodes, a negative feedback takes place for MOSFET 3, as a result of which the peak loads that occur in the abovementioned short-circuit situation are effectively limited. After a delay time T, MOSFET 2 is also turned on and the emitter potential of the bipolar transistor 1 decreases. As a result the current i.sub.1 passes as a control current to the transistor 1 and causes it to conduct, so that the major part of the load current flows through the transistors 1 and 2 as current i.sub.2.

In order to ensure that the bipolar transistor 1 is not over-loaded, it is desirable for the delay time T to be greater than the time that it takes the diode 7 to recover. In this case a significant flow of current through the transistor 1 does not take place until the voltage u.sub.3 has significantly dropped. These conditions are shown in FIGS. 3A-3D. If the Zener voltage of the Zener diode used as the threshold element is made larger than the sum of the base-emitter voltage of the transistor 1 and the source-drain voltage of the MOSFET 2, then the current i.sub.1 flows entirely as control current to the transistor 1. If one or more diodes are used, the threshold voltage of these diodes must be greater than the sum of the abovementioned voltages.

If the control voltage u.sub.1 is equal to zero, MOSFETs 2 and 3 are cut off simultaneously. The emitter of the bipolar transistor 1 is without current, in which case the current i.sub.2 flows through the collector-base junction due to th storage charge. As a result the beginning of the rise in the output voltage u.sub.3 is delayed. The rise itself, however, takes place very quickly, since only the capacitance of the base-collector-PN-junction has to be discharged. Since with a diode the case of the so-called second breakthrough, which is based on a thermal overload, cannot take place, the bipolar transistor 1 can be exposed to a cut-off voltage that corresponds to the highest permissible base-collector cut-off voltage. This is higher than the highest permissible emitter-collector voltage.

The circuits described have the advantage that they need no additional voltage sources to drive the bipolar transistor. In addition, the switch is impervious to reverse currents, if one or more diodes are used as a threshold element. This means that, for example, in the case of bridge circuits recovery diodes can be connected in antiparallel with no further equipment being needed. Since the MOSFETs will not conduct reverse currents, the familiar du/dt problems that arise during commutation are eliminated.

In connection with FIG. 2 it was explained that the control voltage is applied to MOSFET 2 through a delay element 8. However, it can also be driven through a separate master clock with a corresponding time lag.

The circuits shown in FIGS. 1 and 2 can be varied by connecting to MOSFET 2, instead of the individual bipolar transistor 1 arranged in series, a Darlington amplifier consisting of two or three bipolar transistors in series. A circuit of this kind, with a Darlington amplifier consisting of two bipolar transistors 1 and 10 is shown in FIG. 4. Here the emitter-collector section of the second (last) bipolar-transistor is connected in series to the drain-source section of MOSFET 2 . The series circuit consisting of the other MOSFET 3 and the threshold element 4 is arranged in parallel to the series circuit consisting of bipolar transistor 1 and MOSFET 2. The threshold element 4 is placed between the base terminal of the first transistor 10 of the Darlington amplifier and the source terminal of MOSFET 2.

In both the arrangement shown in FIG. 4 and the arrangement shown in FIG. 2 a delay element 8 can be introduced, which is effective in the presence of a control signal of the same polarity that causes MOSFET 2 to conduct.

There has thus been shown and described a novel electronic switch which fulfills all the object and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.