DC/DC converter which ensures damping of voltage overshoots of a semiconductor switch

Abstract

A DC/DC converter has an active energy store, such as an inductance, which can be periodically charged and discharged by one or more semiconductor switches, such as transistors. To avoid voltage overshoot, an RCD element is provided for at least one semiconductor switch, wherein a capacitor and a diode of the RCD element are connected in series, and a resistor of the RCD element can be connected either in parallel with the diode or disconnected from the diode by a switch. The diode of the RCD element is arranged so as to be blocking in the conducting direction of the semiconductor switch.

Claims

1. A DC/DC converter, comprising an active energy store having an inductance, a first controllable semiconductor switch configured to periodically charge the active energy store, and a second controllable semiconductor switch configured to periodically discharge the active energy store, with at least one of the first and second controllable semiconductor switches having an RCD element to prevent voltage overshoots for the at least one of the first and second controllable semiconductor switches, said RCD element comprising a capacitor and a diode connected in series and a resistor, and furthermore a switch having a first switching position connecting the resistor in parallel with the diode and a second switching position disconnecting the resistor from the diode, wherein the diode is arranged in series with the capacitor so as to be conducting in a direction of a current flowing from an emitter to a collector of the at least one of the first and second controllable semiconductor switches and to be blocking the current when the at least one of the first and second controllable semiconductor switches is conducting.

2. The DC/DC converter of claim 1, wherein the resistor is connected in parallel to the capacitor in the second switching position.

3. The DC/DC converter of claim 1, wherein the RCD element is connected in parallel with the first controllable semiconductor switch.

4. The DC/DC converter of claim 1, wherein the RCD element is connected in parallel with the second controllable semiconductor switch.

5. The DC/DC converter of claim 1, wherein the RCD element is connected in parallel with a freewheeling diode.

6. The DC/DC converter of claim 1, wherein the switch is constructed as a relay.

7. The DC/DC converter of claim 1, wherein the switch is constructed as a transistor.

8. The DC/DC converter of claim 1, further comprising a charging circuit for charging the capacitor of the RCD element.

9. The DC/DC converter of claim 1, wherein the first controllable semiconductor switch comprises a first RCD element having a first diode and a first capacitor and the second controllable semiconductor switch comprises a second RCD element having a second diode and a second capacitor, with the first RCD element and the second RCD element having a shared resistor connected to a shared switch which connects the shared resistor in a first switching position in parallel with the first diode and in a second switching position in parallel with the second diode.

10. The DC/DC converter of claim 1, wherein the first controllable semiconductor switch comprises a buck converter transistor.

11. The DC/DC converter of claim 1, wherein the second controllable semiconductor switch comprises a step-up converter transistor.

12. The DC/DC converter of claim 1, wherein the first and second controllable semiconductor switches comprise transistors.

13. A method for operating a DC/DC converter having an active energy store in form of an inductance, a first controllable semiconductor switch constructed as a buck converter transistor configured to periodically charge the active energy store and a second controllable semiconductor switch constructed as a step-up converter transistor configured to periodically discharge the active energy store, with at least one of the first and second controllable semiconductor switches having an RCD element to prevent voltage overshoots for the at least one of the first and second controllable semiconductor switches, said RCD element comprising a capacitor and a diode connected in series and a resistor, and furthermore a switch having a first switching position connecting the resistor in parallel with the diode and a second switching position disconnecting the resistor from the diode, wherein the diode is arranged in series with the capacitor so as to be conducting in a direction of a current flowing from an emitter to a collector of the at least one of the first and second controllable semiconductor switches and to be blocking the current when the at least one of the first and second controllable semiconductor switches is conducting, the method comprising: for operation as a buck converter, switching only the buck converter transistor on, connecting the resistor in the RCD element, that is connected in parallel to the buck converter transistor, in parallel with the capacitor of the buck converter transistor, and selectively connecting the resistor, that is connected in parallel with the step-up converter transistor, in parallel with the diode in the RCD element of the step-up converter transistor or disconnecting the resistor from the diode.

14. A method for operating a DC/DC converter having an active energy store in form of an inductance, a first controllable semiconductor switch constructed as a buck converter transistor configured to periodically charge the active energy store and a second controllable semiconductor switch constructed as a step-up converter transistor configured to periodically discharge the active energy store, with at least one of the first and second controllable semiconductor switches having an RCD element to prevent voltage overshoots for the at least one of the first and second controllable semiconductor switches, said RCD element comprising a capacitor and a diode connected in series and a resistor, and furthermore a switch having a first switching position connecting the resistor in parallel with the diode and a second switching position disconnecting the resistor from the diode, wherein the diode is arranged in series with the capacitor so as to be conducting in a direction of a current flowing from an emitter to a collector of the at least one of the first and second controllable semiconductor switches and to be blocking the current when the at least one of the first and second controllable semiconductor switches is conducting, the method comprising: for operation as a step-up converter, switching only the step-up converter transistor on, connecting the resistor in the RCD element, that is connected in parallel to the step-up converter transistor, in parallel with the capacitor of the step-up converter transistor, and selectively connecting the resistor, that is connected in parallel with the buck converter transistor, in parallel with the diode in the RCD element of the buck converter transistor or disconnecting the resistor from the diode.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For further elucidation of the invention reference is made in the following part of the description to the figures, from which further advantageous embodiments, details and developments of the invention can be taken, in which:

(2) FIG. 1 shows a schematically illustrated, inventive DC/DC converter with relay for commutating the resistors in the RCD elements,

(3) FIG. 2 shows a schematically illustrated, inventive DC/DC converter with transistors for commutating the resistors in the RCD elements,

(4) FIG. 3 shows a variant circuit to FIG. 2,

(5) FIG. 4 shows the variant circuit from FIG. 3 with electronic charging circuit for the capacitors of the RCD element,

(6) FIG. 5 shows the variant circuit from FIG. 3 with a further embodiment of the electronic charging circuit for the capacitors of the RCD element,

(7) FIG. 6 shows an inventive DC/DC converter embodied as a downward converter (buck converter) with an electronic charging circuit for the capacitor of the RCD element, which corresponds to the one in FIG. 5,

(8) FIG. 7 shows a schematically illustrated, inventive DC/DC converter with two half-bridges,

(9) FIG. 8 shows a schematically illustrated, inventive DC/DC converter only with a relay and resistor for commutating the RCD elements.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(10) FIG. 1 shows an inventive upward and downward converter. A basic conventional upward and downward converter merely comprises the input capacitor C.sub.1, the output capacitor C.sub.2, the so-called buck converter transistor T.sub.T, the step-up converter transistor T.sub.H, and the coil (inductance) L.sub.1.

(11) Connected in antiparallel fashion in each case to the buck converter transistor T.sub.T and the step-up converter transistor T.sub.H is a freewheeling diode. In buck converter operation the current changes (commutates) from T.sub.T to the freewheeling diode, which is antiparallel to T.sub.H. In step-up operation the current changes (commutates) from T.sub.H to the freewheeling diode, which is antiparallel to T.sub.T. These diodes are mostly integrated into the housing of an IGBT, but can also be arranged antiparallel to the IGBT and have a separate housing.

(12) The inventive RCD element is not arranged in the immediate vicinity of the transistor to be protected, but where the so-called “freewheeling diode” would be arranged if the DC/DC converter were implemented only as a downward converter (buck converter) or only as an upward converter (step-up converter): the RCD element which protects the buck converter transistor T.sub.T consists of the following components: resistor R.sub.T, capacitor C.sub.T and diode D.sub.T, and is connected in parallel to the step-up converter transistor T.sub.H, or in parallel to the series circuit consisting of coil L.sub.1 and output capacitor C.sub.2. In this case capacitor C.sub.T and diode D.sub.T are connected to one another in series, and the capacitor C.sub.T is arranged in the current flow direction of the step-up transistor T.sub.H (here from the top positive pole to the bottom negative pole) upstream of the diode D.sub.T. The resistor R.sub.T is connected between capacitor C.sub.T and diode D.sub.T, and is implemented by means of a switch S.sub.T, which is here implemented as a relay, and can be connected either in parallel to the capacitor C.sub.T or to the diode D.sub.T. It would also be conceivable for the resistor R.sub.T to be completely disconnected in another variant of the embodiment.

(13) The RCD element, which protects the step-up converter transistor T.sub.H, consists of the following components: resistor R.sub.H, capacitor C.sub.H and diode D.sub.H, and is connected in parallel to the buck converter transistor T.sub.T In this case capacitor C.sub.H and diode D.sub.H are connected to one another in series, and the capacitor C.sub.H is arranged in the current flow direction of the buck converter transistor T.sub.T (from the emitter to the collector) downstream of the diode D.sub.H. The resistor R.sub.H is connected between capacitor C.sub.H and diode D.sub.H, and by means of a switch S.sub.H, which is here implemented as a relay, can be connected either in parallel to the capacitor C.sub.H or to the diode D.sub.H. It would also be conceivable for the resistor R.sub.H to be disconnected completely in another variant of the embodiment.

(14) If the DC/DC converter as illustrated in FIG. 1 works as a downward converter (buck converter), for instance if a battery connected at the output is to be charged, it works from left to right. In this case only the buck converter transistor T.sub.T is clocked (switched on), the step-up converter transistor T.sub.H remains switched off, the resistor R.sub.H in the RCD element connected in parallel to the buck converter transistor T.sub.T is connected in parallel to the capacitor C.sub.H, and the resistor R.sub.T in the RCD element connected in parallel to the step-up converter transistor T.sub.H is connected in parallel to the diode D.sub.T. This is illustrated by the switching positions “T” of the switches S.sub.H, S.sub.T.

(15) If the DC/DC converter in FIG. 1 is to work as an upward converter (step-up converter), it works from right to left. In this case only the step-up converter transistor T.sub.H is clocked (switched on), the resistor R.sub.T in the RCD element connected in parallel to the step-up converter transistor T.sub.H is connected in parallel to the capacitor C.sub.T, and the resistor R.sub.H in the RCD element connected in parallel to the buck converter transistor T.sub.T is connected in parallel to the diode D.sub.H. This is illustrated by the switching positions “H” of the switches S.sub.H, S.sub.T.

(16) The embodiment according to FIG. 2 corresponds to the one according to FIG. 1, with the difference that for the switches S.sub.T and S.sub.H not relays but semiconductor switches are provided, which are connected such that they can connect the resistor R.sub.H or T.sub.H either only in parallel to the diode D.sub.H or R.sub.H or can disconnect it completely.

(17) If the DC/DC converter in FIG. 2 works as a downward converter (buck converter), only the buck converter transistor T.sub.T is clocked (switched on), the resistor R.sub.H in the RCD element connected in parallel to the buck converter transistor T.sub.T is disconnected, and the resistor R.sub.T in the RCD element connected in parallel to the step-up converter transistor T.sub.H is connected in parallel to the diode D.sub.T. If the DC/DC converter in FIG. 2 is to work as an upward converter (step-up converter), only the step-up converter transistor T.sub.H is clocked (switched on), the resistor R.sub.T in the RCD element connected in parallel to the step-up converter transistor T.sub.H is disconnected, and the resistor R.sub.H in the RCD element connected in parallel to the buck converter transistor T.sub.T is connected in parallel to the diode D.sub.H.

(18) The embodiment according to FIG. 3 corresponds to the one in FIG. 2, with the difference that in FIG. 3 the position of the capacitor C.sub.H is swapped with that of the diode D.sub.H, i.e. is arranged in the current flow direction of the buck converter transistor T.sub.T of the capacitor C.sub.H upstream of the diode D.sub.H. The resistor R.sub.H can however likewise be connected by means of the switch S.sub.H either in parallel to the diode D.sub.H or can be disconnected from it completely. The advantage lies in the use of a potential-free supply voltage for the driver for T.sub.T and S.sub.H and a second potential-free supply voltage for the driver for T.sub.H and S.sub.T. A bootstrap circuit for the supply of T.sub.T and S.sub.H is also possible.

(19) FIG. 4 illustrates an electronic charging circuit for the capacitors C.sub.H, C.sub.T of the RCD elements. Thus it is possible to establish the voltage to which the capacitors C.sub.H, C.sub.T are charged. The voltage to which the capacitors C.sub.H, C.sub.T are charged is determined by the switch-on duration and the switch-on instant of the charging circuit.

(20) Based on FIG. 3 the resistors R.sub.H and R.sub.T are replaced by coils L.sub.H and L.sub.T. In addition the output of the coil L.sub.H is fed back by way of a diode D.sub.H2 to the input of the DC/DC converter (upstream of the buck converter transistor T.sub.T). The output of the coil L.sub.T is fed back by way of a diode D.sub.T2 directly upstream of the step-up converter transistor T.sub.H. The diodes are in this case fitted in the non-conducting direction—against the current flow direction of the transistors T.sub.T and T.sub.H from the emitter to the collector.

(21) In the variant of the embodiment according to FIG. 5—in contrast to FIG. 4—the output of the coil L.sub.T is fed back by way of the diode D.sub.T2 upstream of the buck converter transistor T.sub.T, i.e. to the input of the DC/DC converter.

(22) FIG. 6 illustrates an inventive downward converter (buck converter). Thus in comparison to FIGS. 1-5 the step-up converter transistor T.sub.H and the RCD element assigned thereto (R.sub.H or L.sub.H, C.sub.H and D.sub.H) can be omitted. Instead of the step-up converter transistor T.sub.H there is a freewheeling diode D.sub.3, which is fitted against the current flow direction of the step-up converter transistor T.sub.H. In this embodiment it is also possible to leave out the switch S.sub.T.

(23) It is also possible to position a second “half-bridge” (T.sub.T2 and T.sub.H2) at the output of the coil (between coil L.sub.1 and capacitor C.sub.2), see FIG. 7. Thus it is possible for current to flow in both directions regardless of the size of the input voltage U.sub.1 and the size of the output voltage U.sub.2.

(24) In FIG. 8 the positions of the diodes and capacitors in the RCD element are swapped in comparison to FIG. 1. Thus it is possible to save a relay and one of the RCD resistors. In addition, high-impedance resistors R.sub.HE and R.sub.TE are still arranged in parallel to the capacitors C.sub.H and C.sub.T of the RCD elements. These ensure that the capacitors C.sub.H and C.sub.T are discharged in the disconnected state of the RCD element. It is also possible to omit these resistors, so that the step-up converter transistor T.sub.H and the buck converter transistor T.sub.T can consume the stored energy in the capacitor of the RCD element at switch-on.

(25) In principle the switches for the resistors of the RCD elements can be implemented in all variants of the embodiments shown as electromechanical switches (relays) or as electronic switches,