Circuit arrangement with half-bridge

Details Circuit arrangement with half-bridge Circuit arrangement with half-bridge

2000-04-05

2001-07-31

Zweizig, Jeffrey (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Utilizing three or more electrode solid-state device

C327S110000

Reexamination Certificate

active

06268758

ABSTRACT:

The present invention relates to a circuit arrangement having a half-bridge arrangement comprising a first and a second switch, each switch having, between its operating and reference electrodes, an internal diode, which is reverse-connected in parallel with the main current flow direction of the respective switch, and also an internal capacitance, a series diode being arranged in series with the respective switch in the main current flow direction in such a way that current can flow in the main current flow direction, and a freewheeling diode being connected in parallel with the respective series circuit formed by the switch and the series diode, and having a load circuit, which has at least one inductive component and is connected to the midpoint of the half-bridge arrangement.
Such a circuit arrangement, disclosed in the prior art, is illustrated in FIG.
1
. It will be used to illustrate the problem on which the invention is based.
FIG. 1
shows the part which is of interest, in the context of the present invention, of a circuit arrangement which can be found in a wide variety of fields appertaining to electronics, for example in step-down controllers, DC motor power converters and three-phase motor power converters, etc. It comprises a half-bridge arrangement with two switches T
1
and T
2
, in this instance MOSFET transistors, which are fed from a DC voltage source U
G
, for example an intermediate circuit voltage. In order to comprehend the considerations which lead to the circuit illustrated in
FIG. 1
, the circuit illustrated in
FIG. 1
will first be considered without the series diodes DS
1
and DS
2
arranged in series with the switches T
1
and T
2
, and the freewheeling diodes DF
1
and DF
2
connected in parallel with the respective combination formed by series diode and switch. A load circuit is connected to the midpoint of the half-bridge, said load circuit having an inductive component, schematically illustrated as a series circuit formed by an inductance L and a non-reactive resistance RL, through both of which the load current I
L
flows. The load circuit is furthermore connected to the midpoint of two series-connected capacitors C
1
, C
2
, which are arranged in parallel with the half-bridge switches T
1
and T
2
. The half-bridge switches T
1
and T
2
are driven by a control device S, illustrated schematically.
FIG. 2
shows an equivalent circuit diagram of a MOSFET, revealing that the drain and source have arranged between them a so-called body diode DB, which is reverse-connected in parallel with the main current flow direction from the drain to the source, and an output capacitance C
OSS
in parallel with said diode.
FIG. 3
shows the time characteristic of the load current I
L
for exemplary driving of the half-bridge arrangement by the control device S. During phase
1
, the transistor T
2
is switched off and the transistor T
1
is alternately switched on and off, whereas this is reversed in phase
2
. During the phase section
1
a,
which is shown enlarged, the current flows in the mesh—U
G
T
1
L RL C
2
, and, during the phase section
1
b,
the load current flows further in the mesh L RL C
2
T
2
after T
1
has been switched off, T
2
remaining switched off, on account of the inductive component of the load circuit. This is made possible by the body diode DB of the switch T
2
. With regard to its electric properties, the body diode DB inherent to the transistor T
2
is characterized by a very slow turn-off time and a low forward voltage. If T
1
is then switched on again after phase
1
b,
in other words the voltage U
&mgr;
equal to U
G
is present at the midpoint of the half-bridge, then a current flows briefly via the body diode DB of the transistor T
2
, which current could be referred to as a short-circuit current, which leads to a high power loss converted in the transistor. This leads to internal and external interference with such a circuit arrangement, for example in the EMC range.
In order to improve the reliability of such a circuit and in order to avoid the necessity of shields for suppressing interference, the prior art has provided for arranging a series diode DS
1
or DS
2
, respectively, in series with each switch, said diodes preferably being realized as Schottky diodes since these diodes typically have a low forward voltage, and which diodes are oriented in such a way as to enable current to flow in the main current flow direction of the respective switch. As a consequence, therefore, the respective series diode DS
1
or DS
2
, respectively, and the associated body diode DB
1
or DB
2
, respectively, (also see
FIG. 4
in this respect) are reverse-connected in series, which is why current is prevented from flowing through the respective body diode during the freewheeling process (phase
1
B of FIG.
3
). Since the function of the body diode inherent to the MOSFET is accordingly prevented by the respective series diode, it is necessary to provide a freewheeling diode DF
1
or DF
2
, respectively, which is arranged in parallel with said combination and now undertakes the original function of the respective body diode.
In practice, however, it has now been shown that, in the case of a circuit arrangement of this type, one of the two diodes DS
1
or DS
2
frequently fails.
The object of the present invention, therefore, is to improve a circuit arrangement of the type mentioned in the introduction, as is illustrated in
FIG. 1
, in such a way that the failure rate of the series diodes DS
1
and/or DS
2
is reduced.
This object is achieved by virtue of the fact that a load-relieving capacitance is connected in parallel with each series diode. This measure, the function of which is explained in detail further below, leads not only to a significantly reduced failure rate of the diodes DS
1
and/or DS
2
, but also to a reduction in the power loss converted in the circuit. It also enables the use of smaller and thus less expensive diodes for the series diodes DS
1
and DS
2
.
The solution according to the invention can preferably be applied to half-bridge arrangements whose switches are realized by a MOSFET, in which case, as mentioned, the respective internal diode is formed by the respective body diode and the respective internal capacitance is formed by the respective output capacitance C
OSS
. This fact notwithstanding, the solution according to the invention can also be used in circuit arrangements in which these components, which are parasitic components of a MOSFET, are engendered by a different combination.
Each load-relieving capacitance is preferably dimensioned in such a way that the voltage dropped during operation of the circuit arrangement across the load-relieving capacitance of the respective voltage divider essentially formed from the load-relieving capacitance and the internal capacitance is always smaller than the reverse voltage of the respective series diode. The load-relieving capacitance is preferably 100 pF to 10 nF.
It goes without saying that the application of the invention is not limited to half-bridge arrangements, but rather can be realized with the use of two further switches in a full-bridge arrangement, the load circuit being coupled between the midpoints of the switches.
The solution according to the invention can also be realized, with the half-bridge arrangement being supplemented by four further switches, in a three-phase bridge to which the load circuit is coupled. In this respect, diverse areas of use emerge, mention being made, by way of example, of step-down controllers, as are used in ballasts for operating discharge lamps, DC motor power converters and three-phase motor power converters.
Further advantageous developments of the invention are defined in the subclaims.


REFERENCES:
patent: 3770986 (1973-11-01), Drehle
patent: 5994929 (1999-11-01), Sano et al.

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