Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2001-12-06
2003-09-02
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S332000
Reexamination Certificate
active
06614281
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and an apparatus for turning off a cascode circuit comprising a series circuit formed by a low-blocking-capability and high-blocking-capability semiconductor switch.
BACKGROUND OF THE INVENTION
In power electronic circuits, because of internal switching actions or external mains overvoltages, voltage values can occur across power semiconductors which exceed their rated blocking capability. Since the occurrence of such operating states cannot be precluded, an elementary requirement for protecting power semiconductor elements is the ability to withstand overvoltages, in order to avoid destruction of the semiconductor components or more extensive damage.
For this problem, two possible solutions currently exist, namely passive and active overvoltage limiting.
Passive overvoltage limiting employs a capacitor which is electrically connected in parallel with the power semiconductor. This protective circuit is also referred to as a clamp circuit. By means of this clamp circuit, the energy of the overvoltage is conducted into the capacitor, thereby limiting a voltage rise at the power semiconductor switch. This capacitor may additionally be augmented by a resistor and a diode to form a so-called RCD protective circuit, which is also referred to as a snubber circuit. The disadvantage of the clamp protective circuit or of the snubber circuit is that the capacitor has to be designed for the maximum voltage that occurs. Such a capacitor is costly and occupies a comparatively large structural volume. Furthermore, the leakage inductance of the protective circuit will increase because of the large structural volume and the longer connection lines resulting therefrom. If no further outlay is to be expended, the energy stored in the capacitor is converted into heat by means of the protective circuit resistor, as a result of which this positive overvoltage protection causes a non-negligible power loss.
In the case of active overvoltage protection, an overvoltage that occurs at the power semiconductor is detected and compared with a limit value which is less than a maximum reverse voltage of the power semiconductor, and the power semiconductor is actively driven as soon as the overvoltage exceeds the predetermined limit value. As a result of the active driving, the power semiconductor is able to convert the energy of the overvoltage into heat by an elevated current flowing through the power semiconductor at high voltage. An overvoltage can be identified by a voltage-limiting component, for example a zener diode, which carries a current in the reverse direction when its zener voltage is exceeded. This current can be passed directly or via an amplifier into the control terminal of the power semiconductor in order to turn the latter on. In this arrangement, a high-voltage zener diode is required. Instead of a high-blocking-capability zener diode, it is also possible to use a high-impedence voltage divider for identifying the overvoltage. The disadvantage of the active overvoltage protection described is that the detection elements have to be designed for the entire reverse voltage of the power semiconductor. Moreover, a high-impedence voltage divider continually causes a power loss, whereas a high-blocking-capability zener diode is thermally endangered by the power loss converted in it. Moreover, high-blocking-capability components are expensive.
German patent specification 196 10 135 C1 discloses a cascode circuit of two voltage-controlled semiconductor switches which are electrically connected in series. This cascode circuit is described below with reference to
FIG. 1
of the drawings.
In
FIG. 1
, cascode circuit
2
has a low-blocking-capability semiconductor switch
4
and a high-blocking-capability semiconductor switch
6
, which are electrically connected in series. A normally off n-channel MOSFET, in particular a low-voltage power MOSFET, is provided as the low-blocking-capability semiconductor switch
4
, and an n-channel junction FET is provided as the high-blocking-capability semiconductor switch
6
. This high-blocking-capability junction FET
6
is also referred to as Junction Field-Effect Transistor (JFET). The two FETs
4
and
6
are electrically connected in series in such a way that the source terminal of the JFET
6
is electrically conductively connected directly to the drain terminal D′ of the MOSFET
4
and the gate terminal of the JFET
6
is electrically conductively connected to the source terminal S of the MOSFET
4
by means of a gate resistor R
GJ
. This electrical interconnection of two semiconductor components is referred to as a cascode circuit, as is known. Since respective FETs are used as the semiconductor switches
4
and
6
of the cascode circuit
2
, this cascode circuit
2
is also referred to as a hybrid power MOSFET. The low-blocking-capability MOSFET
4
of this cascode circuit
2
has an internal biplar diode D
IN
, which is reverse-connected in parallel with the MOSFET
4
and is generally referred to as an inverse diode or internal freewheeling diode. The normally off n-channel MOSFET
4
is made of silicon, whereas the normally off n-channel JFET
6
is preferably composed of silicon carbide. This hybrid power MOSFET
2
is designed for a high reverse voltage of more than 1000 V and nevertheless has only small losses in the on-state range.
FIGS. 2 and 3
illustrate blocking characteristic curves of the normally on n-channel JFET
6
and of the normally off n-channel MOSFET
4
, respectively, in a plot of reverse current against reverse voltage. Since the low-blocking-capability and high-blocking-capability semiconductor switches
4
and
6
are electrically connected in series in the cascode circuit
2
, the current through both semiconductor switches
4
and
6
must be of the same magnitude. Moreover, the reverse voltage U
DSA
of the low-blocking-capability semiconductor switch
4
is present as gate voltage at the high-blocking-capability semiconductor switch
6
of the cascode circuit
2
. If a reverse voltage U
DSA
is then present at the turned-off cascode circuit
2
, it will be divided between the two semiconductor circuits
4
and
6
of the cascode circuit
2
. This division will be effected such that the same reverse current I
DA
is established for both semiconductor switches. A stable operating point AP will be established in this way.
If, from a stable operating point AP of the cascode circuit
2
, the value of the reverse voltage U
D′SA
of the low-blocking-capability semiconductor switch
4
should shift to low values, then the reverse current I
DA
would likewise have to decrease in accordance with the blocking characteristic curve according to FIG.
3
. For the high-blocking-capability semiconductor switch
6
, this means only a marginal change in its reverse voltage U
DSA
, since the latter is significantly greater than the reverse voltage U
D′SA
of the low-blocking-capability semiconductor switch
4
. A decrease in the reverse voltage U
D′SA
of the low-blocking-capability semiconductor switch
4
of the cascode circuit
2
likewise means a decrease in the magnitude of the gate voltage of the high-blocking-capability semiconductor switch
6
. A reduced-magnitude gate voltage of the high-blocking-capability semiconductor switch
6
means an increased reverse current I
DA
(FIG.
2
). However, this increased reverse current I
DA
can only be carried by the low-blocking-capability semiconductor switch
4
of the cascode circuit
2
if said switch takes up a larger reverse voltage. Consequently, the previously conceived decrease in the reverse voltage U
D′SA
of the low-blocking-capability semiconductor switch
4
of the cascode circuit
2
is cancelled.
This fact can be utilized for overvoltage identification, a low-voltage signal being used to detect an overvoltage at high potential.
SUMMARY OF THE INVENTION
The invention has as its object, the ability to detect an overvoltage at high potential by means of a low-voltage signal.
By virtue of the fa
Baudelot Eric
Bruckmann Manfred
Mitlehner Heinz
Weis Benno
Baker & Botts LLP
Siemens Aktiengesellschaft
Wells Kenneth B.
LandOfFree
Method and device for disconnecting a cascode circuit with... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for disconnecting a cascode circuit with..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for disconnecting a cascode circuit with... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3097639