Semiconductor protection device and power converting system

Details Semiconductor protection device and power converting system Semiconductor protection device and power converting system

1999-06-24

2001-07-31

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific voltage responsive fault sensor

C361S091500, C361S111000, C327S310000

Reexamination Certificate

active

06268990

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor protection device for protecting a semiconductor switch which generates a surge voltage at the time of current interruption and a power converting system using the same.
As a power electronics system such as a motor driving power converting system, an inverter is known. As a switching element used in the inverter, recently, an IGBT (Insulated Gate Bipolar Transistor) is widely used. In this type of inverter, a protection circuit is used for protecting the IGBT from a surge voltage generated at the time of turn-OFF of the IGBT which is turned ON/OFF at the frequency of 50 Hz to 20 kHz.
As the above type of protection circuit, a clamp type snubber circuit (charging type RCD snubber circuit) shown in
FIG. 35
is conventionally used. The clamp type snubber circuit is provided for a series circuit including a filter capacitor Cf for supplying a circuit voltage, a wiring inductance Lm and two IGBTs
1
and
2
which are used as switching elements. More specifically, series circuits including snubber capacitors C
1
and C
2
and snubber diodes D
1
and D
2
are respectively connected in parallel with the IGBTs
1
and
2
. One-side ends of snubber resistors R
1
and R
2
are respectively connected to nodes between the capacitors C
1
and C
2
and the diodes D
1
and D
2
. The other ends of the resistors R
1
and R
2
are respectively connected to the emitter of the IGBT
2
and the collector of the IGBT
1
.
That is, the clamp type snubber circuit has a construction in which the capacitors C
1
and C
2
are cross-coupled and previously charged to a circuit voltage. At the turn-OFF time, energy stored in the wiring inductance Lm is discharged, and when an instantaneous excessively high voltage (which is hereinafter referred to as a surge voltage) higher than the circuit voltage is applied to the IGBTs
1
and
2
, charges are stored (excessively charged) in the capacitors C
1
and C
2
to clamp the surge voltage. This operation occurs at each turn-OFF time. Since the energy loss corresponds to an excessively charged amount of charges, the clamp type snubber circuit has an advantage that the energy loss is small in comparison with a completely discharging type snubber circuit.
However, the surge voltage suppressing function of a snubber circuit using a capacitor, i.e., either of the completely discharging type snubber circuit and the clamp type snubber circuit, has a disadvantage that the magnitude of the generated surge voltage varies, depending on the magnitude of an interruption current. For example, a surge voltage of 400V is generated when the interruption current is 100 A as shown in
FIG. 36 and a
surge voltage of 800V is generated when the interruption current is 200 A as shown in FIG.
37
. Thus, the surge voltage varies according to the magnitude of the interruption current.
In this case, 100 A corresponds to 100% of a normally used current area of the switching element and 200 A is an excessive current set value such as an accident current and corresponds to 200% of the normal current. In the above application method, a circuit a voltage (1000V)+surge voltage (800V)+marginal amount (200V)=2000V is used as the breakdown voltage of the element. That is, the breakdown voltage of the element becomes approximately 200% of the circuit voltage.
The above relation can be expressed by use of a reverse bias safe operating area (RBSOA) which is necessary for the characteristic of the element, as shown in FIG.
38
. That is, it is necessary to safely interrupt the maximum excessive current at the circuit voltage (1000V) and safely interrupt the steady-state current at the maximum voltage (1800V). As is clearly seen from
FIG. 38
, the safe operation is required in an area higher than the circuit voltage (1000V). However, in the high breakdown voltage area, holes and electrons are easily generated by the avalanche breakdown so as to abruptly make it difficult to attain the safe operation. Therefore, particularly, in order to develop IGBTs which can be used in an area higher than the circuit voltage 1000V, some restrictions for reducing the current used are required, for example.
Further, the clamp type snubber circuit has a cross-coupled wiring and cannot be applied to a circuit in which one arm includes serially connected two or more IGBTS. Therefore, in a high voltage converting system having a plurality of IGBTs serially connected, there is a problem in that a usable snubber circuit of small loss is not realized, but, as a result, a non-charging type snubber of large loss is used.
On the other hand, there is a circuit system having a combination of a snubber circuit and a protection circuit against the excessive voltage, as shown in FIG.
39
. The operation of the above circuit is explained with reference to
FIGS. 40 and 41
.
First, if IGBTs
1
and
2
interrupt a normal current i
1
(=100 A at maximum), a surge voltage is generated and it is suppressed to V1 (=1400V at maximum) by a clamp snubber. V1 is suppressed to 150% (=1500V) of the circuit voltage at maximum if the normal current i
1
is interrupted. However, if an excessively large current ioc (=200 A at maximum) generated at the time of accident of load short circuit is interrupted, a high surge voltage (1800V) is generated in the clamp snubber circuit. Therefore, it reaches an excessive voltage protection level Vzd (for example, 170% of the circuit voltage=1700V) and is clamped at Vzd, as shown in FIG.
41
.
Thus, the element breakdown voltage in a case where the temporary excessive voltage protection circuit of
FIG. 39
is attached becomes 1700V+marginal amount 200V=1900V. In this case, an element whose breakdown voltage is lower than the element breakdown voltage 2000V, attained when only the clamp snubber circuit of
FIG. 35
is used, can be used, but the difference in the breakdown voltage is small. Since electric energy indicated by the oblique line in the voltage suppressing period is discharged at the turn-ON condition, a current of an amount larger than that of the current flowing at the normal turn-OFF time flows as shown in FIG.
41
. Like the case of
FIG. 38
, the RBSOA of this case is required so that the steady-state current can be safely interrupted at the time of maximum voltage 1700V, and therefore, significant improvement cannot be expected.
Further, an excessive voltage protection circuit different from the excessive voltage protection circuit shown in
FIG. 39
is disclosed in Japanese Patent No. 2622524 and Jpn. Pat. Appln. KOKAI Publication No. 7-288456. In the construction disclosed in the above publications, an auxiliary switching element such as an FET or IGBT is connected in parallel with a main switching element and an excessive voltage sensor is provided for the auxiliary switching element. If an excessive voltage is applied, the auxiliary switching element is operated to protect the main switching element from the excessive voltage.
However, the protection method in the above cases is the same as that used in the conventional construction shown in
FIGS. 40 and 41
and no improvement can be expected from the viewpoint of utilization factor of the element breakdown voltage. That is, an excessive voltage protection level which is sufficiently higher (for example, 170% as explained in the former case) than the circuit voltage is set and a design is made such that the surge voltage does not exceed the level. Further, the significant improvement cannot be expected from the viewpoint of the RBSOA.
Further, in a case where IGBTs
1
and
2
are connected in parallel instead of using the single IGBT, the characteristics of the IGBTs
1
and
2
are different from each other and the currents flowing therethrough are unbalanced. At this time, a high surge voltage is applied to the snubber circuit by one of the IGBTs which interrupts a larger current. Further, if the IGBTs
1
and
2
are connected in series, surge voltages generated in the IGBTs
1
and
2
at t

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