Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
1998-02-25
2001-08-14
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S432000, C327S440000, C361S089000, C361S091300, C361S091800, C361S018000
Reexamination Certificate
active
06275093
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to insulated gate bipolar transistors (IGBTs) and, in particular, to an apparatus and method for protecting an IGBT when it is subjected to a short circuit, or like, condition.
To better explain the problems faced by Applicants and Applicants' invention, reference will first be made to
FIG. 1A
which shows a standard symbol for an N-conductivity type IGBT and to
FIG. 1B
which shows its simplified equivalent circuit. Thus,
FIG. 1A
shows that an IGBT, T
1
, has a gate electrode
1
, a collector electrode
2
(which may also be referred to as the anode), and an emitter electrode
3
(which may also be referred to as the cathode). The equivalent circuit of
FIG. 1B
shows that the IGBT may be represented by an insulated-gate-field-effect transistor (IGFET) portion, F
1
, which controls the turn-on and turn-off of a semiconductor structure which may be represented by a vertical PNP bipolar transistor P
1
and a parasitic NPN bipolar transistor N
1
. The IGFET portion includes a gate electrode (Gate), connected to a terminal
1
. The potential applied to the gate electrode controls the conductivity of a conduction channel (R-channel) defined at one end by a (drain) region
21
and at its other end by a (source) region
22
. The gate electrode overlies the conduction channel formed within a substrate region
23
which, in
FIG. 1B
, is shown connected to the source region
22
. The potential applied to the gate electrode
1
is intended to control the turn-on and turn-off of transistor P
1
. The emitter of P
1
is connected to terminal
2
which defines the “collector” of the IGBT. The base of P
1
is connected to the collector of N
1
and via a resistor (MOD) to region
21
, which defines one end of the conduction path (R-channel) of the IGFET portion of F
1
. Region
22
of the FET, which defines the other end of the conduction path of the channel, is connected to the substrate region
23
of the FET and to the emitter of N
1
at the IGBT “emitter” terminal
3
. The collector region of P
1
is connected to the base of N
1
. A parasitic resistor Rp which has a very low value of resistance is shown connected between the base and emitter of N
1
. The parasitic resistance normally shunts most of the P
1
collector current (I
1
). However, if the emitter-to-collector current I
1
through P
1
becomes very large, as may be the case under short circuit condition, then N
1
may be driven into a high conductive state and the thyristor portion (P
1
, N
1
) of the IGBT may latch up and the FET portion (F
1
) may lose control.
In normal operation, for any N-type IGBT, a turn-on voltage applied to the gate of the IGBT must be positive relative to the emitter and must exceed the threshold voltage (VT) of the IGBT. By way of example, in the description to follow, it is assumed that VT is about 5 volts. However, VT may be more or less than 5 volts. In fact, the VT of presently available IGBTs ranges from 4 to 8 volts. When a turn-on voltage is applied to the gate of the IGBT, the base of transistor P
1
is supplied (flows) via the channel of the FET portion F
1
. The base current of P
1
is multiplied by the current gain of P
1
which then conducts a current I
1
from the emitter electrode of P
1
(terminal
2
) along the main conduction path of P
1
and via parasitic resistor Rp (which is extremely small) into the electrode defined as terminal
3
(the “emitter” electrode of the IGBT in the drawing). However, if the current I
1
rises dramatically, and the voltage at the base of transistor N
1
rises above the 0.7-1.0 volt range, some of the I
1
current will be supplied into the base of N
1
. The base current into N
1
causes N
1
to conduct a collector current (
1
2
) which is drawn out of the base of P
1
. If I
1
and I
2
are allowed to become too high, the FET portion of the IGBT (i.e., the gate) may lose control and P
1
and N
1
may latch up. Latch-up occurs when I
1
plus I
2
exceeds the latching current of the parasitic thyristor formed by P
1
, N
1
and Rp.
As shown in
FIG. 2
, an IGBT, T
1
, is typically connected at its drain to one end of a load whose other end is connected to a power supply voltage (VBUS). VBUS may range from a few volts to several thousand volts and the load current (IL) through the load and the IGBT may range from the milliampere region to a hundred or more amperes. In order to minimize the power dissipation across the IGBT, a large turn-on voltage (e.g., 12-15 volts) is normally applied to the gate of the IGBT to cause the IGBT to be operated in the saturation region, with its collector-to-emitter voltage (Vce) equal to Vcesat, which is in the order of a few volts. However, a problem occurs if, when the IGBT is fully turned on and carrying a large current, the load is shorted. The IGBT is then subjected to an excessive power dissipation condition, due to the high current through the IGBT and the rising voltage developed across the collector-to-emitter of the IGBT. If the short circuit condition exists (or develops) and persists, the IGBT will fail due to the excessive power dissipation. As the IGBT is heated by the simultaneous presence of a large current through it and a large voltage across it, the voltage needed across the shunt resistor Rp to turn-on transistor N
1
is dramatically reduced. This value may be 0.3-0.4v at 150° C. Furthermore, since carrier mobility degrades with temperature, the resistance Rp will increase as the device gets hotter. These factors and others conspire to increase the chances of latch-up if the short-circuit condition persists for too long before the device is turned-off.
It is therefore necessary to turn off an IGBT if, and when, a fault condition, such as a short circuit, develops or exists. Prior art schemes for turning-off an IGBT subjected to a short circuit condition rely on applying a relatively sharp turn-off voltage to the gate of the IGBT when a short circuit condition is sensed. However, it has been discovered that trying to turn-off an IGBT sharply and rapidly when the IGBT is carrying a very large current and is subjected to a short circuit condition, may cause the IGBT to lose control and fail. Thus, if the IGBT loses control over the load current flowing through it while its Vce keeps increasing, the IGBT will fail due to excessive power dissipation. Another scheme for turning-off an IGBT subjected to a short circuit condition, maintains the full turn-on voltage applied to the gate for a time interval of several microseconds after the detection of a short circuit condition in order to maintain control. During this time interval, the Vce of the IGBT rises towards the load supply voltage (VBUS). Following the time interval the turn-on voltage is removed. However, while the IGBT is on, its power dissipation is very high and the IGBT may fail due to excessive power dissipation.
The problems present in the prior art are significantly reduced in circuits embodying the invention.
SUMMARY OF THE INVENTION
Applicants' invention resides, in part, in the recognition that, an insulated gate bipolar transistor (IGBT) is designed to carry very large currents and that it is rated to withstand a very high power dissipation for a very short period of time. Applicants' invention also resides, in part, in the recognition that during this very short period of time, when an IGBT is subjected to a short circuit condition, it is safer to turn off the IGBT by first decreasing the gate drive of the IGBT from a high turn-on to a lower turn-on level (which is above the threshold voltage of the IGBT), so that the current through the IGBT is decreased, and to then slowly (but still during a very brief period of time) decrease the gate drive to the IGBT until it is shut off completely.
A gate driver circuit embodying the invention includes means for detecting when the collector-to-emitter voltage (Vce) of a turned-on IGBT, intended to be operated in the saturation region, increases above a preset level, indicative of a fault condition such as a short circuit, or similar, c
Bhalla Anup
Gladish Jon
Shekhawat Sampat Singh
Callahan Timothy P.
Intersil Corporation
Jaeckle Fleischmann & Mugel LLP
Nguyen Minh
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