Circuit and method for improving short-circuit capability of IGBTs

A simple, off-chip circuit and method for endowing high efficiency IGBTs with short-circuit capability, that is essentially transparent to the user. The invention involves adding an external common emitter resistor to reduce the effective gain of an IGBT under short circuit. Under normal operating conditions, the voltage across the resistor is small, such that the modifying effect on the normal operating gate-emitter voltage is almost negligible.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to short-circuit protection circuits and, 
more specifically, to a circuit for protecting high efficiency IGBTs in a 
motor controller circuit. 
2. Description of the Related Art 
A common requirement of IGBT inverters for motor control is that the IGBTs 
must be able to withstand short-circuits for periods that are in the range 
of 5 to 10 .mu.s. 
The capability of an IGBT to withstand short-circuit for a given period of 
time is essentially determined by the gain under short-circuit. 
At present, two basic types of IGBTs are generally available, namely 
"short-circuit" types which are designed primarily for motor control 
applications, and "high-efficiency" types, which are designed for 
applications where short-circuit capability is not needed, such as 
switching power supplies. An inherent trade-off exists between these two 
types of devices. Short-circuit IGBTs (typically designed to withstand 
short-circuits for up to 10 .mu.s) are inherently less efficient than 
high-efficiency IGBTs, but the latter have more limited short-circuit 
capability. 
As illustrated by the plots shown in FIG. 1, Gen 4 high-efficiency IGBTs 
manufactured by the assignee of the present application, International 
Rectifier Corporation, have about twice the short-circuit current of 
short-circuit rated types. The greater short-circuit current of the 
high-efficiency type restricts its short-circuit withstand time to less 
than a half of that of the short-circuit rated type. 
It would be desirable for semiconductor manufacturers such as the present 
assignee to eliminate the manufacture of short-circuit rated components, 
in favor of high efficiency types only. 
The potential advantages would be: 
1. Simplified manufacturing logistics, inventory control, etc., through the 
manufacture of one basic high-efficiency type of IGBT, instead of two 
different types; and 
2. Better system solutions for applications that require short-circuit 
capability, with greater design flexibility and improved system 
performance. 
SUMMARY OF THE INVENTION 
The present invention achieves the above-described objectives and 
advantages by providing a circuit and method for improving the 
short-circuit capability of high-efficiency IGBTs in a strikingly simple 
off-chip solution which is essentially transparent to the user. 
More specifically, the present invention, in its various embodiments, 
comprises the addition of an external common emitter resistor to a high 
gain high efficiency type of IGBT to increase the short-circuit capability 
of the IGBT. 
Other features and advantages of the present invention will become apparent 
from the following description of the invention which refers to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A comparison of FIGS. 2A and 2B shows the addition of an external common 
emitter resistor R to the high gain high efficiency type of IGBT, in 
accordance with the present invention. More specifically, the addition of 
resistor R in the main emitter current carrying path as shown in FIG. 2B 
reduces the effective gain under short-circuit, by virtue of the voltage 
I.sub.sc .times.R, which subtracts directly from the net gate-emitter 
voltage. 
The oscillograms shown in FIG. 3 demonstrate an example in which a 20 
m.OMEGA. common emitter resistor, for a size 7 1200V IGBT, permits a 
short-circuit time of 20 .mu.s at 850V, as compared with a short circuit 
withstand time of about 5 .mu.s without the resistor. 
Advantageously, under normal operating conditions, the voltage developed 
across R is small, and the modifying effect on the normal operating 
gate-emitter voltage is almost negligible, as evidenced by the following 
calculated estimates. 
1. Circuit of FIG. 2B with 600V, Size 4, Gen 4 High-Efficiency IGBT, 
provided with 33 m.OMEGA. common emitter resistor R: 
Normal operating output current (3 hp)=12 A rms 
Normal operating current in R=8.4 A rms 
Normal operating loss in R=8.4.sup.2 .times.0.033 
=2.3 W. 
Estimated losses in IGBT at 12 A output: 
______________________________________ 
Frequency Loss Percent Losses in R 
______________________________________ 
4 kHz 13.5 W 17.6 
8 kHz 15.4 W 14.9 
12 kHz 17.8 W 12.9 
______________________________________ 
2. Circuit of FIG. 2B with 1200V, Size 4, Gen 4 High-Efficiency IGBT, 
provided with 66 m.OMEGA. common emitter resistor R: 
Normal operating output current=6 A rms 
Normal operating current in R=4.2 A rms 
Normal operating loss in R=4.2.sup.2 .times.0.066 
=1.15 W. 
Estimated losses in IGBT at 6 A output: 
______________________________________ 
Frequency Loss Percent Losses in R 
______________________________________ 
4 kHz 11.3 W 10.6 
8 kHz 18.1 W 6.6 
12 kHz 24.8 W 4.8 
______________________________________ 
The above calculations show that the projected losses in the common emitter 
resistor at full load, for a 600V rated high efficiency IGBT, are between 
13 and 17.5% of the IGBT losses, for a common emitter resistor that 
provides 10 .mu.s short-circuit time. This estimated added loss in the 
resistor actually is about the same as the estimated added loss for a 10 
.mu.s short-circuit rated 600V IGBT, over that of a high efficiency type. 
The estimated full load losses in the common emitter resistor that provides 
10 .mu.s short-circuit capability for a 1200V rated high efficiency IGBT 
are between 5 and 11% of the IGBT losses. Significantly, these added 
losses are about 30% less than the loss which would be added if a 10us 
short-circuit rated 1200V IGBT were used instead of a high-efficiency type 
with a common emitter resistor. 
Although the use of a common emitter resistor in accordance with the 
present invention probably does not afford any significant reduction of 
overall losses, versus the losses of short-circuit rated types, when 
viewed at the component level, the invention does in fact offer reduced 
losses at the system level, as well as other application advantages, in 
addition to the manufacturing advantages discussed above. The application 
advantages are as follows: 
1. Short-circuit time can be tailored to the specific value required for a 
particular application without changing the IGBT design, simply by 
selecting the value of the common emitter resistance. The lower the 
required short circuit time, the lower the resistance value and the lower 
the added losses. 
2. The added losses in the common emitter resistor are dissipated 
externally to the IGBT die. The IGBT remains a high efficiency type with 
minimum losses, allowing more output for a given IGBT die size than can be 
obtained from short-circuit rated IGBTs; i.e., given that power has to be 
dissipated, it is better to dissipate power in a resistor than in the 
IGBT. 
3. In a three-phase inverter for motor control, a common emitter resistor 
for only three of the six IGBTs can endow the entire inverter with the 
necessary short-circuit capability (e.g. 10 .mu.s), as discussed in 
further detail below. Moreover, a single common emitter resistor can serve 
all three IGBTs in the lower section of a three phase inverter circuit. 
The total loss contribution of the common emitter resistor or resistors 
reduces the incremental losses of the total inverter to about 7.5% versus 
15% total incremental losses for six short-circuit rated IGBTS. It should 
be recognized, however, that it would be possible to use three 
short-circuit-rated IGBTs and three high efficiency types, and no common 
emitter resistors, to achieve a circuit with about the same overall 
losses. However, such a circuit would require two different types of IGBTs 
and would therefore not achieve the stated objective of the invention to 
eliminate the need for short-circuit IGBTs. 
4. If a resistive current measuring shunt is a part of the total 
powertrain, as will normally be the case for drives up to a few 
horsepower, the shunt resistor (or resistors) can serve the dual functions 
of common emitter resistance for short-circuit current limiting, and 
current signal feedback. This is shown in FIGS. 4A and 4B. The present 
invention therefore makes it feasible to substitute high efficiency IGBTs 
into existing powertrains that include a single lower shunt resistor (or 
individual "vector control" shunt resistors for the lower IGBTs), obtain 
10 .mu.s short-circuit capability, and reduce the total inverter losses by 
about 15% for the same output current. 
Various embodiments of the common emitter resistor circuit of the present 
invention are as follows: 
1. The preferred method and circuit is to use existing current sensing 
shunt (or shunts) if such shunts are already present in the powertrain. 
2. An alternative embodiment is to use bonding wires with the desired 
resistance. 
3. A further embodiment is to mount external thick film resistors on the 
substrate of a powertrain circuit board. 
4. A still further embodiment is to use connectors from a power level 
circuit board to a separate driver board having the desired resistance. 
5. Another embodiment is to add the desired resistance in a separate driver 
board. 
To obtain lower incremental losses during normal operation and/or increased 
short-circuit time, a non-linear common emitter resistor with a low 
resistance value at normal operating current and a higher value at 
short-circuit current is preferred in yet another possible embodiment of 
the present invention. Such a nonlinear resistor can be realized by: 
1. A polysilicon resistor with a positive temperature coefficient. The 
increased temperature caused by the short-circuit current would give 
higher common emitter resistance when needed, increasing the short-circuit 
withstand time. The increased resistance at higher operating temperature 
would also give greater limiting of short-circuit current when the fault 
occurs at initial high operating temperature. 
2. A device having a resistance which increases with current, such as a 
power MOSFET, e.g. a 20V HEXFET manufactured by the present assignee, 
International Rectifier Corporation. FIG. 5 shows the circuit 
configuration in which a 20V HEXFET (e.g. a Gen 5 size 1 HEXFET) provides 
the common emitter resistance for a high efficiency IGBT (e.g. a Gen 4 
600V Size 4 high efficiency IGBT). The short-circuit current of the IGBT 
is limited to that of the HEXFET. Thus, in the example of FIG. 5, the high 
efficiency IGBT is endowed with a short-circuit time of about 15 .mu.s. 
Since the area of the HEXFET die is about 15% of that of the IGBT, the 
added HEXFET does not add significant cost. Note that, in the 
configuration shown in FIG. 5, it is possible to apply drive pulses only 
to the IGBT or only to the HEXFET, with only a fixed 15V applied to the 
gate of the other device. 
As stated previously, a single common emitter resistor, or individual 
resistors for the bottom IGBTs only of a three-phase inverter, can be used 
to protect all six IGBTs against line-to-line output short-circuit. This 
is because line-to-line short-circuit current flows through a lower and an 
upper IGBT in series. Line-to-line short-circuit protection of the upper 
IGBT is thus provided by the current limiting action of the lower IGBT. 
The use of common emitter resistors for just the upper side IGBTs also 
provides complete inverter protection against line to line short circuits. 
However, where a resistive shunt, or shunts, are already present in the 
bottom side of the circuit, it is simpler and more economical to utilize 
such existing components as the common emitter resistance. 
It should be recognized that, under certain types of faults, it is possible 
for fault current to flow just through the upper side IGBTs, in which case 
the lower side IGBTs are powerless to limit the current in the upper side. 
Two types of fault that can give rise to short-circuit current in the top 
IGBTs only are as follows: 
Type 1 fault--Ground fault at the output of the inverter. 
Type 2 fault--Inadvertent external connection of a short-circuit from an ac 
output terminal to the negative bus terminal, N. 
Both of these types of faults can be handled without resort to common 
emitter resistors for the upper side IGBTs. Specifically, Type 1 faults 
can almost always be withstood naturally by high efficiency IGBTs for a 
period of 10 .mu.s or more without any additional means of protection. As 
shown in FIG. 6, since the path for ground fault current is through the 
input supply impedance, the rate of rise and prospective amplitude of 
ground fault current is severely limited by the impedance of the ac input 
line. The relatively low amplitude ground fault current will not damage 
high efficiency IGBTs within a 10 .mu.s shut-down period. 
Type 2 faults are more severe, because fault current flows from the low 
impedance bus capacitor via the upper side IGBT to the negative bus 
terminal N. The need for common emitter resistors for the upper side IGBTs 
for this type of fault can be avoided by: 
a) Preventing user access to the negative bus terminal N. The reason for 
providing external access to the negative bus N is to provide a connection 
point for an optional external brake circuit. Powertrains that have an 
internal brake transistor therefore do not require an externally 
accessible N terminal. The possibility still exists that a user might 
miswire an inverter ac terminal to the brake terminal, but now the brake 
IGBT, with its own common emitter resistor, will provide short circuit 
protection for the upper side IGBT. 
b) Designing the upper bus current detector to discriminate between 
line-to-line faults and Type 2 faults, the latter having a much higher 
short-circuit current because of the higher effective gain of the upper 
side IGBTs, which do not have common emitter resistors. For example, the 
upper bus may be provided with a resistor/opto circuit which is designed 
to discriminate between the line-to-line and the Type 2, higher current, 
faults. The resistor/opto detection circuit is designed to respond more 
slowly than the lower fault detection circuit to line-to-line fault, 
allowing the latter to control the shut-down time for line-to-line fault. 
The upper detection circuit is preferably designed to respond as rapidly as 
possible to the higher amplitude Type 2 faults, so that it shuts down the 
inverter and protects the upper IGBTs as quickly as possible, e.g. within 
1 or 2 .mu.s. 
A conceptual schematic of the upper fault detector is shown in FIG. 7. For 
line-to-line short-circuit, the voltage developed across R.sub.s is 
insufficient for Zener diode Z (or some other type of threshold diode) to 
conduct. Capacitor C charges through resistor R1. The time constant R1C is 
such that the opto does not deliver an output during the trip time set by 
the lower detection circuit (not shown). It should be noted that Zener 
diodes may not be available with a sufficiently low threshold, in which 
case Zener diode Z can be replaced by two or three diodes in series, or 
possibly an LED, which has about a 2V threshold. 
For a high amplitude Type 2 fault, the voltage developed across R.sub.s is 
sufficient for Z to conduct. Capacitor C now charges rapidly via R2, which 
has a much lower resistance value than R1. The opto-isolator delivers a 
trip pulse within 1 or 2 .mu.s, protecting the upper IGBTs. A trip time of 
1 or 2 .mu.s for Type 2 faults only should be completely noise-free, thus 
avoiding "nuisance trip" problems with very fast shut-down time for this 
type of fault. 
For normal levels of fault current, the top trip circuit actually has more 
filtering and more noise immunity than the bottom one. A fast trip can 
only be initiated by abnormally high current in the upper shunt, which 
itself can only be caused by a bona fide Type 2 fault. 
c) Adding a circuit that detects a low impedance between any ac output 
terminal to the negative bus terminal N, to prevent the occurrence of Type 
2 faults as a result of miswiring between terminals prior to powering-up 
the inverter. If a low impedance is detected from either U, V or W to N 
during the power-up period, while the IGBT gates are still inhibited, 
inhibition of the driver (and hence the driver signal to the IGBT gates) 
at the end of power-up sequence is maintained, thereby preventing 
initiation of a type 2 fault. A conceptual implementation is shown in FIG. 
8. 
The present invention, in the various embodiments set forth above, offers a 
way of focusing manufacturing of high efficiency IGBTs only, with all the 
ramifications relative to cost savings thereby entailed. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. It is preferred, 
therefore, that the present invention be limited not by the specific 
disclosure herein, but only by the appended claims.