Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2002-08-05
2004-03-30
Sircus, Brian (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
Reexamination Certificate
active
06714397
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a configuration for protecting a Schottky diode against momentary over-current pulses.
In numerous applications of diodes in which the latter are operated in a rapid sequence alternately in the reverse direction and in the forward direction, a minimum storage charge is sought in the diodes since switching losses are caused by the charge during recommutation of the diodes. In this respect, then, Schottky diodes are distinctly superior to diodes with a PN junction, that is to say PN diodes and PIN diodes, since, as is known, the Schottky diodes have no flooding charge in the semiconductor body. For this reason Schottky diodes are preferably used per se in applications in which diodes are operated in a rapid sequence alternately in the reverse direction and in the forward direction.
However, owing to the band structure of the customary semiconductor material silicon and of the Schottky contacts applied thereto, only a maximum reverse voltage of about 200 V can be applied to Schottky diodes with tenable reverse currents. Compared with Schottky diodes made of silicon, Schottky diodes made of semiconductor materials with a large band gap, such as silicon carbide (SiC), for example, have the advantage of a considerably higher reverse voltage. Thus, the reverse voltage can reach values in excess of 1700 V in the case of SiC.
A method for producing a SiC diode is described in U.S. Pat. No. 5,789,311, for example.
In Schottky diodes, given a constant current I, a diode forward voltage Vf increases with the temperature T of the Schottky diode, in which case, by way of example, in the case of SiC, a resistance R thereof is approximately proportional to T
2.5
. As a result, the power loss rises in the Schottky diode, which leads to further heating. When a critical current applied over a fixed time is exceeded, the result is thus a strong non-linear increase in the diode forward voltage Vf.
This relationship is shown in
FIG. 5
, in which the diode forward voltage Vf is plotted as a function of the diode forward current If for three different diodes at a temperature of 25° C. and twice at a temperature of 100° C. In this case, the diodes used were SiC Schottky diodes configured for 4 A (DC or direct current), to which sinusoidal over-current pulses were applied for a time of 10 ms in each case and the peak values of the diode forward voltage Vf were measured. Current values at which destruction of the diode occurred are indicated by black points.
As is known, momentary aperiodic over-current spikes can occur in a wide variety of electronic circuits. This is the case, for example, if backup capacitors are charged when such an electronic circuit is switched on. The components that are stressed by such over-current spikes must be configured for these aperiodic loads. They are therefore generally over dimensioned, which, however, impairs the further dynamic properties of the components and leads to higher costs for the latter.
An unprotected power factor correction (PFC) stage of a switched-mode power supply shall be mentioned as an example. In the case of such a PFC stage, after the power supply has been switched on, the MOSFET switches are initially turned off, the backup capacitor being charged in this state. As an example, the capacitance of the backup capacitor may be up to about 220 &mgr;F in the case of a switched-mode power supply configured for 300 W. The current flowing via the diode of the PFC stage then reaches a maximum of up to 70 A, and the I
2
t load can assume values of the order of magnitude of in excess of 4A
2
s, as can also be seen from
FIG. 6
, in which the input voltage of a 300 W power supply and the diode current of the Si-PIN diode of the power supply are plotted as a function of the time t after switch-on.
At the present time, various measures are usually taken into consideration in order to protect Si-PIN diodes or the power supply system against switch-on spikes.
First, it is conceivable to subject the diodes to over dimensioning, so that they at all events withstand current spikes that are to be expected. However, such over dimensioning results in an increase in the storage charge, which entails increased switching losses during operation and higher device costs.
Furthermore, a NTC thermistor can be connected in series with the diodes. By way of example, it is possible to use NTC thermistors with cold resistances of about 10 &OHgr; in order to reduce switch-on current spikes in power supplies with a higher power above 250 W, in order that power supply disturbances and triggering of automatic protection devices are avoided. However, when such NTC thermistors are used, a residual resistance of about 0.5 &OHgr; for a 300 W power supply results in the operating state, which leads to an additional steady-state power loss of approximately 1 W at the NTC thermistor.
In higher quality power supplies, it is also possible to use so-called “bootstrap” resistors which, in a manner similar to NTC thermistors, are connected in series with the diodes. A semiconductor switch connected in parallel with the bootstrap resistors, such as, in particular, an insulated gate bipolar transistor (IGBT) or a thyristor, then short-circuits the bootstrap resistor after a switch-on. Such a measure is relatively complicated, but practically no additional power loss occurs in normal operation.
As has already been mentioned in the introduction, Schottky diodes have the advantage of an extremely low recovery charge, “Wide Band Gap” Schottky diodes, based on semiconductor materials with a large band gap, additionally achieve high reverse voltages with small leakage currents. For this reason, the use of, for example, SiC Schottky diodes in PFC stages of switched-mode power supplies is a possibility—which has already been taken into consideration for a relatively long time—for realizing operation alternating in a rapid sequence in the reverse direction and in the forward direction in the case of such switched-mode power supplies. This leads to minimized dynamic losses in comparison with conventional PIN diodes.
However, since such Schottky diodes react extremely sensitively to high switch-on currents, as has already been mentioned in the introduction, they should always be protected against such switch-on current spikes.
The protection measures taken into consideration above for PIN diodes pose particular problems, when used for Schottky diodes.
A high degree of over dimensioning of the diode area is critical not so much because of the increasing storage charge but more because of the costs of the basic material for the Schottky diodes. In particular, this applies to compound semiconductors such as SiC, for example.
With the use of an NTC thermistor, by way of example, for the protection of an SiC Schottky diode with a continuous current of 4 A for a 300 W power supply, an NTC thermistor with a cold resistance of 20 &OHgr; is required in order that the Sic diode can be effectively protected, as experiments have shown. However, such an NTC thermistor significantly increases the power loss in normal operation. Moreover, an NTC thermistor would always have to be incorporated even in lower-power power supplies. Furthermore, with the use of an NTC thermistor, it is necessary to comply with a delay of approximately 10 s when switch-on is effected again, during which delay the NTC thermistor can cool down.
The use of a bootstrap resistor is readily possible in theory. In practice, however, it is ruled out for cost reasons in most applications.
Finally, in particular in the case of semiconductor materials with a large band gap, it would also be conceivable to surround the Schottky diode by, for example, a P
+
-doped protective ring. However, this measure is associated with a significant increase in the area requirement for the chip of the Schottky diode by about 20%. The high wafer costs for such semiconductor materials and the need for further process steps restrict the applicability of this solution.
SUMMARY OF THE IN
Griebl Erich
Mauder Anton
Rupp Roland
Infineon - Technologies AG
Kitov Z
Sircus Brian
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