Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-06-09
2001-05-29
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S020000, C363S097000, C363S065000
Reexamination Certificate
active
06239988
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to providing gate drive to switching devices. More specifically, the present invention provides a flyback energy storage technique to provide a current pulse with a fast rise time for driving fast semiconductor switching devices.
Certain types of high power semiconductor switching devices require gate drive pulses with very fast rise times for proper operation. For example, a high power pulse thyristor or a silicon controlled rectifier (SCR) may require a 30 amp pulse with a 200 nanosecond rise time. One technique for driving a stack of such devices is described in commonly assigned, copending U.S. patent application Ser. No. 09/007,574 for COMPACT SOLID STATE KLYSTRON POWER SUPPLY filed on Jan. 15, 1998, the entirety of which is incorporated herein by reference for all purposes. In that application, each of the gates of an SCR stack are driven by a corresponding secondary winding of a multiple secondary, single primary pulse transformer.
The “hard switch” approach favored by many in the industry uses a high voltage source which employs a specially crafted ultra-low-inductance storage capacitor and charging supply. When a gate pulse is required, the high voltage supply is switched onto the primary of the gate drive pulse transformer. Unfortunately, the inductance of such a pulse transformer circuit, while relatively small, is typically large enough to necessitate a very high voltage power source on the primary winding which also must be capable of sourcing a lot of current. That is, the bulk inductance of the wires leading to the primary winding from the power source and the combined leakage inductances of the multiple secondary windings reflected back through to the primary are such that a very large amplitude, high current drive must be provided to overcome the effective primary inductance and generate the required amplitude and rise time, i.e., di/dt, on each of the secondary windings.
An example of the power source necessary for driving a stack of pulse thyristors which require 30 amps in 200 ns should be illustrative. Assuming a typical primary lead inductance of 2 &mgr;H and a 1:5 primary-to-secondary turns ratio, a 3000 volt supply able to source 150 amps must be used. Where the number of device gates being driven (and therefore the number of secondary windings) is high, leakage inductances must be taken into account, making the voltage requirement significantly higher.
The disadvantages of such an implementation are well known to those of skill in the art. High voltage power supplies and their associated circuitry require special design considerations, present safety issues, and are typically less reliable and more expensive than their lower voltage counterparts. For example, pulse transformers in such high power applications must be robust, fast, and efficient, making them both bulky and expensive. In addition, stray reactances in such designs contribute to the generation of formidable transients which must be suppressed.
It is therefore desirable to provide improved gate drive circuitry for fast semiconductor devices which employs a relatively low voltage power supply and which is simpler, more reliable, and less expensive than current solutions.
SUMMARY OF THE INVENTION
According to the present invention, a gate drive circuit is provided for fast semiconductor devices which employs a flyback energy storage technique to deliver a large current pulse with a fast rise time to the gates of a stack of such devices. The technique uses a transformer having multiple secondary windings, one connected to each gate being driven. The primary of the transformer is connected in series with a switch which controls the flow of current through the primary from a relatively low voltage source. When the primary switch is closed, current ramps up in the primary winding which is representative of energy being stored in the transformer core. When a gate pulse is needed, the primary switch is opened, and the energy stored in the core of the transformer is commutated to the secondary windings and thus to the gates of the stack of devices. The only thing limiting the di/dt of the secondary current pulses is the leakage inductance of the secondary circuits which can be made to be quite low. According to specific embodiments of the present invention, 30 amp current pulses with 200 ns rise times can be achieved with a 100 volt primary supply.
Moreover, the decay of the current pulse delivered to the device gates can be set by manipulation of the L/R of the secondary winding to accommodate a variety of rep rates and pulse droop requirements. Of course, it will be understood that there is a tradeoff between pulse rise time (di/dt) and the decay rate.
Several of the advantages of the present invention will be immediately apparent to those of skill in the art. For example, a much lower voltage source may be used as compared with previous techniques with all the attendant advantages. That is, the gate drive circuit of the present invention is smaller, cheaper, easier to design (less critical layout), safer, and simpler (more reliable). In addition, the pulse transformer used may employ relatively inefficient and lossy (and therefore less expensive) core materials.
Thus, the present invention provides circuits and methods for providing gate drive to a gate terminal of a first switching device. The circuit includes a transformer having a core, a primary winding, and at least one secondary winding. The primary winding of the transformer is for coupling to a power source. The at least one secondary winding is for coupling to the gate terminal of the first switching device. A second switching device is connected in series with the primary winding of the transformer. The second switching device controls conduction of current in the primary winding which causes energy to be stored in the core of the transformer. The second switching device is operable to stop the current from flowing in the primary winding thereby causing a current pulse to be generated in the at least one secondary winding from the energy stored in the core. The current pulse is used for driving the gate terminal of the first switching device.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
REFERENCES:
patent: 4297594 (1981-10-01), Onda et al.
patent: 4356525 (1982-10-01), Konrumpf et al.
patent: 4535400 (1985-08-01), Itani
patent: 5438294 (1995-08-01), Smith
patent: WO88/05978 (1988-01-01), None
Froelich Robert J.
Marziale Michael
Han Jessica
Siemens Medical Systems Inc.
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