In an electrosurgical generator for generating radio frequency power, the generator includes a radio frequency output stage having two or more output connections, and a power supply coupled to the output stage for supplying power to the output stage. A controller is operable to cause the generator to supply a blended output signal alternating constantly between a first output signal across the output connections in which the radio frequency output voltage developed across the output connections is limited to at least a first predetermined threshold value for cutting or vaporisation of tissue, and a second output signal across the output connections in which the radio frequency output voltage developed across the output connections is limited to a second threshold value for coagulation. There is also provided adjustment means, operable by a user of the electrosurgical generator, for changing between various preset settings for the blended signal, the preset settings each having a predetermined duty cycle of the blended signal that is limited to the first threshold value for cutting or vaporisation, and a predetermined duty cycle of the blended signal that is limited to the second threshold value for coagulation.

Electrosurgical generator and system

An electrosurgical system including an electrode assembly having two electrodes for use immersed in an electrically conductive fluid has a generator with control circuitry for rapidly reducing the delivered radio frequency output power by at least 50% within at most a few cycles of the peak radio frequency output voltage reaching a predetermined threshold limit. In this way, tissue coagulation can be performed in, for example, saline without significant steam generation. The same peak voltage limitation technique is used in a tissue vaporization or cutting mode to limit the size of the steam pocket at the electrodes and to avoid electrode burning.

Electrosurgical generator method

Various embodiments for improved power devices as well as their methods of manufacture, packaging and circuitry incorporating the same for use in a wide variety of power electronic applications are disclosed. One aspect of the invention combines a number of charge balancing techniques and other techniques for reducing parasitic capacitance to arrive at different embodiments for power devices with improved voltage performance, higher switching speed, and lower on-resistance. Another aspect of the invention provides improved termination structures for low, medium and high voltage devices. Improved methods of fabrication for power devices are provided according to other aspects of the invention. Improvements to specific processing steps, such as formation of trenches, formation of dielectric layers inside trenches, formation of mesa structures and processes for reducing substrate thickness, among others, are presented. According to another aspect of the invention, charge balanced power devices incorporate temperature and current sensing elements such as diodes on the same die. Other aspects of the invention improve equivalent series resistance (ESR) for power devices, incorporate additional circuitry on the same chip as the power device and provide improvements to the packaging of charge balanced power devices.

Power semiconductor devices and methods of manufacture

A power transistor having high breakdown voltage and low on-state resistance includes a vertical field effect transistor in a semiconductor substrate having a plurality of source, channel, drift and drain regions therein. A trench having a bottom in the drift region and opposing sidewalls which extend adjacent the drift, channel and source regions is also provided in the substrate, at a face thereof. The trench preferably includes an insulated gate electrode therein for modulating the conductivity of the channel and drift regions in response to the application of a turn-on gate bias. The insulated gate electrode includes an electrically conductive gate in the trench and an insulating region which lines a sidewall of the trench adjacent the channel and drift regions. The insulating region has a nonuniform cross-sectional area between the trench sidewall and the gate which enhances the forward voltage blocking capability of the transistor by inhibiting the occurrence of high electric field crowding at the bottom of the trench. The thickness of the insulating region is preferably greater than 1500 Å along the portion of the sidewall which extends adjacent the drift region and less than 750 Å along the portion of the sidewall which extends adjacent the channel region. To provide low on-state resistance, the drift region is also nonuniformly doped to have a linearly graded doping profile which decreases from greater than about 1×10 17 cm -3 to less than about 5×10 -16 cm -3 in a direction from the drain region to the channel region.

Vertical field effect transistors having improved breakdown voltage capability and low on-state resistance

An electrosurgical instument, which is used for the treatment of tissue in the presence of an electrically-conductive fluid medium, comprises an instrument shaft (10), and an electrode assembly (12) at one end of the shaft. The electrode assembly (12) comprises a tissue treatment electrode (14) and a return electrode (18) which is electrically insulated from the tissue treatment electrode by means of an insulation member (16). The tissue treatment electrode (14) has an exposed end for treating tissue, and the return electrode (18) has a fluid contact surface which is spaced from the tissue treatment electrode in such a manner as to define, in use, a conductive fluid path that completes an electrical circuit between the tissue treatment electrode and the return electrode. The electrode assembly (12) is provided with a plurality of apertures (14a) in the region of the tissue treatment electrode (14), through which apertures vapour bubbles and/or particulate material can be aspirated from the region surrounding the tissue treatment electrode.

Under water treatment

A synchronous rectifier is able to operate at higher frequencies and provides an output having lower noise than prior art FET synchronous rectifier system by using field effect switching devices which contain only one conductivity type of semiconductor material and connecting a high speed, low charge storage diode in parallel. Schottky diodes are preferred whereby there is no junction diode in the structure. Conventional FETs may be used when paralleled with a Schottky diode which prevents the FET's parasitic internal diode from becoming conductive.

Low noise, high frequency synchronous rectifier

This paper demonstrates that controlled electron irradiation of silicon power MOSFET devices can be used significantly improve the reverse recovery characteristics of their integral reverse conducting diodes without adversely affecting the MOSFET characteristics. By using 3 MeV electron irradiation at room temperature it was found that the reverse recovery charge in the integral diode could be continuously reduced in a well controlled manner from over 500 nC to less than 100 nC without any significant increase in the forward voltage drop of the integral diode under typical operating peak currents. The reverse recovery time was also observed to decrease from 3 microseconds to less than 200 nsec when the radiation dose was increased from 0 to 16 Megarads. The damage produced in gate oxide of the MOSFET due to the electron radiation damage was found to cause an undersirable decrease in the gate threshold voltage. This resulted in excessive channel leakage current flow in the MOSFET at zero gate bias. It was found that this channel leakage current was substantially reduced by annealling the devices at 140°C without influencing the integral diode reverse recovery speed. Thus, the electron irradiation technique was found to be effective in controlling the integral diode reverse recovery characteristics without any degradation of the power MOSFET characteristics.

Improving the reverse recovery of power mosfet integral diodes by electron irradiation

A variable frequency AC power source drives a gas discharge lamp which is connected in parallel with the capacitance of a series resonant circuit. The polarity of the voltage applied to the resonant circuit is commutated at such times as the rate-of-change of current flow in the resonant circuit is below an adjustable threshold value. The setting of a threshold adjusting circuit determines the brightness of the lamp. The polarity is also commutated at such times as the current flow from the source reaches a predetermined level. Lamp current is thus independently and variably controlled in the variable power mode.

Frequency shift inverter for variable power control

A hybrid DMOSFET-Schottky (FastFET) device has been developed that exhibits the excellent on-resistance and gate control properties of the DMOSFET along with improved internal diode switching characteristics. The 45 V and 30 V devices fabricated use an integrated Schottky device that consumes less than 15% of the active area while returning a 35% improvement in reverse recovery performance. The leakage of the FastFET is higher than that of the standard DMOSFET but is still within the tolerances set by most manufacturers' data sheets. Future improvements will include process modification that will reduce this leakage. >

High performance power DMOSFET with integrated Schottky diode

A power semiconductor device incorporates in its active, or current-carrying, region a main current section and an emulation current section. The active region is surrounded by a common device termination region. This is accomplished through provision of respective separate cathodes for the main and emulation current regions, while the device anode is common to both the main and emulation current sections. The current level in the emulation current section provides an accurate representation of the current level in the main current section since the main and emulation current sections are closely coupled both thermally and electrically and, further, are formed in the same fabrication process. The current level in the main current section can be economically determined with low power circuitry by way of sensing the current level in the emulation current section.

John P. Walden

Papers

Low noise, high frequency synchronous rectifier

Improving the reverse recovery of power mosfet integral diodes by electron irradiation

Frequency shift inverter for variable power control

High performance power DMOSFET with integrated Schottky diode

Power semiconductor device with main current section and emulation current section