Y. Mizutani

High-frequency drive-power and frequency control for ultrasonic transducer operating at 3 MHz

This paper describes RF drive-power and frequency control for the ultrasonic cleaning system operating at a frequency of 3 MHz. The RF output power is maximized by the automatic frequency tuning of the PLL built in the full-bridge MOSFET DC-to-RF power inverter to the series resonating frequency of the ultrasonic transducer operating at 3 MHz. In addition, the full-bridge DC power controller utilizing the pulse-width control of the drive voltage is used to control the drive power for the ultrasonic transducer. Since the ultrasonic transducer has a series impedance of 1 to 2 ohms at the resonating frequency, the quality factor of the ultrasonic transducer is high at the resonating frequency. So, the power conversion efficiency is generally low at 3 MHz. In order to keep the quality factor of the load composed of the sensor and ultrasonic transducer high, a new circuit technique established using an RF current transformer and a digital clipping circuit has been employed to sense the current flowing through the ultrasonic transducer for the PLL circuit. The system provides a total power conversion efficiency of greater than 80% at an RF output power of up to 60 watts at a frequency of 3 MHz when the ultrasonic transducer having a resonating impedance of 1 to 2 ohms is driven.

Full-bridged MOSFET DC-to-RF inverter for high frequency ultrasonic transducer at 3 MHz

Described is a full-bridged MOSFET DC-to-RF inverter operating at a frequency of 3 MHz for driving a high frequency ultrasonic transducer for cleaning material surfaces. The full-bridged MOSFET DC-to-RF inverter generates an RF power of 60 W with a power conversion efficiency of 79 to 87% at 3 MHz when an ultrasonic transducer with a very low impedance in the order of 1 to 5 ohms is connected to the inverter. The ultrasonic transducer was satisfactorily operated when the ultrasonic transducer was driven from the full-bridged MOSFET DC-to-RF inverter.

Power maximizing of ultrasonic transducer driven by MOSFET inverter operating at 1 MHz

Since the ultrasonic transducer for cleaning the semiconductor wafer or other industrial materials is driven at its own resonating frequency, the operating frequency of the MOSFET DC-to-RF inverter should be tuned to the resonating frequency. As one method to make the operating frequency of the inverter tune to the resonating frequency of the ultrasonic transducer, automatic frequency control (AFC) using the phase-locked loop (PLL) is presented. The ultrasonic transducer has the minimum impedance and maximum power supply at its own resonating frequency. From the view point of the safety operation of the inverter, the power supply system should be operated at the resonating frequency of the load. Since the ultrasonic transducer has a complicated frequency dependent impedance, the RF power fed to the load can be maximized by new type of PLL tuning circuit even if the resonating frequency of the ultrasonic transducer changes due to change of the load impedance. The ultrasonic transducer has poles at 1.04 MHz, 1.8 MHz, and so on. The generating frequency range of VCO was to cover 0.8 MHz to 2.7 MHz so that the resonating frequency of the ultrasonic transducer was set at 1 MHz which was the center frequency of the VCO output. The operation of the system for feeding an output power of 60 W at 1 MHz was satisfactory.

Frequency control of MOSFET full bridge power inverter for maximizing output power to megasonic transducer at 3 MHz

The frequency of the MOSFET DC-to-RF power inverter is precisely set at the series resonant frequency of a megasonic transducer by controlling the oscillation frequency of the PLL when the MOSFET DC-to-RF power inverter is used to drive the megasonic transducer having an impedance of as low as 5 to 6 ohms or less. The frequency control system features a current sensor having an effective internal impedance of as low as 0.25 ohm which is negligible compared with the impedance of the megasonic transducer at the series resonant frequency. The impedance of the inverter load composed of the output transducer, matching capacitor, coaxial cable, megasonic transducer, and current sensor is calculated to determine the frequency control range of the PLL. In accordance with the result of calculation, the performance of the phase-frequency comparator, low pass filter, and VCO is decided. When the RF output power of the MOSFET power inverter was 60 W with a power conversion efficiency of 95%, the frequency control range was 2.8 MHz to 3.2 MHz. The PLL covers the range of frequency shifts and variations in the megasonic transducer and VCO.

Automatic frequency control for maximizing RF power fed to ultrasonic transducer operating at 1 MHz

For driving an ultrasonic transducer for cleaning semiconductor wafers or other industrial materials, the ultrasonic transducer having an input power of 60 W and operating at 1 MHz is driven from a prototype of the DC-to-RF inverter with an automatic frequency tuning function. Though the automatic frequency tuning circuit using a PLL circuit is not easy to build since the ultrasonic transducer has a complicated frequency dependent impedance, the RF power fed to the load can be maximized by a new type of PLL tuning circuit even if the resonating frequency of the ultrasonic transducer changes due to change of the load impedance. The ultrasonic transducer has a pole at 1 MHz. The frequency range of a VCO was designed to cover 0.8 MHz to 1.2 MHz so that the resonating frequency of the ultrasonic transducer was set at 1 MHz which was the center frequency of the VCO output. The operation of the system for feeding an RF power of 60 W to the ultrasonic transducer at 1 MHz was satisfactory.

Single-ended compact MOSFET power inverter with automatic frequency control for maximizing RF output power in megasonic transducer at 3 MHz

A single-ended compact MOSFET DC-to-RF power inverter is built to drive a megasonic transducer having an impedance of as low as 5 to 6 ohms or less at 3 MHz. The operating frequency of the MOSFET DC-to-RF power inverter is automatically set at the resonating frequency of the megasonic transducer using the PLL, so that the RF power in the megasonic transducer is maximized. The impedance of the inverter load consisting of an output transformer, a matching capacitor, a coaxial cable, a megasonic transducer, and a current sensor is calculated to determine the frequency control range of the PLL. In accordance with the result of calculation, the performance of the PLL consisting of a phase-frequency comparator, a low-pass filter, and a VCO is determined. The output power of the single-ended compact MOSFET power inverter was 25 W with a power conversion efficiency of 70%, when the operating frequency was set at the resonating frequency of the megasonic transducer.

MOS-FET RF inverter with auto-tuning function for driving ultrasonic transducer at 1.8 MHz

When an ultrasonic transducer operating at 1.8 MHz for use in cleaning semiconductor wafers or other materials used for industrial purposes is driven from a MOSFET DC-to-RF inverter, the output power severely depends on the frequency of operation, since the quality factor of the transducer is high. In order to tune to the resonating frequency of the ultrasonic transducer, the drive signal frequency of the MOSFET power inverter is automatically pulled in until the frequency is set at the resonating frequency of the ultrasonic transducer. A PLL is used to set the operating frequency at the resonating point so that the output power of the ultrasonic transducer is maximized. The control circuit consists of an output power sensing circuit, a PLL controller, a frequency standard and other peripheral circuits. Although a transducer having many resonating frequencies was used, the operation was satisfactory when the transducer having an output power of 60 W at 1.8 MHz was used.

Dispersion of water drops by induction heating using MOSFET inverter

A single-ended MOSFET DC-to-RF inverter connected to the load consisting of a capacitor and a planar coil, which are connected in serial, is described to remove water drops from the surface of a small mirror. The planar coil is made of a patterned printed circuit board resembling a swirl. Induction heating is accomplished on a sheet of iron by an RF current flowing through the planar coil. When the surface temperature of the mirror attached to the iron sheet was 40/spl deg/C, the power of the MOSFET DC-to-RF inverter operating at a resonating frequency of 52.3 kHz was supplied from a DC power supply at an output power of 22 W. This temperature is high enough to remove water drops from the surface of the mirror, even if the output power is small.

Operating frequency control for maximizing output power of MOS-FET power inverter to drive megasonic transducer at 3 MHz

The operating frequency of the MOSFET DC-to-RF power inverter is controlled using the PLL when the MOSFET DC-to-RF power inverter is used to drive a megasonic transducer having an impedance of as low as 2 to 3 ohms or less. The impedance of the inverter load (composed of the output transducer, matching capacitor, coaxial cable, megasonic transducer and current sensor) is calculated to determine the frequency control range of the PLL. In accordance with the results of calculations, the performance of the phase-frequency comparator, lowpass filter and VCO is decided. When the output power of the MOSFET power inverter was 60 W with a power conversion efficiency of 95%, the frequency control range was 2.8 MHz to 3.2 MHz. The PLL covers the range of frequency shifts and variations in the megasonic transducer and VCO devices used in the system.