In many practical applications such as ultrasonic cleaning, ultrasonic emulsion, ultrasonic suspension and ultrasonic welding, ultrasonic waves are obtained by applying high frequency sinusoidal voltage to bolted Langevin type transducer (BLT) and by mechanical vibration.
Resonant inverters are used to generate high-frequency sinusoidal currents that drive BLTs because of their high power conversion efficiency.
Among the various types of resonant inverters, the class-D resonant inverter is suitable as a power source for high-power ultrasonic devices due to the possibility of high power drive. Power control of class-D resonant inverters is generally used in a way that controls the magnitude of the input dc voltage of the inverter, i.e. the pulse-amplitude modulation (PAM) scheme. This scheme has the advantage of high operating stability because all the switch elements are ZVZCS (zero voltage zero current switching) turn-on and-off when the inverter is operating at the resonant frequency of the load. However, the use of another ac/dc converter is inevitable, which leads to a cost increase and reduction of the power efficiency of the inverter.
For class-D resonant inverters, the use of phase-shifting control has been widely investigated in many applications, including large power induction heaters, because of the possibility of power control without additional converters. However, it is in principle impossible to realize ZVZCS turn-on and-off for all switch elements of the inverter when controlling the inverter output using a phase-shift control method, such as the PAM scheme. Even when the phase-shift control inverter operates at the resonant frequency of the load, the switch elements of the left-branch of the full-bridge circuit are ZVZCS turning-on, but non-ZVZCS turning-off(forced turn-off) and the switches of the right-branch that is controlled by phase-shift modulation (PSM) are ZVZCS turning-off, but non-ZVZCS turning-on (forced turn-on).
Since the forced turn-off of the left-branch switch elements reduces the efficiency of the inverter by increasing the heat loss in the switch elements, some research on soft turn-off is being done. But in particular, the generation of shock current due to forced turn-on of the right-banch switches increases the power loss in the switch elements, and furthermore, it can cause severe damage to the inverter operation when the inverter without PSM control(PAM-mode inverter) works under capacitive load, which can lead to severe unfavorable consequences of the inverter operation and, in serious case, to the breakdown of the switches.
Thus, in this work our objective is to study and analyze the operation of a BLT-loaded PSM controlled inverter and the power control principle, and to find a reasonable power control strategy for the converter without differential current (shock current) generation during its operation.
The power control by the proposed method fundamentally eliminates the generation of shock current by controlling the operating frequency so that the starting zero points of the inverter output voltage and load current at the corresponding phase-shift angle coincide.
While the power output of the ultrasonic wave is controlled by the proposed scheme, there is a weakness in that the power conversion efficiency decreases because the power supply is operating slightly off the resonant frequency of the BLT. However, the power loss due to forced-on is also eliminated and the breakdown-hazard of switch-elements because of the shock current is also eliminated because the switching elements of the right-branch being forced turned on are ZVZCS turned on.
A strategy to operate the PSM inverter in a steady state where shock currents do not occur is to match the zero points of the output voltage and current of the inverter by giving the phase delay of the current with the change of operating frequency for the corresponding phase shift as well as the control of the phase shift Φ.
In the proposed method, when the power control, the operating frequency is selected in left section of the resonant frequency (frequency domain lower than the resonant frequency). The grade of deviation from the resonant frequency is more and more increased according to decrease in the power. That is, lower power moves frequency towards lower band. Therefore, our proposed power control scheme can be simply represented as "phase-shift + operating frequency control".
When the inverter is operating at the operating frequency satisfying above condition at the phase-shift angle Φ, there are four states in one-cycle operation. The phase-shift angle control range for ultrasonic power control is 0° to about 90°, and the power control range is from 12% to 100%.
The details were published in the "Journal of Acoustical Society of America" under the title of "A safe power control method for a phase-shift migration inverter to drive a high-power ultrasonic transducer"(https://doi.org/10.1121/10.0021054).
