Electrosurgical Power Supply with Harmonic Control and Component Integration
Electrosurgical devices are promising tools for advanced surgical performance compared with traditional manned scalpels, due to their precise resolution and fast response. However, several different electrosurgical devices, which can only generate single ac frequency, are demanded for varied operations such as dissection, sealing or coagulation, and thereby, multiple independent instrument are employed, which increases system costs and volumes. Another issue of current electrosurgical supplies is undesired harmonics inherent from the pulse width modulation (PWM), which may bring about unexpected phenomenon such as muscle contraction or overheating of human tissues. In this project, a new modulation scheme is first proposed to generate multiple ac frequencies from a single inverter, which not only significantly reduces number of electrosurgical supplies, but also facilitate combination of advanced surgical operation, dissection and seal simultaneously for instance. Moreover, advanced component integration along with this new modulation scheme are dedicated for further reduction of overall size of state of art electrosurgical devices. Additionally, undesired harmonics are addressed when using proposed converter modulation schemes, where several low order harmonics can be largely suppressed or even totally eliminated from output spectrums.
(a) Schematic circuit of the proposed electrosurgical power supply. (b) Output voltage waveforms of the inverter, low frequency output and high frequency output. (c) Spectrum of the output voltages.
In this research, a new modulation scheme, which can simultaneously generate two different ac frequencies while suppressing undesired harmonics in between, is first proposed and systematically investigated, and then the proposed modulation is verified on a 100 W prototype. Second, the band-pass filters that are employed to separate different frequencies from the inverter are designed and compared to achieve better filtering effect. Next, advanced filter network involving planar transformer design will be conducted to further minimize the system volume as well as to modeling the parasitic effect of filtering network. Finally, the leakage current caused by high order harmonics will be addressed using the proposed modulation scheme, and corresponding modification and tradeoffs will be studied in the stage.
Modulation Scheme Study
Two different modulation schemes, unipolar and bipolar dual-frequency selective harmonic elimination (DFSHE), are investigated. Compared to traditional PWM modulation, which controls only the fundamental frequency of the output waveform through modulation of duty cycle at constant frequency, selective harmonic elimination (SHE) modulation varies both switching frequency and duty cycle per switching period in order to generate an output in which a large range of the output spectrum is directly controlled. The SHE method uses Fourier analysis, based on the desired output spectrum, to synthesize a pulse train, consisting of a number of discrete switching instances. Each switching instance is defined by switching angles relative to the fundamental period. In this work, the SHE method is extended to a dual-frequency scheme. Rather than generating a single fundamental frequency and cancelling n harmonics, the DFSHE approach regulates the amplitude of the fundamental and a kth harmonic to controlled values, while cancelling all harmonics in between and, possibly, a number of harmonics above the kth.
Passive Component Design and Integration
Two band-pass filters (LC resonant tank) are first adopted in the prototype to separate the fundamental and the kth harmonic, while attenuating other frequency element out of their resonant frequencies. The load variation will influence the quality factor of such two-order filters, and therefore, higher order filters are studied and further simulated to compare with the performance of the two-order filters. On the other hand, parasitic capacitance and ac resistance of inductor windings will bring impacts on the tuning of the appropriate frequencies of individual filters. As a result, a compact planar magnetic design will be conducted to precisely model and control such parasitic effect. Also, such component integration will decrease overall volume of the system.
How WBG Can Help
One goal of this project is to reduce the overall size of the prototype and to increase the power density of the converter. The passive components are design and optimized to achieve such goal. On the other hand, the GaN devices are also adopted in the prototype, which has smaller profile and lead to smaller PCB layout. Second, the volume of heat sink attached to the devices are also reduced due to their superior switching characteristic in several kilohertz operations compared with silicon devices. One impact of the GaN devices is the device capacitances, which relates to the resonant operation of the inverter as well as the dead time determination. The behavior of the GaN device output capacitance will be studied to achieve desired frequency and to set appropriate dead time to minimize losses.
- Chongwen Zhao