Power control strategies evaluation of a series resonant inverter for atmosphere plasma applications M. T. Tsai Department of Electrical Engineering, Southern Taiwan University, Tainan, Taiwan
Outline I. Atmosphere plasma Introduction II. The research system configuration III. Control system design IV. Experimental results V. Conclusion
I. Induction Examples of industrial plasma applications： • Low-pressure (vacuum pressure (<10 mTorr), moderate pressure (~ 1 Torr)) • Glow discharge plasmas: non-thermal plasmas generated by the application of DC or low frequency RF (<100 kHz) electric field to the gap between two metal electrodes. Probably the most common plasma; this is the type of plasma generated within fluorescent light tubes. • Capacitively coupled plasma(CCP): similar to glow discharge plasmas, but generated with high frequency RF electric fields, typically 13.56 MHz. These differ from glow discharges in that the sheaths are much less intense. These are widely used in the microfabrication and integrated circuit manufacturing industries for plasma etching and plasma enhanced chemical vapor deposition.
Low-pressure • Inductively coupled plasma (ICP): similar to a CCP and with similar applications but the electrode consists of a coil wrapped around the discharge volume which inductively excites the plasma. • Wave heated plasma: similar to CCP and ICP in that it is typicallyRF (or microwave), but is heated by both electrostatic and electromagneticmeans. Examples are helicon discharge, electron cyclotron resonance (ECR), and ion cyclotron resonance (ICR). These typically require a coaxial magnetic field for wave propagation.
Atmospheric pressure (760 Torr) • Arc discharge: this is a high power thermal discharge of very high temperature ~10,000 K. It can be generated using various power supplies. It iscommonly used in metallurgical processes. For example it is used to melt rocks containing Al2O3 to produce aluminium. • Corona discharge: this is a non-thermal discharge generated by the application of high voltage to sharp electrode tips. It iscommonly used in ozone generators and particle precipitators.
Atmospheric pressure (760 Torr) • Dielectric barrier discharge (DBD): this is a non-thermal discharge generated by the application of high voltages across small gaps wherein a non-conducting coating prevents the transition of the plasma discharge into an arc. It is often mislabeled 'Corona' discharge in industry and has similar application to corona discharges. It is also widely used in the web treatment of fabrics. The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere. A RF generator (20-100 kHz, 0-2.4 kV peak) is used for discharges in DBD tube • Capacitive discharge: this is a nonthermal plasmagenerated by the application of RF power (e.g., 13.56 MHz) to one powered electrode, with a grounded electrode held at a small separation distance on the order of 1 cm. Such discharges are commonly stabilized using a noble gas such as Helium or Argon.
II. The research system configuration Fig. 1(b) Inverter stage • Fig. 1(a) PFC stage
The research system configuration A simplified series resonant inverter model (b) (a) Fig. 2 Reactor model in different operation status, (a) before plasma, (b) plasma operation.
The research system configuration • For the inverter output voltage and frequency at which the value of voltage amplitude on capacitor can not reach the gas discharge value, the system has the maximum resonant frequency of . (1) When the inverter output voltage and frequency at which the value of voltage amplitude on capacitor reaches the gas discharge value, , the system resonant frequency will be reduced to minimum situation of . (2)
The research system configuration • The value of can be shown as follows: (3) Thus, the voltage at the capacitor can be obtained as follows: (4)
The research system configuration • Assuming as the gas discharge starting voltage, then the switching boundary frequencies at which gas discharge starts can be obtained as follows: (5) The switching frequency of the inverter should meet the above equations, and is dependent on the parameters including transformer, plasma reactor, and the values of the inverter input voltage and the inverter output pulse-width.
III. Control system design • A. PFC stage • The PFC stage uses the UC3854 based average-mode controller to accomplish fixed frequency current control with stability and low distortion. Unlike peak current-mode, average current control accurately maintains sinusoidal line current without slope compensation and with minimal response to noise transients.
Control system design • B. Inverter stage • The inverter has four stages, determined by the power switching elements of the two legs.The stages in which two diagonally opposite power switches are conducting are called active . On the contrary, the stages in which two switches on the same site of power switches are conducting are called passive.The switching of the leg can moves the inverter from active stage to passive stage is called the leading leg (QC，QD). The other leg which switches only from passive stage to active is called the trailing leg (QA，QB).
Control system design A experimental result
Control system design • For phase shift control and with the lossless snubbing capacitor, the ZVS can be achieved in the leading leg for all the load conditions, but can be achieved in the trailing leg only in the case that the inverter operates with a lagging load current. For a RLC series, it means the inverter switching frequency should be higher than the load resonant frequency.
Power control strategies • PAM control • The PAM controls the inverter input voltage by controlling the PFC stage output voltage to achieve the adjusting inverter output power. • Considering the power elements stress and wide voltage range, it is preferred buck-boost scheme to boost scheme as the PFC stage. • As the instantaneous electrode voltage should be large than the gas breakdown level so as to form a sustain discharge procedure, thus the inverter input voltage can not be lower than a certain level,so it leads to a result that discharge power is difficult to less that half of the full range. • A low voltage operation may be accompanied a partial or local discharge due to inequality of the gas between the electrode and dielectric. • The inverter output power factor decreases as the output increases, if the operation frequency is constant.
Power control strategies • PWM control • The PWM controls the pulse width of the inverter output voltage to achieve the adjusting output power. • By shifting the phase difference of the control phase with respect to the standard phase, the output power can be varied from full power to low power, therefore it is feasible to regulate the inverter output power. • Lossless snubbing capacitors can enable the inverter to perform ZVS function when it operates with a lagging load current. • For discontinuous load current or leading load current, the ZVS function can not be achieved in the trailing leg, resulting in an increasing switching loss. • Similarly to the PAM scheme, the inverter output power factor also decreases as the pulse width increases in the PWM control when the switching frequency is constant. • A small pulse width tends to a discontinuous load current or leading load current, which is adverse to the switching loss, thus it is disapproved for a low pulse width control.
Power control strategies • PFM control • The PFM controls the frequency of the inverter output voltage to achieve the adjusting output power. • To realize zero voltage switching, the inverter output frequency should be large than the load resonant frequency. Thus, one can see that the inverter power fact should decline in low power range. • The inverter output power will have a steep increase when the frequency is approaching the load resonant frequency. This will increase the difficulty to control the inverter output power stably. • As the same as the PAM and PWM situations, it is difficult for the PFM applied to the voltage-source series-resonant inverter to adjust the discharge power to less than half of the full power, as the electrodes voltage would be lower than the gas discharge breakdown voltage.
Power control strategies • PDM control • The PDM controls the output power by controlling the number of inverter output voltage pulses, in other words, it is to repeat “run and stop”, in accordance with the desired output power. For example, if a working cycle represents a time interval of 40 working pulses, thus by varying the number of these working pulses from 10 to 40 will have a regulated output power from 25% to 100%. •  has shown this method can work well over a range of pulse densities from 3/30 to 1. • The PDM scheme can have lower switching loss than other schemes as it achieves quasi-ZCS and ZVS functions . However, if a shorten rising time when the next discharge period starts is desired, it was found useful to apply pulses with reduced width to the plasma reactor during zero-power periods, which is used to prevent deionization of gas . Thus, the control signal will consist of full-width pulses during the discharge period and reduced-width pulses during the zero-power period.
Power control strategies UC3895 structure
Power control strategies • PDM control PDM control signals
Power control strategies • PDM control PDM implementation –method 1
Power control strategies • PDM control PDM implementation –method 2
IV. Experimental results • CH1-20A CH2-200V M-5ms/div • Fig. 3(b) The source voltage and current for a load case of 1000W
IV. Experimental results • CH1：5V CH2：200V M：10us • Leading leg switching performance
IV. Experimental results • CH1：5V CH2：200V M：10us • Lagging leg switching performance
IV. Experimental results • CH1：400V CH2：5A M：5us • ZVS switching performance
IV. Experimental results • Fig. 4(a) The experimental waveforms of 50% PWM control
IV. Experimental results • Fig. 4(b) The experimental waveforms of 75% PWM control
IV. Experimental results • Fig. 5(a) PFM control for 50 kHz switching frequency.
IV. Experimental results • Fig. 5(b) PFM control for 40 kHz switching frequency.
IV. Experimental results • Fig. 6(a) PDM control for a case of during operation at a pulse density of 25% (10/40).
IV. Experimental results • Fig. 6(b) PDM control for a case of during operation at a pulse density of 50% (20/40).
V. Conclusions As the experimental results, some conclusions can be made as follows: • The only PAM control is not encouraged as it needs a complicated front stage to achieve voltage regulation function, and is hardly used to less that half of the full range due to the required gas breakdown voltage level • The PWM control can fulfill the full load range conditions. However, a small pulse width tends to a discontinuous load current or leading load current, which is adverse to the switching loss, thus it is disapproved for a low pulse width control. • The only PFM control is also not encouraged as it should be large than the load resonant frequency to realize zero voltage switching. Thus, one can see thatthe inverter power fact should decline in low power range, and it is difficult to adjust the discharge power to less than half of the full power as the electrodes voltage would be lower than the gas discharge breakdown voltage. • The PDM can work well over a range of pulse densities from 3/30 to 1, however, the environment temperature fluctuations should disturb the stability of the inverter output power. To compensate this influence, a hybrid control such as PDM plus PFM or PDM plus PWM is suggested.