2.1. Plasma Lighting Control System
The entire plasma lighting system is divided into a control area, an RF energy transmission area, and an emitter. The control area, taking a Micro Control Unit (MCU) as the core, is composed of various detection and control circuits. The RF energy transmission area, controlled by the MCU, is responsible for the power amplifier and output of the RF signal. As the plasma load, the emitter includes a resonator and a bulb. It absorbs RF energy to excite the gas and luminescent elements in the bulb and maintain high-intensity light emission.
The structure of the plasma lighting system is shown in
Figure 1. The microwave signal within a band of 430–460 MHz is generated by the digital PLL and finally output to the plasma load through a voltage-controlled attenuation network and multi-stage power amplifiers. The attenuator is used to improve the resolution of the power adjustment. With the signal conversion of the peripheral detection circuits, the MCU utilizes analog-to-digital conversion channels (ADC) to monitor various parameters continuously and control the startup of the plasma lamp.
2.2. The RF Source Circuit
Generally, a fractional-N PLL provides the high-frequency carrier for communication and is used as the RF source for plasma lamps in this design. An integration chip named CMT2380F32 from HOPERF Corporation is adopted as the signal generating and control unit for compact structure. Besides a fractional-N PLL, it also includes an ARM Cortex-M0 MCU and an RF transmitter. As shown in
Figure 2, the fractional-N PLL consists of a PFD, a charge pump, a loop filter, a VCO, a dual-modulus prescaler (D-DIV), and a 20-bit fractional frequency division modulator (M-DIV). The reference signal is from an outside crystal oscillator, and the feedback signal is formed by dividing the frequency of the VCO with the M-DIV and D-DIV. The error signal between the reference signal and the feedback signal is outputted by the PFD, integrated by the charge pump, and filtered by the low pass loop filter. The RF output signal of the VCO is locked at the desired frequency by the error signal of the PFD when the frequency of the reference signal and the feedback signal are the same. Under the 26 MHz input reference frequency, the output signal of the PLL can achieve a resolution of 25.8 Hz over a frequency range of 127–1020 MHz.
When the solid-state RF driver is used for the plasma lighting system, the sweeping frequency signal is very useful for fast startup. The change in the frequency can be realized by modifying the frequency division ratio
N, which is calculated by:
where
fREF is the reference frequency,
fVCO is the output frequency and
CDIV is the frequency division coefficient.
D and
is the integer part and the fractional part of
N, respectively.
CDIV = 4 is chosen in this design. For example, if the desired output frequency
fVCO is 450 MHz, then
The frequency of the output signal can be adjusted to 450 MHz by writing D and M, respectively, into the registers controlling the D-DIV and M-DIV.
The RF transmitter has two modulation modes, On-Off Keying (OOK) [
17] and Gauss Frequency Shift Keying (GFSK). Additionally, OOK is a special case of Amplitude Shift Keying (ASK). As shown in
Figure 3, the base-band signal
m(
t) adopts a unipolar NRZ code that “1” is a high level and “0” is a zero level. As a switch signal,
m(
t) controls the continuity of the high-frequency carrier
f(
t), and the output signal
s(
t),
when
, we obtain
During the startup process, plasma light sources usually work in the continuous wave mode to absorb enough energy and achieve final arc discharge as soon as possible. Using the modulation principle of the digital signal, the relevant registers in the PLL are configured to force the transmitter work in the OOK and DIRECT mode, and DIRECT means that data are sent directly without encapsulation. As shown in
Figure 4, when the DATA pin is kept at a high level, the PA pin can output a continuous wave with the desired frequency. The input frequency for the plasma load circuit could be changed by continuously modifying the frequency division ratio
N of the PLL.
The RF transmitter has two states, STBY and TX. During frequency hopping, it is necessary to enter the STBY state to modify the RF frequency registers, and then switch back to the TX state to output this signal. Unfortunately, the PLL stops working at the STBY state, but the state is necessary for registers to be accessed and modified, causing a cut-off time with no RF power output. In addition, there is also a period of 370 µs for PLL calibration and stabilization from STBY to TX. Therefore, one frequency hopping must go through the state transition of TX-STBY-TX, which can be shortened to 0.8 ms at least. This phenomenon may cause the plasma light source to extinguish due to the lack of the continuous injection of RF energy.
In order to study the effect on plasma, the maximum cut-off times of maintaining the plasma after a gas breakdown are tested in the glow discharge stage. The reflected power and the light signal measured under the condition indicate that the peak incident power is 30 W, the pulse period is 5 ms, and the duty cycle is 60%. As shown in
Figure 5, it is found in experiments that the light signal emitted by plasma will disappear as soon as the energy injection stops. However, when the next energy injection pulse comes again after the cut-off time, the plasma will light up immediately. The allowable interval of the cutoff time is up to 2.9 ms at the peak power of 30 W and will lengthen with the increase in the incident power. The 0.8 ms of time required for one frequency hopping is shorter than the maximum cut-off time, so the startup of the plasma light source will not be affected.
The stability of the output frequency is important for plasma lamp startup in different environments. One of the main reasons for PLL temperature drift is that the reference oscillation frequency of the crystal fluctuates under different temperatures [
18]. The working temperature of the plasma lamp is −40–85 °C, where the temperature coefficient of frequency (TCF) of the RF crystal used is 10 ppm. The temperature drift ∆
f1 for the 26 MHz oscillation frequency
fREF could be obtained according to Formula (7),
Therefore, the theoretical temperature drift ∆f2 for the highest working frequency fMAX of the RF source is 4.6 kHz, calculated by the Formula (8).
2.3. The Control Circuit
The RF control module is an important part of the RF driver for plasma lighting systems, providing a signal source with adjustable power and frequency to control the power amplifier, excite the gas discharge and complete the startup of the plasma light.
The MCU collects the consumption DC of whole circuits, the temperature, incident power, and reflected power of the plasma load, respectively, through four analog-to-digital conversion channels (ADC). According to these measured parameters, MCU controls the output RF power by adjusting the attenuation intensity of the voltage-controlled attenuator and the working state of the power amplifier module, respectively, through two digital-to-analog conversion channels (DAC). In addition, pulse modulation for the RF signal is realized through another PWM channel to improve the system luminous efficacy of the plasma lamp.
A four-PIN diode π attenuation network is used as the attenuator. The resistance value of the PIN diode could be adjusted continuously under the control of the current and shows linear resistance to microwave signals, which can achieve better matching and flatter attenuation in a wide range of frequencies [
19]. The voltage of the attenuator is controlled by the MCU to prevent sparking between the bulb electrodes due to current overshoot and insufficient energy injection due to low current. Hence, damage to the bulb or plasma extinguisher could be avoided. Besides that, it is also necessary to stabilize the circuit current for the plasma light startup process and adjust the light intensity during the steady operation phase through the attenuator.
A high-frequency triode is used as the medium power amplifier and the maximum output power of 21 dBm is obtained. It has been reported that the conversion efficiency from electric to light energy could be improved by using a modulated RF pulse signal [
20]. The data rate of the transmitter is too low for pulse modulation with a frequency of 87 kHz and a duty cycle of 90%. Therefore, another way for pulse modulation is needed. As shown in
Figure 6, a PWM signal output by the MCU is used as the switch signal to control the switch circuit, then the following DC power supply of the medium amplifier will be modulated, and an RF pulse signal could be obtained. The pulse period and the duty cycle of the RF pulse signal are determined by the MCU.
In the initial stage of the startup, the plasma load is mismatched, and the reflection coefficient is very high, so high incident power is required for the gas breakdown. With the warm-up of the plasma, the reflection coefficient continues to decrease and then enters a stable arc discharge stage when the reflection is low enough to indicate the completion of the startup. In the final stage, the controlling method aims to improve the conversion efficiency of the amplifier. The quiescent operating point of the power amplifier is adjusted by the gate voltage through DAC so that the working type of the amplifier changes into class C from class AB in the early stage [
21].
2.4. Design of Control Flow
During the startup of the plasma light source, the dielectric material inside the bulb changes into plasma from a low-pressure vacuum. Additionally, the relative permittivity
εp of the plasma is determined by the following formulas [
22]:
where
ωp is the oscillation frequency of the plasma,
ν is the collision frequency of electrons with other particles,
ne is the electron density in the plasma, and
e and
me are the charge and the mass of a single electron. With the increase in the electron density in the warm-up process, it can be predicted that the input impedance of the resonator and the optimal frequency for RF energy absorption are constantly moving under the mutual influences of these parameters. In order to analyze the general trend of impedance change and find a control method for quick startup, it is assumed that
ne is 1.8 × 10
19/m
3 and
ν is 5.0 × 10
11 Hz after the gas is instantly broken down. According to Formulas (9) and (10),
εp = 0.77−41.46 i is obtained. These material values are used in a high-frequency structural model of the resonator and the bulb to carry out electromagnetic simulation. Then, both the return losses
S11 of the resonator before and after the gas breakdown in the bulb are calculated over different frequencies. As shown in
Figure 7, the curve shifts obviously to the lower frequency once the plasma forms. Note that the curve of the return loss will change continuously with the injection time of microwave energy.
Therefore, a method of changing the driving frequency is used in this design for the fast absorption of the RF energy. The startup of the plasma light source is performed in three stages: ignition, preheating, and operation. In these stages, the status of the plasma light is evaluated according to the various measured parameters described in the control circuit section. The output power of the RF amplifier and the input driving frequency are controlled for quick RF coupling so that the plasma absorbs enough energy, transitions to the final arc discharge from glow discharge, and achieves complete impedance matching as soon as possible.
In the ignition stage, a PWM signal with a full duty cycle is used to the switch circuit so that the RF driver outputs a continuous wave. As shown in
Figure 7, the input frequency is controlled to sweep downward rapidly from the point F0, which is above the frequency F1 with a minimum return loss before breakdown. After arriving at the frequency F1, the DC of the circuit at this point is acquired from the ADC. Then, the driving signal continues down to the point F2, and the DC at this point is also acquired. Whether the plasma forms can be determined by comparing the values of the two currents. If the current at F2 is larger than that at F1, it means that the curve in
Figure 7 has moved to the lower frequency and the gas filled in the bulb has been successfully broken down into plasma. Then, the startup process enters the preheating stage, and the plasma begins to glow discharge. Otherwise, the ignition is failed, and the startup process should be restarted. Due to the high reflected power in the ignition stage, the power amplifier needs to work at class AB with full power output to ensure successful breakdown.
In the preheating stage, the plasma warms up and transitions to the high-intensity arc discharge gradually. The control method at this stage is shown in
Figure 8. The frequency continues to sweep down until it crosses over the new frequency point with the lowest return loss established by the plasma. Then, the load mismatch of the RF driver will aggravate, and the consumed DC will decrease rapidly. When the current cannot be maintained within the rated working range by controlling the attenuator, the frequency should stop downward immediately and begin to move upward in order to prevent the plasma from extinguishing. The frequency is shifted up for maximum incident power at a safe rated current, and then the incident driving signal is kept waiting at this status. With the microwave heating, the plasma density increases, and the load matching of the RF driver improves. When the reflection coefficient measured by the ADC drops to a value lower than 0.45, the frequency starts to move up and track the frequency with the maximum incident power, and the illumination of the plasma lamp will gradually heighten. If a reflected power detected is low enough and below 1 W, it means that the preheating stage has been completed, and the plasma has converted into an arc discharge stage from the glow discharge. Then, the plasma lamp enters the steady operation stage.
During the operation of the arc discharge stage, the impedance matching between the RF driver and the resonator is well and the reflected power is small enough. The amplifier is able to output sufficient RF power under the matched load to maintain the plasma discharge. To improve the drain efficiency of the amplifier, the gate voltage of the power transistor is reduced to zero biased, making the power amplifier in the RF driver work in the class C mode.
Affected by temperature, the frequency with the minimum reflected power will sway slightly, so it is necessary to adjust the driving frequency up and down and follow the optimal reflection. After several minutes, the pulse modulation is turned on, and the RF driver outputs the amplified power pulses with several hundred watts so as to improve the system luminous efficacy. Moreover, the temperature of the RF driver and reflection coefficient are monitored at the same time for the reliability of the system. A high temperature will reduce the lifetime of the RF driver, and the output of the RF energy should be stopped to prevent the amplifier from being damaged. A high reflection coefficient means the failure of the startup and the startup process of the plasma lamp should restart after cooling down.