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RF-CO2激光器正常工作时,由射频电源对输入信号进行功率放大,经过对应的匹配输出给激光器负载,如图1所示。
但射频电源系统不是绝对稳定的,由于射频能量的影响,传输线将不再遵循基尔霍夫理论。如图2所示,平行导线或电缆(图2(a))在这种情况下会等效成电容和电感(图2(b)),射频能量以电磁场的方式传输。
由于传输线的长度不同,转换成的阻抗大小也不尽相同,阻抗匹配网络也并非处于最佳匹配状态。于是,实际的RF-CO2激光器结构示意图可以等效成图3,整个RF-CO2激光器系统处于非稳定状态。
如图3所示,输入匹配与功率管功放以及输出匹配与功率管功放之间存在耦合关系,表现为RF信号源反射系数(
${\varGamma _{\rm{s}}}$ )、输入反射系数(${\varGamma _{{\rm{in}}}}$ )、输出反射系数(${\varGamma _{{\rm{out}}}}$ )、负载反射系数(${\varGamma _{\rm{l}}}$ )。图3中,
${\varGamma _{{\rm{in}}}}$ 和${\varGamma _{{\rm{out}}}}$ 分别反映输入匹配和输出匹配对功率管功放的反射功率大小。由于功率管功放本质上是一个工作在饱和状态的场效应管元件,其漏极电流和栅源电压间关系如图4所示。图 4 饱和区功率管的漏极电流与栅源电压关系
Figure 4. Relation between drain current and gate source voltage of power tube in saturated region
当反射功率过大时,功率管放大后的输出功率与反射功率相互作用形成自激振荡,产生一个较高的峰值电压,由于瞬间的峰值电压远大于功率管正常工作时的电压
${V_{\rm GS}}$ ,漏极电流${i_{\rm DS}}$ 激增,功率管就会被烧损。为保证射频电源正常运行,功率管不被烧毁,功率管需满足稳态放大要求[13]:
$$ \begin{split} \\ \left| {{\varGamma _{{\rm{in}}}}} \right| < 1 \end{split} $$ (1) $$ \left| {{\varGamma _{{\rm{out}}}}} \right| < 1 $$ (2) $$ \varGamma = {V_{\rm{r}}} /{{V_{{\rm{in}}}}} < 1 $$ (3) -
根据射频电源系统原理和功率管的特性,选择以下参量作为监控对象。
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如图4所示,要保证功率管不被击穿,需要控制功放电路在稳定条件下工作,即
$\varGamma < 1$ ,根据$$ \varGamma = V_{\rm{r}}/V_{\rm{in}} $$ (4) 式中:
$\varGamma $ 为反射系数;${V_{\rm{r}}}$ 为反射电压;${V_{{\rm{in}}}}$ 为入射电压。要使得$\varGamma < 1$ ,即要求反射电压小于入射电压,且反射系数越小,匹配效果越好,射频电源也越稳定。反之,当
$\varGamma > 1$ ,即${V_{\rm{r}}} > {V_{{\rm{in}}}}$ 时,射频电源系统处于不稳定状态,此时极易产生自激振荡致使功率管被反向击穿。因此,入射电压、反射电压与反射系数作为重点监测对象。 -
反射信号过大时会与功率管的输出信号不断叠加,产生自激振荡现象,如图5所示。
自激振荡期间,功率管的漏极电流和温度激增,致使功率管熔断。而自激振荡产生到功率管熔断会经过一段时间
$\Delta {{t}}$ ,这段时间内功率管漏极电流和温度会有明显变化,因此,功率管的漏极电流和温度作为第二个监测对象。 -
结合设计原理和平台设计结构,完成应用层、传输层和设备层的软件设计和电路搭建,其实物图如图7所示。
平台采用型号为MRFE6VP61K25H系列的功率管作为主放大芯片设计射频电源,工作频率为81.36 MHz,其特性参数如表1所示。
表 1 功率管特性参数
Table 1. Power tube characteristic parameters
Characteristic Value VGS/V 10 IDS/A 30 Pout/W 1250 ${\eta _D}$ 74% GPS/dB 27 Frequency/MHz 81.36 Temperature/℃ −60-150 从表1中可知,功率管的反向击穿电压为10 V,漏极电流极限为30 A,正常工作温度范围为−60~150 ℃。为保证射频电源功率管处于正常工作范围,根据功率管的特征参数和稳定条件,将系统参数配置和阈值设定如表2所示。
表 2 系统配置参数
Table 2. System configuration parameters
Parameter Value Baud rate/bit 115200 Serial port COM5 Traffic rate/μs 0.5 Current threshold/A 13 Temperature threshold/℃ 100 Reflected voltage/V 5 Incident voltage/V 5 Reflection coefficient 1 其中反射系数
$\varGamma $ 为1,反射电压阈值${V_{{\rm{r}}th}}$ 为5 V,功率管漏极电流阈值${I_{Dth}}$ 为13 A,温度${T_{th}}$ 为100 ℃。启动RF-CO2激光器射频电源监控平台,监控平台软件界面显示如图8所示。可以看到,左侧监测点数据显示区不断更新,图形区实时绘制当前反射电压、入射电压、漏极电流和功率管温度的变化曲线。此时采集到的入射电压为2.489 V,反射电压为2.216 V,漏极电流为8.997 A,芯片温度为19.343 ℃,输出功率为773.16 W。将测试数据与设定阈值参数进行比较得:
$$ {\varGamma }_{{\text{测}}}=\frac{{V}_{r}}{{V}_{in}}=\frac{{V}_{{\text{反测}}}}{{V}_{{\text{入测}}}}=\frac{2.216\;{\rm{V}}}{2.489\;{\rm{V}}}=0.89 < 1 $$ (5) $$ {V}_{r{\text{测}}}=2.216\;{\rm{V}} < 5\;{\rm{V}} $$ (6) $$ {I}_{d{\text{测}}}=8.997\;{\rm{A}} < 13\;{\rm{A}} $$ (7) $$ {T}_{{\text{测}}}=19.343\; {\text{℃}}<100 \;{\text{℃}} $$ (8) 所监测的数据均在正常范围,此时,射频电源处于正常状态,RF-CO2激光器射频电源监控平台的射频电源状态区绿灯亮起。
不断调节阻抗,测得警报触发、保护电路自动响应时的监控平台数据如图9所示。
此时,图形区的反射电压曲线超过了入射电压曲线,漏极电流曲线超过了设定的阈值界线,通过监测点数据区可以读出,此时漏极电流为13.94 A,反射电压为3.803 V,入射电压为3.690 V。可以看出,此时:
$$ {\varGamma }_{{\text{测}}}=\frac{{V}_{r}}{{V}_{in}}=\frac{{V}_{{\text{反测}}}}{{V}_{{\text{入测}}}}=\frac{3.803\;{\rm{V}}}{3.690\;{\rm{V}}}=1.031 > 1 $$ (9) 反射系数
${\varGamma }_{{\text{测}}} > 1$ ,漏极电流${I}_{d{\text{测}}}=13.94\;{\rm{A}} > 13\;{\rm{A}}$ ,超过了设定的阈值界线,达到了射频电源不稳定的条件,监控平台自动响应保护电路,关断射频电源,其关断响应时间如图10所示。从图10中可以看出,平台系统关断响应时间为
$ 0.96 $ μs,远高于人为反应的时间。检查射频电源功率管器件及相关电路器件,未出现烧毁情况。最后,利用该平台进行RF-CO2激光器开发调试,该平台能够自动处理数据,大幅提高RF-CO2激光器的开发效率。
RF-CO2 laser RF power supply monitoring platform
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摘要: 射频电源是RF-CO2激光器中的一个重要部件。在开发一个新的RF-CO2激光器时,由于激光器负载和射频电源匹配失调,在调试过程中容易造成射频功放功率管击穿、烧损。为解决这一问题,设计了一套集数据采集、自动保护控制和可视化技术为一体的RF-CO2激光器射频电源监控平台。平台采用软件和硬件相结合的方式,在射频电源中嵌入控制模块,搭建数据采集系统和自动保护系统,并结合电路设计软件平台完成对射频电源的数据信息采集和保护控制。经过实验测试,平台可以有效避免射频电源功率管的损坏,实现射频电源的自动保护,并完成射频电源系统的数据采集和远程保护控制,缩短了RF-CO2激光器的调试周期,提高了整机的开发效率。Abstract: RF power supply is an important component in RF-CO2 laser. During the development of a new RF-CO2 laser, due to the mismatch between the laser load and the RF power supply, the power tube of the RF power amplifier is easy to break down and burn out in the debugging process. In order to solve this problem, a RF power supply monitoring platform for RF-CO2 laser, which integrated data acquisition, automatic protection control and visualization technology, was designed. The platform adopted a combination of software and hardware, a control module was embedded in the RF power supply, a data acquisition system and automatic protection system were set up, and combined with the circuit design software platform, the RF power supply data acquisition and protection control was completed. The experimental test shows that the platform can effectively avoid the damage of the power tube of the RF power supply, realize the automatic protection of the RF power supply, complete the data acquisition and remote protection control of the RF power supply system, shorten the debugging period of the RF-CO2 laser, and improve the development efficiency of the whole machine.
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Key words:
- RF-CO2 laser /
- RF power supply /
- power monitoring /
- automatic protection
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表 1 功率管特性参数
Table 1. Power tube characteristic parameters
Characteristic Value VGS/V 10 IDS/A 30 Pout/W 1250 ${\eta _D}$ 74% GPS/dB 27 Frequency/MHz 81.36 Temperature/℃ −60-150 表 2 系统配置参数
Table 2. System configuration parameters
Parameter Value Baud rate/bit 115200 Serial port COM5 Traffic rate/μs 0.5 Current threshold/A 13 Temperature threshold/℃ 100 Reflected voltage/V 5 Incident voltage/V 5 Reflection coefficient 1 -
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