电子管阴极专用电源的工作原理 电子管, 稳定性, 工作原理, 电视, 加热电源

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    电子管阴极专用电源的工作原理：电子管阴极（灯丝）加热电源是电子管正常工作的前题。而灯丝电源的质量将直接影响到电子管特别是大功率发射管的寿命，因此灯丝加热电源应有一定程度的稳定性和精确度。一般大功率发射管的灯丝电源无较高精度的稳压措施，无平滑加电压的功能，起动时有浪涌电流冲击，效率低。

    例：10kW电视发射机末级大功率发射管灯丝的供电，就是采用变压器降压，二极管整流，LC滤波的电路。这类电路的电压输出稳定度取决于电网电压的稳定度，目前一般工业用电电网电压的稳定度约在10％～20％左右，电网电压的波动直接影响着灯丝变压器的输出电压，而灯丝电压的不稳定就导致灯丝加热温度的变化，进而影响电子管的工作状态的稳定。对于电网电压波动较大的地区和使用三相电源的中功率发射差转机，可以给灯丝电源采取单独交流稳压措施。

    有的地方采用交流稳压器对交流电源进行稳压，此时的稳压精度大约是2％～3％，只是相对改善了供电的稳定度，对于大功率发射管来说，不稳定的供电无论对发射管的工作状态还是灯丝的寿命都会产生一定程度的影响。另外，处在真空状态下的电子管灯丝在常温或低温状态下，其电阻很小，而在高温状态下，其电阻要大得多，因此电子管起动的瞬间，稳定工作的灯丝电压加在尚处于冷态甚至低温状态的发射管的灯丝上，会产生一个比正常工作电流大得多的浪涌电流。虽然这个电流很快会因灯丝温度的升高而下降，浪涌电流的冲击会对电子管灯丝本身的寿命造成危害。加之变压器降压整流供电电路的效率一般在50％以下，多余的能量以热的形式释放出来，而过多的热量又会造成别的器件的老化或氧化，金属的氧化层会引起触点之间接触电阻增加，从而增加损耗，并引发其他的故障。

    为了减小浪涌电流的冲击，避免发射管提前损坏，已在大功率电视发射机上采取了一些改进的措施。主要有：采用分档上灯丝电压的方法，以解决灯丝冷态电阻小（往往是正常工作时热阻阻值的1/7），开机时冲击电流大，影响电子管灯丝寿命。分档加灯丝电压的方法可以分两档或三档，也可以缓慢增加灯丝电压至额定值。在灯丝供电系统作的另一项改进是采用了“黑灯丝”的方法。所谓黑灯丝是指发射机停播以后，只降低而不断开灯丝供电电源的电压，保持额定值的10％～20％左右的电压，灯丝呈暗红色，微温使灯丝有较大的电阻。

    灯丝电压对电子管寿命的影响可以从跟踪调查的一批进口钨阴极电子管使用情况中得知，当灯丝电压超过额定值5％，电子管在国内条件下使用寿命由4000h以上降到了2500h左右。这个方法的缺点首先是浪费能源，其次带电停机给例行维修带来不便，再者发射管长期使用在欠压状态下，会造成阴极中毒，不利于发射管正常的工作特性的发挥。

    发射管灯丝专用稳压电源有两个特点：一是采用高精度的开关电源稳压技术，稳压精度高，效率高，节能；二是采用软起动，从根本上解决浪涌电流冲击的问题。

    将开关稳压电源电路与软起动电路组合，开关稳压电源提供高精度的稳压输出，软起动电路采取从零开始平滑提升PWM输出脉冲宽度的办法，利用其输出电压随PWM的脉宽相应改变的原理，达到输出电压从零平滑升高的目的。

    随着集成电路技术的不断发展，脉宽调制（PWM）电路技术也得到很大提高，大部分开关电源的控制电路都采用PWM来作为控制电路。

    它们的工作原理是：单相交流输入电源直接经整流滤波得到300V直流高压，然后分两路输出，一路为功率驱动电路提供电源，另一路产生12V～25V的辅助电源供PWM及其它电路。PWM电路产生一系列脉冲宽度可调的脉冲电压，作为驱动电压控制功率管的导通与截止，再经高频变压器隔离降压后整流输出。输出电压及电流信号经反馈电路送回PWM来自动调整输出脉冲的宽度，从而稳定输出电压。保护电路负责检测输出电压，电流的情况，当电压及电流出现设计所不允许的异常值时，保护电路则给出控制信号，使PWM电路停止工作，以保护功率电路本身及负载。

    软起动电路的作用是对PWM电路进行控制，使输出电压逐步形成一个上升的曲线，上升时间的长短可以借助RC参数的调整而在较大的范围内改变，刚开机时软起动电路控制PWM输出脉宽为零，从而使输出电压为零，随着电容C的充电，PWM电路的脉宽开始逐步增加，输出电压也在逐步提高，当电容充满电后，软起动电路会自动断开对PWM电路的控制。由于控制接入点的逻辑与关系，使得软起动电路完全不影响电源反馈回路对PWM电路的控制作用。

    根据不同的发射管工作特性，软起动曲线可以调整为线性型、指数型、对数型等，软起动时间一般在30s～300s左右，也可根据要求进行具体调整。

本文来自       http://www.glspower.org/c1330.html

yidodid09

October 26, 2011

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