TL431分流稳压器电压输入限制

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TL431分流稳压器电压输入限制
使用输入限制电路，可以在不损害供电器件的前提下，增加输入电压到有效值440V。
　　图1中电路限制或箝住大于有效值260V的输入交流电压到功率MOSFET工作的安全等级。电路使用MOSFET Q1，如100Hz开关分流稳压器IC1 TL431CZ，通过分压的R2和R4设置箝位高压等级。电路使用图中所示器件值。箝位输出电压为直流360V，输入电压为有效值260V，最大输入电压为有效值440V。经测试电路消耗功率5到10W。
　　输入电压小于有效值260V，点C小于2.5V,，IC1闭合，最小关闭状态阴极电流下降。齐纳二极管D2到15V崩塌，确保Q1的稳定状态。这个操作是Q1在输入电压小于有效值260V的正常状态。从而，在这些电压等级，电路作为容性负载C3下的标准全桥整流器工作。
　　有效值260V或更大的输入电压时，点C大于2.5V，IC1打开，D2电流换向且降低。门源电压Q1降到约2V，Q1关闭。现在，即使D1桥整流二极管处于正向偏压，也没有电流流到电荷泵电容C3。整流输入交流电压大于通过C3电压，但是Q1关闭，回路中断，没有电流流过。从而，由于没有合适的放电电流，C3上输出直流电压得到限制。
　　整流交流输入电压开始减少时，其最终到点C的2.5V极限水平，Q1再次打开。但由于整流桥二极管反向偏置，没有电流流动；整流输入交流电压小于通过C3的电压。这个电压由输出功率等级决定的速度减小。最终，当整流桥二极管变为前向偏置时，电压和整流输入交流电压在同一水平。Q1仍然打开；因此，放电电流开始流动。瞬间跟随，通过Q1和D1引导。短放电脉冲补充能量损耗，增加电压到限制等级。当输入电压高于有效值260V时，Q1再次闭合，重复整个过程。
　　Q1消耗能量少。每个开关周期，MOSFET只闭合450 μs，导致此高电压限制电路高效率。可以使用它作为带STMicroelectronics SuperMesh MOSFET STP4NK50Z的MOSFET开关。其有TO-220封装，但也可以使用Dpak封装节省空间，因为MOSFET不是一个消耗电压限制器。当50/60Hz整流二极管前置偏置，通过Q1的电流中断。电流中断引起漏源电压。依照EN 55022 B级，使用峰值和均值检测，箝位电路通过管理EMI（电磁干扰）测试。1-mH, 0.2A阻塞，L1和L2抑制EMI。通过D1桥的整流管，220-nF, 440V交流电容C1为简单的缓冲器元件。
　　英文原文：
　　Use a TL431 shunt regulator to limit high ac input voltage
　　This circuit clamps input-ac voltages to levels safe for the operation of the power MOSFET in an SMPS.
　　Todor Arsenov, STMicroelectronics, Prague, Czech Republic; Edited by Charles H Small and Fran Granville -- EDN, 10/25/2007
　　Most isolated, offline SMPSs (switched-mode power supplies), including flyback, forward, and resonant, must operate at input voltages of 90 to 260V rms. Some cases even use line-to-line voltages of 400V rms±10%, leading to increased component-voltage ratings and, thus, increased cost of the overall design. In such cases, it is preferable to use input-limiting circuits, allowing you to increase the input voltage to 440V rms without damaging the power-supply componentsThe circuit in Figure 1 limits, or clamps, input-ac voltages higher than 260V rms to levels safe for the operation of the power MOSFET in an SMPS. The circuit employs MOSFET Q1 working as a 100-Hz switch and shunt-regulator IC1, a TL431CZ, setting the clamped high-voltage level by divider R2 and R4. The circuit uses the component values shown. The clamped output voltage is 360V dc, the input voltage is 260V rms, and the maximum input voltage is 440V rms. The circuit was tested at power levels of 5 to 10W.

　　At an input volt
age of less than 260V rms, Point C is less than 2.5V, and IC1 is off, sinking the minimum off-state cathode current. Zener diode D2 breaks down to 15V, ensuring a stable on-state for Q1. This operation is the normal condition of Q1 at input voltages lower than 260V rms. Accordingly, at these voltage levels, the circuit works as a standard full-bridge rectifier under capacitive load C3.
　　At an input voltage of 260V rms or greater, Point C becomes higher than 2.5V, and IC1 turns on, diverting and sinking the current from D2. The gate-to-source voltage of Q1 drops to approximately 2V, and Q1 switches off. Now, no current flows to charge bulk capacitor C3 even if the D1 bridge-rectifier diodes are forward-biased. The rectified input-ac voltage is higher than the voltage across C3, but Q1 is off, the loop is interrupted, and no current flows. Accordingly, the output-dc voltage across C3 gets limited because no charging current is available.
　　When the rectified ac-input voltage starts decreasing, it eventually hits the 2.5V threshold level of Point C, and Q1 again switches on. But current does not flow because the rectifier bridge’s diodes are now reverse-biased; the rectified input-ac voltage is less than the voltage across C3. The voltage across C3 decreases at a rate that the output-power level determines. Eventually, the voltage across C3 and the rectified input-ac voltage intersect at a level when the rectifier bridge’s diodes get forward-biased. Q1 is still on; therefore, charging current starts flowing. A short interval follows, during which both Q1 and D1 conduct. The short charging pulses replenish the energy loss, increasing the voltage to the limited level. When the input voltage gets higher than 260V rms, Q1 again switches off, and the whole process repeats.
　　Q1 has small power dissipation. During every switching period, the MOSFET is on for only 450 μsec, resulting in high efficiency for this high-voltage-limiting circuit. You can use it as a MOSFET switch with the STMicroelectronics SuperMesh MOSFET STP4NK50Z, which comes in a TO-220 package, but you can also use a Dpak to save space because the MOSFET is not a dissipative-voltage limiter. The current through Q1 gets interrupted when the 50/60-Hz rectifying diodes are forward-biased. This current interruption causes ringing on the drain-to-source voltage. The clamping circuit passed the conducted EMI (electromagnetic-interference) tests, according to EN 55022 Class B, using peak and average detection. The 1-mH, 0.2A chokes, L1 and L2, suppress EMI. The 220-nF, 440V-ac capacitor, C1, is a simple snubber element across the rectifying diodes of the D1 bridge.