2020 年 3 月 18 日
降低钳位电压
In part one, we learned transient voltage suppressor (TVS) clamping voltage is a key protection parameter. TVS manufacturers are constantly working to lower clamping voltage to keep up with ever shrinking IC geometries and increased sensitivity to overvoltage events. One method to achieve lower clamping is to engineer the device to “snapback” to a lower voltage once triggered. The snapback can be shallow, or deep depending on the intended application. One method of achieving a deep snapback is to design a self-triggering thyristor TVS structure (Figure 4).
图 4 – 深折回的 TVS 结构 © Semtech Corporation
使 PNP 晶体管的集电极-基极在特定电压下击穿，便可实现自触发，并可通过 TVS 结构精确控制。在瞬态事件中，TVS 开始导通，电流流入 NPN 晶体管的基极。随后电流被驱动到 PNP 晶体管的基极，依此类推，直到两个晶体管都被驱动到饱和状态。这样两个端子之间的电压会突然折回或降至低电平。一旦瞬变消退，只要移除受保护线路上的电压，器件就会返回到高阻抗状态。这种保护解决方案的一个缺点是在某些应用中可能发生“闩锁”。瞬态电压事件触发 TVS 时会发生闩锁，并在瞬态消散后保持导通。虽然深折回器件提供了一些可行的最低钳位电压解决方案，但出于闩锁考虑，可能需要使用非折回或浅折回器件。图 5 是典型的浅折回 TVS 电路。
图5 – 浅折回的结构 © Semtech Corporation
这里，TVS 与具有浮动基极的晶体管的集电极-发射极并联。基极被设计为一旦 TVS 开始导通，晶体管集电极-发射极两端的电压将迫使基极“穿通”。结果是轻微或“浅”折回特性。图 6 为 TVS IV 特性曲线的对比。
图 6 – TVS IV 特性 © Semtech Corporation
降低电容
TVS 必须提供针对瞬态事件的保护，而又不影响电路的正常工作。这很有挑战性，因为参数通常相互冲突。例如，高速电路上使用的 TVS 必须具有低电容以避免信号衰减。但器件吸收能量的能力与结尺寸有关。结点越大，吸收的能量越多。不过结尺寸越大，器件电容也会增加，这是不利的一面。降低有效电容的常用方法是将低电容转向二极管电路与 TVS 结合使用（图 7）。
图 7 – 低电容 TVS © Semtech Corporation
The capacitance of the TVS is effectively “hidden” from the circuit. The design of the compensation circuit requires diodes with sufficient area to conduct transient current under forward bias conditions, and a reverse breakdown voltage greater than the breakdown voltage of the TVS. The resulting TVS is usually a monolithic device. Emerging standards such as USB4 and Thunderbolt 3 have strict insertion loss allocations for ESD protection. Semtech’s RailClamp® series devices rely on the aforementioned low capacitance structure to provide high levels of ESD protection on high-speed lines while meeting critical insertion loss requirements.
封装效率
Packaging is an important component in TVS construction. Most “diodes” utilize a wire bond structure to connect the die to the lead. The wire bond adds inductance and adds to the peak clamping voltage. Recall that V = L di/dt. For an 8kV ESD pulse, 1nH of inductance can add 30V to the peak clamping (V = 1nh 30A/ns). Many TVS devices still utilize wire bond structures, but higher performance devices minimize the series inductance by eliminating the wire bond. For example, Semtech’s Z-Pak™ series are chip scale devices with a proprietary epoxy coating. This structure provides the absolute minimum inductance without sacrificing structural integrity like traditional, uncoated chip scale packages. Multi-line devices are available that utilize chip-on-lead technology to eliminate wire bonds and minimize inductance.
新兴的标准、减少的片上保护和缩小的 IC 尺寸，这些因素推动着 TVS 器件的研发。现代 TVS 组件不是“普通二极管”，而是用于保护当今日益敏感器件的复杂构件。选择器件前需要仔细考虑电路参数。
30 多年来，Semtech 一直是 TVS 二极管开发的先驱。我们拥有用于保护传统端口和下一代端口的各种器件组合。
Semtech、Semtech 徽标和 RailClamp 是 Z-Pak 是 Semtech Corporation 或其附属公司的商标或服务标志。
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