过压保护及瞬态电压抑制电路设计

　　利用电池供电的移动设备通常需要通过外置的 AC适配器对系统电池进行充电。而不同供电电压的设备间往往共用着相似的电源插座和插头，这些不同电压标准的适配器往往会给用户带来潜在的错插风险，可能导致设备因过高的电压而烧毁。另一方面，来自 AC适配器前端的浪涌或者电网的不稳定也有可能导致适配器的输出电压超越设备所能承受的范围。因此，在移动设备设计中就有必要加入充电端口的过压保护电路，以避免上述情况对设备后端电路的破坏。 

　　本文介绍的过压保护电路由过压保护开关（OVP Switch）和瞬态电压抑制器 (TVS)组成（如图1），可实现完善可靠的抗持续高电压和瞬间冲击电压的功能。

图1

　　在整个方案中，核心部分器件为过压保护开关，以美国研诺逻辑科技有限公司（AATI）的过压保护开关 AAT4684为例，过压保护开关的内部主要是由控制逻辑电路和 PMOS管组成，当 OVP端的检测电压高于特定电压阈值之后，逻辑电路就会通过栅极关断 PMOS的沟道。由于该 PMOS管拥有较高的持续性耐压（28V），因此可以保护后端的元器件不会因前端电源输入异常高压而烧毁（其内部原理如图2所示）。

　　　　　　　　　　　　　　　　图2：AAT46842 内部原理图。

　　通过以下实验可以说明当过压保护开关的输入端出现过高电压时它对后端电路所起到的保护作用。

　　图3所示为测试所用电路原理图，输入端为 12V平稳直流源，电源通过一段长度为 1米的导线与 AAT4684的输入端相连， CH1为 AAT4684输入电压的测试点， CH 2为 AAT4684输出电压的测试点，CH3为其输出电流探测点。将 AAT4684的 OVP保护电压设为 6V（即当电压超过 6V后，开关管立刻关闭，以保护输出端的电路）。为体现实际应用中 AC适配器的插拔情况，对系统的上电过程通过导线和电源的机械性拔插来实现。

　　　　　　　　　　　　　　　　　图3：测试所用电路原理图。

[page]

　　由图4所示的波形中可以到，在电路上电的时刻，输入端的电压很快超过了 6V并最终稳定在了 12V左右，而输出端电压由于 OVP开关的作用，始终维持在 0V电压，即 AAT4684输出端之后的电路不会因过高的输入电压而受到影响，后端电路器件在此时受到了 AAT4684的过压保护。

　　　　　　　　　　　　　　图4：经示波器测得的各通道的电压及电流波形。

　　但是在这同时却发现当电源电压插入的瞬间， AAT4684输入端的电压呈现了一个超过 20V的尖峰。如果进一步调高输入电压（如将电压调整到 16V），在拔插电源时会发生 OVP开关烧坏的现象，但是电源所提供的输出电压却远小于 OVP开关的最高耐压 28V。如何解释此现象呢？

　　原因就出在从电源输出到 AAT4684输入的这段导线上。任何一段有长度导线具有一定的等效电感。等效电感的存在相当于在理想导线上串联了一个分立电感器，同时由于芯片的输入端存在的输入电容，接合起来就相当于一个如图 5所示的 LC振荡电路；而这个电路当输入一个阶跃时在输入电容上最大可出现 2倍于输入的振荡电压。

　　　　　　　　　　　图5: 输入端输入电容与导线电感构成的LC振荡电路。

　　由于这些等效器件的存在，就会在系统上电的瞬间于 OVP开关输入端产生一个高于电源的电压。过高的瞬间电压就类似静电放电电压，虽然总能量不大，但是如果其电压值在瞬间高过了 OVP开关的最高耐压范围，就足以将 OVP开关内部的 MOSFET击穿，使得芯片输入端对地发生短路，失去作用。因此在考虑过压保护设计时，还应考虑对电路输入端可能出现的瞬态高压的防护。

　　为解决以上问题，可以在 AAT4684的输入端放置TVS来实现对瞬间冲击电压冲击的防护。TVS是一种二极管形式的高效能保护器件。当 TVS二极管的两极受到反向瞬态高能量冲击时，它能以纳秒级的速度，将其两极间的高阻抗变为低阻抗，吸收高达数千瓦的浪涌功率，使两极间的电压箝位于一个预定值，有效地保护电子线路中的元器件免受各种浪涌脉冲损坏。由于它具有响应时间快、瞬态功率大、漏电流低、击穿电压偏差小、箝位电压较易控制、无损坏极限、体积小等优点，目前已广泛应用于各类电子设备之中。

　　由于 OVP保护开关虽然可以持续地长时间承受耐压范围内的电压，但是却无法经受超过其耐压范围的瞬时电压冲击，而TVS结构的二级管，虽然无法承受长时间的导通电流，但是却可以在瞬时吸收很高的电压冲击，通过自身的雪崩导通来限制其两端的最高电压，对电压起到钳位的作用。因此将 TVS管置于 OVP开关电路之前，就可以有效地防止瞬时高压对 OVP开关的破坏，同时 OVP的持续受压能力又可以保护后端电路免受前端电源持续高电压的破坏。电路逻辑结构如图 6所示。

　　　　　　　　　　　　　　　　　　图6：耐高压电路逻辑图。

　　由于 TVS本身就是属于 ESD保护器件，可以同时提高设备在接口端的静电保护能力（通常的 TVS管都可以耐受 2KV以上接触式静电放电），这样的设计就可以在真正意义上实现端口的保护功能，有效地提高了器件的使用寿命和可靠性。（关于详细的 TVS选用可参阅具体文献。）

[page]

　　另一方面，当 OVP开关导通并存在一个持续较大工作电流流过时，此时如果突然关闭开关（比如启动了 OCP过流保护或 OTP过温保护），因导线电感中的电流不会突变，导线电感中的瞬时电流的变化会在 AAT4684的输入端产生一个高于电源的电压，这就使得 OVP开关会在一个很短的时刻需要承受一个极高的电压，其原理有些类似于开关升压电路（如图 7所示）。

　　　　　　　　　　　　　　　　　　　　　图7：开关升压电路.

　　以下实验为了说明这类现象所可能产生的实际输入电压的突变，当过压保护开关有大电流流过并正常工作时将 AAT4684加温使之自动进入过温保护（ OTP）状态来观察此时输入端可能产生的波形变化。该实验电路依旧如前文所述的图3所示，电源以5.5V电压供电，负载电流约为 1.5A。

　　实验时对 AAT4684进行加热至芯片过热保护功能启动，内部的 MOSFET立刻关断。可以看到，在大约 400ns的时间里，由于流过开关管的电流被快速关断，在 OVP的输入和输出端瞬间确实出现了一个超过 15V以上峰峰值的冲击电压，假如电源的输入电压更高一些或者负载电流更大一些，这个冲击电压也同样会更高，虽然持续的时间极短，但是完全有可能在尖峰时刻突破 OVP开关的最高耐压，从而破坏其内部的 PMOS。

　　如果用同样的原理进一步分析 OVP开关接通电源时的过程，可以发现，因 OVP开关内部的控制电路在刚刚上电的瞬间需要建立状态，所以在初始的极短时间里， PMOS栅极电压没有立刻置高，因此 PMOS沟道还没来得及关断（这个时间大约会持续 0.1us），虽然对后端电路不会有什么影响，但是这个时间产生的导通电流在 PMOS关断的时刻同样会产生类似前文所述的问题，即在 OVP的输入端产生的一个时间极短的过高电压冲击可能会危及 OVP开关正常工作。

　　　　　　　　　　　　　　图8：开关关闭时各种电压变化的测量结果。

　　为了避免上面所述的这两种情况带来的瞬间高压对 OVP开关的冲击，在其前端放置合适的瞬态电压抑制器同样可以很好地解决该问题。由于 TVS管和 OVP开关具有其各自的功能特点，当电路在正常工作时，OVP开关导通，TVS处于反向截止状态，当输入电压高于 OVP保护电压又低于 OVP正常耐压时，OVP就起到了对高压很好地持续阻断的作用，保护了后端器件的安全，而当电路的输入端因前文所述几种情况而导致瞬时高压冲击出现时， TVS管的瞬间导通机制又能很好地吸收冲击电压的能量，保护了 OVP开关的安全。其两者的共同作用就可以有效地实现抑制瞬态和持续高压的功能，完善地保护了整个电路系统的接口免受异常高压的破坏。

　　本文特别感谢美国研诺逻辑科技有限公司中国区应用工程部总监谭磊（Taylor Tan）先生及华东区应用工程部经理毛铮（Frank Mao）先生对本文的修改和指正。在文章最后部分将附上使用 TVS管前后实验测得的波形，以供读者对照分析，可以进一步理解 TVS加 OVP开关在电路端口保护设计中的必要性。

[page]

　　附录 :

　　以下实验对 AAT4684输入端使用 TVS和不使用 TVS管前后的波形进行比较，供读者参考。

　　CH1为 AAT4684的输入电压，CH2为 AAT4684的输出电压。

　　将电源电压 Vin设为12V，输出电容为0.1uF，图A1为前端没有TVS管的波形，图A2为前端加了TVS管的波形（TVS导通电压为19V）

　　用5.5V作为电源输入，负载电流为1.5A时将OVP加热进入过温保护瞬间波形，图B1为前端没有TVS的波形，图B2为前端加了TVS时的波形（TVS导通电压为16V）

　　　　　　　　　　　　　　　　　图A1: 无TVS时输入端波形

　　　　　　　　　　　　　　　　　　图A2: 有TVS时输入端波形。

　　　　　　　　　　　　图B1: 前端没有TVS时过温保护瞬间。

　　　　　　　　　　　　　　　图B2: 前端加TVS时过温保护瞬间。

[page]

英文版本：

Transient Voltage Suppression and Overvoltage Protection

Steven Zhou　　Field Application Engineer of AATI Shanghai

A battery based mobile device is normally charged by an external AC adapter. It is very common that different adapters have similar electronic plugs. Different adaptors have different voltage/amperage outputs which may lead to the risk of burning the device due to high voltage if users misuse it. On the other hand, the inrush current from AC adaptor or the unstableness of the electronic grid can also cause the output voltage of the adaptor higher than the allowable input range of the device. Thus, in order to protect the mobile device, it is very necessary to have an overvoltage protection circuit between the charging circuit and the AC adaptor.

This article will introduce an overvoltage protection circuit composed of Overvoltage Protection Switch (OVP Switch) and Transient Voltage Suppressor (TVS) shown in Fig. 1. This protection circuit is able to protect the devices from both constant and transient high voltage.

The core part of the design is the OVP Switch. Take the OVP Switch AAT4684 from Advanced Analogic Technologies, Inc. (AATI) as an example, the OVP Switch is composed of control logic circuits and an internal P MOSFET. When the OVP Switch detects the voltage higher than a certain threshold value, the logic circuit will cut off the channel of PMOS by the gate. Since PMOS logic has higher input voltage allowance (28V), it will protect the parts even the input of the front part is unusual (the system diagram of AAT4684 is shown in Fig. 2).
 

Further performance of the overvoltage protection circuits will be illustrated by the following experiments.

Fig. 3 shows the testing circuit. The input end is a 12V DC Voltage Power, which is connected to the AAT4684 Vin Pin by a 1m wire. As shown in the figure, Channel 1 is the testing point of the AAT4684 input voltage, Channel 2 is the testing point of the AAT4684 output voltage and Channel 3 current is its output current testing point. The overvoltage protection threshold voltage of AAT4684 is now set to be 6V (as soon as the voltage reaches 6V, the switch will turn off so as to protect the output circuit). In order to adapt to the fact that AC adaptor is pluggable, this system is charged by plugging the wire end manually to the AAT4684 Vin Pin.
 

[page]

The result shown in Fig. 4 is obtained by using oscilloscope to grasp the voltages and current of different channels.
 

It can be seen from Fig. 4 when the power is plugged in, the input inrush voltage soon exceeds 6V and it finally stables at around 12V, however, due to the OVP Switch, the output voltage is always around 0V. It shows that the circuit behind the AAT4684 output is not affected by high input voltage, which is then protected this way.

It is also seen that when the moment the power is on, the voltage at the input Pin of AAT4684 has a peak voltage over 20V. If we further higher the input voltage (e.g. adjust the input voltage to 16V), the OVP Switch will be burnt when we plug it off, though the output voltage of the power supply is less than the voltage allowance of OVP Switch 28V. How can it be explained?

The reason lies on the wire which connects the DC power and AAT4684. A wire with any length can be equivalent to a certain inductor. The equivalent inductor is like a series connection of an inductor to the wire, meanwhile, the input Pin of the chip has a certain value of equivalent input capacitor, which will make a equivalent LC circuit shown in Fig. 5. Such circuit is able to generate two times of the input voltage in maximum with a step phase voltage input.

Due to the mechanism explained in Fig. 5, the system is able to generate a voltage which is much higher than the power supply at the OVP input end when the system is plugged on. A high transient voltage spike does not have a high energy, however, if the peak voltage value exceeds the allowance voltage of OVP Switch, it will breakdown the MOSFET in the OVP Switch which will lead to the short circuit from the input to the output. Thus, when we design an overvoltage protection circuit, it is also necessary to take the transient high voltage spikes into consideration.

The above problem can be solved by using a Transient Voltage Suppressor (TVS) in front of the OVP to eliminate the transient high voltage spikes from happening.

TVS is a high performance protection part in a form of diode. When the two electrodes of the TVS diode have two opposite transient high energy impulse, it can convert the high resistance into low resistance within nanoseconds, and absorb the surge power which can be as high as few thousand watts. It fixes the voltage between two terminals at a predetermined value, and effectively protects the parts in the backend circuit from the damage of wave impulses. TVS has the advantages of fast response, high transient power, lower leakage current, high breakdown voltage accuracy, predetermined voltage controllable and flexibility. It has been widely used in various electronic equipments.

OVP has the advantage of being able to endure the constant voltage within its allowance, but it can not take the transient voltage spikes beyond that. TVS, on the other hand, is not able to endure the large current for a long time but can absorb the high voltage impulse in very short moment. Thus, using TVS logic before OVP Switch can prevent the damage from transient high voltage spikes to OVP Switch as well as protecting the backend circuit from constant high voltage. The block diagram of the circuit is shown in Fig. 6.
 

[page]

TVS is also an Electrostatic Discharge (ESD) protection device, this enhances the electrostatic protection feature of the connectors of the devices (a normal TVS can endure at least 2KV electrostatic discharge). By this way, the designs can achieve the connector protection function which increases the reliability and the lifetime of the devices. (Please refer to other documents for the selection of TVS models).

Besides, in the scenario when the OVP Switch is on and has a constant large current, the sudden turn off of the power (e.g. due to the Over Current Protection (OCP) or Over Temperature Protection (OTP)) will not lead to the sudden change of the current in the wire due to its equivalent inductor. The wire inductor will generate a voltage which is higher than the power at the input of AAT4684, causing the OVP Switch to take a very high voltage within that moment. The mechanism is similar to a boost converter circuit. (shown in Fig. 7).
 

The next experiment illustrates the sudden change of the input voltage due to the above reason. We first let the OVP Switch work under high current, then heat AAT4684 to the OTP mode so as to observe the change at the input Pin of AAT4684. We still use the circuit which is shown in Fig. 3. The input of voltage is set to 5.5V and the input current 1.5A.
 

In the experiment, the AAT3683 is heated till it enters the OTP mode. As soon as it starts the OTP mode, the MOSFET turns off. It can be seen from Fig. 8 that the current in the OVP Switch is cut off within 400ns. During the 400ns, there is a 15V impulse voltage at the input and output of the OVP Switch. The impulse voltage could even be higher with a higher input voltage or higher input current. Although it only lasts for a very short time, it is easy to exceed the OVP Switch allowance voltage and break it down.

Similar mechanism also applies to the time when OVP Switch turns on. It takes time for the control circuit in the OVP Switch to work normally when powered on, thus, there is a very short period (around 0.1us) that the gate of PMOS does not have any voltage and the channel of PMOS is on. During this time, the system has a start-up current which will lead to a transient high voltage at the input of OVP Switch, and it can potentially prevent OVP Switch from working properly.
In order to protect the OVP Switch from the transient high voltage which can happen in the above two scenarios, a TVS in front of the OVP switch is necessary.

Conclusion

TVS and OVP have their own properties. When the circuit works properly, OVP Switch is on and TVS is off. When the input voltage is higher than the OVP protection voltage Switch but lower than its allowance voltage, OVP Switch can protect the circuit from constant high voltage. When the circuit has transient extra high voltage due to the above reasons, TVS can absorb the impulse energy and protect the OVP Switch. The collaboration between OVP and TVS can effectively suppress the transient and constant high voltage so as to protect the circuit from the damage.

The author of this article thanks Taylor Tan, FAE Director of AATI China and Frank Mao, FAE Manager of East China for their suggestions to this article. The results of the other two experiments of the circuits without and with TVS will be shown in the appendix.

[page]

Appendix
The following experiments show the waveforms in the input of AAT4684 with and without using TVS.

CH1 is the input voltage of AAT4684, CH2 is the output voltage of AAT4684.

In the system setup, the power voltage Vin is 12V and the output capacitor is 0.1uF. Fig. A1 is the waveform without using TVS, while Fig. A2 shows the waveform after using it (the TVS breakover voltage is 19V).
 

The following experiment shows the result when we heat the OVP to the OTP mode. The power voltage is 5.5V and input current is 1.5A. Fig. B1 shows the waveform without using TVS, while Fig. B2 shows the result after using it (the TVS start-up voltage is 16V).