How to Choose the Right Protection for Your Circuit

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是什么推动了对保护设备的需求以及实施它们的挑战。
Traditional types of protection devices.
Different approaches to protection, including surge stoppers.

所有行业的制造商都不断推动尖端的绩效，同时试图平衡这种创新与久经考验的强大解决方案。设计师面临着平衡设计复杂性，可靠性和成本的艰巨任务。特别是一个子系统，电子保护，拒绝由于其性质而变为创新。这些系统保护敏感且昂贵的下游电子设备（FPGA，ASIC和微处理器），需要零失败速率。

Many traditional and historically proven protection methodologies—e.g., diodes, fuses, and TVS devices—retain their go-to status, though these are often inefficient, bulky, and require maintenance. To address these deficiencies, active, intelligent protection ICs have proven they can match the protection requirements of traditional methods, but in many ways they’re more robust. Because of the wide range of devices available, the most difficult problem for the designer is simply choosing appropriate solutions. To help designers narrow their choices, this article compares traditional protection methods.

为什么考虑电压和当前保护设备？

The increase in the amount of electronics used in all industries, and the expansion of functions handled by expensive FPGAs and processors, has elevated the need to protect these devices from the harsh environments in which they operate. Layer on top of this the need for small form factors, high reliability, and fast response to overvoltage and overcurrent surge events. Let’s take a look at the challenges and traditional protection methodologies, and compare them to newer, alternative solutions that offer better accuracy, reliability, and design flexibility.

Automotive, industrial, communications, and aviation electronic systems must operate through a range of power-supply surges(Fig. 1)。In each of these markets, transient events are defined in a number of industry specifications. For example, automotive transients are covered by theISO 7637-2和ISO 16750-2specifications, which outline both the details of expected transients and test procedures to ensure that these are consistently validated.

增加事件的类型和他们的能量能够vary depending on the area in which the electronic device is used; circuitry can be exposed to overvoltage, overcurrent, reverse-voltage, and reverse-current conditions. Ultimately, many electronic circuits would not survive, let alone operate, if directly facing the transient conditions shown inFigure 1。因此，设计师必须考虑所有输入事件，并实施保护电路免受这些电压和电流冲浪的保护机制。

Design Challenges

There are many different causes of transient voltage and current surges in electronic systems, but some electronic environments are more prone to transient events than others. Applications in automotive-, industrial-, and communications-based environments notoriously experience potentially harmful events, wreaking havoc on downstream electronic devices. However, surge events aren’t limited to these environments.

Other possible candidates for surge-protection circuitry include any application that requires high-voltage or high-current supplies, or those featuring supply connections that are hot plugged, or systems that have motors or may be exposed to potential lightning-induced transients. High-voltage events can occur over a wide range of time bases, from microseconds to hundreds of milliseconds, so a flexible and reliable protection mechanism is imperative to ensure the longevity of costly downstream electronic devices.

For instance, automotive load dump can occur when the alternator (charging the battery) is momentarily disconnected from the battery. As a result of this disconnect, the full charge current from the alternator is placed on the power rail, which raises the rail voltage to very high (>100 V) levels for hundreds of milliseconds.

Communications applications can have a number of possible surge causes, ranging from hot-swapping communication cards to outdoor installations that can be exposed to lightning strikes. Inductive voltage spikes are also possible with long cables used in large facilities.

Ultimately, the environment in which the device must operate must be understood along with meeting published specifications. This helps the designer put together an optimal protection mechanism that’s both robust and unobtrusive, but allows downstream electronics to operate within safe voltage levels with minimal interruption.

Traditional Protection Circuitry

有这么多不同类型的电气事件要考虑，电子工程师的武器库应该是什么来保护敏感的下游电子产品？

传统的保护实施依赖于几种设备，而不仅仅是一种设备。示例包括用于过电压保护的瞬态电压抑制器（TVS），用于过电流保护的在线保险丝，用于反向电池/电源保护的系列二极管以及电容器和电感器的混合物，以过滤降低能量的尖峰。(Fig. 2)。While discrete setups can meet published specs—protecting downstream circuits—they result in complex implementations, requiring multiple selection iterations to correctly size the filtering.

让我们仔细看看这些设备中的每一个，探讨了该实现的优势和缺点。

Transient voltage suppressor

The TVS is a relatively simple device that helps to protect downstream circuitry from high-voltage spikes on the power supply. It can be broken into several different types, which have a wide range of characteristics (thetablebelow is in order of response time, smallest to largest).

尽管它们具有一系列的构造和特性，但它们都以类似的方式工作：当电压超过设备阈值时，将多余的电流分流。电视在很短的时间内将输出的电压固定在额定水平上。例如，电视二极管的响应能力低于Picseconds的时间，而气体排出管（GDT）可能需要几微秒来做出响应，但能够处理更大的潮流。

Figure 3shows the simple implementation of a TVS diode to protect a downstream circuit. Under normal operating conditions, the TVS is high impedance and the input voltage simply passes to the output. When an overvoltage condition occurs at the input, the TVS becomes conductive and responds by shunting the excess energy to ground (GND), clamping the voltage seen by the downstream load. The rail voltage rises above the typical operation value but is clamped to a value at a safe level for any downstream circuitry.

Although TVS devices are effective in suppressing very-high-voltage excursions, they’re not immune to damage when faced with sustained overvoltage events, resulting in a requirement for regular device monitoring or replacement. Another concern is that a TVS can fail short and thus crowbar the input supply.

Also, depending on the energy involved, they can be physically large to match with margin, increasing the solution size. Even when a TVS has been correctly sized, the downstream circuitry must be able to handle the clamped voltage, resulting in increased voltage rating requirements downstream.

在线保险丝

可以使用无处不在的在线融合以高于名义的边缘的保险丝打击等级来实现过电流保护 - 例如，比最大额定电流高20％（百分比将取决于电路的类型以及典型的操作性操作预期的负载）。当然，保险丝的最大问题是，一旦爆炸，必须更换它们。

Time and cost savings resulting from a fuse’s simple design can be incurred later because of relatively complex maintenance, especially if the application is physically hard to reach. Maintenance requirements can be reduced with alternate fuses, such as resettable fuses, which utilize a positive temperature coefficient to open the circuit when a larger-than-normal current passes through the device (the increased current level increases the temperature, resulting in a sharp increase in resistance).

Maintenance issues aside, one of the biggest problems with fuses is their reaction time, which can vary widely depending on the type of fuse selected. Fast-blow fuses are available, but clearing time (time to open the circuit) can still range from hundreds of microseconds to milliseconds. Therefore, the circuit designer must consider the energy released over these extended times to ensure that downstream electronics can survive.

Series diode

In some environments, circuits are exposed to supply disconnection and reconnection—for example, in a battery-operated environment. In such instances, correct polarity isn’t guaranteed when reconnecting the supply.

Polarity protection can be achieved by adding a series diode on the positive supply line of the circuit. While this simple addition is effective in protecting against reverse polarity, the voltage drop of the series diode results in commensurate power dissipation. In relatively low current circuits, the tradeoff is minimal, but for many modern high-current rails, an alternative solution is required.Figure 4shows an update toFigure 3, showing both the TVS and the added series diode to protect against the reverse-polarity connection.

Filters using inductors and capacitors

The passive solutions discussed to this point all limit the amplitude of the events passed through, but they generally capture larger events while leaving some smaller spikes to pass. These smaller transients can still cause damage to downstream circuitry, so additional passive filters are required to clean the line. This is achievable using discrete inductors and capacitors, which must be sized to attenuate the voltage at the unwanted frequencies.

Filter design requires test and measurement before design to ascertain the size and frequency before the filter can be correctly sized. The drawbacks of this path are the cost of BOM and real-estate requirements—the board area and cost of the components required to achieve the level of filtering—as well as the need for overdesign, which means rating the component tolerances to compensate for changes over time and temperature.

Active Protection Using a Surge Stopper

One way to overcome the challenges and disadvantages of the passive protection solutions described is to instead utilize a surge-stopper IC. A surge stopper eliminates the need for bulky shunt circuitry (TVS devices, fuses, inductors, and capacitors) with a controller IC and a series N-channel MOSFET. Surge-stopper controllers can greatly simplify system design since there are few components to size and qualify.

A surge stopper continuously monitors the input voltage and current. Under nominal operating conditions, the controller drives the gate of an N-channel MOSFET pass device fully on, providing a low-resistance path from the input to the output. When an overvoltage or surge condition occurs—with a threshold dictated by a feedback network at the output—the IC regulates the N-channel MOSFET’s gate to clamp the output voltage of the MOSFET at the level set by the resistor divider.

Figure5shows a simplified schematic of a surge-stopper implementation, along with the results of a 100-V input surge on a nominal 12-V rail. The output of the surge stopper circuit is clamped to 27 V for the duration of the surge event. Some surge stoppers also monitor for overcurrent conditions using a series sense resistor(the circuit breaker in Figure 5)和adjust the gate of the N-channel MOSFET to limit the current presented to the output load.

There are four broad types of surge stopper, classified by their response to an overvoltage event:

Linear surge stopper
Gate clamp
交换surge stopper
Output disconnect protection controller

The choice of surge stopper depends on the application, so let’s compare their operation and advantages.

Linear surge stopper

A linear surge stopper(Fig. 6)drives the series MOSFET much like a linear regulator, limiting the output voltage to the pre-programmed safe value, dissipating excess energy in the MOSFET. To help protect the MOSFET, the device limits the time spent in the high dissipation region by implementing a capacitive fault timer.

Gate-clamp surge stopper

The gate-clamp surge stopper（图7）通过利用内部或外部夹具（例如31.5 V或50 V内部或可调节的外部夹具）来操作，以将栅极引脚限制在该电压上。然后，MOSFET的阈值电压确定输出电压极限。例如，使用内部31.5-V门夹和5V的MOSFET阈值电压，输出电压限制为26.5V。另外，外部栅极夹具允许选择更宽的电压范围。

交换surge stopper

For higher-power applications, a switching surge stopper is a good choice（图8）。Like linear and gate-clamp surge stoppers, a switching surge stopper fully enhances the pass FET under normal operation to provide a low-resistance path between the input and output (minimizing power dissipation).

The main difference between a switching surge stopper and a linear or gate-clamp surge stopper appears when a surge event is detected. In the event of a surge, the output of a switching surge stopper is regulated to the clamp voltage by switching the external MOSFET much like a switching dc-dc converter.

Protection controller: Output disconnect

A protection controller isn’t officially a surge stopper, but it does stop surges. Like a surge stopper, a protection controller monitors for overvoltage and overcurrent conditions, but instead of clamping or regulating the output, the protection controller disconnects the output immediately to protect downstream electronics.

This simple protection circuit can have a very compact footprint, suitable for battery-operated, portable applications. For example,Figure 9shows a simplified schematic of protection controller (in this case, theLTC4368), along with its response to an overvoltage event. Protection controllers are available in a number of variants.

A protection controller operates by monitoring the input voltage to ensure that it remains within a voltage window configured by the resistor divider on the OV/UV pins, disconnecting the output via back-to-back MOSFETs when the input is outside this window(Fig. 9, again)。The back-to-back MOSFETs can also protect against a reversed input. The sense resistor at the output enables overcurrent protection capability by continually monitoring the forward current, but without a timer-based ride-through operation.

Surge-Stopper Features

To select the most suitable surge stopper for your application, you will need to know what features are available and the challenges that they’re helping to solve. The devices can be found at the parametric table。

Disconnect vs. ride-through

Some applications require disconnecting the output from the input when a surge event is detected. In this instance, overvoltage disconnect would be required. If you needed the output to remain operational in the face of surge events, thus minimizing the downtime of downstream electronics, you would require the surge stopper to ride through the surge event. In this case, a linear or a switching surge stopper can achieve this functionality (provided the power levels were reasonable for the topology and FET selected).

Fault timer

乘车操作需要对MOSFET进行一些保护，以防止持续的潮流。要留在FET的安全操作区域（SOA）内，可以实施计时器。计时器本质上是要接地的电容器。当发生过电压条件时，内部电流源开始为该外部电容器充电。

Once the capacitor reaches a certain threshold voltage, a digital fault pin pulls low to indicate the pass transistor will soon turn off due to the extended overvoltage condition. If the timer pin voltage continues to rise to a secondary threshold, the GATE pin pulls low to turn off the MOSFET.

计时器电压的变化速率随MOSFET的电压而变化，即较大电压的较短计时器，对于较小电压的较长计时器。这项有用的功能使设备能够穿越短电压事件，从而使下游组件能够保持运行，同时通过长期持久的过电压事件来保护MOSFET免受损害。一些设备具有重试功能，使设备能够在冷却后再次打开输出。

Overcurrent protection

Many surge stoppers have the ability to monitor current and protect against overcurrent events. This is achieved by monitoring the voltage drop across a series sense resistor and responding appropriately. Inrush current also can be monitored and controlled to protect the MOSFET. The response may be similar to an overvoltage condition, as it either disconnects by latching off or riding through the event if the circuitry can handle the power levels.

Reverse-input protection

Reverse-input protection is possible due to the wide operating capabilities of the surge-stopper devices (capable of withstanding up to 60 V below ground potential on some devices).Figure 10shows a back-to-back MOSFET implementation of reverse-current protection. During normal operation, Q2 and Q1 are turned on by the GATE pin, and Q3 doesn’t have any impact. However, when a reverse-voltage condition exists, Q3 turns on, pulling Q2’s gate down to the negative input and isolating Q1, protecting the output.

Reverse-output-voltage protection also is achieved with robust device pin protection. Up to 20 V below ground potential is possible, depending on the device selected.

For applications that require wide input-voltage ranges, a floating topology surge stopper can be used. When a surge event occurs, the full surge voltage is seen by the surge-stopper IC; therefore, the internal transistor technology limits the voltage range of the IC.

With a floating surge stopper such as theLTC4366, the IC floats just below the output voltage, giving it a much wider operating voltage range. A resistor is placed in the return line (V_SS), which allows the IC to float up with the supply voltage. The result is an input-voltage limitation set by the voltage capability of the external components and MOSFETs.Figure 11shows an application circuit capable of operating from a very high dc supply while protecting the downstream load.

Choosing the Right Device for My Application

在许多方面，由于其固有的稳健设计，使用电涌塞子简化了保护电路设计。数据表可以极大地帮助尺寸组件，并且已经显示了许多可能的应用程序。最难的部分可能是选择最合适的设备。按照以下几个步骤缩小范围：

Consult a protection parametric table。
Choose input-voltage range.
Choose the number of channels.
Filter on features to narrow the possible options.

As with all product selection, it’s important to understand your system requirements before looking for the correct device. Some important considerations are the expected supply voltage and the voltage tolerance of downstream electronics (important for deciding the clamp voltage), as well as any particular features that are important for the design.

Some filtered parametric table examples are listed below for reference—these can further be modified on the website to include some other parameters:

可以找到高压潮湿的设备这里。
可以找到具有OV断开功能的保护控制器这里。

不管surge-stopper类型的实现,active, IC-based surge-stopper designs eliminate the need for bulky TVS diodes or large profile inductors and capacitors for filtering. This results in an overall smaller area and a lower profile solution.

的输出电压钳比T更准确VS with 1% to 2% accuracy possible. This prevents overdesign and allows for downstream devices with tighter tolerances to be selected. Taking this approach enables designers to implement reliable, flexible, and small-form-factor protection for downstream devices—especially those facing harsh overvoltage and overcurrent events, which can occur in many industrial-, automotive-, aerospace-, and communications-based designs.