What active circuit protection solutions can replace TVS diodes and fuses?

Many traditional reliable protection solutions such as diodes, fuses and TVS devices can remain protected, but they are often inefficient, bulky and require maintenance. In order to solve these shortcomings, active intelligent protection ICs emerged. They can meet the protection requirements of traditional methods, and in some aspects, they are more reliable. But there are so many different types of devices that the most difficult problem designers face is choosing the right solution. To help designers narrow down their choices, this article compares traditional protection methods with the ADI protection product line to demonstrate the characteristics of these products and recommended applications.

As the use of electronic devices continues to increase across all industries, and the processing capabilities of costly FPGAs and processors continue to expand, there is an increasing need to protect these devices operating in harsh environments. In addition, they are required to be compact, highly reliable, and able to respond quickly to overvoltage and overcurrent surge events. This article explores the challenges faced by many applications and why protection is needed, comparing traditional protection methods with newer alternative solutions that offer greater accuracy, reliability and design flexibility.

Automotive, industrial, communications, and avionics systems are subject to a range of power surges, such as those shown in Figure 1. In these markets, transient events are defined by many industry specifications. For example, the ISO 7637-2 and ISO 16750-2 specifications define automotive transients, providing a detailed overview of expected transients, as well as test procedures to ensure continued verification of these transients.

The type and energy content of a surge event will vary depending on the area in which the electronic device is used; circuits may be subject to conditions such as overvoltage, overcurrent, reverse voltage, and reverse current. Finally, many circuits cannot sustain, let alone operate independently, if they are to be directly subjected to these transient conditions shown in Figure 1, so designers must consider all input conditions and take steps that can protect circuits from voltage and current surges. Mechanisms.

Figure 1. Overview of some of the more stringent ISO 16750-2 tests.

There are many different causes of voltage and current transients in electronic systems, but some electronic environments are more prone to transient events than others. Applications in automotive, industrial and communications environments are known to experience potentially hazardous events that can cause severe damage to downstream electronics, but surge events are not exclusive to these environments. Other situations where surge protection circuitry may be required include applications requiring high voltage or high current power supplies, applications with hot-swap power connections, or systems that contain motors or may be affected by lightning-induced transients. High-voltage events can last from a few microseconds to hundreds of milliseconds, so flexible and reliable protection mechanisms must be used to ensure the service life of costly downstream electronic devices.

For example, a car load dump occurs when the alternator (which charges the battery) is momentarily disconnected from the battery. After this disconnect occurs, the full-load charging current provided by the alternator is transferred to the power rail, causing the rail voltage to climb to extremely high (>100 V) levels within hundreds of milliseconds.

Surges can occur in communications applications for a variety of reasons, ranging from hot-swappable communications cards to outdoor installations that may be affected by lightning. Long cables used in large facilities can also produce induced voltage spikes.

Ultimately, designers must fully understand the environment in which the device will be used and meet existing specifications. This helps them combine the best protection mechanisms that are reliable and non-interfering, but allow downstream electronics to operate within safe voltage ranges. Runs within the system with minimal interruption.

With so many different types of electronic issues to consider, how should electronics engineers protect sensitive downstream electronics?

Traditional protection methods provide protection based on multiple devices instead of one. For example, transient voltage suppressors (TVS) are used to provide overvoltage protection, line fuses are used to provide overcurrent protection, and series diodes are used to provide reverse battery/power supply protection. and a mix of capacitors and inductors to filter out lower power spikes. While a discrete configuration can meet established specification requirements (protecting downstream circuitry), it is cumbersome to implement and requires multiple selections to determine the appropriate filtering specifications.

Figure 2. Traditional protection devices.

Let's take a closer look at these devices and clarify the advantages and disadvantages of this implementation method.

This is a relatively simple device that protects downstream circuitry from high voltage spikes on the power supply. They can be divided into several different types with a wide range of characteristics (Table 1 is arranged in order from shortest to longest response time).

Table 1. Response time of different transient voltage suppression devices

Although their structures and characteristics vary, they are used in a similar manner: to shunt excess current when the voltage exceeds the device threshold. TVS can fix the output voltage at the rated level within a very short time. For example, TVS diodes can have response times as low as picoseconds, while GDTs can have response times of microseconds but can handle much larger surges.

图3显示了用于保护下游电路的TVS二极管的简单配置。在正常工作条件下，TVS具有高阻抗，输入电压会直接传输至输出。当输入端出现过压时，TVS开始导电，并将多余的电能分流到接地(GND)，从而箝位下游负载电压。电源轨电压升高到典型操作值以上，但被箝位到保证下游电路可以安全运行的值。

虽然TVS器件在抑制极高电压偏移方面很有效，但在遭受持续过压时，也不能避免损坏，因此需要定期监测或更换。另一个担心是TVS可能短路，导致输入电源断开。此外，根据涉及的电能大小，它们的尺寸可能需要很大才能满足裕量要求，导致解决方案的尺寸相应增大。即使TVS的尺寸正确，下游电路也必须要能够处理箝位电压，对下游的电压额定要求也随之增高。

Figure 3. Protection against voltage surges with traditional TVS solutions.

Overcurrent protection can be achieved using common line fuses with a higher-than-nominal blowing rating, for example, 20% above the maximum current rating (the percentage depends on the circuit type and expected typical operating loads). Of course, the biggest problem with fuses is that they must be replaced once they blow. The design of fuses is quite simple, but maintenance is relatively complicated, especially in hard-to-reach locations, so it will still consume time and cost later. Maintenance requirements can be reduced by using a backup fuse, such as a resettable fuse, which uses a positive temperature coefficient to open the circuit when a higher than nominal current flows through the device (increased current will increase the temperature, causing a sharp increase in resistance) .

Aside from maintenance issues, one of the biggest problems with fuses is their reaction time, which can vary greatly depending on the type of fuse chosen. We can use fast-acting fuses, but the fusing time (the time it takes to open the circuit) can still take hundreds of microseconds to milliseconds, so circuit designers must consider the amount of power released during these times to ensure that downstream electronics are not damaged.

In some environments, a circuit may be disconnected and then reconnected—for example, in a battery-powered environment. In this case, there is no guarantee that the polarity is correct when the power is reconnected. We can achieve polarity protection by adding a series diode to the positive supply line of the circuit. While this simple addition effectively prevents reverse polarity, the voltage drop across the series diode results in a corresponding power loss. In circuits with relatively low currents this trade-off is small, but for many modern high-current rails another solution is required. Figure 4 is an update to Figure 3 showing the use of TVS and the addition of series diodes to prevent reverse polarity connections.

Figure 4. Adding a series diode can prevent reverse polarity connections, but in high-current systems, the voltage drop across the diode can be a problem.

The passive solutions discussed so far limit the amplitude, but usually only capture larger amplitudes and miss smaller spikes. These smaller transients can still cause damage to downstream circuitry, requiring the use of additional passive filters to clean the lines. This can be accomplished by using discrete inductors and capacitors, sized so that they attenuate voltages beyond the frequency range. Before designing, filter designs need to be tested and measured to determine their size and frequency before the filter can be correctly sized. Disadvantages of this approach are the need to consider bill of materials cost and area requirements (how much board area and cost do the components need to achieve filtering levels), and the need for over-design (tolerating the components so that they can operate over time and temperature Provide compensation when changes occur).

One way to overcome the challenges and shortcomings of passive protection solutions described is to switch to surge suppressor ICs. The surge suppressor uses an easy-to-use controller IC and a series N-channel MOSFET, eliminating the need for complex shunt circuits (TVS devices, fuses, inductors, and capacitors). Surge suppressor controllers can greatly simplify system design because only a few components need to be sized and qualified.

浪涌抑制器持续监测输入电压和电流。在额定工作条件下，控制器驱动N通道MOSFET通路器件的栅极完全开启，提供一条从输入到输出的低阻抗路径。在发生过压或浪涌时（阈值由输出端的反馈网络给出），IC调节N通道MOSFET的栅极，将MOSFET的输出电压箝位到电阻分压器设定的电平。

图5显示了浪涌抑制器配置的简化示意图，以及标称12 V电源轨上出现100 V输入浪涌时的结果。在浪涌发生期间，浪涌抑制器电路的输出被箝位到27 V。一些浪涌抑制器也使用串联感应电阻（图5中的断路器）来监测过流情况，并调整N通道MOSFET的栅极，以限制输出负载端的电流。

Figure 5. Detailed schematic of surge suppressor configuration.

Surge suppressors can be divided into four broad categories based on their response to overvoltage events:

• Linear surge suppressor

• 栅极箝位

• Switching surge suppressor

• Output disconnection protection controller

Surge suppressors should be selected based on the application, so let’s compare their operation and benefits.

The way a linear surge suppressor drives a series MOSFET is similar to a linear regulator, limiting the output voltage to a preset safe value and dissipating excess energy in the MOSFET. To protect the MOSFET, the device limits the time spent in the high-dissipation region by employing a capacitive fault timer.

Figure 6. LT4363 linear surge suppressor.

栅极箝位浪涌抑制器利用内部或外部箝位（例如，31.5 V或50 V内部箝位，或可调的外部箝位）将栅极引脚的电压限制到这个电压值，然后，由MOSFET的阈值电压决定输出电压限值。例如，在使用内部31.5 V栅极箝位，且MOSFET阈值电压为5 V时，输出电压限制为26.5 V。或者，外部栅极箝位允许更广泛的电压选择范围。栅极箝位浪涌抑制器的示例如图7所示。

图7.LTC4380栅极箝位浪涌抑制器。

对于更高功率的应用，开关浪涌抑制器是一个很好的选择。与线性和栅极箝位浪涌抑制器一样，开关浪涌抑制器在正常操作条件下可以充分增强调整FET，以在输入和输出之间提供一个低阻路径（最小化功率损耗）。开关浪涌抑制器和线性或栅极箝位浪涌抑制器之间的主要区别出现在检测到浪涌事件时。在浪涌事件中，开关浪涌抑制器是通过开关外部MOSFET（比较类似于开关DC-DC转换器），将输出调节到箝位电压。

Figure 8. LTC7860 switching surge suppressor.

保护控制器不是真正的浪涌抑制器，但它确实能停止浪涌。和浪涌抑制器一样，保护控制器监测过压和过流条件，但它不会箝位或调节输出，而是通过立即断开输出来保护下游电子器件。这种简单保护电路的布局紧凑，非常适合由电池供电的便携式应用。LTC4368 保护控制器的简化示意图，以及它对过压事件的响应如图9所示。保护控制器有许多版本。

Figure 9. LTC4368 protection controller.

The protection controller monitors the input voltage to ensure that the voltage remains within the voltage range configured by the resistor divider of the OV/UV pin. When the input voltage exceeds this range, the back-to-back MOSFET is used to disconnect the output, as shown in Figure 9. Back-to-back MOSFETs can also be used to protect against reverse input. A sense resistor at the output provides overcurrent protection by continuously monitoring forward current, but does not require timer-based ride-through operation.

In order to choose the most appropriate surge suppressor for your application, you need to know what features are available and what challenges they can help solve. You can find these devices in the parameter tables.

Some applications require that the output and input be disconnected when a surge event is detected. In this case, the overvoltage connection needs to be disconnected. If you need your output to remain operational during a surge event, thus minimizing downtime for downstream electronics, you need a surge suppressor to ride through the surge when it occurs. In this case, using a linear or switching surge suppressor can accomplish this (provided, for the topology and FETs chosen, the power levels are reasonable).

When implementing ride-through, the MOSFET needs to be protected from sustained surges. To ensure that the FET remains within the safe operating area (SOA), a timer can be used. The timer is essentially a capacitor to ground. When an overvoltage occurs, the internal current source begins to charge this external capacitor. When the capacitor reaches a certain threshold voltage, the digital fault pin is pulled low, indicating that due to the influence of a prolonged overvoltage, the pass tube will soon turn off. If the timer pin voltage continues to rise to the secondary threshold, the gate pin will be pulled low to turn off the MOSFET.

The rate of change of the timer voltage changes with the voltage across the MOSFET, that is, the higher the voltage, the shorter the time, and the lower the voltage, the longer the time. This useful feature allows the device to ride out short-duration overvoltage events, allowing downstream components to remain operational while protecting the MOSFET from damage during longer-duration overvoltage events. Some devices have a retry feature that allows the device to turn the output back on again after cooling down.

Many surge suppressors are capable of monitoring current flow and protecting devices from overcurrent events. This is accomplished by monitoring the voltage drop across the series sense resistor and responding appropriately. MOSFETs can also be protected by monitoring and controlling inrush current. The response may be similar to an overvoltage condition in that if the circuit is capable of accepting this power level, it will either disconnect via latch-up or via a ride-through event.

Surge suppressors provide reverse input protection due to their wide operating capabilities (capable of withstanding subsurface voltages up to 60 V on some devices). Figure 10 shows a back-to-back MOSFET configuration that provides reverse current protection. During normal operation, Q2 and Q1 are turned on by the gate pins and Q3 has no effect. However, when a reverse voltage connection occurs, Q3 turns on, pulling Q2's gate down to the negative input and isolating Q1 to protect the output.

Reverse output voltage protection is also available through reliable device pin protection, which can withstand underground voltages up to 20 V depending on the device selected.

Figure 10. LT4363 reverse input protection circuit.

For applications requiring a wide input voltage range, floating topology surge suppressors can be used. When a surge event occurs, the surge suppressor IC monitors the entire surge voltage and uses internal transistor technology to limit the IC's voltage range. When using a floating surge suppressor such as the LTC4366, the IC floats just below the output voltage, giving it a wider operating voltage range. A resistor (VSS) is included in the return line to allow the IC to float with the supply voltage. Thus, the input voltage limit is set by external components and the voltage function of the MOSFET. Figure 11 shows an application circuit that can operate normally with extremely high DC supplies while protecting back-end loads.

Figure 11. LTC4366 high-voltage floating topology.

Since the surge suppressor itself adopts a reliable design, it can simplify the design of the protection circuit in many aspects. The data sheet shows many possible applications and can be of great help when sizing the component. The hardest part may be choosing the most appropriate device. Here are a few steps you can follow to narrow it down:

• Visit ADI’s protection device family parameter sheet.

• Select input voltage range.

• Select the number of channels.

• Filter functionality to narrow down available options.

和所有产品选型一样，在查找正确的器件前，您需要了解您的系统要求，这点非常重要。一些重要的考虑因素包括：预期的电源电压和下游电子器件的电压容限（在决定箝位电压时非常重要），以及对设计而言非常重要的一些特性。

无论采用哪种类型的浪涌抑制器，基于IC的有源浪涌抑制器设计都无需使用繁杂的TVS二极管，或使用大尺寸电感和电容来进行滤波。所以，解决方案的整体面积更小，体积也更小巧。相比TVS，其输出电压箝位精度可能高出1%至2%。如此可以防止过度设计，且能够选择公差更严格的下游器件。

ADI's family of system protection devices enables designers to protect downstream devices with reliable, flexible, and compact solutions, especially for industrial, automotive, aerospace, and communications designs that may face severe overvoltage and overcurrent events. device.