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可靠电源路径设计：集成MOSFET、二极管、TVS器件与电容

Reliable Power-Path Design: Integrating MOSFETs, Diodes, TVS Devices, and Capacitors

中文摘要

本文介绍24V电源路径的可靠设计，涉及MOSFET、二极管、TVS器件和电容的选型，以应对浪涌、反极性和浪涌电流。

原文深度解读

本文由Unikey Electronics Pte. Ltd.于2026年8月26日发布，主题为24V工业电源路径的可靠设计，涵盖MOSFET、二极管、TVS器件和电容的选型与集成。文章强调各组件需协同工作以应对反极性、浪涌、反向电流和浪涌电流等威胁，并提供了设计步骤、计算示例和常见错误。与电子元器件供应链相关，涉及Schottky二极管、P/N沟道MOSFET、TVS二极管、MLCC和铝电解电容等元器件的选型参数和类别。

文章基于TI对电源路径威胁的分类，包括过流、过压、反压、反向电流、浪涌电流和汽车负载突降。
Schottky二极管VF较低但漏电流大，标准整流二极管VF较高但漏电流小；导通损耗公式为P ≈ I × VF。
P-channel MOSFET栅极驱动简单但RDS(on)较高，N-channel MOSFET RDS(on)较低但需电荷泵；体二极管方向决定电流路径。
TVS二极管的关键参数包括VRWM、VBR、VC、IPP和峰值脉冲功率；VC通常高于VBR，设计时需确保最大正常输入电压低于VRWM，最坏VC低于下游器件绝对最大额定值。
MLCC在直流偏压下有效电容下降，铝电解电容用于储能但寿命受高温影响；大输入电容会导致浪涌电流。
工作示例中，24V±10%输入，3A负载，1kV/42Ω浪涌，下游最大电压40V；计算得Schottky损耗1.5W，MOSFET损耗0.27W，TVS IPP约22.9A，PPP约870W，MLCC有效电容8.8µF，浪涌电流2.4A，结温约98.5°C。

供应链影响

设计需求可能影响对低VF Schottky二极管或低RDS(on) MOSFET的采购，具体取决于系统效率要求。
TVS二极管的选择可能需匹配特定电压窗口，供应商需提供精确的VC和IPP数据，取决于下游器件耐压。
MLCC和铝电解电容的选型可能受直流偏压降额和温度影响，采购时需考虑实际工作条件，取决于应用环境。
组件协同设计可能增加对多品类元器件的需求，供应链需确保各组件参数匹配，取决于设计验证结果。
文章未推荐具体料号，采购决策可能需基于参数筛选，取决于工程师对供应商目录的评估。

系统解读，仅作为判断线索，不构成备货、出货、涨价或投资建议。

正文内容 · AI翻译

中文英文

作者：Unikey Electronics Pte. Ltd.

2026年8月26日

1. 引言

许多工业机器使用24V直流电源。它用于控制面板、通信设备以及边缘AI硬件，如工业相机和现场推理盒。

在实际使用中，不同问题可能同时发生。这些包括反极性、热插拔、电压浪涌、反向电流和突然的负载变化。

一个组件可以通过其测试，但这并不意味着整个电源路径是安全的。当所有部件正确协同工作时，系统才是可靠的。单独一个部件是不够的。

图1显示了电源路径的工作原理。电源从保险丝或PTC器件进入，然后通过TVS二极管和反极性保护。接下来是输入电容，最后运行DC/DC转换器和负载。

由Unikey Electronics Pte. Ltd. 2026年8月26日

由Zackary Glazewski，ChipAgents创始AI工程师 2026年8月26日

由Colin Duggan，BG Networks CEO兼联合创始人 2026年8月26日

图1：24V工业电源路径信号链

2. 在选择组件前定义电气威胁

在选择组件之前，工程师应首先了解可能的威胁。不同的威胁需要不同的保护和组件。下表显示了电源路径中的主要威胁。

此威胁列表遵循TI对电源路径威胁的分类，涵盖过流、过压、反压、反向电流、浪涌电流和汽车负载突降。

3. 反极性保护：二极管还是MOSFET？

3.1 二极管方案

两种常见的二极管可用于反极性保护。它们是标准整流二极管和肖特基二极管。

这两种类型在正向压降（VF）、反向电压额定值、漏电流和热损耗方面有所不同。

肖特基二极管的VF低于整流二极管，因此导通损耗较低。但它们通常有更大的漏电流。漏电流在高温下也会迅速增加。

标准整流二极管具有较高的VF，但其漏电流较低。总热损耗不应仅从漏电流判断——应综合考虑正向压降、漏电流、占空比和热阻，因为如果VF和占空比高，漏电流较低的二极管整体上可能运行得更热。

导通损耗可以用简单公式估算：P ≈ I × VF。在高电流下，这种损耗变得很大，因此二极管的选择对系统效率有实际影响。

比较部件的工程师可以浏览肖特基二极管类别以获取低VF选项。

3.2 MOSFET方案

P沟道MOSFET通常比N沟道MOSFET更容易设计。栅极通常可以用连接到地的低侧晶体管控制。然而，在24V输入下，必须仔细检查栅源电压。VGS电压必须在整个输入范围内保持在MOSFET的额定限值内。栅极电阻有助于限制开关电流，而栅源电阻在栅极未被驱动时保持栅极处于已知电平。VGS齐纳二极管也可以保护栅极免受电压尖峰影响。在某些设计中，可能需要专用的栅极驱动器或热插拔控制器，以在24V±10%时保持VGS在其安全范围内。

N沟道MOSFET通常具有较低的RDS(on)，因此导通损耗较低。但驱动栅极更困难。通常使用电荷泵，因为栅极电压需要高于输入电压。

MOSFET的体二极管方向很重要。它决定了MOSFET导通前的电流路径。因此，MOSFET的方向必须正确以实现所需的保护。

还有其他一些重要事项需要检查。这些包括VDS和VGS电压裕量、高温下的RDS(on)值、安全工作区（SOA）和反向电流路径。RDS(on)也会随着温度升高而增加。

当需要在两个方向阻断电流时，使用背对背MOSFET。一个MOSFET的体二极管允许电流在一个方向流动，因此两个MOSFET一起使用以从两侧阻断电流。

本文不推荐任何具体料号。它侧重于选择组件时要检查的重要参数。工程师可以利用这些要点来比较不同选项，并选择最适合其设计的方案。

工程师可以在选择最终部件之前，根据VDS额定值、VGS限值、RDS(on)、封装类型和热特性比较MOSFET选项。Unikeyic的MOSFET类别提供参数筛选器，可根据这些设计要求帮助缩小可用选项。

4. TVS选择必须匹配整个电压窗口

TVS二极管有几个关键额定值：VRWM（最大工作电压）、VBR（击穿电压）、VC（钳位电压）、IPP（峰值脉冲电流）和峰值脉冲功率。

一个常见错误是将VBR视为实际钳位电压。VBR只是TVS开始导通的点。VC，即钳位电压，通常高于VBR，而VC是下游组件在脉冲期间实际看到的电压。

安全设计必须同时满足两个规则。首先，最大正常输入电压必须低于VRWM。其次，最坏情况下的VC必须低于下游MOSFET或DC/DC转换器的绝对最大额定电压。

其他因素也会影响TVS二极管保护电路的效果。电压浪涌的类型和长度会改变TVS二极管必须承受的应力。高温也会降低TVS二极管能安全处理的浪涌功率。电源和导线中的电阻会影响浪涌期间流过TVS的电流。

选择合适的TVS二极管也很重要。放置在反极性保护之前的单向TVS在输入极性反转时可能变为正向偏置。接下来发生什么取决于保险丝或PTC、限流和电源。电流可能被限制，或者保险丝可能熔断。这并不总是意味着直接短路。双向TVS在输入极性反转时不会变为正向偏置。这意味着TVS本身不会为电流创建低电阻路径。然而，它不能替代反极性保护。您仍然需要二极管或MOSFET来阻止反向电流到达电路的其余部分。

最后，TVS二极管应放置在靠近电源输入和被保护电路的位置。保持PCB走线短有助于减少快速浪涌期间走线引起的额外电压。

TVS器件仅保护短瞬态。在持续过压下，TVS不能替代断开开关或限流器件。如果过压持续时间过长，TVS可能过热并失效。

提供全系列TVS二极管和ESD抑制器，以匹配上述定义的电压窗口。

有关更深入的逐步方法，请参阅东芝的TVS/ESD二极管选择指南。本文在此方法的基础上，增加了对MOSFET和电容级的协调分析，而不是单独考虑TVS器件。

5. 电容：瞬态支持也可能产生浪涌电流

MLCC（多层陶瓷电容）提供良好的高频旁路。但它们的有效电容在直流偏压下会下降，实际值取决于工作电压、介电类型和封装尺寸。

铝电解电容用于存储大量能量。选择电容时，检查其ESR、纹波电流额定值、工作温度和使用寿命。在高温下使用时，其寿命可能缩短。

大的输入电容会在上电时产生高浪涌电流。这种额外应力影响MOSFET的SOA和连接器。它还可能与电缆电感形成振铃电路。

最佳方法是MLCC和电解电容的组合。不应将其视为简单的二选一。

合适的部件可以从陶瓷电容类别中获取用于高频级，从铝电解电容类别中获取用于大容量储能。

6. 工作示例：24V工业输入

为了演示逐步设计过程，让我们考虑一个典型的24V工业系统，规格如下：

- 输入电压：24V DC ± 10%（21.6V至26.4V）

- 标称负载电流：3A（72W）

- 浪涌保护等级：IEC 61000-4-5（1kV / 42Ω浪涌波形）

- 下游最大电压额定值：40V（DC/DC转换器输入限值）

- 工作环境温度：-40°C至+85°C

上述浪涌等级使用基于IEC 61000-4-5的应用特定1kV测试条件。确切的IEC严重等级取决于适用的产品标准、被测端口和耦合模式——在常见参考表中，1kV通常对应2级，而3级通常为2kV，因此这里不声称特定等级。本文中使用的42Ω源阻抗是此示例假设的测试配置，并非所有24V电源端口的通用阻抗；实际阻抗必须根据适用的产品标准和端口类型确认。测试可以使用适当的耦合/去耦网络（CDN）在线对线或线对地应用，使用标准的1.2/50µs开路电压波形和8/20µs短路电流波形。CDN和耦合方法应与被测端口匹配，这就是使测试条件代表真实24V电源输入的原因。

设置这些值后，设计工作遵循六个步骤，使用第3节的方案比较表来权衡此特定24V输入的导通损耗与电路复杂度：

图2：24V工作示例的信号链

6.1 示例计算

以下计算使用一些示例组件值。这些值仅用于解释设计，并非对特定部件的推荐。在实际设计中，使用所选组件的实际规格。

计算 | 假设与公式 | 结果
肖特基二极管损耗 | P = I × VF，假设VF = 0.5V在3A时 | 3A × 0.5V = 1.5W
MOSFET导通损耗（热） | P = I² × RDS(on,hot)，假设RDS(on,hot) ≈ 30mΩ在降额至85°C环境温度加自热后 | (3A)² × 0.03Ω ≈ 0.27W
MOSFET电压裕量 | 比较最坏情况VC（≈38V）与MOSFET VDS额定值（假设60V部件）以及文章所述下游DC/DC转换器绝对最大输入额定值（40V） | MOSFET：60V − 38V = 22V裕量（约为额定VDS的63%，在80%降额准则内）。DC/DC转换器：40V − 38V = 仅2V裕量，无法可靠覆盖TVS容差、温度漂移和PCB寄生过冲→此裕量不足，需要进一步验证（例如更低VC的TVS或更高输入额定值的转换器），而不是“安全”的通过/失败结论。
TVS峰值脉冲电流（IPP） | IPP ≈ (VOC − VC) / ZS，假设VOC = 1000V（浪涌开路电压），VC ≈ 38V（钳位电压，非VBR），ZS = 42Ω源阻抗 | (1000V − 38V) / 42Ω ≈ 22.9A
TVS峰值脉冲功率（PPP） | PPP ≈ VC × IPP，使用VC ≈ 38V | 38V × 22.9A ≈ 870W → 选择额定峰值脉冲功率≥1kW的TVS以获得裕量
有效MLCC电容 | 标称22µF X7R额定部件，假设在24V直流偏压下降额约60% | 22µF × 0.4 ≈ 8.8µF在24V时有效
电容浪涌电流 | I = C × dV/dt，假设总输入电容100µF在热插拔时约1ms充电至24V | 100µF × (24V / 1ms) = 2.4A — 检查MOSFET SOA
热设计（MOSFET结温升） | Tj = Ta + P × RθJA，假设RθJA ≈ 50°C/W，Ta = 85°C（最大环境温度） | 85°C + (0.27W × 50°C/W) ≈ 98.5°C，低于典型150°C Tj(max)额定值

建议的验证清单：

- 在额定脉冲等级下进行浪涌测试

- 在最小和最大输入电压下进行热插拔测试

- 在满载下进行反极性测试

- 负载阶跃响应测试

- 在最大环境温度下进行温升测试

7. 常见集成错误

- 仅根据标称电压而不是整个电压范围选择组件

- 忽略高温下的RDS(on)

- 检查下游保护时使用VBR代替VC

- 忽略直流偏压下的MLCC电容降额

- 增加更多电容而不检查由此产生的浪涌电流

- 将TVS器件放置得离连接器太远

- 仅根据封装尺寸和额定值选择替代部件，而不检查其实际工作条件

常见问题解答

TVS二极管中VBR和VC有什么区别？

VBR是击穿电压。它是TVS开始导通的电压。VC是钳位电压，即在实际脉冲期间到达TVS后部件的电压。VC通常高于VBR，因此在检查保护时很重要。

TVS二极管可以替代保险丝吗？

不能。TVS二极管仅钳位短电压尖峰。它不能限制电流或在长时间过压或过流事件中保持安全。仍然需要保险丝或PTC器件来提供该保护。

对于反极性保护，二极管和MOSFET哪个更好？

二极管使用简单，但在高电流下可能因功率损耗而发热。MOSFET具有较低的导通损耗，可以提供更好的效率。然而，它需要适当的栅极驱动和正确的体二极管方向。最佳选择取决于电流、板空间和所需效率。

为什么MLCC电容在直流偏压下会下降？

这是因为MLCC内部的陶瓷介电材料。随着直流电压升高，有效电容下降。这种效应在小封装尺寸和高K介电材料中更强，因此工作电压下的实际电容通常低于印刷值。

直流电源中什么导致浪涌电流？

当电源打开时，输入电容需要充电。这可能导致浪涌电流。较大的电容可能导致更大的浪涌电流。如果电流过高，可能对MOSFET、连接器和输入保护部件造成应力。

如何保护24V直流工业输入免受浪涌和反极性影响？

电源路径需要不同的组件，而不仅仅是一个部件。保险丝或PTC用于过流保护。TVS二极管用于浪涌保护，二极管或MOSFET用于反极性保护。MLCC和电解电容有助于保持电源稳定。

结论

对于可靠的电源路径，MOSFET、二极管、TVS器件和电容应一起选择。它们作为一个保护系统工作，因此不应被视为单独的部件。

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Unikeyic Electronics是UNIKEY ELECTRONICS PTE. LTD.运营的电子元件分销品牌，该公司总部位于新加坡，通过unikeyic.com为100多个国家和地区的设计工程师、采购员、ODM、OEM和EMS提供商提供服务。拥有超过30万个库存SKU、30万平方英尺的智能仓库，以及与200多个品牌的授权分销合作伙伴关系，Unikeyic支持快速全球发货和高效的电子元件采购。

By
Unikey Electronics Pte. Ltd.

08.26.2026

1. Introduction

A lot of industrial machines use a 24V DC power supply. It is used for control panels, communication devices, and edge AI hardware such as industrial cameras and on-site inference boxes.

In actual use, different problems can happen together. These can include reverse polarity, hot-plugging, voltage surges, reverse current, and sudden load changes.

One component can pass its test, but it does not mean the whole power path is safe. The system is reliable when all parts work together properly. One part alone is not enough.

Figure 1 shows how the power path works. The power enters from the fuse or PTC device, then goes through the TVS diode and reverse-polarity protection. Next is the input capacitor, and finally it runs the DC/DC converter and load.

By Unikey Electronics Pte. Ltd. 08.26.2026

By Zackary Glazewski, Founding AI Engineer, ChipAgents 08.26.2026

By Colin Duggan, CEO and co-founder, BG Networks 08.26.2026

Figure 1: 24V industrial power-path signal chain

2. Define the Electrical Threats Before Selecting Components

Before choosing a component, the engineer should first understand the possible threats. Different threats need different protection and components. The table below shows the main threats in a power path.

This threat list follows TI’s classification of power-path threats, which covers overcurrent, overvoltage, reverse voltage, reverse current, inrush current, and automotive load dump.

3. Reverse-Polarity Protection: Diode or MOSFET?

3.1 Diode Solutions

Two common diodes can be used for reverse-polarity protection. They are standard rectifier diodes and Schottky diodes.

These two types differ in forward voltage drop (VF), reverse-voltage rating, leakage current, and thermal loss.

Schottky diodes have lower VF than rectifier diodes, so the conduction loss is lower. But they usually have more leakage current. The leakage current also increases quickly at high temperature.

Standard rectifier diodes have higher VF, but their leakage current is lower. Total thermal loss should not be judged from leakage current alone — it should account for forward voltage drop, leakage current, duty cycle, and thermal resistance together, since a diode with lower leakage can still run hotter overall if its VF and duty cycle are high.

Conduction loss can be estimated with a simple formula: P ≈ I × VF. At high current, this loss becomes large, so the choice of diode has a real effect on system efficiency.

Engineers comparing parts can browse the Schottky Diodes category for low-VF options.

3.2 MOSFET Solutions

A P-channel MOSFET is usually easier to design with than an N-channel MOSFET. The gate can often be controlled with a low-side transistor connected to ground. However, at a 24V input, the gate-source voltage must be checked carefully. The VGS voltage must stay within the MOSFET’s rated limit across the full input range. A gate resistor can help limit switching current, while a gate-source resistor keeps the gate at a known level when it is not being driven. A VGS Zener can also protect the gate from voltage spikes. In some designs, a dedicated gate driver or hot-swap controller may be needed to keep VGS within its safe range at 24V ±10%.

N-channel MOSFETs usually have lower RDS(on), so they can have lower conduction loss. But driving the gate is more difficult. A charge pump is often used because the gate voltage needs to be higher than the input voltage.

The body diode direction of the MOSFET is important. It decides the current path before the MOSFET turns on. So, the MOSFET orientation must be correct for the required protection.

There are some other important things to check. These include the VDS and VGS voltage margin, the RDS(on) value at high temperature, the Safe Operating Area (SOA), and the reverse-current path. RDS(on) also increases when the temperature increases.

Back-to-back MOSFETs are used when current needs to be blocked in both directions. One MOSFET has a body diode that allows current in one direction, so two MOSFETs are used together to block current from both sides.

This article does not suggest any specific part numbers. It focuses on the important parameters to check when selecting a component. Engineers can use these points to compare different options and choose what is best for their design.

Engineers can compare MOSFET options across VDS ratings, VGS limits, RDS(on), package types, and thermal characteristics before selecting a final part. Unikeyic’s MOSFET category provides parametric filters that can help narrow the available options based on these design requirements.

4. TVS Selection Must Match the Entire Voltage Window

A TVS diode has several key ratings: VRWM (maximum working voltage), VBR (breakdown voltage), VC (clamping voltage), IPP (peak pulse current), and peak pulse power.

A common mistake is to treat VBR as the actual clamping voltage. VBR is only the point where the TVS starts to conduct. VC, the clamping voltage, is usually higher than VBR, and VC is the value that downstream components actually see during a pulse.

Two rules must both be true for safe design. First, the maximum normal input voltage must stay below VRWM. Second, the worst-case VC must stay below the absolute maximum voltage rating of the downstream MOSFET or DC/DC converter.

Other factors can also affect how well a TVS diode protects a circuit. The type and length of the voltage surge can change how much stress the TVS diode has to handle. High temperature can also reduce the amount of surge power the TVS diode can safely handle. The resistance in the power source and wires can affect how much current flows through the TVS during a surge.

Choosing the right TVS diode is also important. A unidirectional TVS placed before the reverse-polarity protection can become forward-biased when the input polarity is reversed. What happens next depends on the fuse or PTC, current limiting, and the power supply. The current may be limited, or the fuse may blow. It does not always mean a direct short circuit. A bidirectional TVS does not become forward-biased when the input polarity is reversed. This means the TVS itself does not create a low-resistance path for the current. However, it does not replace reverse-polarity protection. You still need a diode or MOSFET to block reverse current from reaching the rest of the circuit.

Finally, the TVS diode should be placed close to the power input and protected circuit. Keeping the PCB traces short helps reduce extra voltage caused by the traces during a fast surge.

A TVS device only protects against short transients. Under sustained overvoltage, a TVS cannot replace a disconnect switch or a current-limiting device. If overvoltage lasts too long, the TVS can overheat and fail.

A full range of TVS Diodes & ESD Suppressors is available for matching against the voltage window defined above.

For a deeper step-by-step method, see Toshiba’s TVS/ESD diode selection guide. This article builds on this method by adding coordinated analysis of the MOSFET and capacitor stages, rather than considering the TVS device alone.

5. Capacitors: Transient Support Can Also Create Inrush

MLCCs (multi-layer ceramic capacitors) give good high-frequency bypassing. But their effective capacitance drops under DC bias, and the actual value depends on operating voltage, dielectric type, and package size.

Aluminum electrolytic capacitors are used to store a large amount of energy. When choosing a capacitor, check its ESR, ripple-current rating, operating temperature, and service life. Their life can become shorter when they are used at high temperature.

Large input capacitance can create high inrush current at power-on. This extra stress affects the MOSFET’s SOA and the connectors. It can also form a ringing circuit together with cable inductance.

The best approach is a combination of MLCCs and electrolytic capacitors together. It should not be treated as a simple either-or choice.

Suitable parts can be sourced from the Ceramic Capacitors category for the high-frequency stage, and from Aluminum Electrolytic Capacitors for bulk energy storage.

6. Worked Example: A 24V Industrial Input

To demonstrate the step-by-step design process, let’s consider a typical 24V industrial system with the following specs:

- Input Voltage: 24V DC ± 10% (21.6V to 26.4V)

- Nominal Load Current: 3A (72W)

- Surge Protection Level: IEC 61000-4-5 (1kV / 42Ω surge waveform)

- Downstream Max Voltage Rating: 40V (DC/DC converter input limit)

- Operating Ambient Temperature: -40°C to +85°C

The surge level above uses an application-specific 1kV test condition based on IEC 61000-4-5. The exact IEC severity level depends on the applicable product standard, the port under test, and the coupling mode — in common reference tables, 1kV typically corresponds to Level 2, while Level 3 is usually 2kV, so no specific level is claimed here. The 42Ω source impedance used in this article is the test configuration assumed for this example, not a universal impedance for all 24V power-supply ports; the actual impedance must be confirmed against the applicable product standard and port type. The test can be applied line-to-line or line-to-earth using the proper coupling/decoupling network (CDN), with the standard 1.2/50µs open-circuit voltage waveform and 8/20µs short-circuit current waveform. The CDN and coupling method should match the port being tested, and this is what makes the test condition representative of a real 24V power-supply input.

With these values set, the design work follows six steps, using the solution-comparison table from Section 3 to weigh conduction loss against circuit complexity for this specific 24V input:

Step | What To Do
1 | Set the protection goals and the voltage window
2 | Compare Schottky diode loss vs MOSFET loss
3 | Check MOSFET voltage rating, RDS(on), VGS, and SOA
4 | Pick a TVS device and check worst-case clamping voltage (VC)
5 | Set the MLCC and electrolytic capacitor values
6 | Test surge, hot-plug, reverse polarity, load step, and temperature rise

Figure 2: Signal chain for the 24V worked example

6.1 Example Calculations

The calculations below use some example component values. These values are used only to explain the design and are not a recommendation for a specific part. In a real design, use the actual specifications of the selected components.

Calculation | Assumptions & Formula | Result
Schottky diode loss | P = I × VF, assuming VF = 0.5V at 3A | 3A × 0.5V = 1.5W
MOSFET conduction loss (hot) | P = I² × RDS(on,hot), assuming RDS(on,hot) ≈ 30mΩ after derating to 85°C ambient plus self-heating | (3A)² × 0.03Ω ≈ 0.27W
MOSFET voltage margin | Compare worst-case VC (≈38V) against BOTH the MOSFET VDS rating (60V assumed part) AND the article’s stated downstream DC/DC converter absolute maximum input rating (40V) | MOSFET: 60V − 38V = 22V margin (≈63% of rated VDS, within an 80% derating guideline). DC/DC converter: 40V − 38V = only 2V margin, which cannot reliably cover TVS tolerance, temperature drift, and PCB parasitic overshoot → this margin is insufficient and requires further verification (e.g. a lower-VC TVS or a converter with higher input rating), not a pass/fail conclusion of “safe.”
TVS peak pulse current (IPP) | IPP ≈ (VOC − VC) / ZS, assuming VOC = 1000V (surge open-circuit voltage), VC ≈ 38V (clamping voltage, not VBR), and ZS = 42Ω source impedance | (1000V − 38V) / 42Ω ≈ 22.9A
TVS peak pulse power (PPP) | PPP ≈ VC × IPP, using VC ≈ 38V | 38V × 22.9A ≈ 870W → select a TVS rated ≥ 1kW peak pulse power for margin
Effective MLCC capacitance | Nominal 22µF X7R rated part, assuming ~60% derating at 24V DC bias | 22µF × 0.4 ≈ 8.8µF effective at 24V
Capacitor inrush current | I = C × dV/dt, assuming 100µF total input capacitance charging to 24V over ~1ms at hot-plug | 100µF × (24V / 1ms) = 2.4A — check against MOSFET SOA
Thermal design (MOSFET junction rise) | Tj = Ta + P × RθJA, assuming RθJA ≈ 50°C/W and Ta = 85°C (max ambient) | 85°C + (0.27W × 50°C/W) ≈ 98.5°C, below a typical 150°C Tj(max) rating

Suggested validation checklist:

- Surge test at the rated pulse level

- Hot-plug test at minimum and maximum input voltage

- Reverse-polarity test at full load

- Load-step response test

- Temperature-rise test at maximum ambient temperature

7. Common Integration Mistakes

- Choosing components based only on the nominal voltage instead of the full voltage range

- Ignoring RDS(on) at high operating temperature

- Using VBR in place of VC when checking downstream protection

- Ignoring MLCC capacitance derating under DC bias

- Adding more capacitance without checking the resulting inrush current

- Placing the TVS device too far from the connector

- Choosing alternative parts based only on package size and rated values without checking their actual operating conditions

Frequently Asked Questions

What is the difference between VBR and VC in a TVS diode?

VBR is the breakdown voltage. It is the voltage at which the TVS starts conducting. VC is the clamping voltage, which is the voltage that reaches the parts after the TVS during a real pulse. VC is normally higher than VBR, so it is important when checking the protection.

Can a TVS diode replace a fuse?

No. A TVS diode only clamps short voltage spikes. It cannot limit current or stay safe under a long overvoltage or overcurrent event. A fuse or PTC device is still needed for that protection.

Which is better for reverse-polarity protection, a diode or a MOSFET?

A diode is simple to use, but it can get hot at high current because of power loss. A MOSFET has lower conduction loss and can give better efficiency. However, it needs proper gate drive and correct body-diode direction. The best choice depends on the current, board space, and required efficiency.

Why does MLCC capacitance drop under DC bias?

This happens because of the ceramic dielectric material inside the MLCC. As DC voltage rises, the effective capacitance falls. This effect is stronger in small package sizes and high-K dielectrics, so the real capacitance at operating voltage is often lower than the printed value.

What causes inrush current in a DC power supply?

When the power is turned on, the input capacitors need to charge. This can cause inrush current. Larger capacitors can cause more inrush current. If the current is too high, it can put stress on the MOSFET, connectors, and input protection parts.

How do you protect a 24V DC industrial input from surges and reverse polarity?

A power path needs different components, not just one part. A fuse or PTC is used for overcurrent protection. A TVS diode is used for surge protection, and a diode or MOSFET is used for reverse-polarity protection. MLCC and electrolytic capacitors help keep the power stable.

Conclusion

For a reliable power path, MOSFETs, diodes, TVS devices, and capacitors should be selected together. They work as one protection system, so they should not be treated as separate parts.

Unikeyic Electronics supplies electronic components for industrial control, automotive electronics, robotics, medical electronics, IoT, communications, and new energy applications.
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