在被动电子组件中，保险丝和二极管是众所周知的，众所周知的，广泛使用的，对于许多电路和系统必不可少。尽管如此，每个人都有固有的缺点，使它们不足或限制，尤其是在当今的低压电路中。
By reimagining them via active components like op amps and MOSFETs, they can be enhanced with attributes that make them a good fit for many situations. These include overcurrent transients and protection against reverse-polarity connection of the battery in the automobile or LED lighting strings (which does happen and would cause serious damage to the electronics).
The Electronic Fuse: Eliminating Thermal Activation
150多年来，标准的热激活保险丝（也称为熔融链接）已用作电流限制的保护组件，其简单性是其优点之一。功能很简单：设计限制的电流会加热内部线，然后融化并打开电流路径，从而将电流流量降低到零。它有效，相对简单且价格便宜，并且可以使用从放大器的一部分到数百个放大器的评分。
但是，热保险丝有几个缺点，从反应时间开始。根据与阈值相比的过电流值，它可能需要从数十毫秒到数十秒钟的时间来反应和打开电路。在当今的低压设计中，过电流通常是一个适中的值，因此保险丝反应时间可能太慢而无法保护敏感电路。同样，标准保险丝在打开后必须物理替换，这在许多（但不是全部）应用中是不利的。
坏蛋e-fuse提供了一个可选择的方法rent limiting and cut-off with unique advantages, as it’s an all-electronic fuse that doesn’t depend on the thermal heating and subsequent open circuit of an in-line element. An e-fuse is built from several analog components: a precision current-sense resistor; an amplifier with accurate scaling resistors for capturing and “gaining-up” the voltage across the resistor; a comparator circuit to “switch” at the preset value; and a MOSFET to allow/break the path of current flow in the line being monitored(Fig. 1)。除了电路保护应用程序外，电子融合还广泛用于热扫描控制器中，其中当前的Inrush（在将卡插入活总线中）时发生的电流inrush必须受到监控。
Typically, the resistor value is chosen so that the voltage drop across it will be between 50 to 100 mV at maximum current. The e-fuse is connected between the power rail (or supply source) and the load to be protected.
The circuit function is fairly straightforward. The current to be monitored passes through the resistor, and the resultant voltage across this resistor is sensed and scaled by the current-sense amplifier (CSA). Since the resistor value is known, it’s easy to utilize analog circuitry to set a threshold on the current passing through it using basic Ohm’s Law: I = V/R. If the threshold is exceeded, the comparator that controls the FET turns the FET off to interrupt the current. Response time is on the order of microseconds, far faster than a thermal fuse.
For the e-fuse to be truly useful, possible error sources must be understood. First, the resistor and scaling circuit must be accurate and have minimal temperature drift. The resistor value will deviate from nominal value due to changes in ambient temperature, as well as unavoidable self-heating of the resistor (which can cause significant errors). Therefore, the resistor power rating must be commensurate with the dissipation. Also, the on/off FET must be sized for the situation and may need its own gate driver, depending on its current and voltage rating. Second, the circuit which “trips” on the overcurrent must be properly designed to not have false positive or negatives. Usually, some hysteresis is added to prevent this chatter.
在几乎所有设计中，CSA都不是一个简单的操作放大器，而是差异放大器（diff amp）。这是因为在大多数配置中，感觉电阻器没有连接在负载的接地侧（假设负载甚至接地）。取而代之的是，它连接在高侧，因此CSA必须在没有任何接地连接的情况下测量电压。这也意味着必须将CSA额定为在未接地电阻的共同模式电压（CMV）处进行操作。CSA IC可用，可以轻松测量100毫秒范围内的微伏，但可以耐受50-100 V（以及负CMV）的共同模式电压，而不会降低其准确性或基本功能（甚至损坏）。
While it’s possible to build an e-fuse from individual components, most users instead opt for a complete IC-based e-fuse that incorporates the needed circuitry including the FET. Some even have an internal sense resistor. IC-based e-fuses can also include additional functions and features, such as user-programmable undervoltage lockout, overvoltage clamp, and auto retry, as well as a startup time that can be set by means of external components(Fig. 2)。最后一个功能对于在启动和热汇操作过程中控制刷新电流很有用，这就是为什么在该应用程序中找到此类电子融合的原因。
幸运的是，尽管它们具有内部复杂性和其他功能，但这些电子融合非常易于使用。一些电子融合已获得UL（以前称为承销商实验室）和可比的世界边缘安全机构的认可，这进一步扩大了其适用性。
理想二极管：削减前压滴
A reverse-connected battery or power rail can cause havoc and damage to sensitive electronic circuits. The problem is especially acute in cars, where the battery must often be disconnected and reconnected manually, without keyed connectors. Even a momentary reversal can damage or weaken sensitive circuit components.
显而易见的解决方案是使用一个标准的PN semiconductor diode in series with the supply rail or rails to the subcircuits. This ubiquitous diode is an essential component with a simple basic function: It blocks the flow of current in one direction (reverse bias) but allows it in the opposite direction (forward bias).
但这是一种释放新问题的解决方案。在向前偏置的方向进行操作时，在整个标准硅二极管上不可避免的电压下降为0.6至0.8 V。虽然在某些设计中，这种下降可能微不足道，但它的范围从在其他设计中有些意义，到在操作电压本身只是几伏（例如，大多数汽车的12-V标称导轨或下部汽车的12-V标称导轨）的系统中不可接受。电压子电路）。不仅有电压损失，而且由于该二极管的耗散而导致功率耗散和有害的自加热。
One solution is to go with more expensive Schottky diodes. They have a much lower forward drop of about 0.3 V, and are often used in place of conventional diodes for this reason. However, they have other less desirable attributes, such as higher reverse-leakage current, which makes them more challenging to use. Even so, their ~300-mV drop is still a significant fraction of the available voltage in a 12-V system (and even worse in a lower-voltage subcircuit).
A better solution is to use an “ideal diode” built with active devices for reverse-polarity protection. Note that the term “ideal” is somewhat of a misnomer, since a truly ideal diode would have a forward drop of zero volts(Fig. 3)。However, the active “ideal” diode comes fairly close with its forward drop about 30 to 40 mV, which is an order-of-magnitude less than even the Schottky diode. Regardless of the accuracy of the designation, the industry refers to these as ideal diodes.
To understand the ideal-diode operation, we can use the Analog Devices LTC4358 as an example(Fig. 4)。This 12-V/5-A IC contains an internal 20-mΩ MOSFET as the pass element and doesn’t require any external components. The IN pin connects to the source of the MOSFET and functions as the anode of a diode, while the drain connection functions as the cathode. When power is first applied, the load current initially flows through the MOSFET’s body diode, and the MOSFET’s gate is enhanced and turns on to maintain a 25-mV forward-voltage drop.
If the load current results in a forward-voltage drop of more than 25 mV, the MOSFET is driven to its fully on state, and the forward drop equals RDS（ON）× I加载。If the load current is reversed and the FET thus reverse-biased (as may occur during a short circuit at the input), the LTC4358 turns the internal MOSFET off in less than 0.5 μs.
The on-resistance of the MOSFET is the key parameter that determines how close to “ideal” this ideal diode can become. As with nearly all semiconductors parameters, RDS（ON）是温度的函数，这是任何电路建模，仿真和分析中必须使用的因素。请注意，理想二极管的内部电路有些复杂（图5）, but this complexity is invisible to the user. The ideal diode is connected like a simple two-terminal device plus ground.
理想的二极管不仅减少了正压降低，而且还降低了相关的功率耗散。例如，将LTC4358与表面座分组器（SMC）软件包中广泛使用的B530C Schottky二极管进行比较。LTC4358的4-×3毫米DE14套件具有SMC套件的四分之一，而电压下降和功率耗散也少得多（图6）。Furthermore, the much lower power dissipation at 5 A current in the Schottky diode (2 W versus 0.5 W for the LTC4358) greatly increases system efficiency, simplifies board layout, and trims BOM and cost as no heat sink is required.
Both e-fuses and ideal diodes are active, more sophisticated versions of the venerable thermal fuse and PN diode, respectively. While they may seem more complex, both are easy to use and overcome many of the shortcomings of these older components at little or no cost. They usually will also reduce overall system-level complexity and cost when looking at the “bigger picture” of the design.
References
模拟设备，LTC1153 Auto-Reset Electronic Circuit Breaker Datasheet
STMicroelectronics,STEF01 8 V to 48 V Universal Electronic Fuse Datasheet
Texas Instruments,TPS25925x, TPS25926x Simple 5-V/12-V eFuse Protection Switches
Texas Instruments, SLVA862A, “灵言的基础”
Maxim Integrated, Design Solutions No. 50, “Cut Your Losses—With an Ideal Diode”
Maxim集成，设计解决方案第51号，“Design a Cooler, Safer Smart-Home Hub—Using an Ideal Diode”
Analog Devices.LTC4358 datasheet
模拟设备，“Ideal Diode Betters a Schottky by a Factor of Four in Power and Space Consumption”
Texas Instruments, “Ideal diode & ORing controllers
Electronic Design, “Reverse-Polarity Protection in Automotive Design”
