Make the Switch from Discrete to Integrated Load Switches, Ideal Diodes, and eFuses

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What you’ll learn:

• How to replace MOSFETs with load switches, and the impact of on-resistance.
• Swapping out diodes with ideal diodes, and the use of ideal-diode controllers.
• Why replacing fuses and resettable fuses with eFuses is a smart play.

对更大的功率密度和较小的印刷电路板（PCB）的需求继续升级，例如建筑自动化和汽车信息娱乐。为了跟上这些趋势，有一个努力将离散电路转换为完全或部分集成的解决方案。

Discrete components such as diodes, fuses, and metal-oxide semiconductor field-effect transistors (MOSFETs) are being replaced by integrated devices that not only replicate their functionality, but also provide improved performance—including up to 5X higher power density—and other features benefiting the overall application. This article highlights some of the common discrete circuits used for power distribution and protection, as well as the most appropriate integrated solutions to replace them.

Replacing MOSFETs with Load Switches

In analog power distribution, discrete MOSFETs are used to connect and disconnect a power supply to a load. The most common type of MOSFET used for these applications is a P-type metal-oxide semiconductor (PMOS) due to its ease of use and low cost. By implementing three resistors, a capacitor, and a control FET, it’s possible to create a simple active-high switching solution with a controlled turn-on for limiting capacitive-load inrush current(Fig. 1).

虽然该电路可以电气开关打开和关闭下游负载，但它的缺点使得在现代设计中很难使用。最大的问题是能够增加系统功率密度以实现更高功率而不会影响解决方案尺寸的能力。具有六个离散组件，平均溶液尺寸为17毫米², it’s very likely this circuit can be improved through integration. The solution also has no protection or additional features to benefit the rest of the system, such as thermal shutdown or power good indication.

负载开关将MOSFET开关解决方案的功能与添加功能（例如当前限制和输出放电）相结合。使用启用引脚，负载开关能够使用线性控制的电压坡道连接并断开输入到输出的连接，以管理Inrush电流。通过使用负载开关代替离散MOSFET电路，可以将溶液尺寸降低到4 mm²or smaller. This results in a 3.5X improvement in power density.图2compares the solution size of a discrete implementation to theTPS22919, an integrated load switch from德州仪器公司(TI).

On-resistance is the specification that determines how efficiently power is transferred from supply to load. Thus, it’s used to compare performance between two switching solutions. Equations 1 and 2 show how a higher on-resistance directly results in a larger voltage drop across the switch and increase in power loss:

力量loss = I_LOAD²× R_ON(1)

Voltage drop = I_LOAD× R_ON(2)

The on-resistance of a MOSFET is proportional to the gate-source voltage (V_GS). In the discrete solution, this V_GS电压通常使用电阻和电容器（RC）电路控制。RC控制方法在输出处导致非线性坡道。非线性坡道会导致较高的峰值泛滥电流，从而为输入源负担，并引起供应下降的可能性。

A load switch linearly controls the V_GSvoltage, resulting in a linear voltage ramp at the output and thus lowering the peak inrush current(Fig. 3).

In addition to inrush current control, the load switches integrate other features such as thermal shutdown, quick output discharge, short-circuit protection, adjustable current limit, and rise time. These features eliminate the need for a safe-operating-area (SOA) curve since the load switch will protect itself during high-stress events. Features like adjustable slew rate and adjustable output discharge allow for greater control over the turn-on and turn-off profile of the load switch, enabling better inrush current management and more controlled power-off. For power-sequencing applications, load switch features like power good can enable downstream devices or turn on the next rail in a sequence.

用理想的二极管代替二极管

Typically, diodes are used for reverse-current blocking and reverse-polarity protection. Diodes are placed in series with power rails to protect the power supply during a fault event（图4）.

However, placing a diode in series to a power-supply output causes a voltage drop across the diode (300 mV for a typical Schottky diode and 700 mV for a traditional diode). Therefore, the power loss in the Schottky diode can be as high as 300 mW for a 1-A load current—see Equation 3. To minimize the power loss, a higher-power diode is often used for high-power systems.

力量dissipation = I_LOAD× V_FWD（3）

另外，可以使用理想二极管控制器来驱动像理想二极管一样起作用的外部MOSFET。MOSFET与动力导轨串联放置，以便其身体二极管从供应到负载。例如，LM74700, an ideal-diode controller from TI, regulates the forward voltage from supply to load to 20 mV（图5）.

The GATE pin is tied to the gate of the MOSFET and sources or sinks current to regulate the MOSFET resistance and maintain the constant forward voltage（图6）.

这门电压调节provides a low voltage drop in the forward direction and blocks reverse current within 0.75 µs when the output voltage rises above the input voltage. The 20-mV voltage drop also results in a significant reduction in power dissipation, as shown in the comparison between the LM74700 with an external MOSFET and a Schottky diode(Fig. 7).

In addition to the reduced power dissipation, the solution size using an ideal-diode controller and a discrete MOSFET is 5X smaller than a high-power DDPAK Schottky diode.

Replacing Fuses and PTCs with eFuses

保险丝是保护电源免受超载或短路事件的保护设备。它们被放置在电源和负载之间。在故障事件（超载或短路）期间，保险丝断开打开，从负载上断开电源。在断层条件下，传统的融合永久破裂。因此，电路可操作需要替换保险丝(Fig. 8).

A discrete fuse requires an overcurrent level that’s 6X higher than the rated current for the fuse to break open. The overload current for these fuses isn’t very accurate; therefore, fuse selection for an application requires considerable analysis to avoid fuse break under nominal inrush-current conditions.

A resettable fuse is a positive-temperature-coefficient (PTC) device that increases its on-resistance with temperature. During an overload event, the excessive load current increases power loss, thereby increasing its on-resistance. The higher on-resistance helps limit the overload current and protects the circuit.

Unlike a traditional fuse, PTCs allow current to flow after the fault is cleared without replacing the device. PTCs are activated by the heating effect of an overcurrent load. As a result, the reaction time of the PTC is limited to several milliseconds due to the dependency on the ambient temperature(Fig. 9).

此外，每次重置后，PTC的耐药性都会增加。因此，PTC在长时间的使用中表现出不可重复的行为。

Efuse是一种具有积分MOSFET的活动电流保护设备，用于在故障条件下将电流限制为安全水平。效用的共同元素是调节负载电流，电流传感元素和控制逻辑的电源开关（图10）.

The current-limit threshold can be set to a desired value through an external resistor, providing system flexibility in adjusting the current limit to a wide range. In a steady-state condition, the integrated power switch is fully turned on (similar to a discrete MOSFET switch) to minimize voltage drop in the power path.

During an overcurrent condition, the gate control transitions the integrated power switch into linear mode. The on-resistance of a MOSFET increases in the linear mode; therefore, the corresponding voltage drop across the eFuse increases to provide a constant output current.

图11shows the overload response of TI’sTPS2595eFuse with a current-limit setting of 4 A.

eFuses快速响应过载事件(典型ly hundreds of microseconds) by regulating the output current to safe levels. Once the fault is removed, they recover back to normal operation without needing to be replaced. An eFuse uses a current mirror circuit to measure the load current without requiring an external sense resistor. This approach not only saves board space, but also eliminates the power loss in sense resistors. In addition, the eFuses provide better current-limiting accuracy (±5% to ±8%) over traditional fuses and PTCs.

此外，Efuses集成了过电压保护，以防止设备输入上看到的高电压以通过负载。结合其他功能，例如反向电流阻止和当前监视，EFUS能够在包装中处理多种保护需求，甚至比Fuse或PTC小。

结论

MOSFET，二极管和保险丝已经在电路中使用了数十年，但是整合使这些组件过时了，而现代电路则集中在小型外形和高功率密度上。尽管离散电路使用多个被动组件来实现其功能，但集成解决方案可以减少简化供应链管理所需的组件总数。希望在其行业中进步的工程师需要在其设计中利用集成解决方案来保持竞争力并最大化系统性能。

MOSFET开关解决方案可以通过负载开关来简化，以获得线性转机和其他功能的好处。通过降低二极管的电压下降到理想二极管的电压下降，可以显着降低系统功率损失。并且可以用函数代替PTC或保险丝，以进行更准确的过载保护和更快的响应时间。所有这些集成的解决方案提供的功能与其离散对应物具有相同的功能，具有改进的性能，其他功能和较小的解决方案尺寸。