对于许多低功率和中等功率轨，在给定的汽车系统中，异步降压转换器仍然是成本和效率之间的最佳折衷。
这种方法的隐含和关键是降压二极管的选择。
尽管如此，思考二极管并不会占用很多，但错误的选择会导致低效率，高功耗和高电磁辐射（EME）或更糟！
我们如何定义有问题的二极管？
二极管操作可以通过自然或强制换向进行分类。
SMPS通常使用连续导通模式（CCM）或不连续导电模式（DCM）。
CCM采用强制换向，有时称为硬开关，其中主开关导通迫使二极管关断，尽管二极管在该点承载负载电流。
DCM采用自然换向，有时称为软开关，其中二极管关断通过内部复合自然发生。
典型的汽车异步降压转换器专为CCM操作而设计，因此具有硬切换功能。
二极管执行两个功能：整流和续流。
作为整流器，它将电流脉冲传递到输出滤波器;
作为一个续流实体，它允许电流在主开关关闭后继续在电感中流动。
因此，我们可以将降压二极管定义为续流肖特基整流二极管。
我们暂时把它称为肖特基整流器。
图1 - 由二极管直流特性引起的肖特基整流器功率损耗。
用于CCM降压转换器的肖特基整流器的选择是并列的 - 实际上是一系列相互矛盾的困境......
传统上，选择似乎是对直流参数的直接考虑 - 正向和反向：选择反向电压电平高于开关节点预期的最高电压电平的肖特基整流器;
峰值正向电流能力高于最低工作周期的峰值电流;
最低的反向漏电流（尽管除非在非常轻的负载下工作，否则这种损耗通常是微不足道的）;
当然还有可以优化效率的最低正向电压。
前向条件参数虽然导致第一个并置 - 而最低正向电压优化效率，功耗，正向电压本身是芯片尺寸的反函数。
因此，需要的正向电压与芯片尺寸成反比，整流器的价格随芯片尺寸而变化，所需的正向电流要求随芯片尺寸而变化，反向漏电流也需要权衡。
与正向电压成反比，最终也是封装尺寸，与芯片尺寸和热阻抗要求成比例。
技术选择也可以发挥作用 - 沟槽技术通常提供比平面技术更好的正向电压和漏电流品质因数。
式。
1 - 续流二极管功耗（分别为正向损耗，反向损耗和开关损耗的乘积之和）。
在发现EME问题之前，肖特基整流器的交流特性通常会被忽略。
显然，开关越快，交流特性变得越重要，许多汽车SMPS在频率高达2 MHz的情况下切换，通常是为了最小化功率级元件的尺寸，最大限度地减少交流开关损耗。
最小化肖特基整流器结电容是降低EME和功耗的关键 - 多兆赫兹开关节点振铃可能转化为多兆赫兹的发射。
然而，我们面临着进一步的并置 - 芯片面积越大，结电容越高。
如果你有一个低负载电流应用可能很好，但如果你需要更高的正向电流能力可能不是那么好，需要减少EME或最小化交流损耗。
技术再次发挥作用 - 沟槽技术在温度范围内提供比平面技术更稳定的开关特性，这在高环境温度下运行的汽车应用中非常重要。
图2 - 由二极管AC特性引起的肖特基整流器功率损耗
用于汽车SMPS的续流肖特基整流二极管选择是并置的。
哪些因素最重要取决于最大占空比，开关频率，负载电流要求，以及最终的环境温度和散热限制。
但是，选择正确的二极管可能会陷入两难境地！
以上来自于谷歌翻译
以下为原文
For many low and medium power rails, within a given automotive system, the asynchronous buck converter remains the optimum compromise between cost and efficiency. Implied and critical to this approach is the choice of buck diode. Despite this, pondering the diode doesn’t pre-occupy many, yet the wrong choice can lead to low efficiency, high power dissipation and high electro-magnetic emissions (EME) or worse!
How do we define the diode in question? Diode operation can be categorised by either natural or forced commutation. SMPSs typically use either continuous conduction mode (CCM) or discontinuous conduction mode (DCM). CCM employs forced commutation, sometimes called hard-switching, where the main switch turn-on forces the diode to turn-off, despite the diode carrying the load current at that point. DCM employs natural commutation, sometimes called soft-switching, where the diode turn-off occurs naturally by internal recombination. The typical automotive asynchronous buck converter is designed for CCM operation and is hard-switching as a result. The diode performs two functions: rectification and free-wheeling. As a rectifier it passes current pulses to the output filter; as a free-wheeling entity it allows the current to continue to flow in the inductor, after the main switch turns off. So we can define our buck diode as a free-wheeling schottky rectifier diode. Let’s just call it a Schottky rectifier for the moment.
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Fig. 1 – Schottky Rectifier Power Loss Due To Diode DC Characteristics.
The choice of Schottky rectifier for the CCM buck converter is juxtaposition – indeed a series of conflicting dilemmas…
Conventionally the choice is seemingly a straight forward consideration of dc parameters – forward and reverse: Choose a Schottky rectifier with a reverse voltage level higher than the highest voltage level expected at the switch node; a peak forward current capability higher than the peak current at the lowest duty cycle; the lowest reverse leakage current (although such losses are generally insignificant unless operating at very light loads); and of course the lowest forward voltage possible to optimise efficiency. The forward condition parameters though lead to the first juxtaposition - while the lowest forward voltage optimises efficiency, power dissipation, the forward voltage itself is an inverse function of die size. So there’s a trade-off between the required forward voltage, which inversely scales with die size, the price of the rectifier, which scales with the die size, the required forward current requirement, which scales with the die size, the reverse leakage current which inversely scales with forward voltage, and ultimately the package size too, which scales with the die size and thermal impedance requirements. Technology choice can play a role here too – a trench technology typically offers an improved forward voltage and leakage current figure of merit, than a planar technology.
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Eq. 1 – Free-wheeling Diode Power Dissipation function (the sum of the products of forward loss, reverse loss and switching loss respectively).
The ac characteristics of the Schottky rectifier typically get ignored until an EME problem is found. Clearly the faster one switches then the more important the ac characteristics become, and many automotive SMPSs switch at frequencies of anything up to 2 MHz, usually to minimise the size of the power stage components, placing a premium on the minimisation of ac switching losses. Minimising the Schottky rectifier junction capacitance is key to reducing EME, as well as power dissipation – multi-megahertz switch node ringing is likely to translate to multi-megahertz emissions. However, we’re faced with a further juxtaposition - The greater the die area the higher the junction capacitance. Possibly great if you have a low load current application but perhaps not so great if you need a higher forward current capability, need to reduce EME or minimise ac losses. Technology again can play a role here – a trench technology offers a more stable switching characteristic over temperature, than a planar technology, important in automotive applications operating at high ambient temperatures.
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Fig. 2 – Schottky Rectifier Power Loss Due to Diode AC Characteristics
The free-wheeling Schottky rectifier diode choice for automotive SMPS is juxtaposition. Which factors prove most significant depends upon the maximum duty cycle, switching frequency, load current requirements, and ultimately the ambient temperature and heat sinking constraints. However, choosing the correct diode can prove to be a dilemma!
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