那么，对于汽车驾驶舱应用，如导航，音频和信息娱乐系统，什么是正确的调制方案？
这确实重要吗？
设计人员经常面临两种基本方案的SMPS IC之间的选择：脉冲宽度调制（PWM）或脉冲频率调制（PFM）。
经典PWM利用恒定周期和可变导通时间，而PFM利用恒定导通时间和可变关断时间。
图1 - 对于PWM TK是恒定的。
对于PFM，aK是常数。
对于两种方法，ɛK通常为零
PWM的恒定周期给出固定频率。
固定频率给出了已知的基波和可识别的谐波。
然而，相对于PFM，折衷是效率较低，因为适用于PFM的延长关断时间和较低频率的时间段不适用于PWM。
虽然PFM具有较低静态电流的优点，但在低占空比和频率下仍然存在问题。
PFM的可变关闭时间相当于每个周期的可变周期;
换句话说，变频。
未知数量的基本频率在EMC方面很难解决，因此无法发现（尽管在某些情况下它可以帮助进行频谱扩展）。
这在汽车驾驶舱中很重要，但在无线电调谐器附近尤为重要。
因此，在这种环境中，在系统接通状态下使用利用PWM的SMPS IC，其谐波将避免接收频带，这是确保接收完整性的必要条件。
使用PFM只会导致EMC和音频质量问题。
什么跳脉冲？
有时会遇到利用脉冲跳跃的控制方案，通常称为滞后，突发模式或砰砰声控制。
它们采用不同的控制方案（没有误差放大器来集成输出电压误差），但会导致脉冲跳跃行为。
脉冲跳跃意味着可变周期，因此关于EMC和音频接收的基频控制再次成为关注点。
这不仅适用于纯脉冲跳跃方案，例如滞后控制。
PWM拓扑通常达到低占空比限制，由其最小导通时间确定，超过该限制，它们将跳过开关周期或折回其频率以延长低占空比操作。
一些PWM方案可以提供足够小的最小导通时间，使得在通常的汽车电池范围内不会遇到这种脉冲跳跃。
例如，安森美半导体NCV890xxx系列2 MHz降压转换器将允许在2 MHz下进行18V：3.3V转换，无需跳脉冲或频率折返。
始终检查最短的开关时间！
那么，汽车驾驶舱SMPS的调制方案的选择是否重要？
是!
虽然PWM是首选，但这是买家要小心的情况。
并非所有的PWM方案都是平等的。
以上来自于谷歌翻译
以下为原文
So, what is the right modulation scheme for automotive cockpit applications such as navigation and audio and infotainment systems? Does it indeed matter? Designers often face a choice between SMPS ICs with two fundamental schemes: Pulse Width Modulation (PWM) or Pulse Frequency Modulation (PFM). The classic PWM utilises a constant period and a variable on time, while PFM utilises a constant on-time and a variable off-time.
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Fig. 1 – For PWM TK is constant. For PFM aK is constant. ɛK is typically zero for both approaches
PWM’s constant period gives a fixed frequency. That fixed frequency gives a known fundamental and identifiable hARMonics. The trade-off, relative to PFM though, is lower efficiency as periods of extended off-time and lower frequencies, which apply for PFM, don’t apply for PWM. While PFM offers benefits of lower quiescent current, at low duty cycle and frequency, there is a catch. PFM’s variable off-time equates to a variable period, per cycle; in other words variable frequency. A fundamental frequency of an unknown quantity is liable to be difficult to resolve in terms of EMC, catching out the unaware (although in some circumstances it can help with spectral spreading). This matters in the automotive cockpit but it matters particularly near a radio tuner. So, in this environment the use of an SMPS IC utilising PWM, at a frequency where the harmonics will avoid reception bands, in the system on-state is a necessity to ensure reception integrity. Use of PFM will only lead to EMC and audio quality headaches.
What of pulse-skipping though? Sometimes control schemes that utilise pulse-skipping are encountered, often known as hysteretic, burst-mode or bang-bang control. These utilise a different control scheme (with no error amplifier to integrate the output voltage error) but result in pulse-skipping behaviour. Pulse-skipping implies a variable period, so control of the fundamental frequency with respect to EMC and audio reception becomes a concern again. This doesn’t just apply to pure pulse-skipping schemes, such as hysteretic control. PWM topologies typically reach a low duty cycle limit, determined by their minimum on-time, beyond which they will either skip switching cycles or fold-back their frequency to extend that low duty operation. Some PWM schemes can offer a sufficiently small minimum on-time that such pulse skipping is not encountered within the usual automotive battery range. The ON Semiconductor NCV890xxx family of 2 MHz buck converters for example, will allow 18V:3.3V conversion at 2 MHz, without pulse skipping or frequency fold-back. Always check the minimum on and off times!
So, does the choice of modulation scheme for automotive cockpit SMPS matter? Yes! While PWM is preferable, it’s a case of buyer beware. Not all PWM schemes are born equal.
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