A snubber circuit limits voltage spikes in power converters. Snubbers are also used in the output ofD类音频放大器for the same reasons(Fig. 1)。当您使用用于关闭电流的晶体管时，它会生成一个高压尖峰，可能会损坏设备。该尖峰还将产生电磁干扰（EMI）。请注意，EMI是由循环中电流引起的。如果您的Snubber网络以较大的循环路线流动，则它将增加而不是减少EMI。
It’s All About Inductance
The voltage spikes in your circuit are caused by discrete inductors that you’re switching. The leakage inductance of transformers looks like a series inductance with the ideal transformer winding, and that will generate voltage spikes. These discrete inductances are obvious looking at your schematic. A further problem is all of the stray inductance in the wires and printed-circuit-board (PCB) traces. Any wire in space has inductance. Power electronics design is like RF design, where your circuit board is a component.
PCB更改电感的布局和构建的事实意味着您需要构建和测试功率设计以优化它们。这通常比使用现场求解器to figure out the stray inductance while in the design phase. Looking at similar designs can give you an idea of the sizes of the snubber components to start with.
How to Snub Your Spikes
The simplest snubber network is a series RC circuit at the switch node(Fig. 2)。The capacitor prevents dc currents from flowing. When the transistor switches, the capacitor then tends to look like a short circuit and the resistor can shunt current out of the switch node. One disadvantage of the RC snubber is that it also adds to the current the transistor must carry when it turns on—it doesn’t distinguish between the switch node voltage rising or falling.
For higher power application, you can put a diode in series with the RC to create an RCD snubber(Fig. 3)。The diode blocks any current in the network as the transistor turns on. When the transistor turns off, the diode forward-biases, and current flows through the resistor and capacitor.
Snubber Design
Cornell Dubilier has设计Snubber网络的好指南。The guide has a quick design section with the very analog suggestion, “Plan on using a 2-watt carbon composition resistor.” The guide notes that wire-wound resistors will cause problems since they have higher inductance. Even a metal film resistor might have problems if trimming is done in a spiral shape that has inductance. Since this is analog, you can be confident a 2-W resistor isn’t appropriate for a 1-W wall-wart or a 10-kW power inverter. But it’s a nice place to start.
该指南通过快速设计方法运行，然后进行优化的设计，将电阻器额定值降低5倍，而Snubber电容器值则为3.5倍。这些收益是针对具有特定PCB布局的特定设计；您的一定会有所不同。
One valuable observation in the guide is that the switch node you’re trying to snub will have a characteristic impedance, just like a transmission line. You want to size the snubber resistor no larger than that characteristic impedance so that there’s no voltage transient when the transistor switch opens.
Cornell Dubilier also has a much更详细的指南设计阻尼器。该指南具有更多的数学，理论和图表。但是，请记住，如果您不建立，测试和测量您设计的Snubber电路，那么所有理论都是没有用的。
RC网络中的电阻器的一件好事是，您可以使用差分示波器探头对其进行测量，以查看电容器中也是瞬时电流。使用RCD网络，您必须在电容器引线周围进行当前的探测。对于表面安装设备，这将很棘手，但是您通常可以在电线环中砍入电路，以便在电路中获得当前的探针。这些都是快速信号，因此请确保具有足够带宽的示波器和探针，以测量真实的峰值电流和电压。
The Problem with Electrolytics
经验丰富的模拟工程师知道，电容器类型有很多差异。没有物理电容器可以完美地表示您的原理图或香料模拟中模型的符号。电解电容器的价值很高，而且便宜。使用RCD Snubber网络中的该二极管，您将能够解决电解电容器具有极性的问题，如果您使它们具有反向极性，它们将爆炸。
Despite this, electrolytic capacitors aren’t suitable for snubber networks since snubbers have very high peak currents that would self-heat and damage an electrolytic capacitor. Worse yet, electrolytic capacitors have terrible reliability, worse than most any passive or discrete component, with the possible exception of a potentiometer. This make electrolytic or tantalum capacitors a poor choice for snubber circuits.
Mica and Film Capacitors
A nice feature of the Cornell Dubilier guide is that the company is semi-agnostic about the type of capacitor you use. The guide initially recommends you look at a mica capacitor. Mica does get close to the perfect capacitor for a lot of parameters. Unfortunately, they only come in low values and tend to be expensive.
In lieu of mica capacitors, your snubber could use film capacitors. These are both wound foil film types and metallized film types(Fig. 4)。箔类型具有较高的峰值电流。当塑料膜从短路针孔融化时，胶片类型将在过电压后“治愈”。一些箔类型的铝箔厚度不同，以进一步增加当前能力，同时降低物理尺寸(Fig. 5)。聚丙烯膜是首选的，因为聚酯损失较高，因此不适合Snubber网络。
Dielectrics: Friend and Foe
尺寸较小和坚固，您可以考虑在Snubber网络中的陶瓷电容器。小心电容器的峰值电流等级，即使是瞬间，您将永远无法超过。陶瓷和金属化膜也将具有瞬时电压限制，可能低至50 V/NS。随着硅卡比德（SIC）和甲硝基（GAN）功率晶体管的更快切换时间，您应该确保不会超过Snubber电容器的短暂性承受能力。
Another problem with ceramic capacitors is that they can lose capacitance with temperature and applied voltage. This is a function of the dielectrics used by the companies. A C0G dielectric has very good temperature stability, but it’s only available in small values and costs significantly more than other dielectrics.
陶瓷电容器确实有效
Despite the peculiarities of ceramic capacitors, you can use them in snubbing networks as long as you understand their advantages and limitations.穆拉塔（Murata）描述不同的电介质如何适用于不同的冷静电容器。请记住，高容量介电的温度性能较差，因此它们的尺寸较小，电感较低的是需要更高的值才能在升高温度下工作。
The transistor manufacturers want you to properly snub your circuits. Accordingly, ROHM has关于使用胶片与陶瓷电容器的比较(Fig. 6)。Here they design around the voltage and value limitations of ceramic capacitors by putting two banks of five ceramic capacitors in series. These 10 capacitors give the voltage rating and value commensurate with a single film capacitor.
The results achieved by ROHM show how much better the ceramic capacitors damp ringing（图7）。Note that these results are for ROHM’s SiC transistors, which switch very fast and need the optimum snubber network. Also note that the company did not test at the elevated temperatures that power circuits always operate at. It’s your responsibility to ensure that the snubber works well at high temperatures when the ceramic capacitance value drops off.
You should also test for acoustic problems from ceramic capacitors. They work as both speakers and microphones. If your converter is operating under 20 kHz, the ceramic snubber capacitors might make objectionable noise. Be sure to find a young person to listen for the noise; anyone over 35 can’t hear above 15 kHz.
Made for Snubbing
AtAPEC 2019，TDK推广了CeraLink capacitorsfor power conversion. These ceramic capacitors use a lead-lanthanum-zirconium-titanium dielectric (PLZT). This dielectric works in a higher voltage and larger value niche compared to other ceramic types（图8）。CeraLink电容器电容会增加with applied voltage. They do share the same temperature problems as conventional ceramics. TDK notes this might be an advantage since they won’t current hog; that is, the hottest paralleled capacitor will not increase in value and take more current—until it burns up.
This current sharing is critical since the CeraLink capacitors come stacked in an even tighter configuration than conventional stacked MLCC capacitors（图9）。Intended for the dc link capacitor in switching converters, the CeraLink’s high ripple current capacity may also serve in your snubber circuits. TDK’sCeraLink documentation还男人tions the physical stresses a voltage pulse will put on a ceramic capacitor. If fast and high enough, it can fracture the capacitor. This is another reason you have to test and measure everything about your snubber design to ensure that it will work at the temperatures and times you need it to.
Pick Your Design Method
当我是一名汽车工程师时，普遍认为，通用汽车将设计首先有效的东西，然后该公司从设计中付出了成本，直到它运行得足够好为止。福特，我可以亲自证明，设计便宜，然后增加成本，直到它起作用为止。两种方法都有有效性。现在，我在多层电路板上进行原型的所有转换器设计。我可以在随后的旋转中取出成本。另外，您可能会在经过验证的胶片电容器责备中最好地设计设计，因为您知道可以使用较小的陶瓷电容器增加性能和成本。
It’s一个棘手的决定。The smaller capacitors mean you will have less stray inductance—the very thing that requires larger snubbers. If you can tighten up the entire design, especially with SiC and GaN designs, it might make sense to start with ceramics. Then again, if you need some space to get the heat out of the design, and that space gives room for a film capacitor, well, maybe that’s your best bet. “Maybe” and “analog” go hand-in-hand. Every design and every circuit is unique, as is the situation it’s used in. That’s what makes analog design so challenging, and so rewarding when you get it right.
