An introduction to snubber circuits and snubber capacitors

When an electrical or mechanical switching device opens, the inductance in an electronic circuit can cause voltages spikes. These voltage spikes can significantly affect the performance of a circuit, and it is necessary to suppress them. Snubbers are used in a broad range of electronic circuits to absorb energy and suppress such voltage spikes. These circuits are also commonly used to prevent oscillation and ringing effects in circuits. In this article, we will explore some common types of snubbers and the importance of capacitors in these circuits. 

Implementation of snubber circuits

Semiconductor devices are commonly used in today’s power electronic circuits for switching purposes. In such circuits, snubbers are placed in parallel with these devices to improve the overall performance of a circuit.  A simple snubber can be implemented by combining a capacitor and a resistor (resistor-capacitor network). For some applications, complex passive or active snubbers are required.

Although snubbers are mostly used for suppressing voltage and current spikes, they are also used for limiting dV/dt and dI/dt and minimizing EMI in circuits. Snubbers are also commonly used in circuits for transferring power dissipation to the desired load and maintaining them within the safe operating area. These circuits also help to reduce the overall losses caused by switching operations.

Snubbers are widely used in power circuits such as power converters and motor drives, and we will consider various waveforms which are common in such circuits. Before we start exploring some of the common types of snubbers and how they are implemented, it is important we understand how voltage and current spikes are generated in circuits.

Although there are different types of power circuits, nearly all of them are based on common networks and their waveforms have similar characteristics. The most common topologies in today’s power circuits include buck, boost, and buck-boost networks. For power circuits with the same switch-diode-inductor network, it is easy to analyze the characteristics of one circuit, design a snubber and use the same design in all circuits. Designing a snubber involves analyzing the behavior of a circuit during the short switch transition time.  Figures 1-3 show some of the most common converter topologies. 

Snubber circuits can be broadly categorized into active or passive networks. Passive snubbers are based on passive components (resistors, capacitors, inductors, and diodes) while active snubbers have active components such as transistors. For this article, we will focus on two of the most common passive snubbers: resistor-capacitor (RC) snubbers and resistor-capacitor-diode (RCD) snubbers. 

There are two categories of passive snubbers: lossy and non-lossy snubbers. Lossy snubbers consume power, and they are unsuitable for use in circuits where high efficiency is desired. On the other hand, ideal non-lossy snubbers do not consume power, and they are ideal for applications that demand high efficiency. However, practical non-lossy snubbers dissipate a small amount of power. Unlike lossy snubbers, non-lossy snubbers are complex and harder to design. 

Capacitors are essential components in snubber circuits, and one of the key factors to consider when selecting a component for use in your circuit is its peak current capability. Mica capacitors can tolerate high peak currents, and this is the reason why they are widely used in snubbers. However, these components have low capacitance values, and they are therefore unsuitable for applications that demand high capacitance values, typically more than 0.01 μF.

Polypropylene film/foil capacitors have an impressive peak current capability and are available in high capacitance values. This makes them a suitable option for snubber circuits where high capacitance values are desired. Polypropylene film/foil capacitors are also suitable for applications that demand high voltage and high capacitance values. 

Whereas metallized film capacitors have impressive electrical characteristics, they are rarely used in snubbers because of their limited peak current capability. In addition, these components have low transient withstanding capability. Use of high-k ceramic capacitors in snubbers is limited because of the same reasons. In addition, the capacitance of a typical high-k ceramic capacitor changes significantly with variation in temperature.

When selecting a resistor for your snubber, it is vital to consider the parasitic inductance of a component. Wirebound resistors are inductive and this makes them unsuitable for most snubber applications. Comparatively, carbon composition resistors such as carbon film resistors have very low parasitic inductances, and this makes them an ideal choice for snubber applications.  

Resistor-capacitor (RC) snubbers

For a typical switching circuit, the output is a DC voltage and there are minimal changes during the short switch transition time. This means that we can replace the combination of a filter capacitor and a load with a battery. Similarly, since the change in current during this period is minimal, we can use a current source to replace the inductor. With these assumptions, we can represent the circuit of a typical boost converter with a simplified circuit such as the one shown in Figure 4 above.  

There are two methods that are commonly used in the implementation of snubber circuits. The first approach involves using simple design techniques to select suitable components for the snubber circuit. This approach does not yield precise values of resistance and capacitance. The second approach entails using a more complex technique to determine precise resistance and capacitance values for the circuit.

The simplified representation shown in Figure 4 assumes that ideal components and board are used and ignores stray capacitive and inductive components. In the case of practical circuits, it is necessary to consider parasitic capacitance and inductance because the two are unavoidable. Circuit mounting, circuit layout and junction capacitances are the main contributors of stray capacitance. 

On the other hand, the parasitic inductance is mainly contributed by the layout of the circuit and the leads of the components used. These stray components (parasitic inductance and parasitic capacitance) can be minimized by using good circuit layout practices. Adding the two stray components to our simplified circuit yields the circuit shown in Figure 5.

Simple RC Snubbers

This method entails using a simple technique to estimate values of resistance and capacitance to use in a snubber circuit. The method is widely used in circuits where power dissipation is not a factor of critical importance. 

In order to dump the voltage spikes, a capacitor (Cs) with a capacitance value that is greater than the stray capacitance is required. It is common for circuit designers to use a component with a capacitance value that is twice that of the combined stray capacitance of the circuit.

For the resistor, a component with a value of resistance that ensures that the same current continues to flow even when the switch opens is required. To ensure that current flows through the component without requiring a voltage overshoot, the resistance value of the component must be less than or equal to E1/I1. The amount of energy dissipated by this resistor is independent of the component’s resistance. A typical RC snubber is shown in Figure 6 above.

Optimized RC Snubbers

This approach is mostly used in cases where the power dissipated by a circuit is a factor of critical importance. This method helps to minimize the peak voltage, and entails using I1,E1,  Lpvalues to determine the parameters that yield optimum performance. One of the most common approaches involves obtaining the optimized values from performance curves. 

To start with, the values of I1,E1, Lp are determined and a maximum peak voltage chosen. A normalized peak switch voltage is then calculated by dividing peak switch voltage (E) by E1. Curves are then plotted from which the values of initial current factor and damping factor are determined. The optimized capacitance and resistance parameters are calculated using the initial current factor and damping factor.

Resistor-capacitor-diode (RCD) snubbers

As compared to a typical RC snubber, this snubber is highly effective in reducing the overall loss of a circuit. Apart from its impressive performance in minimizing snubber and switching losses, the RCD snubber also achieves better load lines.  A typical RCD snubber is shown in Figure 7 above. The RCD snubber is commonly used in circuits where better performance is desired.

Conclusion 

A snubber is used in a circuit to eliminate harmful voltage spikes by providing an alternative current path. These spikes are caused by the inductance of a circuit when a mechanical or semiconductor switch is opened. In power circuits, snubbers are used for a broad array of applications including maintaining semiconductor switching devices within their safe operating areas, removing energy from semiconductor switches, reducing EMI, and minimizing ringing. Using a snubber helps to improve reliability, efficiency, and switching frequency of a circuit. Mica and polypropylene capacitors have excellent peak current capabilities, and they are widely used in today’s snubbers.