Snubber Capacitors Functionality and Selection Guide

Paul Rako at Electronic Design’s article describes snubber capacitors functionality to reduce the spikes in converter design, protecting the transistors and reducing EMI. The article also provides some recommendations on snubber capacitor type selection.

A snubber circuit limits voltage spikes in power converters. Snubbers are also used in the output of Class-D audio amplifiers for the same reasons (Fig. 1). When the transistor you use to switch the current turns off, it generates a high-voltage spike that could damage the device. This spike will also generate electromagnetic interference (EMI). Be aware that the current causes EMI in a loop. If your snubber network routes current in a large loop, it will increase rather than reduce EMI.

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 a good guide on designing snubber networks. The guide has a quick design section with the very analogue 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.

The guide runs through a quick design method and then does an optimized design that reduces the resistor power rating by a factor of 5, and the snubber capacitor value by a factor of 3.5. Those gains are for a specific design with a particular PCB layout; yours is sure to differ.

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 more significant than that characteristic impedance so that there’s no voltage transient when the transistor switch opens.

Cornell Dubilier also has a much more detailed guide on designing snubbers. That guide has a lot more math and theory and charts. Remember, though, all of the method is useless if you don’t build, test, and measure the snubber circuits you design.

A nice thing about the resistor in the RC network is that you can measure across it with a differential scope probe to see the instantaneous currents that are also in the capacitor. With an RCD network, you have to put a current probe around the capacitor lead. This will be tricky with surface-mount devices, but you can usually hack in a wire loop to get a current probe in the circuit. These are all fast signals, so be sure to have an oscilloscope and probes with sufficient bandwidth to measure the true peak currents and voltages.

The Problem with Electrolytics

Experienced analogue engineers know there are many differences in capacitor types. No solid capacitor is a perfect representation of the symbol of your schematics or the model in your Spice simulation. Aluminium electrolytic capacitors come in large values and are cheap. With that diode in your RCD snubber network, you will be able to get around the problem that electrolytic capacitors have a polarity, and they will explode if you subject them to reverse polarity.

Despite this, electrolytic capacitors aren’t best suitable for snubber networks since snubbers have very high peak currents that would self-heat and damage an electrolytic capacitor. Worse yet, aluminium wet electrolytic capacitors have terrible reliability, and conventional MnO2 tantalum capacitors are sensitivity to high power surges. This makes aluminium and tantalum electrolytic 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 manual 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.

Figure 4. Film capacitors can be either be a foil or metallized type. Foil types use a discrete layer of metal, whereas the metallized types have the metal plated on the film. (Courtesy of Wikimedia)

Instead of mica capacitors, your snubber could use film capacitors. These are both wound foil film types and metallized film types (Fig. 4). The foil types take higher peak currents. The film types will “heal” after an overvoltage when the plastic film just melts away from the short-circuit pinhole. Some foil types have varying thickness of the foil to increase current capacity further while reducing the physical size (Fig. 5). Polypropylene film is preferred since polyester has higher losses, making it unsuitable for snubber networks.

Figure 5. Some foil film capacitors use thicker foil near the electrodes to allow greater peak currents. (Courtesy of Wikimedia)

Dielectrics: Friend and Foe

With small size and ruggedness, you might consider ceramic capacitors in snubber networks. Be careful of the capacitor’s peak current rating, which you can never exceed, even momentarily. Ceramics and metallized film will also have transient-voltage limitations, perhaps as low as 50 V/ns. With the much faster switching times of silicon-carbide (SiC) and gallium-nitride (GaN) power transistors, you should be sure you’re not exceeding the transient withstanding capability of the snubber capacitor.

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 component type. A C0G dielectric has excellent temperature stability, but it’s only available in small values and costs significantly more than other dielectrics.

Ceramic Capacitors Do Work

Despite the peculiarities of ceramic capacitors, you can use them in snubbing networks as long as you understand their advantages and limitations. Murata describes how different dielectrics are suitable for different snubber capacitors. Just remember that high-capacity dielectrics have worse temperature performance, so their small size and low inductance comes at the cost of needing higher values to work at elevated temperatures.

The transistor manufacturers want you to snub your circuits properly. Accordingly, ROHM has a comparison of using film vs ceramic capacitors (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 ten capacitors give the voltage rating and value corresponding with a single film capacitor.

Figure 6. ROHM has experimented with both film and ceramic capacitors for its SiC power transistor snubbers. (Courtesy of ROHM)

The results achieved by ROHM show how much better the ceramic capacitors damp ringing (Fig. 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.

Figure 7. The red trace shows the better attenuation that ceramic capacitors give the ROHM snubber network from Figure 6. (Courtesy of ROHM)

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

At APEC 2019, TDK promoted its CeraLink capacitors for 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 (Fig. 8). CeraLink capacitors will increase in capacitance with applied voltage. They do share the same temperature problems as conventional ceramics. TDK notes this might be an advantage since they won’t get 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 (Fig. 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’s CeraLink documentation also mentions 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

When I was an auto engineer, it was generally agreed that GM would design things that worked first, and then the company took cost out of the design until it worked well enough. Ford, I can personally attest, designed things cheap, and then added cost until it worked. Both methods are valid. I now do all my converter designs on multilayer circuit boards for prototypes. I can take out the cost on a subsequent spin. Alternatively, you might feel best designing in proven film capacitor snubbers, knowing you can add performance and cost with smaller ceramic capacitors.

It’s a tricky decision. 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 “analogue” go hand-in-hand. Every design and every circuit is unique, as is the situation it’s used in. That’s what makes analogue design so challenging, and so rewarding when you get it right.

Source: Electronic Design