The Choices We Make

Arguing is easy—just look at our two political parties. Agreeing on the best part for a design project is harder: often, even after we’ve discovered precisely what specs a given component has and which characteristics are most important for the application at hand, the deciding differences can be marginally slim with the final outcome sometimes based on a factor or factors we didn’t even think of at the outset of the project.

Consider, for example, choosing between different types of capacitor for use in a solar inverter. By way of background, as the interface between solar panels and the mains grid a DC to AC inverter is a key factor in the operation of these alternate energy devices. Solar inverters efficiently convert the DC power generated by the panels into useable AC power. Inside the inverter an energy buffer absorbs the difference in power flow between a DC/DC converter handling the maximum power point tracking of the solar panel and the DC/AC converter that transfers this power to the mains grid.

The success of the inverter depends to a large extent on the capacitors selected for the energy buffer. Inverters require AC filtering (to reduce the harmonic components) and a DC-link supplying periodically high currents and handling a high frequency ripple signal. In addition to the DC-link position, there are other capacitors traditionally used in an inverter; a DC snubber capacitor is often added whose requirements include high peak current capability and extremely low inductance. Snubbers are simple energy absorbing circuits used to eliminate voltage spikes caused by circuit inductance when a switch opens Connected in parallel with semiconductor components its primary function is to damp dangerous voltage spikes locally. The most popular snubber circuit is a capacitor and a series resistor connected across a switch.

Modern inverter designs demand reliable DC-link capacitors that can be subjected to a main DC voltage accompanied by a high frequency ripple signal. The response to this high frequency signal and the existing peak voltage in the application determines the suitability of the capacitor. Other important parameters are low inductance, a large range of working temperature, long expected life time, a low dissipation factor and low ESR, high stability versus time for the capacitance value, the flexibility to adapt to the shape of the available space and low total cost.

Generally speaking, either a film or aluminum capacitor—two variations on the parallel plate capacitor can be used. How you go about selecting the right capacitor or capacitors, however, is not a simple matter. Film and aluminum capacitors have limitations that impact service life and reliability of the solar inverter. As noted, important parameters are component ambient temperature, operating voltage, ripple current and duration. It should be noted here that the purpose of this column is not to choose between one or the other, but to point out a typical situation where the engineer earns his or her paycheck by thoroughly understanding the selection factors and trade-offs involved in a not-necessarily-straightforward component selection.

Film capacitors used as the energy buffer in solar inverters most often consist of a winding of two layers of metalized polypropylene. The thickness of the polypropylene determines the voltage rating. The capacitance of a film capacitor doesn’t change significantly with temperature. Because of its construction current paths are short, resulting in low inductance and allowing its use over a wide frequency range of up to several MHz.

Aluminum capacitors are more complex and as the electrolyte interacts with the other materials in the aluminum capacitor the electrical characteristics change over time, leading to an increased failure rate after its end of life. For electrolytic caps, given that the typical maximum output voltages for solar panels is somewhere around 600V to 1000V, a pair of aluminum capacitors connected in series is needed to cover this voltage range. This would further require balancing the voltage by connecting resistors in parallel with each capacitor. Surge voltage is another consideration. Aluminum electrolytic capacitors can withstand surge voltages of only about 1.2 times the nominal voltage. Some solar systems will require using banks of several aluminum capacitors, not because a higher capacitance value is required, but simply to handle the current.

Capacitor lifetime is mainly determined by the applied ripple current and ambient temperature in normal operation mode. A commercial solar power implementation can be expected to work for 20 years or more with high ambient temperatures. One advantage of film capacitors over aluminum electrolytic designs is life expectancy. Unlike electrolytic capacitors, film capacitors cannot dry out and will therefore last longer. They are also less affected by temperature and also have the ability to overcome internal defects.

The latest dielectric films used for DC-filter capacitors are coated with a thin metallic layer. Should a defect occur, caused by, say, an impurity in the dielectric, the metal at that point in question evaporates and isolates the defect, effectively self-healing the capacitor. With the the weak point insulated the capacitor can continue working.

Still, film caps are vulnerable to failure such as a result of exceeding the repetitive rate of change of voltage (dV/dt). Metallized polyester snubber caps across switching semiconductors, for example, have been found to fail due to excessive dV/dt (in such cases the use of polypropylene, ceramic or foil film would have been preferable). End of life criteria is generally considered to be a decrease of capacitance value of 2%. However, this is a theoretical end of life, because, the capacitor can still be used beyond this point. If the application can allow 5% capacitance decrease, lifetime will be significantly increased.

One clear advantage aluminum capacitors have is lower cost. The same μF/ V capacitor in film technology costs several times more than its aluminum counterpart. Also weighing in in behalf of aluminum capacitors is the fact that they are definitely more space efficient than film capacitors. Again, a similar μF/ V aluminum capacitor requires about 15% of the volume of an equivalent film capacitor.

So there you are. Benefits and trade-offs on both sides. The point we are trying to make is that in many cases how you go about selecting the right part for a design is one that requires a knowledge of all aspects of the application environment, from mechanical to thermal to electrical. But then that’s why engineering is an interesting way to earn a living.