Abstract - For these years engineers have chosen electrolytic capacitor technology for use as the bus link capacitor on inverter designs. The main attraction has always been the low cost per farad associated with electrolytic capacitors.
This paper will present a practical mathematical approach on how to properly size a bus link capacitor for a high performance hard switched DC to AC inverter using film capacitors and will show how film capacitors are advantageous over electrolytic capacitors in terms of size, weight, lifetime, inverter efficiency and cost.
What’s the difference of Electrolytic Capacitor and Film capacitor ?
Electrolytic capacitors have been the workhorse technology for hard switched inverter bus link capacitors for many years. Until now, the greatest benefit in using electrolytic capacitors for bus link capacitors in inverters has been their cost. Electrolytic capacitors due however suffer from low ripple current capability due to high ESR and ESL, high thermal resistances limiting the ability to get the heat out, clumsy packaging sizes and mounting difficulties, weight, temperature range and limited lifetime.
Many high performance inverters such as electric vehicles, aircraft, and alternative energy to name a few are now turning to film capacitors because they do not have the limitations of electrolytic capacitors. Moreover, modern film capacitors not only perform better but can be a cost effective technology as well if applied correctly.
What’s the bus capacitance requires for inverters?
The first step in sizing capacitors for inverter bus link applications should be to understand how much bus link capacitance is required for a given inverter design. The biggest design limitation for electrolytic capacitors in inverter applications has been the amount of ripple current that the electrolytic capacitor can sustain. This limits the design criteria of the designer to figuring out how many individual capacitors are required for a given design rather than the total amount of capacitance that is required.
For example, let’s say for a given inverter, the bus link capacitor maximum ripple current requirement is 56 Arms. A 5,000uF / 450V electrolytic capacitor typically will only be able to sustain 28 Arms for a given package size. Therefore the designer must use two 5,000uF electrolytic capacitors totaling 10,000uF to meet the required 56 Arms capability. As will be seen further in this paper, 10,000uF is many times more capacitance then is required to meet the design performance for an inverter.
Having an excessive amount of bus link capacitance also presents some safety concerns. Once the inverter is powered down, a large amount of energy is stored in the bus link capacitor and this energy can be depending on the voltage, lethal if touched by an unsuspecting repair person. Most inverter designs with large amounts of capacitance add circuitry to discharge the bus link capacitors in a quick and safe manner upon power down so as not to present a safety concern. This of course adds complexity and cost to an inverter’s overall design.
Film capacitors do cost more per uF than electrolytic capacitors. It will be shown in this paper that the amount of capacitance needed for an inverter bus link capacitor design is much less for a film capacitor than an electrolytic capacitor since the film capacitor is not limited by ripple current rating like the electrolytic capacitor is. This is why when film capacitors are considered as direct uF replacements in an existing inverter design, the film capacitors will look much more expensive if you simply use the same amount of uF for film capacitors as you would with electrolytic capacitors.
The capacitor voltage rating must exceed the worst-case peak bus voltage as might arise under “high-line” mains conditions, maximum solar-panel output voltage, etc. Low-ESR aluminum electrolytic capacitors are rated only up to 500 Vdc, so they may need to be connected in series with balancing resistors. Film capacitors are rated to much higher voltages than aluminum electrolytic capacitors and generally do not require a series connection. Aluminum electrolytic capacitors are less expensive per unit of nameplate energy, but they don’t handle as much ripple current per unit of stored energy, so the ripple current handling needs to be investigated.
What are the factors influence the bus link capacitance value?
There are other factors that can affect and ultimately determine the bus link capacitance value. Some immediate examples are bus ripple voltage due to the AC to DC input rectifier circuit, regenerative energy, source impedance and load dynamics.
We will not and cannot address all the system affects on the bus link capacitor since they are unique to the application and depend on each specific design. We can provide however the opportunity to size the bus link capacitor by taking into consideration the effects of the bus voltage, load inductance and switching frequency.
We had presented a discussion on how to properly size a bus link capacitor for a high performance hard switched DC to AC inverter using film capacitors and has shown that film capacitors are advantageous over electrolytic capacitors in terms of size, weight, lifetime, inverter efficiency and cost.
The supporting equations to determine the capacitance and ripple current requirements for an inverter were shown to be based primarily on bus voltage, load inductance and inverter switching frequency. The film capacitor technology has been shown to be smaller, lighter, have longer life and be cost competitive compared to the electrolytic capacitor technology for high performance inverter applications. Design examples were presented showing the advantages of film capacitors for a general inverter design and also a windmill inverter application.
Although this paper does not attempt to address every system consideration when designing a high performance inverter, it has discussed how to effectively size film capacitors in terms of ripple current, bus ripple voltage and load inductance as capacitor performance factors in selecting bus link capacitors. The result as compared to an equivalent design using electrolytic capacitors shows film capacitors used for bus link capacitors in hard switched inverters as being superior in terms of size, weight, lifetime and cost.
