Thermal Management Application Notes
This application note primarily addresses thermal issues as they relate to polypropylene film capacitors, however the same concepts apply to other capacitor types as well. Because thermal environments vary considerably our approach is to provide RMS voltage versus frequency performance curves (see specific data sheet for details) based on the following extreme parameters:
- +85°C ambient temperature;
- Pure convection cooling;
- All heat dissipation from the lead wire is added to the capacitor;
- Increased lead wire dissipation at high frequencies is included [skin effect losses];
- “Worst-case” capacitor material parameters are assumed.
The ambient temperature is the air temperature adjacent to the capacitor within the application enclosure at the highest exterior temperature permitted by the end user. Since the maximum allowable temperature for polypropylene is +105°C, our performance curves are based on a power level that raises the temperature +20°C from ambient to the hot spot.
Warning: Before comparing capacitor performance curves between suppliers it is critical to ensure that they represent similar conditions!
We realize that the environment assumed by our performance curves is highly unlikely to represent that of a real application. We also understand that a “pure convection” environment is also highly improbable. There will almost always be a circuit board and other adjacent components that will impede convection based air circulation, and the capacitor under discussion will not be the only “heat generating” device present! Because of all this “variability” our performance curves are calculated very conservatively. Therefore you can always reliably specify a capacitor for your application if:
- The AC voltage across the capacitor is less than that indicated by our published curves;
- There are no adjacent “really hot” parts;
- The capacitor’s ambient environment is less than +85°C;
- Air can circulate by means of some natural convection.
In MANY cases a capacitor can be operated reliably above the values shown in our performance curves since our performance data is calculated under “worst-case/extreme” thermal conditions. The trick is to remove heat from the capacitor by means other than convective air motion. Most of the following suggestions are relatively easy and inexpensive to implement if considered early on in the design cycle.
- Reduce the ambient temperature (if possible);
- Separate hot(ter) components from the capacitor if electrical/mechanical environment permits. (we understand loop inductance, creepage/clearance requirements, and mechanical “keep away’s”);
- Minimize the I2R heat added to the capacitor by the board etch or other connection methods;
- Size the pads around the capacitor vias as large as possible to remove heat from the leads. Add as much extra copper around the capacitor pads as layout and design rules permit;
- Specify larger diameter copper capacitor leads (works even better when copper added at vias);
- Forced air helps, but there are some cautions. Case to hot spot rise is fixed by power level!
- Heat can be removed to a “heat sink”, even if by hot glue to a cooler adjacent object or the board.
The application power level can be compared to the performance curves at a given frequency by the following calculation:
(Actual VAC/Rated VAC)² = Power level relative to performance curve power
Temperature rise is directly related to power; power is related to the voltage squared.
A final thought. It may be tempting to obtain sample capacitors (from ANY supplier) and make temperature rise measurements; making choices based on the results. The temperature rise measurements are certainly useful information (and highly recommended for tough applications) but this method does not take into account the capacitor material parameter variation over which we (and other capacitor suppliers) have no control.
Please contact us for more detailed information and methods to estimate allowable voltage in your electrical/thermal environment. We always welcome an engineer-to-engineer discussion of your particular situation!