MMC's and Their Small Lead Size
Chris Maness, Thu Jul 16 2015, 02:45PMI have calculated I need 25 Cornell-Dubillier 942C20P15K-F's in series. That seems like a lot of small leads in series with a lot of added resistance and uncoupled induction. I am concerned about a reduction in performance to the point where I would just be better off sticking with my salt water caps. I guess if I routed my caps in a serpentine fashion the self inductance would cancel.
Any thoughts?
Thanks,
Chris
Re: MMC's and Their Small Lead Size
Wolfram, Thu Jul 16 2015, 03:10PM
The capacitor leads are usually made from copper, which is over two million times as conductive as saturated salt water solution, so even if they are quite thin, resistance will be a lot lower. Still, you want to keep the parasitic resistance as low as possible, so make the leads as short as practical.
It is important to minimize the inductance. Anything that will reduce the area of the current loop will help. Arranging the caps in a serpentine fashion like you suggested is a nice approach.
Keep in mind that one terminal of each capacitor is connected to a foil that sits just inside the outer plastic foil of the capacitor. Therefore, it is important to keep some distance between caps and metal objects, and also between capacitors and other components, like bleeder resistors or other capacitors in the string. Lastly, the MMC design should allow some free airflow over the caps so that they can get rid of the dissipated heat.
Wolfram, Thu Jul 16 2015, 03:10PM
The capacitor leads are usually made from copper, which is over two million times as conductive as saturated salt water solution, so even if they are quite thin, resistance will be a lot lower. Still, you want to keep the parasitic resistance as low as possible, so make the leads as short as practical.
It is important to minimize the inductance. Anything that will reduce the area of the current loop will help. Arranging the caps in a serpentine fashion like you suggested is a nice approach.
Keep in mind that one terminal of each capacitor is connected to a foil that sits just inside the outer plastic foil of the capacitor. Therefore, it is important to keep some distance between caps and metal objects, and also between capacitors and other components, like bleeder resistors or other capacitors in the string. Lastly, the MMC design should allow some free airflow over the caps so that they can get rid of the dissipated heat.
Re: MMC's and Their Small Lead Size
Ash Small, Thu Jul 16 2015, 03:46PM
There comes a point where using several parallel strings of smaller value capacitors makes sense, as the leads are normally the same size. This can considerably reduce the ESR of the MMC.
It can also improve the parasitics, I think.
Ash Small, Thu Jul 16 2015, 03:46PM
There comes a point where using several parallel strings of smaller value capacitors makes sense, as the leads are normally the same size. This can considerably reduce the ESR of the MMC.
It can also improve the parasitics, I think.
Re: MMC's and Their Small Lead Size
Mads Barnkob, Thu Jul 16 2015, 03:53PM
Approximately 2/3 generated heat rise moves out axial and 1/3 radial. So it is most important to cool a capacitor at its terminals as it does not radiate the heat evenly from all over its surface.
The thermal resistance (Rth) from case to ambient is given for still air in most datasheets, so if forced air cooling is used the thermal resistance can be de-rated. Some manufacturers supply equations to calculate a exact thermal resistance in regard to capacitor surface and forced air speed velocity.
As a rule of thumb ESL is about 1.6 nH per millimetre of lead distance between the capacitor itself and the rest of the circuit. This also includes the leads of the capacitor itself. This only applies to well designed capacitors.
Mads Barnkob, Thu Jul 16 2015, 03:53PM
Wolfram wrote ...
Lastly, the MMC design should allow some free airflow over the caps so that they can get rid of the dissipated heat.
Lastly, the MMC design should allow some free airflow over the caps so that they can get rid of the dissipated heat.
Approximately 2/3 generated heat rise moves out axial and 1/3 radial. So it is most important to cool a capacitor at its terminals as it does not radiate the heat evenly from all over its surface.
The thermal resistance (Rth) from case to ambient is given for still air in most datasheets, so if forced air cooling is used the thermal resistance can be de-rated. Some manufacturers supply equations to calculate a exact thermal resistance in regard to capacitor surface and forced air speed velocity.
As a rule of thumb ESL is about 1.6 nH per millimetre of lead distance between the capacitor itself and the rest of the circuit. This also includes the leads of the capacitor itself. This only applies to well designed capacitors.
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