[SOLVED] Capacitors: bulk VS large, bypass VS smaller?

Hi,

In reading the DDR2 memory controller specification of a CPU I encounter the term “bulk capacitor” in addition to “bypass capacitors”. A related document Power Supply Design for PowerPC™ Processors, between page 7 and 13, basically give the following information on the two types of capacitors, but in the context of power supply, NOT DDR2 memory controller:




The concept of “bulk capacitor” seems similar to a typical parallel configuration of bypass capacitors: A big one, like 10µF, in parallel with a smaller one, like 0.1µF. Such configuration are usually described as have two benefits:

1. Large capacitance capacitors usually have larger ESR and inductance, and response slower than small capacitance capacitors. However, they stores more energy and could help to maintain the voltage of the system when the supply voltage temporarily falls.
2. Small capacitance capacitors doesn’t serve the above functionality much since their capacitance is too small; however, they responses fast and are good at shunting current (voltage) fluctuations to the ground, thereby contributing to a more stable power supply.

Although there are many similarities between the two pairs: (bulk & Large capacitance capacitors), (bypass & Small capacitance capacitors), I wonder whether the principle are actually the same? Does the rationale of (1) and (2) above applies also to “bulk capacitor” and “bypass capacitor” for

1. Power supply
2. Memory controller

?

Matt

I see bulk capacitors mainly as a part of the power supply, which is mostly a SMPS, e.g. a secondary buck converter. The requirements for the buck capacitor are layed by the switching frequency, output current waveform and acceptable voltage ripple of the SMPS. The SMPS can be expected to supply a clean DC voltage. The acceptable transient response on load steps also influences the bulk capacitor dimensioning.

FvM,

Thanks for the points on SMPS bulk capacitor. Do you have any comments on bulk and bypass converter for DDR2 and its controller?

Matt

Bypassing is both required for DDR2 power suppy (e.g. 1.8V) and reference voltage, if you utilize the latter. 1.8V will use a switched mode converter in most cases, for reference supply both linear and switched mode regulators are in use.

Bulk capacitor dimensioning is also affecting voltage regulator stability. For reference voltage supply, I would mainly follow the suggestion in the respective datasheet, that is already considering the specific requirements for RAM operation.

FvM,

I still need to study more, but thanks for explanation.

Matt