The Role of Capacitors in Computer Power Supplies
Introduction
This small note is for those that like to modify their motherboards to improve performance or reliability. Hopefully it will improve understanding.
Definitions
Computer power supplies including the VRM (for CPU) are all closed loop switching voltage regulators. At the output of each an electrolytic capacitor plays some important roles in conjunction with an output filter inductance.
The output inductance carries out the role of current smoothing and this assists with ripple voltage reduction. The output capacitor in the case of a VRM is usually a number of capacitors connected in parallel.
Electrolytic output capacitor roles.
It is clear that capacitor ESR plays an important role dealing with the high frequencies outside of the regulator response time. There are harmonics of the switching frequency that are higher than an electrolytic can handle with ESR. Fortunately very little, if any, gets past the output inductance.
It is unlikely that ceramic capacitors will be found in a typical computer power supply or VRM because the power supply filtering method is sufficient. Ceramic capacitors are found on the supply lines at the CPU; they are applied to supply the very fast supply transient needs of the CPU and must be placed very close to the CPU socket to avoid ESL problems.
What the impacts of replacing capacitors?
Ratio of Load to ESR example.
CPU power is 60 watts at 1.6 volts (V core). Calculated load resistance E2/W = 42.6 milliohm
Six Capacitors in parallel @ 15 milliohm each = 2.5 milliohm
A ratio of 17: 1
This means the load will receive 5.88% of the available supply ripple.
Now change the capacitors to 9 milliohm each. Result is 1.5 milliohm
The ratio is now 28:1
Load now receives 3.57% ripple.
The absolute value of the ripple voltage available depends on the power supply components and switch frequency.
What about adding ceramic capacitors?
Great care must be taken in that ceramic capacitors have a very low ESR and if any ESL is present in the motherboard traces a high Q circuit can be established eg the power line from the VRM to the CPU can create a very small ESL. With a ceramic at the CPU there is no problem, however if a ceramic is connected at the output of the VRM (in parallel with an electrolytic capacitor) a high Q circuit can be formed by the ESL between the two ceramics, this high Q will magnify a small very high frequency signal of the same frequency as the resonant frequency formed by these components. This does not happen with electrolytic capacitors at the output because at these very high frequencies they are lossy (a low Q factor).
Motherboard manufacturers can control the value of stray capacitance and ESL to suit a particular need. Most often they try to minimize both. Sometimes ESL may be introduced to improve current sharing in parallel connected devices.
Adding ceramics to the output of the main computer power supply has been tried and found to produce spikes. Here a high Q capacitor is reacting with inductance and there is plenty in the power supply leads.
If improvement is to be tried the best place is at the CPU. An increase in ceramic capacitance may help.
Closing comment
All the above relates to the current and common types of switching power regulators in computers. New technology may emerge that operates at very high switching speeds (Mhz range). The designers claim that response is so fast that large electrolytic bulk storage capacitors are not needed, ceramics will be used.
Introduction
This small note is for those that like to modify their motherboards to improve performance or reliability. Hopefully it will improve understanding.
Definitions
- VRM = Voltage Regulator Module
- ESR = Equivalent Series Resistance
- ESL = Equivalent Series Inductance
- CPU = Central Processor Unit
- Mhz = millions of cycles per second
Computer power supplies including the VRM (for CPU) are all closed loop switching voltage regulators. At the output of each an electrolytic capacitor plays some important roles in conjunction with an output filter inductance.
The output inductance carries out the role of current smoothing and this assists with ripple voltage reduction. The output capacitor in the case of a VRM is usually a number of capacitors connected in parallel.
Electrolytic output capacitor roles.
- The capacitor/s function as part of the regulation of the DC output in that they affect the regulator response time. These regulators respond relatively slowly to change (in milliseconds) this is largely controlled by the value of output capacitance. With no output capacitance there would be many problems (even with no load) including a fast response that would create instability. Increasing the capacitance will lengthen the regulator response time. In this role the capacitor helps maintain stability of regulation.
- The capacitance is an energy store (bulk storage) that delivers transient energy to the load where the speed required to respond is outside the capability of the regulator. Later the regulator tops up the capacitor. ESR is important in that a high ESR will cause an excessive voltage drop during a transient load.
- The regulator does not respond to the switching speeds of the power MOSFETs. The output capacitance together with the series inductance form a filter to remove ripple frequencies. ESR is important in bypassing the ripple current so that ripple at the load is minimal.
It is clear that capacitor ESR plays an important role dealing with the high frequencies outside of the regulator response time. There are harmonics of the switching frequency that are higher than an electrolytic can handle with ESR. Fortunately very little, if any, gets past the output inductance.
It is unlikely that ceramic capacitors will be found in a typical computer power supply or VRM because the power supply filtering method is sufficient. Ceramic capacitors are found on the supply lines at the CPU; they are applied to supply the very fast supply transient needs of the CPU and must be placed very close to the CPU socket to avoid ESL problems.
What the impacts of replacing capacitors?
- Increasing the value of output capacitance will slightly slow the regulator response and will improve the energy storage.
- Using capacitors with lower ESR. This will not normally affect the regulator response or stability. Here it should be understood that ESR plays important roles.
- It must be lower than the load resistance and to draw the bulk of ripple current.
- Being a resistance it acts as a means of removing the ripple energy in terms of heat. If the ESR was reduced to zero there would be no way of removing the energy, if this were possible a near lossless output filter would have what is termed a high Q factor and damped oscillation is likely to occur (the ripple energy needs to be absorbed), this would be at a frequency outside of the regulator control. It is therefore possible that if ESR is reduced to a very low value voltage spikes will develop on the regulator output. This would be critical in a VRM. Output capacitance total ESR should remain at an effective ratio to the load resistance. See the following Ratio of Load to ESR example.
Ratio of Load to ESR example.
CPU power is 60 watts at 1.6 volts (V core). Calculated load resistance E2/W = 42.6 milliohm
Six Capacitors in parallel @ 15 milliohm each = 2.5 milliohm
A ratio of 17: 1
This means the load will receive 5.88% of the available supply ripple.
Now change the capacitors to 9 milliohm each. Result is 1.5 milliohm
The ratio is now 28:1
Load now receives 3.57% ripple.
The absolute value of the ripple voltage available depends on the power supply components and switch frequency.
What about adding ceramic capacitors?
Great care must be taken in that ceramic capacitors have a very low ESR and if any ESL is present in the motherboard traces a high Q circuit can be established eg the power line from the VRM to the CPU can create a very small ESL. With a ceramic at the CPU there is no problem, however if a ceramic is connected at the output of the VRM (in parallel with an electrolytic capacitor) a high Q circuit can be formed by the ESL between the two ceramics, this high Q will magnify a small very high frequency signal of the same frequency as the resonant frequency formed by these components. This does not happen with electrolytic capacitors at the output because at these very high frequencies they are lossy (a low Q factor).
Motherboard manufacturers can control the value of stray capacitance and ESL to suit a particular need. Most often they try to minimize both. Sometimes ESL may be introduced to improve current sharing in parallel connected devices.
Adding ceramics to the output of the main computer power supply has been tried and found to produce spikes. Here a high Q capacitor is reacting with inductance and there is plenty in the power supply leads.
If improvement is to be tried the best place is at the CPU. An increase in ceramic capacitance may help.
Closing comment
All the above relates to the current and common types of switching power regulators in computers. New technology may emerge that operates at very high switching speeds (Mhz range). The designers claim that response is so fast that large electrolytic bulk storage capacitors are not needed, ceramics will be used.
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