Secondary Power Supplies
Ecos Consulting and the Electric Power Research Institute (EPRI) have been funded by the California Energy Commission’s Public Interest Energy Research (PIER) program to measure the energy efficiency of secondary power supplies and determine whether opportunities exist to cost-effectively improve their efficiency. Through 2007 and 2008, the organizations will develop a secondary power supply efficiency test procedure and will use the procedure to measure the efficiency of a wide variety of secondary power supplies found in common products such as computers and televisions.
What are secondary power supplies?
All electronic products require primary power supplies that convert ac power from the grid to dc power used in electronic circuitry. Some products require secondary power supplies that convert dc electricity from one voltage to another. For example, secondary power supplies inside computers convert dc power from the primary power supply from 12 Vdc to lower voltage dc power used by components on the motherboard, such as the CPU and memory. Our discussion here will concentrate on secondary power supplies in desktop computers, but secondary power supplies are also found in other electronics products, such as televisions, set top boxes, and telecommunications equipment.
In computers, secondary power supplies are distributed across the motherboard and in general are located close to the circuits they power. Secondary power supplies used in computers can be further categorized into the following application-specific types, described in greater detail below:
- Point-of-Load (POL) Converter
- Voltage Regulator Module (VRM)
- Voltage Regulator Down (VRD)
What are POL Converters, VRMs, and VRDs?
A Point-of-Load Converter (POL) is a term used to describe a secondary power supply that provides power to circuits on the motherboard other than the CPU, such as memory or on-board graphics processor. A POL converter can have a module form or, more commonly, can be directly mounted on the printed circuit board (embedded).
A Voltage Regulator Module (VRM) is a secondary power supply that provides power to the main processor (CPU). VRMs are significantly more complex than POLs, as they often consist of several converters in parallel and have special controls that respond to signals from the CPU. VRMs have modular form factors and are sold separately as stand-alone products for integration into any computer motherboard design. A VRM is plugged into the motherboard using either an edge connector or solder connection.
A Voltage Regulator Down (VRD) is similar in function and performance to a VRM, but consists of discrete components connected directly to the motherboard, rather than installed on a separate circuit board. Manufacturers appear to be moving away from VRMs towards VRDs as a cost saving measure. The VRD can be found near the CPU on a motherboard.
The output voltage from a VRM or VRD is programmed by the CPU using a Voltage Identification Code (VID). Other secondary power supplies, such as POL converters, do not have this feature. VRM and VRD voltage and power requirements will vary according to the needs of different computer systems.
In most computers with motherboard-based graphics processors, approximately 85% of the motherboard power is consumed by the VRM/VRD – exclusively for the CPU.