Every year, technology advances the capabilities of the PC. These advances can be seen either externally or functionally as display types, sound capability and operating speed. And they may be related to operating voltages or low-power modes of operation, making them somewhat invisible to the user. To maintain compliance and interoperability, designers in the PC industry must be aware of specifications such as ACPI, PS 98 and PC 98.
As PCs race down the technology highway, power management becomes more important. Every added feature increases the overall power consumption of the system. At the same time, specifications like Energy Star, White Swan and Blue Angel are trying to reduce it. One of the latest initiatives introduced to help manage system power consumption is the Vaux, the auxiliary voltage from a standby power supply. The Vaux requirement is referenced in the Microsoft PC 99, Intel PS 98, PCI Local Bus and PC Card specifications. Although currently only a recommendation, it will soon be a required implementation on many desktop systems.
From the most basic perspective, Vaux is an auxiliary 3.3-V rail for devices on the PCI bus that need to support wake-up events even when the rest of the system has been powered down. The application is simply to allow switching from a high-power 3.3-V rail to a low-power 3.3-V rail, optimizing system efficiency and power consumption. This functionality lets power supplies operate more efficiently by allowing the designer to effectively narrow the range of loads that the supplies must support. It also enhances the system's instant-on capability supporting the Intel and Microsoft Instantly Available Power-Managed PC and OnNow initiatives. The problem with implementing Vaux power management is that none of the specifications referencing the Vaux requirements discuss the implementation and not all of these specifications define Vaux in exactly the same manner.
The common section of the definition among all of the specifications is that the normal 3.3-V Vcc power rail and the 3.3-V Vaux power rails are independent and completely isolated. Under normal operation the device will use the standard 3.3-V power rail. When the system drops into a D3 low-power state, the system controller will transition all devices needing to support wake-up from the Vcc power rail to the Vaux power rail. After this transition is completed, the main power supply will shut down, taking the Vcc rail with it. At this point everything is powered by the Vaux rail. That allows the PC subsystems to maintain their register contents, which shortens the power-up sequence of the PC.
Until now there have not been many elegant ways to support both low-power modes and instant-on capabilities in desktop computers and servers. Many systems do support low-power states during idle-operation periods, but power savings are minimal. Supporting Vaux allows the desktop and server to support significantly lower power states. System support of Vaux is currently optional, but if a system chooses to support Vaux it must meet these requirements:
- 3.3-V Vaux must be connected to all PCI expansion slots;
- The system must be able to deliver 375 mA for Vaux per enabled PCI slot;
- The system must be able to deliver 20 mA for Vaux per disabled PCI slot;
- The system auxiliary power source must have enough capacity to support at least one enabled PCI slot when the PCI bus is in B3 or off.
Basically, D0 through D3 are the four power states defined for PCI functions, with D0 defined as on and D3 normally defined as off. Two D3 states are actually defined: D3Hot, off with Vcc still applied; and D3Cold, off with Vcc power removed. The intention of the Vaux auxiliary power source is to support wake-up from devices supporting power-management wake-up events (PME) from the D3Cold state. A device that supports PME-generated wake-ups informs the system of its Vaux power requirements using the Aux-Current register. This register allows a device to define its current consumption between 0 and 375 mA in approximately 50-mA increments, which further enables the overall system to optimize power management during low-power states.
The Intel Power Supply 98 specification defines many of the attributes of the PC power supply, one of which is a dual-mode 3.3-V power rail. This output provides power for both the main 3.3-V rail and the Vaux rail. By defining both rails separately, more-efficient supplies can be built to handle these two very different load requirements.
In desktop computers and severs, Vaux is supported on pin 14 A on the PCI Slot connectors for add-in cards. Since both the 3.3-V main and 3.3-V Vaux supplies are connected to the card, it is the responsibility of the card designers to keep the supplies electrically isolated from one another. If a design supports logic paths from Vaux to the main 3.3-V rail, care must be taken to ensure that devices are not damaged when the main 3.3-V supply is removed. The cost and responsibility of supporting Vaux has been pushed onto the card because not all cards will require Vaux support. This minimizes the cost impact to the system as well as the implementation difficulty.
One way to minimize the amount of circuitry needed to support Vaux while 3.3-V Vcc is still in use is with some type of device to switch between the two voltage rails on the card. This device could be some diodes, MOSFETS or even a relay or analog switch. Each of these solutions has both advantages and disadvantages and must be selected based on the specific requirements and limitations of the application. Of all the design hurdles in implementing Vaux support, one of the most critical might be maintaining the operating voltage on the output side of the switching circuitry during the transition.
Using some diodes to create an "OR" gate for the two power rails is by far the simplest solution, eliminating switching time constraints, current flow back across the diodes from one supply to another and the need for a digital interface to the system. The transition from one supply to another is completely automatic. This is also the solution with the greatest number of drawbacks. A typical diode will have approximately a 700-mV drop from cathode to anode; this might drop the supply rail to the Vaux-supported circuitry below its operation level. Since no digital-enable is possible, the Vaux-supported circuitry will receive power even if PME is not enabled. Although the simplest, this is not the ideal solution.
While using discrete MOSFETs is another simple solution, it too has both advantages and disadvantages when compared with the diode implementation. The main advantages are that a MOSFET-based circuit can be set up to use a signal from the system telling it when to transition from one rail to another, and the fact that MOSFETs tend to have low on-resistance values. A 200-mohms FET will only drop 100 mV when a 500-mA load is applied, which is a dramatic improvement over the diode-based solution. Unfortunately, a discrete MOSFET solution has disadvantages of its own. The first is that most discrete MOSFETs do not use TTL- or CMOS-level signals as their gate-drive voltages and therefore additional circuitry or very specific MOSFETs may need to be selected.
The most critical shortcoming is that almost all discrete MOSFETs have a back-gate diode connecting the drain to the source. The problem is that the MOSFET will freely conduct current backward across the device whenever the output is higher than the input. Since the two 3.3-V rails may not be at exactly the same potential, and even more importantly, since either of them may be shut down during operation, this limitation poses a severe risk to proper system operation. One way to correct this would be to place two MOSFETS back to back, effectively eliminating the diode from the current path.
Another key concern with the solution is the transition time the circuit displays when switching between power rails. If there is an excessive amount of time between one switch turning on and the other turning off, a large droop will be seen on the output side of the circuit. The circuit must also ensure that both MOSFETs are not on at the same time. Of all discrete solutions, this is one of the better ones.
Relays provide true switching capability and many have logic-level enables. Since both supplies never get connected together, they effectively isolate the supplies from one another. The problems with a relay-based solution are the inductive kick the many relays generate during switching and the spikes and surges that may be observed because of the rapid transition of which relays are capable. Another problem that may be encountered with a relay solution is the inherent voltage drop. The contact resistance of a good relay is typically in the range of 1 to 10 ohms, while some may be much higher. Using the 500 mA from the previous example, a 10-ohms relay would drop 500 mV from the supply rail under normal operation. In Vaux mode where current demands are smaller, this resistance becomes less of an issue.
Several companies are releasing devices suited to the Vaux application, some targeting desktops and servers while others target notebook applications. The devices share many common features demanded by the Vaux applications. These include:
- Low voltage drop under full load;
- Controlled switching/transition speeds;
- Maintaining isolation between the supplies;
- Low quiescent-current devices.
Another question in selecting the appropriate device to handle the Vcc/Vaux switching is, exactly what size and type of load does it have to switch? By providing the appropriate hold-up capacitor on the output side of the device, the application will be able to maintain a tighter tolerance on the power-supply rails on both sides of the switching device.
Maximizing run-time in notebook computers is an ongoing and never-ending effort. Notebook computers support several power-down modes and standby conditions. Vaux support was added, on top of all these efforts, by proposal 194 to the PC Card specification. This proposal was accepted and approved by the PCMCIA committee and will be released in the PC Card specification next printing.
Basically, proposal 194 is broken into two distinct sections, defining the Vaux requirements for the PCI-to-Cardbus bridges and the PC Card sockets separately. As in the desktop and server applications, this is a functionality that is supported from D3Cold. In the PCI-to-Cardbus bridge section of the specification, the Vaux power consumption of the PCI controller is defined as a maximum of 10 microamps, but is allowed current peaks of up to 5 mA for 15 ms for recognition of card insertion and removal events. However, the power consumption of a PC Card socket from the Vaux power rail is a significantly larger amount of current, with a maximum of 200 mA per supported socket. In notebooks, as in desktops and servers, if the controller and card sockets must support a wake-up event from D3Cold, PME must be enabled, and the controller and sockets must transition from the 3.3-V Vcc power rail over to the 3.3-V Vaux power rail.
And, while most of the specifications that contain Vaux requirements are PC-related, this trend is spreading from the PC industry into television and other major power-consuming devices.