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24 July 2014

How Many Channels is Enough?

Figure 1: A switch-mode power supply driving a fixed load
can be designed and optimized specifically for that load.
The bulk of oscilloscope applications are well served by instruments with four analog input channels. Most basic debugging and design-related work involves probing of only one signal at a given time, and occasionally more than one, especially when differential signals are concerned. Thus, many users may never see a need for an oscilloscope with more than four channels.

Having said that, there are some applications that by their very nature surpass four channels. Moreover, some of these applications concern circuits and devices that are produced in extremely high volumes. A case in point is switch-mode power supplies, such as those typically found in notebook PCs, tablets, or embedded systems.

Figure 2: When power-supply loads vary, so does
efficiency, a scenario common in computing tasks.
Consider the basic embedded switch-mode power supply. A simplified implementation might require only a single-phase current controller such as ON Semiconductor's
NCP81141, a device with an Serial VID (SVID) interface for desktop and notebook CPU applications. SVID, by the way, is the communications protocol  Intel concocted for its VR12/VR12.5 specification for PWM control.

In a fixed-load scenario, a switch-mode power supply may be purpose-built for the specific application and thus be highly efficient, on the order of >90% (Figure 1). A device like the NCP81141 would be employed to regulate the current delivered to the supply while maintaining a constant voltage.

Figure 3: A multi-phase switch-mode controller dynamically
switches in more phases are required to service
a variable load.
However, in computing and other embedded applications, the load is anything but fixed, but rather is widely variable (Figure 2). Power control devices have a tough time optimizing the supply's efficiency with large load variations. The "sweet spot" for maximum efficiency is now a moving target. When the load changes and efficiency varies, the result is increased heating and stress on the power supply's components. Moreover, battery performance is compromised in portable systems.

The answer to this is a multi-phase controller such as ON Semiconductor's NCP81140, which dynamically switches multiple phases in (or out) depending on load changes (Figure 3). The response from such a device to changing loads is very fast, whether it is adding phases to shore up current to increasing loads, or shedding them when a single phase is able to keep up with the power requirements. In this fashion, high efficiency is maintained across the load spectrum with a huge corresponding reduction in heat stress on the supply's components. Moreover, the circuit is scalable in terms of the number of phases and the output per phase.

The debugging of a multi-phase switch-mode power supply design would be difficult to undertake
Figure 4: Proper debugging of this four-phase power supply
circuit requires six oscilloscope channels
with the typical four-channel oscilloscope. The circuitry is considerably more complex than that of a single-phase power supply (Figure 4). In this case, debugging would call for six probes: four for monitoring the output currents of the four phases, and two more to monitor the overall voltage and current.

It gets even more complex when the power supply is for a server. Such power supplies call for six phases, so you'd be looking at eight channels. There are many other embedded applications for multi-phase switch-mode power supplies as well.

One example of an oscilloscope well suited for an application of this nature is Teledyne LeCroy's HDO8000 series, which sports eight analog input channels as well as 16 digital channels as an option.

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