A power supply with high peak-current capability can support loads that are higher than the nominal continuous power for short periods of time, without the unit shutting down or damage occurring. Typical constraints on this capability include time (duration of the current peak) and the percentage of time the supply must support the higher load (duty cycle).
Products often requiring high peak current include print heads, pumps, motors, and disk drives, found in abundance in factory automation, medical pumping systems, fluid and material handling, robotics, power tools, machining, packaging, test, dispensing systems, and printers.
Anticipated growth in these sectors indicates a growing need for power supplies that can support short-duration high-peak loads. Such units must also be environmentally friendly and efficient. There are significant benefits to utilizing power supplies capable of supporting high peak loads.
First, designers may use a smaller supply, reducing overall system size and weight. In a system needing 800 W for a short duration, a 400-W unit with an 800-W peak will generally mean a smaller footprint.
Second, the lower-power, high-peak unit will cost less. Again, if you need 400 W continuous but there is a short 800-W peak, rather than using a more expensive 800-W supply, a 400-W unit with an 800-W peak may be more cost-effective. To realize these benefits, it is important to define your system with respect to some variables: peak current, duration of peak current, frequency of peak current events (duty cycle), and anticipated power requirements during non-peak current demand (Fig. 1).
There are ways of estimating some of these values and deriving them based on the characteristics of the supply. For applications with peak current requirements, a simple evaluation of the proposed systems’ power budget can help minimize both the size and cost of the system. Properly profiling the power requirement will lead to an optimal solution.
However, there are potential pitfalls. Some supplies described as supporting high peak loads may not actually provide the anticipated benefits. Digging deeper into a manufacturer’s data sheet may be necessary to understand how the performance characteristics relate to the specific application.
PEAK LOAD CAPABILITY Typically, there are three methods for characterizing peak load capability. One common characterization entails testing a supply for a short duration (up to 30 seconds is not unusual) at a duty cycle of 10% to 15% and at a peak load just below the overcurrent protection (OCP) cutoff, usually 12% to 20% above the continuous current rating.
Essentially, this is an off-the-shelf supply, characterized to infer a safety margin or headroom over and above the nominal continuous rating. In practice, this may provide a false feeling of security. Although some applications may require an additional 12% to 20% of power for short durations, most motors, pumps, and print heads, for example, demand much higher current for a shorter duration.
A second practice characterizes the supply with a very high peak, up to 200% of nominal, but for such a short duration that the OCP circuit cannot detect or react to the overcurrent condition. Some data sheets specify peak-current handling based on what the power supply can withstand for 500 µs via this characterization scheme.
Based on this, a 300-W supply can operate at 600 W for 500 µs, but must operate at less than 300 W through a nonpeak period. While this is a common way to characterize power supplies, very few applications require 200% nominal power for such short durations.
The third characterization method specifies the peak rating at high-line, meaning with a greater than 180-V ac input. In this example, a 400-W supply may be able to provide 600 W of continuous power with an input voltage greater than 180 V ac—a real benefit when high-line ac input is available.
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