By John Benatti
Power supplies are employed in various applications. Many have standard load requirements that fall within the rated output current of the supply. In these cases, Vout x Iout will yield a sufficient power rating for the power supply selection. However, some applications will require a high peak load or Ipeak to initialize their operation. Examples of these are electric motors, pumps, fans, disk drives and other devices that need a start-up or peak current. Peak currents can be several orders of magnitude higher than the static (steady state) load. Even though an Ipeak duration tends to be relatively short, typically a few seconds or less, these periods can be problematic for many supplies. Manufacturers rate an acceptable amount of over current, shown as an Ilimit threshold, but peak currents will usually far exceed these limits. When this happens, the peak current will be inhibited by any of several types of current limit protection ranging from hiccup mode to total latching off. Before the evolution of peak rated power supplies, one solution was to overrate the power supply to the high peak load, but this solution unnecessarily increases the size and cost of the power supply.
Fortunately, PSUI carries many high peak load power supplies, such as the Tri-Mag DG160 series (see link for datasheet below)*. The Tri- Mag DG160 can handle high peak loads to 360 watts, while still operating as a traditional power supply. The only provisions of this type of supply are that the high peak load applied not exceed the specified peak load for the duration and duty cycle per the data sheet. This is essential to ensure that the total power, including peak power, does not exceed the nominally rated average output power of the supply. If a high peak load event is within the manufacturer's peak load ratings, then we can calculate how much power is available during the non-peak power period to drive the steady state operation.
Figure 1 illustrates this concept. The basic equation for a high peak load supply is α = [(Wm x T) - (Wp x t)] / (T - t)
α = the available non-peak power (W)
Wm = Maximum Average Output Power (W)
Wp = Peak Pulse Power (W)
T = Total Period (S)
t peak = Ipeak duration (S)
Using the Tri-Mag DG160 data sheet we can examine the example below.
Example: An application needs 300 watts of peak power for 3 seconds every 25 seconds and has static load of 100 watts. Can I use the Tri-Mag DG160? If so, how much available power is there for my static load.
Following the equation:
α = [(160 x 25) - (300 x 3)] / (25-3)
Therefore, 140.9 watts is available during the non-peak period of 22 seconds. This is below the average output power of 160 watts per the data sheet.
This satisfies both the peak load requirement and static load obligation. We can manipulate the high peak load equation to produce various peak, non- peak and duty cycle solutions.