Peak power ratings with convection and conduction cooled power supplies

Many open frame power supplies now offer two power ratings, one with convection or conduction cooling and one where external forced air is applied.  As some applications require low acoustic noise levels, for example in healthcare where patient comfort is important, fan-less cooling is preferred.  This can be achieved through operating a power supply at its convection rating, and if it is also capable of conduction cooling, a combination of both.

If the output load significantly varies with time it may be possible to utilize a power supply with a peak power rating.  This allows a lower power product to be selected, saving cost and reducing its size, as the convection cooled peak power rating is often the same as the forced air continuous rating. 

For example, modern hospital beds are fitted with electric motors that can adjust the angle of the bed to allow the patient to sit upright, or raise the lower portion to elevate the legs. These motors may only operate for short periods of time. The same will apply for dental chairs, thermal warmers for babies and incubators.

Power supply manufacturers will state the maximum peak power level and the length of time it can be drawn.  The datasheet or application notes indicate if the peak power rating is calculated using RMS or average power. Alternatively, examples may be given in the application notes or instruction manual with a list of components to be monitored and their maximum temperatures.

TDK-Lambda’s CUS600M power supply (Figure 1) has a convection cooled and RMS peak rating of 400W. It can deliver 600W peak for a maximum continuous duration of 10 seconds.

Figure 1: TDK-Lambda’s CUS600M power supply 

To determine the RMS power of a repetitive peak load, we shall use the example shown in Figure 2. OPa = minimum output power drawn, OPb = maximum output power, t = peak load time in seconds and T = quiescent load time in seconds.

 

Figure 2: Repetitive peak power load example

To calculate the RMS power we use equation 1.

Equation 1:  RMS Power = √((OPb² x t + OPa²xT)/(t + T))

It OPb=600W, OPa=100W, t=10s and T=20s, the maximum RMS power would be 356W.

Alternatively, if the power supply had an average power rating stated we would use equation 2.

Equation 2:  Maximum Average Power > ((OPb-OPa) x t/T) + OPa

For the previously used values that would calculate as 350W.

TDK-Lambda’s CUS400M power supply (Figure 3) can deliver 400W with forced air or, with a combination of convection and conduction cooling, provide up to 250W continuous output power and a peak power of 400W.

Figure 3: TDK-Lambda’s CUS400M power supply

Derating curves (Figure 4) are provided in the application notes as a guide rather than firm limits with the CUS600M, as it is expected that component temperatures will be measured and confirmed in the end system.

Figure 4: CUS400M peak power performance curves

Whereas many power supplies have peak power durations measured in seconds, the mechanical construction of this series provides additional cooling through the use of a metal baseplate. With a 230Vac input, the 400W peak rating applies up to 30 minutes with a 40^oC ambient, or 5 minutes with 115Vac at 30^oC. This makes this type of product very suitable for powering applications with occasional peak demands.

To ensure long term reliability, convection cooled power supplies should always have their critical component temperatures measured to ensure conformity with the manufacturer’s recommendations. This should be done in the end equipment at the worst-case conditions of ambient, input voltage and orientation, allowing the temperatures to fully stabilize.

For further advice and application support on this, or any other power supply related topic, please contact your local TDK-Lambda office.

Power Guy