Traditionally, the selection of
power supplies is based upon the expected
maximum total system power, calculated
as volts × amps = watts. But for some
applications, a power supply with a peak
current or peak power rating may provide
a significant cost savings. For example, if
the power supply’s load includes electric
motors, disk drives, pumps, fans, actuators,
or other components that require an
initial startup current that’s much higher
than the steady state draw, a peak-rated
power supply should be considered.
Electric motors including those in disk
drives can exhibit a peak startup current
that is anywhere from two to three times
their normal operating current. This peak
startup current may only last for 200 ms or
a few seconds. Therefore, rather than size
the system’s power supply for the worstcase
short-duration peak current, a costsaving
alternative would be to find a power
supply that can handle the peak current,
yet provide the normal (non-peak) system
operating power.
Power supplies with peak power ratings
can exceed their normal ratings for short
specified durations without going into an
overcurrent mode (Fig. 1). For example,
some power supplies can provide a peak
current or power that is two to three times
their normal output ratings. These types of
supplies specify peak ratings for a limited
time period and maximum duty cycle.
PEAK-OUTPUT-POWER VS. PEAK
DUTY-CYCLE CURVES
Figure 2 illustrates the peak-outputpower
versus the peak-duty-cycle curves
for a typical peak-rated power supply with
an output voltage of 48 V dc and 600-W
average output power. The peak power
duty cycle is definable as a percentage of
the total operating time.
With an input voltage of 220 V ac, from
the solid-line curve in Figure 2, we can see
that if we needed to pull 1800 W of peak
output power (three times the rated power)
from this supply, we would be limited to a
bit more than a 10% duty cycle.
Also in this example, from its datasheet,
we know that this supply has a maximum
peak power pulse width of 5 s. When using
high-peak-power supplies, it is necessary
to operate the supply below its continuous
output power rating before the next peakpower
pulse is drawn. This is necessary to avoid exceeding the average-power rating,
which in this case is 600 W.
As another example from the solid-line
curve in Figure 2, we can see that if our
system needed a 35% peak-power duty
cycle, the maximum output power would
be limited to 1300 W, which is still more
than twice the normal rated power. In
many applications, it is not difficult to stay
within the peak-power constraints of this
type of power supply, and the resulting
cost savings can be significant.
PEAK, NON-PEAK, AND AVERAGE POWER CONSIDERATONS
When using a peak-power-rated supply,
we must take care not to exceed its
specified average output power rating. Figure
3 shows a typical peak output power
pulse waveform. Equation 1 below can be
used to determine a, which is the available
non-peak power from a peak-rated supply
when driving peak loads.
a = [(Wm × T) – (Wp × t)] ÷ (T – t) (1)
Definitions and example for Equation 1:
a = available non-peak power (watts –
TBD)
Wm = maximum average output power
(600 W per the power supply’s datasheet)
Wp = peak pulse power (1800 W per this
example)
T = total period (50 s, see *Note below)
t = pulse width during peak power (5 s maximum
per the power supply’s datasheet)
Duty cycle = peak output pulse width during
each period (10% as per Figure 2)
*Note: To calculate “T” from the above,
we know the peak pulse-width time “t” is 5
s maximum (specified). In this example, it
has a duty cycle of 10% of the total period
“T.” Therefore:
T × 0.10 = 5 s
T = 5 s ÷ 0.10 = 50 s
Using Equation 1 with the given data
for this example, we can now calculate a
below:
a = [(Wm × T) – (Wp × t)] ÷ (T – t) =
[(600 W × 50 s) – (1800 W × 5 s)] ÷ (50
s – 5 s)
= (30,000 – 9000) ÷ 45
a = 466.66 W
Consequently, 466 W is the maximum
available output power deliverable to the
system’s load during the non-peak period,
which in this case would be 45 s. Since the
output voltage of the supply in this example
is 48 V dc, the non-peak period output
current would be 9.7 A for 45 s (466 W ÷
48 V = 9.7 A), the peak pulse current would
be 37.5 A for 5 s (1800 W ÷ 48 V = 37.5 A),
and the average current from the supply
would be 12.5 A (600 W ÷ 48 V = 12.5 A).
If we were to reduce the peak-pulse
period, or the required peak power, or the
peak duty cycle, that would allow for more
power to be available during the non-peak
period. The table lists examples of various
combinations of peak-power and nonpeak-
power pulse durations and duty-cycle
conditions for this 48-V/600-W peak-rated
power supply.
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