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WHITE PAPER

How Vicor Power Components

Enable Power Averaging

Power systems generally ﬁt into one of two types: those that need to deliver a continuous output and

those where the energy is driven in short bursts. Using traditional design techniques, pulse load systems

are conﬁgured based on the maximum output power required. For example, if the load is 3kW and is

on for 1ms and off for 5ms, the supply is conﬁgured for 3kW even though the average power in this

case is only 500W.

This approach presents several problems. Because conventional power-design techniques require the

system to be designed around the maximum required power, the size, weight and cost of the system is

determined by the peak, rather than the average power requirement. As system power levels increase,

many power designers are ﬁnding it difﬁcult to stay within space and weight constraints. Support circuitry,

such as bypass caps, heat sinks and system fans takes up system real estate, exacerbating the problem and

making it even more difﬁcult to meet the application’s physical requirements. Designing the power system

for the average power can be a better alternative.

Power averaging

One solution for applications where the load is only on for a short duration and is repetitive, is to use a

current-limiting converter and a capacitor to supply peak power needs. When conﬁguring such a power

system, the designer must take into account the current limit, power limit and stability of the power

supply as well as sizing the capacitor properly to keep the voltage drop at the load within its tolerances.

Applications such as pulsed ampliﬁers, ﬂashing LED lights and reclosures can take advantage of power

averaging to reduce cost, space and weight within the system.

Dave Berry

Principal Applications Engineer

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Power-averaging conﬁguration

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Power-averaging conﬁgurations are very effective when the load can tolerate a wide input voltage

range. This is typically the case where the load is another regulating device (typically a PoL converter) or

several of these regulating devices.

When the PoL or load demands power, the capacitor will deliver a greater portion of the load current

because the DC converter will go into current limit feeding the capacitor and the load. As the capacitor

delivers the PoL or load current, its voltage will begin to fall. The capacitor must be sized so that the

voltage across the capacitor stays within the input voltage range of the PoL converter.

To minimize the capacitance needed the designer can charge the capacitor to the PoL’s maximum input

voltage and allow the power draw over the power duration to operate the capacitor down to the PoL’s

minimum input voltage.

Operation of current limiters in pulsed applications

DC-DC converters are typically designed to regulate voltage up to a maximum power level and therefore

have a maximum current and power rating. If the load tries to draw more than the rated current from

the supply, the supply will typically go into a current-limiting mode that will either fold back the output

voltage of the supply or the supply will shut down and restart.

The current limit is typically set just above the maximum rated current so, at the voltage set point of

the converter, full power delivery can be achieved. A converter rated for 500W at 48V

will have a

maximum continuous current rating of 500W/48V

or 10.4A. The current-limiting feature however,

may not start until the output current is at 13A. Current limiting is typically designed for load faults

where the converter will see current limit only a few times in its life. If the converter isn’t designed to go

into current limit as a normal mode of operation, you can stress components within the converter and

shorten the life of the power system.

If the load draws more than the maximum current but below the current limit at the voltage set point,

then you can overpower the supply and cause eventual power system failure. So a 500W converter at

48V

with a current limit set at 13A will be overpowered at up to 624W before current limiting starts.

Dealing with large bulk capacitance

The large bulk capacitance used in power-averaging applications can cause many complications for

the DC power system. At turn-on, the larger capacitor, which in many cases can be in the thousands

of microfarads, can draw the DC supply into current limit. With the potential problems associated with

built-in current limiter, frequently an external current-limiting circuit is required to keep the supply within

its maximum current and maximum power ratings.

Power designers can pre-charge the capacitor or add series resistance that limits the current at start

up, which can be shorted out once the external current limiting circuit is active. Pre-charging and

current-limiting circuits can be complex and they take up valuable board real estate. The external

current-limiting circuits must also be fast enough to catch an overcurrent event. This can be challenging

because even a few switching cycles can cause an overcurrent event.

Once the DC supply successfully powers up, the power system must be stable. With some DC-DC

converters, the large capacitor can destabilize the voltage control loop, which can cause supply or

system failure. The power designer can overcome this potential stability issue if he has access to the

control loop of the power system, but this requires complex and time-consuming engineering work.