Damaging Power Supplies with Repetitive Peak Current Draws

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There are numerous units that require peak currents when first turned on together with print heads, motors, disk drives and pumps.

Many customers typically don't measure the precise peak present and rely on an empirical methodology whereby they struggle a power provide within the application to see if it should work.  If the power provide can not present sufficient peak present, capacitors are added to the output.  These capacitors will act as momentary vitality storage, sufficient to ship load present for just a few hundred micro seconds.  If that works then the power system resolution is deemed as working, and the Engineer strikes on to the subsequent part of their mission.

Many power provides have the power to provide high peak currents, despite the fact that the datasheet doesn't point out it.  The truth is among the least expensive power provides on the market can ship very massive currents for a brief time period as a result of the output present restrict could be very crude, and is primarily there to guard the unit in opposition to a brief circuit on the output.

In dialogue with TDK-Lambda Engineering, I discovered that this may result in subject failures.  Let me clarify additional.

Under is a schematic of a ahead converter, the power FET is proven as a swap for simplicity.  That “swap” operates at a fee often within the a whole lot of kHz, vitality is transferred from the secondary aspect to the output rectifiers after which is smoothed by the output LC filter.

When a pulsed (peak) load is utilized to the power provide in extra of its rated present, the vitality is first drawn from the output capacitor.  This could add to the capacitor ripple present, elevating the temperature and lowering the element’s life.  Warmth, as I defined in earlier blogs, dries out the capacitor’s electrolyte.

When the vitality saved within the capacitor begins to deplete, the power provide will then attempt to proceed to supply the height present from the primary switching circuit.  This in flip results in repetitive surge currents in each the output diodes which is then mirrored by the transformer to the power FET.  Usually this peak present exceeds the utmost ranking of the semiconductors resulting in latent and erratic subject failures.

Extra heating within the transformer, inductors and printed circuit board traces can also be skilled as a result of, though the typical power drawn from the power provide is lower than the continual ranking, we're dealing with the formulation I2R and the height present is now squared.

TDK-Lambda recommends utilizing a power provide that has a specified peak power ranking like our HWS-P series or working intently with the power provide producer to find out if the product is appropriate for the application