Choosing a DC/DC Converter for Capacitive Startup Loads

Separate running current from the charging interval

A controller can have a modest running demand yet present a difficult starting task. Capacitors at its input must gain charge while the voltage rises. The converter and any protective stage in that path must support the resulting event under their own operating conditions.

This creates three useful requirements: the current after startup, the demand during the voltage rise, and the time by which the equipment must become ready. Putting only the first number on an enquiry can leave the actual constraint unanswered. A successful start without the rest of the cabinet connected also may not represent a simultaneous system start.

Begin with the equipment manufacturer’s input information. Identify the capacitance presented to the supply where available, the permitted startup profile, and any supported internal inrush control. Treat unknown capacitance as a question to resolve, rather than assuming every device with the same running watts starts alike.

Use a bounded charging calculation

For an ideal capacitor, charging current is I = C × dV/dt. If the rise is linear, a hypothetical 1 mF capacitor rising from 0 V to 12 V in 10 ms requires 1.2 A of capacitor-charging current: 0.001 × 12 ÷ 0.010 = 1.2.

That example excludes simultaneous load current, losses, path voltage drop and the converter’s actual control behavior. It is not a Highidea rating, a permitted rise time or a recommended current-limit setting. Its purpose is to show why a small running-current figure cannot describe the whole startup event.

Changing the assumed rise time changes this ideal current, but a slower ramp is not automatically acceptable to the load. Equipment may have its own startup timing or reset requirements. Obtain those conditions before treating a longer rise as the solution.

Read capacitive-load capability with its conditions

RECOM’s REC5K-AW datasheet lists maximum capacitive load separately from output current, startup time and protection behavior. Its capacitive-load figure also has a stated test condition. This is a useful example of how a converter’s specification can answer several distinct questions; its small module’s values do not specify an RP rack converter.

Texas Instruments’ application note on large capacitive loads explains another part of the problem. A power switch charging a capacitor can encounter thermal shutdown and repeated startup attempts. The note discusses particular eFuse and power-switch arrangements. It helps explain the mechanism without identifying which internal design an offered rack converter uses.

Ask the selected supplier how startup is managed and what happens when the allowed task is exceeded. Current limitation, delayed startup, repeated attempts and a stopped output have different consequences for the attached equipment. An automatic recovery statement needs its applicable conditions as well as its name.

Build one startup evidence record

Keep the requested duty and the supplier’s corresponding result in the same record. Use the exact converter model and revision, rather than moving a result between family members.

This record should distinguish a supplier test, a calculation and an untested assumption. If a result covers only a resistive load or a different initial condition, keep that limitation visible when deciding whether further evidence is needed.

Resolve the system task before changing hardware

Consider a hypothetical cabinet that starts correctly with one controller but repeatedly restarts after additional modules are fitted. The added modules change the connected startup task. Review their charging demand, the source and the converter’s documented response together before deciding what has failed.

The correct configuration may involve a different supported converter or an approved startup arrangement. Do not add capacitors, bypass protection or repeatedly force a live system to start as a trial remedy. Any further measurements or functional tests belong to qualified personnel using an approved safe plan.

Our DC/DC input-window guide addresses the source-voltage requirement. The inrush and crest-factor guide explains a separate AC UPS load question. Keep those reviews connected without substituting one for the other.

Prepare an RP configuration enquiry

Highidea’s RP range includes 2U rack power-conversion products for AC/DC supply and DC/DC applications. For a DC load, send the available source range, approved equipment input, running demand, known startup information and cabinet environment through our contact page.

Include the desired equipment-ready time and whether several loads start together. We can use that complete requirement to discuss the appropriate conversion route and the information needed for the selected model.

Primary references

• RECOM: REC5K-AW technical datasheet, capacitive-load conditions, startup and protection tables.
• Texas Instruments: Reliable Start-up with Large and Unknown Capacitive Loads, capacitor charging and power-switch thermal behavior.

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