Checking an Unregulated DC DC Module at No Load

An unregulated isolated DC DC module can produce a higher output voltage when its load becomes very small. A board that works during normal activity may therefore overstress a downstream component during standby or startup. Check the minimum load condition as deliberately as the maximum output current.

Confirm the regulation model

The Murata DC DC converter application notes explain that some unregulated low power modules require a minimum load to keep output voltage within specification. The requirement must be taken from the individual device data rather than assumed for every isolated converter.

Record line regulation, load regulation, no-load behavior, and the applicable input range. A nominal output voltage in the part number is not a guarantee of that voltage at every load.

Find the real minimum load

List downstream operating states, including reset, sleep, disconnected peripherals, and disabled regulators. A regulator that normally draws current may present almost no load while its enable is low.

Measure output voltage at the highest permitted input and lowest expected load. Include startup before downstream circuits become active. A steady-state test with the complete application running can miss the highest voltage condition.

Evaluate a preload explicitly

If a resistor preload is suitable, calculate its dissipation and the current it consumes across the actual output range. Include that consumption in the module’s available power and the product’s standby budget.

For a hypothetical 12 V output requiring a 10 mA preload, a first resistor value is 1.2 kilohms and its nominal dissipation is 0.12 W. At 15 V, the same resistor dissipates about 0.188 W. Select ratings from the worst relevant condition, not only the nominal calculation.

Check alternatives and transition behavior

A regulated module, post-regulator, or controlled clamp may offer a different tradeoff, but each needs its own voltage, current, thermal, and startup assessment. A clamp that absorbs no-load energy must remain within its dissipation limits when input is high.

Step between minimum and normal load and observe overshoot and recovery. Check whether the downstream circuit remains within its recommended operating range throughout the transition, including any interval before its own protection responds.

Save input conditions, complete load-state table, measured voltage envelope, and preload or regulation calculations. Recheck the minimum load after firmware changes that reduce standby current. A good design controls the output voltage in the product’s quietest state as well as its busiest one. Lower power consumption is not automatically harmless when an unregulated converter relies on load to remain within its intended output range.