Siegel's lemma

Load regulation is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in the supply's load (such as a change in resistance value connected across the supply output).

Definitions

Load regulation of a constant-voltage source is defined by the equation:

$\text{<mrow data-mjx-texclass="ORD"><mo>\%</mo><mspace width="0.167em"></mspace>Load<mspace width="0.167em"></mspace>Regulation</mrow>}=100\%\frac{V_{min-load}-V_{max-load}}{V_{nom-load}}$

Where:

• $V_{max-load}$ is the voltage at maximum load. The maximum load is the one that draws the greatest current, i.e., the lowest specified load resistance (never short circuit);
• $V_{min-load}$ is the voltage at minimum load. The minimum load is the one that draws the least current, i.e. the highest specified load resistance (possibly open circuit for some types of linear supplies, usually limited by pass transistor minimum bias levels);
• $V_{nom-load}$ is the voltage at the typical specified load.

For a constant-current supply, the above equation uses currents instead of voltages, and the maximum and minimum load values are when the largest and smallest specified voltage across the load are produced.

For switching power supplies, the primary source of regulation error is switching ripple, rather than control loop precision. In such cases, load regulation is defined without normalizing to voltage at nominal load and has the unit of volts, not a percentage.

$\text{<mrow data-mjx-texclass="ORD">Load<mspace width="0.167em"></mspace>Regulation<mspace width="0.167em"></mspace><mo stretchy="false">(</mo>V<mo stretchy="false">)</mo></mrow>}=V_{min-load}-V_{max-load}$

Measurement

A simple way to manually measure load regulation is to connect three parallel load resistors to the power supply where two of the resistors, R₂ and R₃, are connected through switches while the other resistor, R₁ is connected directly. The values of the resistors are selected such that R₁ gives the minimum load resistance, R₁||R₂ gives the nominal load resistance and either R₁||R₂||R₃ or R₂||R₃ gives the full load resistance. A voltmeter is then connected in parallel to the resistors and the measured values of voltage for each load state can be used to calculate the load regulation as given in the equation above.

Programmable loads are typically used to automate the measurement of load regulation.

Examples

Two examples of load regulation specifications are given for a linear and a switching power supply.

Linear Supply

The old Lambda H Series power supply is a typical linear design and has a load regulation specification of ±0.05% for a 50% load change. Consider an H Series model HSB5-3-OVP supply, which has a single 5 V, 3 A output, with a load of 3.7 Ω. If the load changed to 1.85 Ω, this power supply would have a maximum voltage change of 0.05% × 5 V = 2.5 mV.

Because the load regulation specifies 50% load change and since the nominal and full load must be within the operating range of the supply (maximum 3 A), the heaviest full-load condition at which we can test load regulation is $R_{max-load}=\frac{5V}{3A}=1.67\mathrm{\Omega }$ and the heaviest nominal load is $R_{nom-load}=2\times R_{full-load}=3.33\mathrm{\Omega }$.

Switching Supply

The Sola Hevi SCP series is a typical switching supply with approximately similar output specifications to the Lambda above. The family has a load regulation of ≤ 0.5% at "10…90…10%" nominal I_out. Defining the load change in terms of current is strictly the same as defining load change in resistance change. The "10…90…10%" specification states that the minimum load is 10% of maximum current and the full load is 90% and the specification test includes taking the load back to 10%. If we take the SCP 30D312, which has a 5 V, 3 A output equivalent to the Lambda example above, 10% of maximum load is 10% × 3 A = 300 mA minimum load, and 90% × 3 A = 2.7 A for full load. At 5 V output, full load is 5 V / 2.7 A = 1.85 Ω and minimum load is 5V / 300 mA = 16.7 Ω. The voltage will change by no more than 25 mV when changing the load from 16.7 Ω to 1.85 Ω (or 88% load change).

Note that:

• The input voltage ranges from 85VAC to 265VAC. Switching supplies can often accept a larger range of input voltages than linear supplies without the need for transformer switching; linear supplies would need to physically switch to a different transformer to operate at both 120 VAC and 240 VAC.

• The input voltage for load regulation test is defined as 230VAC. This is a worst case for line regulation because it is the widest change in voltage between input and output. Due to the error introduced by line regulation, this input value will be the worst case for load regulation as well.

• The minimum load remains fairly low in absolute resistance: 16.7 Ω. Typically, a linear supply is capable of handling lower loading (higher resistance). Switching supplies usually cannot operate without some load current.

• The absolute voltage change for the switching power supply is tenfold worse than the linear supply. This is a typical difference. Lambda makes switching supplies and Sola makes linear supplies to demonstrate this further.

See also

• Line regulation
• Linear regulator

Notes

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