Load regulation

Load regulation is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in load [cite web|title=Line and Load Regulation for Programmable DC Power Supplies and Precision DC Sources- Developer Zone - National Instruments|url=http://zone.ni.com/devzone/cda/tut/p/id/3597 070618 ni.com (national instruments)] [cite web|title=Neets Module 06-Introduction to Electronic Emission, Tubes, and Power Supplies|url=http://www.tpub.com/content/neets/14178/css/14178_143.htm 070618 tpub.com] .

Definitions

Load Regulation can be defined as a percentage by the equation [cite web|title=Measuring Line and Load Regulation - Rantec|url=http://www.rantec.com/L2A_Prod/LowVoltage/LVAN/LVAN_HDMA105.pdf] :

$\%\; Load\; Regulation\; =\; 100\%\; frac\{Voltage(Full\; Load)\; -\; Voltage(Minimum\; Load)\}\{Voltage(Nominal\; Load)\}$

Where:
* $Full\; Load$ is the load that draws the greatest current (is the lowest specified load resistance - never short circuit)
* $Minimum\; Load$ is the load that draws the least current (is the highest specified load resistance - possibly open circuit for some types of linear supplies, usually limited by pass transistor minimum bias levels)
* $Nominal\; Load$ is the typical specified operating load

For switching power supplies, the primary source of regulation error is switching ripple rather than control loop inefficiency. In such cases Load Regulation is defined without normalizing to Voltage at Nominal Load and then has the units of volts.

$Load\; Regulation,\; volts\; =\; Voltage(Full\; Load)\; -\; Voltage(Minimum\; Load)$

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 a 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 (depending on how you choose to switch) given the full load resistance. A voltmeter is then connected in parallel as well and the measured values of resistance for each switch state give the inputs to the load regulation equation.

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.

A Linear Supply

The [http://www.lambdapower.com/ftp/Specs/h.pdf Lambda H Series power supply] is a typical linear design and has a load regulation spec of ±0.05% for a 50% load change. This means that if using a H series model HSB5-3-OVP, which has a single 5V, 3A output, with a load of 3.7 ohms would have a worst case voltage change of 0.05% × 5V = 2.5 mV if the load changed to 1.85 ohms.

Note that because the load regulation specifies 50% load change and since the nominal load and full load must be within the operating range (3A) of the supply, the heaviest full load condition we can test load regulation at would be R(full load) = 5V / 3A = 1.67 ohms and thus the heaviest nominal load is R(nominal load) ≥ 2 × R(full load) or 3.33 ohms.

A Switching Supply

The [http://www.sola-hevi-duty.com/products/powersupplies/scp.htm Sola Hevi SCP series] is 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%" is stating 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 5V 3A output equivalent to the Lambda example above, 10% of maximum load is 10% × 3A = 300 mA minimum load, and 90% × 3A = 2.7A for full load. At 5V output, full load is 5V / 2.7A = 1.85 ohms and minimum load is 5V / 300 mA = 16.7 ohms. So the voltage will change by no more than 25 mV when changing the load from 16.7 ohms to 1.85 ohm (or 88% load change).

Somethings to note:

* The input voltage ranges from 85VAC to 265VAC; switching supplies often allow a fuller and wider range of input voltages without transformer switching usually required for linear supplies when switching between 120VAC and 240VAC.

* The input voltage for load regulation test is defined as 230VAC. This is a worst case for line regulation because it's 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 is still pretty low in absolute resistance: 16.7 ohms. Typically a linear counterpart handle lower loading (higher resistance). Switching supplies usually can not operate without some load current.

* Finally the absolute voltage change for the switching power supply is 10× worst 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