What Is Load Regulation?
Load regulation ensures programmed output voltage remains constant despite load variations. It is calculated using the following formula:
Load Regulation (%) = (VLmin-VLmax)/Vprog * 100%
Where VLmin is the voltage at no load and VLmax is the voltage at maximum rated load. It is typically specified in % of full scale voltage. The lower the load regulation spec %, the better. In an ideal case, load regulation would be 0% so changes in load level would have no effect on the output voltage of the supply. That is obviously not practical, but load regulations in the fractional percentile are feasible with good regulation loop design.
How Does Load Regulation Work?
The output amplifier receives an input signal (reference signal). This signal is amplified using a fixed gain to produce a high voltage AC sine wave. The output voltage is sensed by a sense circuit and produces an output reference. This reference is compared to the input reference using an error amplifier. The error amplifier signal is used to adjust the output up or down until the error signal is at zero. A simple schematic of this concept is shown in Figure 1.
Why Does It Matter?
Without good load regulation, output voltage will sag with increases in load or surge when a load is suddenly removed. Test results under such conditions may not be repeatable. How can you ensure that what was programmed is indeed applied if the power supply has poor load regulation.
Good load regulation applies to both DC supplies and AC sources but is particularly important for AC power sources as output frequency can have a big impact on load regulation. In today’s world of switch mode power sources, output filters are required to mitigate output switching noise. These output filters contain series inductance which increases output impedance as the frequency increase. Thus, where load regulation may be ok at 50Hz, at 400Hz or 800Hz it may not be.
To overcome this, a good feedback loop design that incorporates frequency compensation must be used. Another refinement is to incorporate current feedback input into the feedback loop that results in better load compensation.
Historically, power supplies have relied on analog circuits like the one shown in Figure 1. In modern AC power source designs like the all-digital AFX Series® from Pacific Power Source, load regulation feedback loops are implemented in the digital domain. This results in several improvements such as:
- No more reliance on analog components like capacitors and resistors with finite accuracy tolerances
- Ability to incorporate programmed output parameters like frequency and load current in the feedback loop algorithms
- Ability to adapt feedback loops to varying dynamic load conditions since loop parameters can be changed on the fly as needed to maintain optimal load regulation
When selecting an AC source or DC power supply, don’t just look at the load regulation specification number, also consider the technology used.