What is Load Regulation?

Load regulation ensures programmed output voltage remains constant despite load variations It is calculated using the following formula:

AC Load Regulation Formula

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 It Work?

Programmable impedance relies on feeding back signal to the output inverter’s control loop that is proportional to the load current. By summing this signal with the error signal that is used to maintain output voltage, the amount of voltage change is now a function of both programmed set value as well as load current. This concept is illustrated in Figure 2.

Simple Error Amplifier Schematic

Figure 1: Simple Error Amplifier Schematic

At low load currents, there is little boost from this current feedback. At higher load currents, the amount of feedback increases, effectively boosting the output voltage to simulate a lower output impedance. Since the level of compensation can be programmed, the output impedance is now programmable.

Why Does it Matter?

Without good load regulation, output voltage will sags 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 was it 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, good feedback loop design that incorporates frequency compensation must be used. Another refined it to incorporate current feedback input into the feedback look 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 as 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.