Introduction to AC ADAPTERS Performance Evaluation: Measurement Methods and Reference Values for Static Characteristics (Efficiency, Regulation, and Noise)

AC ADAPTERS are essential POWER SUPPLIES supply devices that provide stable power to all electronic devices, including smartphones, laptops, IoT devices, etc. However, simply verifying that the specified voltage and current are output is not enough when it comes to product development.
To ensure product reliability and safety, it is important to correctly understand the characteristics of AC ADAPTERS and quantitatively evaluate their static characteristics, such as stability, noise, and efficiency.
This article provides an easy-to-understand explanation of what the static characteristics of AC ADAPTERS are, as well as the evaluation items and evaluation methods, using actual measurement data.

Why is AC ADAPTERS Evaluation Necessary?

AC ADAPTERS need to be evaluated for three reasons: safety, reliability, and standards.
We will explain each one in detail.

1. Ensuring safety

Because AC ADAPTERS are powered for long periods of time, it is essential to prevent risks such as overvoltage, overcurrent, and overheating.
By evaluation,

Overload protection function (overcurrent protection and overvoltage protection)
Insulation performance and voltage resistance tests
Temperature Rise Characteristics

Check the following to prevent serious accidents such as fires and electric shocks.

2. Reliability and long life

Poor POWER SUPPLIES quality can cause stress to connected devices, leading to malfunctions and shortened lifespans.
In the evaluation,

Output voltage stability
Ripple noise reduction
Appropriate start/stop voltage

to ensure long-term reliability.

3. Compliance with standards and market demands

Compliance with each country's energy-saving regulations and safety standards (DOE, ErP, IEC, USB PD, etc.) is essential for product sales.
Through evaluation,

Conversion efficiency
standby power
Protection circuit operation

Check that it complies with the standard.

What are static characteristics?

Static characteristics indicate the output characteristics when the input or load conditions are changed while AC ADAPTERS output remains unchanged over time.
Simply put, it is an index that evaluates "how stable it is in steady state."

Why are static characteristics important?

Static characteristics indicate whether the output voltage remains within the specified value even when the load and input conditions change.
Precision devices such as smartphones and laptops are sensitive to voltage deviations, and a difference of even a few tens of millivolts can lead to malfunctions or charging problems. Good static characteristics can prevent such problems and ensure stable operation.
In other words, static characteristics are an essential element in measuring the basic performance of POWER SUPPLIES, and are important for preventing equipment malfunctions and shortened lifespans.

Important characteristics and evaluation methods for static characteristics evaluation of AC ADAPTERS

When evaluating the static characteristics of AC ADAPTERS, the following five characteristics are particularly important.

1. Conversion efficiency
2. Load regulation/line regulation
3. Ripple noise
4. standby power
5. Start/Stop Voltage

This time, we will provide an overview of these five characteristics and explain how to evaluate them.
In addition, each evaluation result example is actual measurement data using AC ADAPTERS with a rated input of AC100V-240V / output of 5V 2A.

Preparing for the assessment

To accurately evaluate AC ADAPTERS, it is essential to create a proper evaluation environment.
The evaluation environment is set up so that the input/output voltage, current, and ripple noise can be accurately measured.
In particular, when probing, using short coaxial CABLES to minimize the loop area will suppress high-frequency noise and external interference, enabling accurate ripple noise measurement.
Below is an example of an evaluation environment.

Example of AC ADAPTERS evaluation environment

1. Conversion efficiency

Conversion efficiency indicates the percentage of input power that can actually be used as output when converting power from AC (alternating current) to DC (direct current). Conversion efficiency varies depending on the output current (load), and AC ADAPTERS generally achieve their highest conversion efficiency at around 40-70% of their rated load. If conversion efficiency is low, the unused power that is not converted is released as heat energy, causing AC ADAPTERS to heat up. Excessive heat generation also places stress on electronic components, increasing the risk of failure and shortening the product's lifespan.
Therefore, it is important to check the conversion efficiency of AC ADAPTERS you are using.

The conversion efficiency can be calculated using the following formula (1).

Conversion efficiency guideline

General AC ADAPTERS: Around 80%
High-efficiency AC-DC converter: over 90%
Portable POWER SUPPLIES: 75%-90% (including DC-AC conversion)

Evaluation method

① Measure the input power of AC ADAPTERS using a power meter or similar.

② Measure the output voltage of AC ADAPTERS using a multimeter or similar device, and calculate the output power using the output current (load) and the following (Equation 2).

③Calculate the conversion efficiency using the above formula (1).

Example of evaluation results

Input voltage [V]	Input power [W]	Output current (load) [A]	Output voltage [V]	Output power [W]	Conversion efficiency [%]
100	0.69	0.10	5.19	0.51	74.10
100	3.18	0.50	5.16	2.58	81.20
100	6.29	1.00	5.14	5.14	81.70
100	9.43	1.50	5.12	7.68	81.40
100	12.63	2.00	5.10	10.19	80.70

Conversion efficiency measurement results

A typical flyback AC ADAPTERS has an efficiency curve like the one shown in the graph above. At light loads, conversion efficiency is low due to the bias current and SWITCHES losses of the control IC, and low-frequency losses due to burst/skip mode. At medium loads, the balance between SWITCHES loss and conduction loss is optimal, resulting in maximum efficiency. Most flyback POWER SUPPLIES reach their peak conversion efficiency at a load of 40-70%. At higher loads, conduction losses in the internal MOS FETS and DIODES increase, slightly decreasing conversion efficiency.

2. standby power

Standby power is the power consumed by AC ADAPTERS simply by being plugged into an outlet, even when it is not supplying power to a device. The reason for this standby power is that AC ADAPTERS contains POWER SUPPLIES control circuit, SWITCHES element, FILTERS, etc., which are required to maintain operation while power is being supplied. In particular, SWITCHING POWER SUPPLIES

Control IC operating power
Input FILTERS and rectifier circuit losses
Leakage current

For these reasons, power is consumed even when there is no load.
Although standby power consumption is small, if the appliance is left plugged in, it can add up over the course of a year and result in significant power loss.

Estimated standby power consumption

Old AC ADAPTERS (pre-regulation): 0.5W-1.0W or more
Current energy-saving standard compliant products (DoE Level VI, ErP Directive, etc.): 0.1W or less
Latest high-efficiency models (GaN, etc.): 0.05W or less

Evaluation method

①Set the output current (load) to zero and measure the input power (standby power) using a power meter or similar.

Example of evaluation results

Input voltage [V]	Output current (load) [A]	Standby power consumption [W]
90	0	0.025
100	0	0.026
110	0	0.027

Standby power measurement results (excluding power meter power consumption)

In this measurement, the standby power consumption did not show any significant dependency on the input voltage, remaining almost constant at 0.03W or less.
This indicates that the control IC's power consumption has been reduced and SWITCHES control has been optimized appropriately, and also provides good results in static characteristics evaluation.

3. Load/line regulation

Load regulation is an index that shows how much the output voltage fluctuates when the load current changes. The current consumption of AC ADAPTERS varies depending on the device connected to it. If the output voltage fluctuates significantly, it can adversely affect the operation of the device, so stability is important.

Load regulation can be calculated using the following formula (Equation 3).

Line regulation is an index that indicates how much the output voltage changes when the input voltage of AC ADAPTERS changes.
AC ADAPTERS typically support a wide input range, such as 100-240V AC, but it is important to ensure that the output voltage remains stable even when the input voltage fluctuates.

Line regulation can be calculated using the following equation (Equation 4).

Load/line regulation guidelines (general SWITCHES AC ADAPTERS)

Load regulation: ±2% to ±5%
Line regulation: ±0.5% to ±2%

Evaluation method

①Measure the output voltage when there is no load.

②Measure the output voltage at rated load.

③ Calculate the load regulation using the above (Equation 3).

①Measure the output voltage at the maximum input voltage.

②Measure the output voltage at the minimum input voltage.

③ Calculate the line regulation using the above (Equation 4).

Example of evaluation results

Input voltage [V]	Output current (load) [A]	Output voltage [V]	Load regulation [%]
100	0	5.187	-1.72%
100	2	5.098	-1.72%

Load regulation measurement results

Input voltage [V]	Output current (load) [A]	Output voltage [V]	Line regulation [%]
90	2	5.097	90V - 100V: -0.02%
100	2	5.098	100V - 110V: -0.02%
110	2	5.097	100V - 110V: -0.02%

Line regulation measurement results

The load regulation of this sample is slightly high at no load and drops to the appropriate level as the load increases, which can be said to be a typical characteristic that reflects the design intent of the output stage resistance (Rout) and feedback compensation.In addition, from the voltage difference ΔV = 89mV from 0 to 2A, the output equivalent resistance can be estimated to be Rout = approximately 45mΩ.
In terms of line regulation, the output error is minimal, meaning that it is hardly affected by the AC line's THD (total harmonic distortion) or frequency difference (50/60 Hz), making it an excellent product for wide input compatibility. Expanding the input range and increasing the granularity of the load sweep will lead to further improvements in the quality of the evaluation.

4. Ripple noise

Ripple noise refers to minute AC components and high-frequency noise contained in the output voltage of AC ADAPTERS.
The original purpose of AC ADAPTERS is to supply a stable DC voltage, but due to the effects of SWITCHES operations, rectification, and FILTERS, the voltage does not become completely DC, and slight fluctuations remain. If the ripple noise is large, it may lead to malfunction of connected devices, so it is important to evaluate it carefully.

Ripple noise is strictly divided into ripple and noise, and the two together are called Ripple & Noise, which is an important indicator of POWER SUPPLIES quality.

Ripple: Mainly residual AC components (low frequency) after rectification
Noise: High frequency components caused by SWITCHES operations and external interference

Ripple noise guideline (rated load)

General SWITCHES AC ADAPTERS: 50mVp-p ~ 200mVp-p
For USB CHARGERS and consumer devices: 100mVp-p or less
For audio, communications, and medical equipment: 50 mVp-p or less, and in some cases 10 mVp-p or less

Evaluation method

①Set the oscilloscope probe to AC coupling and measure the output voltage waveform. *20MHz bandwidth recommended.

② Record the peak-to-peak value of the measured waveform (unit: mVp-p).

Example of evaluation results

Input voltage [V]	Output current (load) [A]	Frequency [kHz]	Ripple noise [mVp-p]
100	0	0.8	10
100	0.1	21.0	21
100	0.5	38.0	40
100	2.0	64.0	77

Ripple noise increases monotonically as the load increases, but it remains at a maximum of 77 mVp-p, which is satisfactory for consumer applications. However, countermeasures will need to be considered for audio or communications equipment applications. It can also be inferred that the frequency around 64 kHz is likely to correspond to the control SWITCHES frequency or the fundamental component after passing through the secondary-side LC.

5. Start/Stop Voltage

The start-stop voltage is an indicator of the range of input voltage required for AC ADAPTERS to start and maintain normal operation.
This characteristic directly contributes to the stable operation of equipment in places with unstable POWER SUPPLIES environments or situations where voltage fluctuations are likely to occur.

Start-up voltage: The minimum input voltage at which AC ADAPTERS can start outputting power.
Stop voltage: The voltage at which output cannot be maintained when the input voltage drops.

If the start voltage is too high, the equipment may not operate when POWER SUPPLIES is turned on, and if the stop voltage is too low, the output may be maintained while the input is unstable, which may cause malfunctions or overloads. Therefore, it is an important characteristic to ensure reliable operation when POWER SUPPLIES is turned on and safe shutdown when the voltage is low.
Having proper hysteresis in AC ADAPTERS ensures stable POWER SUPPLIES on/off.

Start/Stop Voltage Guideline (General SWITCHES AC ADAPTERS)

Starting voltage: AC85V or less
Stop voltage: AC75V or less

Evaluation method

1. Gradually increase the input voltage from 0V and measure the voltage at which AC ADAPTERS begins to output.

② With AC ADAPTERS turned on, gradually reduce the input voltage and measure the voltage at which the output stops.

Example of evaluation results

<Starting voltage> Measured value: 40V

Input voltage [V]	Output current (load) [A]	Output voltage [V]
37	0.0	0.000
38	0.0	0.000
39	0.0	0.000
40	0.0	5.187
41	0.0	5.187
42	0.0	5.187

<Stop voltage> Measured value: 11V

Input voltage [V]	Output current (load) [A]	Output voltage [V]
14	0.0	5.187
13	0.0	5.187
12	0.0	5.187
11	0.0	0.000
10	0.0	0.000
9	0.0	0.000

It can be seen that a design with large hysteresis is advantageous for suppressing chattering and provides high stability. Of course, this value may change at rated load or light load, so it is important to conduct a more detailed evaluation when actually conducting an evaluation.

Summary

Evaluating AC ADAPTERS is essential to ensure safety, reliability, and compliance with standards. Thorough evaluation of static characteristics prevents problems caused by excessive heat generation and voltage fluctuations, ensuring a stable, safe, and highly efficient POWER SUPPLIES supply.
After checking the trace of the specified values in the static characteristics, reliability in actual operation is determined by evaluating dynamic characteristics such as transient response and protection functions such as OCP and OVP.
In our next post, we will explain the dynamic characteristics of AC ADAPTERS based on the evaluation results of these static characteristics.