Introduction to AC ADAPTERS Performance Evaluation: Measurement Methods and Reference Values for Dynamic Characteristics (Output Start-Up/Shutdown, Load Response, and Inrush Current) | NEXTY Electronics, a Trading Company for Electronic Components and Semiconductors

AC ADAPTERS are essential POWER SUPPLIES supply devices that provide stable power to all electronic devices, including smartphones, laptops, and IoT devices. To ensure product reliability and safety, it is important to correctly understand and evaluate the characteristics of AC ADAPTERS.

In the previous article, we mainly introduced the evaluation items and methods for static characteristics. Static characteristics are essential indicators for ensuring basic performance, but in reality, electronic devices do not always operate under a constant load.

AC ADAPTERS are required to respond instantaneously to sudden voltage changes when POWER SUPPLIES is turned on, operation mode transitions of connected devices, and sudden current demands from motors and wireless modules.

These dynamic characteristics that change over time are directly linked to the actual usage environment, so it is important to thoroughly evaluate them.
This article provides an easy-to-understand explanation of the dynamic characteristics of AC ADAPTERS, as well as the evaluation items and methods, using actual measurement data.

Previous article: Introduction to AC ADAPTERS performance evaluation: Measurement methods and reference values for static characteristics (efficiency, regulation, noise)

What are dynamic characteristics?

Dynamic characteristics are a characteristic that evaluates how AC ADAPTERS output changes over time, and represent its ability to respond to transient changes. In other words, it is an index that evaluates "how stably POWER SUPPLIES can follow changes."

Why are dynamic characteristics important?

In the actual usage environment of electronic devices, the load current and input conditions are constantly changing.

For example, when a microcontroller wakes up from sleep mode, when a wireless module starts transmitting, when a motor starts, etc., the output current changes significantly. If AC ADAPTERS cannot respond appropriately at these times,

Malfunctions and resets due to momentary voltage drops
Increased IC stress due to abnormal overshoot
Start-up failure due to disturbance in the startup sequence
Excessive inrush current can cause FUSES blowout and TRANSFORMERS saturation

Problems such as these may occur.

Even if AC ADAPTERS has good static characteristics, if its dynamic characteristics are insufficient, problems will often occur after installation, so it is essential to evaluate both static and dynamic characteristics during product development.

Among these, the greatest advantage of evaluating dynamic characteristics is that it can visualize risks during actual use that are difficult to detect on paper, and it can greatly improve the reliability of AC ADAPTERS.

Important characteristics and evaluation methods for evaluating the dynamic characteristics of AC ADAPTERS

When evaluating the dynamic characteristics of AC ADAPTERS, the following three characteristics are particularly important.

1. Output startup/shutdown characteristics

2. Load response characteristics

3. Inrush current

This time, we will explain the overview of these three characteristics and how to evaluate them.

These three items are likely to be the "startup point for actual equipment problems" - whether the equipment can be started (start-up/shutdown), whether it will fall even if the load jumps (load response), and whether the protection malfunctions when powered on (inrush).
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

When evaluating dynamic characteristics, it is important to create a correct evaluation environment, even more so than for static characteristics, because we are dealing with "instantaneous changes in voltage and current."
The evaluation environment has some things in common with static characteristics, but dynamic characteristics place more importance on response speed, so measurements are generally made using an oscilloscope.In addition, when evaluating load response characteristics, an electronic load device is used as the load, which can reproduce the "sudden current demand" of the actual equipment.
When probing, keeping the output line as short as possible and minimizing the loop area will prevent parasitic INDUCTORS and noise from being mixed in, enabling accurate measurement of the waveform of the transient response characteristics.
Below is an example of an evaluation environment.

Example of AC ADAPTERS evaluation environment

1. Output startup/shutdown characteristics

Output rise/fall characteristics are used to evaluate how the output voltage changes over time when AC ADAPTERS is turned on and off. The output voltage of AC ADAPTERS does not rise the instant SWITCHES is turned on; rather, it follows a constant rising waveform influenced by the internal control circuit, rectifier circuit, soft start function, load conditions, and other factors. Similarly, the way the output voltage decays when POWER SUPPLIES is turned off also exhibits specific behavior.

These behaviors have a significant impact on the connected electronic devices, and are therefore important evaluation items from the perspective of normal operation, prevention of malfunctions, and ensuring safety.

Output startup/shutdown characteristics (general SWITCHES AC ADAPTERS 5V to 20V class)

<Output start-up characteristics

Start-up delay: 1.5 seconds or less
The time from when the input voltage is applied until AC ADAPTERS starts to start up the output
Rise time: 100ms or less
The time it takes for the output to reach the specified voltage from 0V

<Output riseturn characteristic><Output riseturn characteristic><Output riseturn characteristic

Hold Up: 10 ms or more
The time from when the AC input is turned off until AC ADAPTERS starts turning off the output
Fall time: 100ms or less *Except when no load is applied
The time it takes for the output to reach 0V from the specified voltage

Evaluation method

1. Operate the AC POWER SUPPLIES and turn on the AC input.

② Use the trigger function of the oscilloscope to observe the input voltage and output voltage waveforms when the output is started up.

③ After measuring the startup delay time from the waveform, adjust the time axis range and measure the rise time.

④ Operate the AC POWER SUPPLIES and turn off the AC input.

⑤ As in ②, observe the waveform when the output falls.

⑥ After measuring the holdup from the waveform, adjust the time axis range and measure the fall time.

Example of evaluation results

Input voltage [V]	Output current (load) [A]	Start-up delay time [s]	Rise time [ms]	Hold Up [ms]	Falling Time [ms]
100	0.00	1.1	2.5	5.19
100	0.50	1.0	2.6	5.16	14
100	2.00	1.0	4.5	5.14	4.2

Start-up delay time measurement waveform

In this sample, the startup delay is stable at about 1 second, with a gentle slope and short convergence during startup, and a stable shutdown mainly due to natural discharge, confirming characteristics that are highly suitable for implementation.

Before mass production, recheck CABLES length, capacitive load, and other characteristics under actual equipment conditions, and adjust output CAPACITORS, etc. as necessary, to achieve even more robust startup and shutdown.

2. Load response characteristics

Load response characteristics evaluate how AC ADAPTERS attempts to maintain voltage when there is a sudden change in output current, such as an "undershoot" where the voltage drops temporarily the moment the current increases, or an "overshoot" where the voltage rises the moment the current decreases. Electronic devices do not always consume a constant amount of power; in actual operation, the load fluctuates instantaneously. If AC ADAPTERS cannot respond quickly enough in these situations, the output voltage will fluctuate greatly, leading to problems such as malfunctions, resets, and communication errors.
Therefore, load response characteristics are an important evaluation item that allows us to confirm stability in the actual usage environment, which cannot be seen in static characteristics.

Load response characteristics (general SWITCHES AC ADAPTERS 5V to 20V class)

Undershoot: within -5%
A phenomenon in which the output voltage drops temporarily when the load suddenly increases.
Overshoot: 3% to 5%
When the load suddenly decreases, the output voltage temporarily rises due to excess energy.
Recovery time: 100us to 1ms
The time it takes for the voltage to return to within the specified voltage tolerance range after an undershoot/overshoot occurs

Evaluation method

① Set the load current (minimum current, maximum current), step response (repetition period, duty), and slew rate on the electronic load device.

② Turn on the AC input and observe the output voltage (AC coupling) and output current waveforms on an oscilloscope.

③ Measure the undershoot, overshoot, and recovery time from the waveform.

Example of evaluation results

Input voltage [V]	Output current (load) [A]	Undershoot [mV]	Overshoot [mV]	Recovery time [us]
100	0 to 2.0	-380	108	330
100	0.2 to 2.0	-326	134	300

In this measurement, the undershoot was a maximum of -380mV, which is higher than the general guideline for 5V systems. However, the overshoot was small at +108 to +134mV, and the recovery time was within 300 to 330us, ensuring stability and damping of the control loop. Since the main cause is thought to be a lack of transient charge, improvement is expected by optimizing the capacitance and ESR of the output CAPACITORS, etc.

3. Inrush current

Inrush current is a sudden current larger than normal that flows the instant AC ADAPTERS is turned POWER SUPPLIES. SWITCHES AC ADAPTERS contain rectifier circuits, smoothing CAPACITORS, TRANSFORMERS, and protective elements, and immediately after POWER SUPPLIES is turned on, these elements are not yet stable, causing an extremely large current to flow for a short period of time.

The mechanism by which inrush current occurs is when a large-capacity electrolytic CAPACITORS on the primary side of AC ADAPTERS is charged from 0V all at once, causing an extremely large current to flow for just a moment, lasting from a few hundred microseconds.In addition, with TRANSFORMERS-type POWER SUPPLIES, the inrush current that flows also varies depending on the phase of the AC input waveform, and tends to peak when the input phase is near the peak of the AC waveform immediately after POWER SUPPLIES is turned on.

Although inrush current is a short-term phenomenon, if it is large it can cause FUSES and breaker malfunctions, affect long-term reliability due to momentary stress on internal components (DIODES, FETs, etc.), and lead to reduced stability when the system starts up, making it a very important item in dynamic characteristics evaluation.

Inrush current guideline

Consumer AC ADAPTERS (5V to 20V class): 10A to 30A
USB CHARGERS (5V to 9V class for smartphones): Approximately 5A to 20A
Notebook PC adapter (45W~100W): Approximately 20A~60A

Inrush current duration

Small AC ADAPTERS (5W~20W): approx. 200us~1ms
Medium-sized AC ADAPTERS (20W~60W): approx. 0.5ms~3ms
Large AC ADAPTERS (100W or more): 1ms to 10ms

Evaluation method

① Connect an outlet or similar device with an AC input on/off SWITCHES between AC ADAPTERS and AC POWER SUPPLIES.

② Operate the AC POWER SUPPLIES and turn on the AC input.

③ Observe the input voltage and input current waveforms using an oscilloscope.

④ Use the manual SWITCHES to turn the AC input on and off multiple times and measure the maximum inrush current.

Example of evaluation results

Input voltage [V]	Output current (load) [A]	Inrush current [A]	Duration [us]
100	0	33.2	264
100	2.0	34.4	2.6

The inrush current peaked at approximately 33-34A and lasted for a short period of several hundred microseconds, a typical example of the acceptable range for a consumer AC ADAPTERS. Considering the difference in inrush phase, a similar range is expected in actual use, and as long as it is consistent with the tolerance design of the breaker, SWITCHES, and rectifier elements, the implementation risk can be evaluated as low. If further reduction of the peak is required, NTC constants or PFC soft charging are effective.

Summary

Evaluating AC ADAPTERS from both static and dynamic characteristics allows you to understand not only steady-state stability but also transient phenomena that occur in actual usage environments, such as startup, load fluctuations, and POWER SUPPLIES on, thereby comprehensively improving the safety, stability, and reliability of AC ADAPTERS. The evaluation methods and judgment criteria introduced in this article can be used directly for selecting, evaluating, and troubleshooting AC ADAPTERS. We hope you will use it for your actual evaluations and product selection.