Power-on and parameters characterizing the behavior of the power supply when turned on

Powering on the AC/DC source can be separated into the following phases:

1. Connecting to the grid, charging the input capacitors
2. Preparing the power supply control circuit for switching on
3. Starting the control circuit
4. Output voltage increase, entering the operating mode

INRUSH CURRENT

Peak input current at full load.

An excessive current value can cause false alarms on protection circuits and interfere with neighboring devices in the power line. Considering that nearby devices at risk could be computers, home automation control systems, etc., the cost of damaging them in such an event can be huge.

The INRUSH CURRENT value depends on the power supply circuit, the instantaneous value of the voltage in the grid at the moment of powering on, and the residual charge of the input capacitors before switching on. To reduce the INRUSH CURRENT value, both parametric (the most common is the introduction of NTC thermistors) and current-limiting circuitry methods to cap the charging rate of capacitors are used.

SETUP TIME

Time from power-on until the output voltage reaches 90% of rated voltage at full load

It affects the readiness time of powered devices, which is essential in many industrial applications: automation, redundancy schemes, etc. This time is not critical for domestic applications and can reach several seconds.

As a common rule, the power-on circuit determines it to the greatest extent and depends on its state at the time of switching on. To a lesser extent, it depends on the input voltage at the moment of switching on and on the residual charge of the input capacitors.

RISE TIME

The time it takes for the output voltage to rise from 10% to 90% of the rated level at full load.

Usually, this is several tens of milliseconds. Mainly depends on the power supply control scheme.

How to measure these characteristics

One can use the power management diagram at the end of this document to obtain the power-on characteristics. You need to connect the AC input to the power source and connect the power source under study to the output.
Connect the channel 1 probe of the oscilloscope to the source output and set the vertical sensitivity so that the expected output voltage of the source conveniently fits on the screen for observation. Set the triggered sweep threshold to 0.7 of the expected voltage. Set the horizontal scan to 20 ms/div. Set the launch moment to position 0.8 from the full horizontal sweep.

Connect the channel 2 probe of the oscilloscope to the measuring output of the power control circuit and set the sensitivity to 0.5 V/div.

Ensure the power control unit's current transformer mode button is pressed.

Keep the circuit off for 5 minutes, switch the oscilloscope to standby mode, set the load to 100% power, and turn on the circuit.

Evaluate the use of the vertical and horizontal dynamic range for the resulting oscillogram: the processes should occupy from half to the total size of the screen; single limitations of the signal "hairpins" along the vertical are allowed. If the display conditions are not met, adjust the gain and/or scan and repeat the power-on procedure from the beginning.

Determine parameters using cursor measurements on the oscilloscope screen.

An approximate view of the oscillogram obtained using the described method is shown in the figure:

These characteristics are consistent with a device having an active PFC circuit and an active inrush current-limiting circuit. It can be seen that the SETUP TIME is 263 ms, and the RISE TIME is 8 ms.

Power-off and parameters characterizing the behavior of the power supply when turned off

Switching off the AC/DC power supply can be separated into the following phases:

1. The supply continues to operate due to the residual charge on the input capacitors until the voltage across them drops to a certain critical level; at this point, maintaining the output voltage at the nominal value becomes impossible.
2. Reducing the output voltage, further stopping the converter, and accelerating the voltage drop.

HOLD UP TIME

Time from disconnecting the power source until the power supply's output voltage reaches 90% of the rated level at full load.

The parameter is essential when powering critical data collection and storage devices. The general requirement is at least 16 ms to ensure sufficient time for the UPS to kick in.
Mainly depends on the total capacitance of the input capacitors.

FALL TIME

Output voltage fall time when switched off from 90% to 10% of the rated level at full load

Mainly depends on the total capacitance of the output capacitors.

How to measure these characteristics

Before recording the oscillogram:
Make sure that the load is at 100%. Switch the oscilloscope to standby and turn off the circuit. Keep an eye on the dynamic range; turn it on again and repeat from the beginning if necessary.

An approximate picture of the oscillogram (the same device was tested), obtained using the described method, is shown in the figure below:

It can be seen that the HOLD-UP TIME is only 14 ms, and the FALL TIME is 18 ms.

Measuring diagram/test bench

To measure parameters, one can use the diagram shown in the figure:

J1 - AC grid input
J3 - power supply output
J2 - oscilloscope connections
SW2 - zero-cross switching on and off
SW3 - instant switching on and off for inrush current measurements
SW1 - circuit sensitivity toggle for inrush current measurements