Basics of switching power supply [Intermediate] Specification items and standards


Someone who knows the basics of power

“I want to know the items in the specifications in detail.
I want to be able to read the specifications myself.
Please tell us about the safety standards. ”

We will answer these requests.

Basics of switching power supply [Beginner] There is an easy-to-understand explanation without circuits and theory.


Main items of specifications

there are items that you should remember in the specifications.

Efficiency and power factor

The power factor indicates how effectively the input power is being used.
It is expressed by a number such as 0.6 or 0.98. If there is no power factor improvement circuit, it will be about 0.6, but if there is a power factor improvement circuit, it will be 0.95 or more.
At “0.6”, 60% of the input power is used and the remaining 40% is unused and returns to the input line. “0.98” means that 98% is used effectively and 2% is returned.

Since the power meter displays the amount of power obtained by subtracting the power returned without being used (reactive power), the reactive power is not counted in the electricity charge.

So what are the benefits of improving power factor?
By reducing the input power, the thickness of the wire and the parts on the input side can be reduced. Not only that, it also affects the input electrical equipment and the amount of contracted power, which may result in a significant cost reduction.

The formula for power factor and efficiency

Power factor = active power ÷ apparent power
Efficiency = output power ÷ active power

Active power is the power (W) consumed by the load device.
Apparent power is expressed as power supply input voltage (V) x input current (A). It is the sum of active power and active power.
The unit is VA because it is not power consumption.

Now, let’s actually calculate the output power from the specifications of the 100W 24V output power supply.
Not many people can calculate this.

  • Input voltage AC200V
  • Input current 0.65A
  • Power factor 0.93
  • Efficiency 89%

The apparent power is 200V x 0.65A = 130VA. The unit is VA because it is not power consumption.
Since the power factor of the apparent power of 130VA is 0.93, which is the active power, 130VA x 0.93 = 121VA (W).
Since the active power of 121W becomes the output power with an efficiency of 89%, it becomes 121W x 0.89 = 108W.

Looking at the output power of the specifications, it was 108W.

The lost power = active power-output power = 121W-108W = 13W.


Inrush current (A)

Also called input surge current.
This is the maximum instantaneous current that flows into the input smoothing capacitor when the input is turned on. The AC200V input is double the AC100V input.

The table below is a rough guide. It has little to do with power capacity or output current.

AC100V AC200V
150W or less 15A 30A
300W or more 20A 40A


Inrush current is a value required when selecting switches, fuses, and breakers, and is often asked by users. The time for the inrush current to flow is 5 ms or less.
There is a table of “current value and time” in the data such as breaker, so check the current value when the time is 5ms. Select a component that can flow a current value larger than the input surge current of the power supply.
This is a rough idea, so please contact each manufacturer for the actual selection.

In addition, the input surge current when multiple power supplies are used is the sum of the inrush currents of each power supply, which is a large value.

Leakage current (mA)

Also called leakage current or leakage current.
This is the value of the current that flows from the input line to the ground through the housing.

There is also the merit of attenuating the noise terminal voltage by flowing it to the ground.
Therefore, the noise terminal voltage of a power supply with low leakage current does not drop.

Leakage current is regulated by the safety regulations of each country from the aspect of electric shock safety.
Especially for medical devices that come into contact with the human body, the standard of leakage current is strict.

Since the leakage current is regulated for the entire medical device, it is the total value of the leakage current of the power supply and the leakage current of other parts.
In addition, the leakage current when using multiple power supplies is the total leakage current of each power supply.

Holding time(ms)

This is the time until the output voltage starts to drop after the power input is cut off.

The holding time is generally 20ms. Retention time when the load factor is 100%.
The lower the load factor, the longer the retention time.
A larger power supply may be used to extend the retention time.

Users use this retention time to back up their data in the event of a power outage or momentary power outage.

Overcurrent protection (OCP)

A function that protects the power supply by limiting the output current and lowering the output voltage so that the output current does not flow beyond the specified value.

When the overcurrent protection is activated, the output voltage drops, and there are three ways to drop it.

・Hanging down method
・foldback current limiting characteristic(フ-shaped)
・no limiting characteristic(へ-shaped)

In the hanging down method, the current value at which the overcurrent protection is activated remains the same, and only the voltage drops straight down.

In the foldback current limiting characteristic, both the current value and the voltage value decrease when the overcurrent protection is activated.

In the へ-shaped method, which is difficult to recover automatically, the voltage drops, but on the contrary, the current value flows indefinitely, resulting in a “short state”. There is a possibility of smoke and ignition due to the method often used for low-cost products. You should check which method the power supply is.

Overvoltage protection (OVP)

This function stops the output when the output voltage rises above the rated voltage. Overcurrent protection is intended to protect the power supply itself, whereas overcurrent protection is intended not to destroy the load.

Overvoltage protection operates above 105% of the rated voltage. When it operates, the output will stop.
The voltage does not drop immediately (directly below), but gradually.

Also, unlike overcurrent protection, it does not automatically recover, so to output, turn off the input voltage and then input again.

Remoto Sensing

This function corrects the voltage drop between the output terminal of the power supply and the load.The + S and -S points on the terminal block and connector are the remote sensing terminals.

For example, suppose that the output terminal of the power supply outputs 5V, while the load side outputs 4.9V.In this example, 0.1V is a voltage drop.

When the remote sensing function is used, the power supply outputs 5.1V plus 0.1V.
Connect the + S terminal to the + terminal of the load and the -S terminal to the-terminal of the load.

Remote (ON / OFF) control

It is also called a remo-con for short.

It is a function to turn on / off the power output by the control signal while inputting.

The control signal method differs depending on the model.
For example, when a voltage of 5V is applied between certain terminals, it turns on, and when a voltage of 1V or less is applied, it turns off.

No inrush current is generated when this function is used.