Terminology to Know When Mounting a Switched-mode Power Supply

When mounting switched-mode power supplies at manufacturing sites, it’s essential to understand the specification sheet. Here we arrange and introduce the basic technical terminology that appears in specification sheets from different perspectives: input, output, functionality, environment, and insulation.

Input	Voltage range (input voltage range) Frequency range Power factor Efficiency Input current Input surge current (inrush current) Leakage current
Output	Rated voltage (rated direct current output voltage) Maximum current (maximum direct output current) Minimum current (minimum output current) Maximum power (maximum output power) Maximum input fluctuation (static input fluctuation) Maximum load fluctuation (static load fluctuation) Maximum temperature fluctuation Ripple noise Retention time (output retention time) Voltage variation range Voltage setting precision
Functionality	Overcurrent protection (OCP) Overvoltage protection (OVP) Remote sensing Remote on/off control Parallel operation Series operation Instantaneous voltage drop protection
Environment	Operating temperature (operating ambient temperature) Storage temperature Operating humidity (operating ambient humidity) Storage humidity Vibration resistance Impact resistance Cooling method
Insulation	Withstand voltage Insulation resistance

Input

Voltage range (input voltage range)

Voltage range expresses the guaranteed input voltage range shown on the specification sheet, and is expressed using the unit V.

• AC input voltage: AC**V–***V (Three-phase AC: øAC**V–***V)
• Direct current input voltage: DC**V–***V

With AC input, input voltage is expressed using root-mean-square (RMS) value. The input voltage defined in the standard specifications is regulated by the voltage between the power supply input terminals.
With an AC power supply, there are two types of AC: single-phase AC and three-phase AC. The differences between the two are shown in Fig. 1 and Fig. 2.
Generally, single-phase (100 V or 200 V) is used for low power supplies and three-phase (200 V) is used for high power supplies.

Fig. 1 Single-phase AC

Fig. 2 Three-phase AC

In three-phase systems, there are three equal electromotive forces of the same frequency with a phase difference (time difference) of 120 degrees (2 π ÷ 3).
Compared to single-phase systems, the three-phase system in Fig. 2 has three electromotive forces, and adjusting these can create a direct current voltage with low ripples.

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Frequency range

Frequency range expresses the guaranteed frequency range of the AC input voltage shown on the specification sheet, and is expressed using the unit Hz.

Fig. 3 Voltage waveforms of commercial power supply (AC 100 V (50 Hz)

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Power factor

Power factor is the ratio of working power to apparent power, and shows how effectively input power is being used. (Only power supply that conforms with high harmonic current regulations is noted in the standard specifications)

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Efficiency

Efficiency is the ratio of output power to working input power, and is expressed using the unit %.

Fig. 4 Approach to efficiency

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Input current

Input current is the current supplied to the power supply (expressed as root-mean-square (RMS) value) and is expressed using the unit A.

Fig. 5 Input voltage and peak current

In switched-mode power supplies, even if input voltage is a sinusoidal AC voltage, input current will not always be a sinusoidal AC current.
As such, it’s important to note that input power does not equal control value × input current.

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Input surge current (inrush current)

Input surge current is the maximum instantaneous current that flows into the smoothing capacitor when power is input, and is expressed using the unit A. Input surge current is important information when selecting switches and external fuses.
There are three types of input surge current prevention circuits: Power thermistor types, thyristor types, and relay types.

Fig. 6 Inrush current

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Leakage current

Leakage current is the current that passes from the input line through the housing and into the ground, and is expressed using the unit mA. There are safety standards for leakage currents in each country to prevent electric shocks and ensure safety in other ways.
Leakage current mainly refers to the current that flows into the ground (earth) from ground capacitors (Y-capacitors) in input filter circuits. To reduce leakage current, users must reduce the capacity of the Y-capacitor on the filter circuit. (Please note that noise attenuation characteristics will change)

Fig. 7 Basic power supply input circuit diagram

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Output

Rated voltage (rated direct current output voltage)

Rated voltage is the direct current voltage generated between power supply output terminals, and is expressed using the unit VDC.

Fig. 8 Correct output voltage measurement method

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Maximum current (maximum direct output current)

Maximum current is the maximum output current that can be continuously supplied from the power supply, and is expressed using the unit A.

Average current lm (average output current)
The output current that can be continuously supplied from the power supply at peak load.

Maximum peak current lp (maximum peak output current)
The maximum output current that can be suppled within the regulated time from the power supply at peak load.

Fig. 9 Output peak current calculation

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Minimum current (minimum output current)

Minimum current is the minimum current required for stable operation of power supply. Minimum current is regulated for some products.

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Maximum power (maximum output power)

Maximum power is the maximum output power that can be continuously supplied from the power supply, and is expressed using the unit W.

Fig. 10 Correct output voltage measurement method

Example:
Single output: 5 V × 20 A = 100 W
Triple output: (5 V × 3 A) + (12 V × 2 A) + (12 V × 1 A) = 51 W

Total maximum output power (multi output power supply only) is the sum of power from each output source that can be continuously supplied from the power supply.

Watt enclosure
If the total output power from each output CH is within the maximum total power on the technical specifications, output current can be combined freely (the output current/power must not exceed each CH).

Example: When using a triple output power supply
Total maximum output power ≧ CH1 output power + CH2 output power + CH3 output power

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Maximum input fluctuation (static input fluctuation)

Maximum input fluctuation is the maximum output voltage fluctuation when input voltage is gently changed within the input voltage range, and is expressed using the unit mV or %.

Fig. 11 Correct output voltage measurement method

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Maximum load fluctuation (static load fluctuation)

Maximum load fluctuation is the maximum output voltage fluctuation when output current is gently changed within the values shown on the standard specifications, and is expressed using the unit mV or %.

Fig. 12 Correct output voltage measurement method

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Maximum temperature fluctuation

Maximum temperature fluctuation is the output voltage fluctuation when changing only the ambient temperature of the power supply. These figures allow the user to confirm whether output voltage is stable even when changing ambient temperature.

Example:
Products that show 0.02%/℃ mean that the output voltage changes at the rated value of 0.02% for every 1℃.

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Ripple noise

Ripple noise is the maximum value of the minute AC voltage superimposed onto output voltage, and is expressed using the unit mVp-p.

Fig. 13 AC-DC power supply ripple noise

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Retention time (output retention time)

Retention time is the time it takes for output voltage to start dropping (for voltage precision to decrease) after shutting down power supply input, and is expressed using the unit ms.
During power outages and instantaneous power failures, users can use this time to protect equipment operation, such as by saving computer memory.

Fig. 14 Output voltage retention time

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Voltage variation range

Voltage variation range is the range within which output voltage can change, and is expressed using the unit VDC or %. Output voltage changing methods differ by product, and so it is important to check the handling instructions for each product.

Please note that with power supplies equipped with overvoltage protection circuits, turning up the output voltage adjustment volume too much (increasing the output voltage too much) can cause overvoltage protection to kick in and shut down the power supply. Further, when increasing output power supply, output current must be reduced to the value regulated in maximum output power.

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Voltage setting precision

Voltage setting precision is the precision of the output voltage set at the time of shipment.

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Functionality

Overcurrent protection (OCP)

Overcurrent protection is a function that controls output current to prevent it flowing above the specified value and at the same time prevents power supply breakdown by reducing output voltage, and is expressed using the unit % or A.

Values shown on the standard specifications show the range at which overcurrent protection kicks in (the output current at which output voltage starts to drop). Please note that continuous operation in an overcurrent state can lead to power supply breakdown (please refer to the handling instructions for each product).

Main characteristics of overcurrent protection
Overcurrent protection is a function that protects load and power supply when excess load current flows due to load short circuits.
There are two main overcurrent protection types: constant current voltage drooping characteristic types and foldback types.

Constant current voltage drooping characteristic type

When output current reaches the overcurrent detection value, the current is maintained while reducing the voltage.
When output current falls below the overcurrent detection value, the output voltage automatically returns. This is recommended when continuous operation is required.

Fig. 15 Constant current voltage drooping characteristic type

Foldback type

The shape of foldback overcurrent protection is shown in Fig. 16. Both output voltage and current fall when the overcurrent detection value is exceeded. With this method, as the output voltage does not automatically return, power must be reinput.

Fig. 16 Foldback type

There are other products that allow for intermittent operation and output shutdown.

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Overvoltage protection (OVP)

Overvoltage protection is a function that shuts down output when output voltage exceeds the specified value and prevents the breakdown of loads connected to the output, and is expressed using the unit % or VDC.
Values shown on the standard specifications show the range at which overvoltage protection kicks in (the voltage at which output voltage starts is shut down).

Fig. 17 Overvoltage protection waveform

Output shutdown manual reset type

This type of overvoltage protection shuts down output when OVP kicks in. To return output, input voltage must temporarily be turned off, and power must be reinput after a certain amount of time. It is a model that can be reset by turning controls on and off.

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Remote sensing

Remote sensing is a function that corrects voltage drops caused by wiring resistors between the power supply output terminal and load.

Remote sensing is only possible with power supplies equipped with +S and -S terminals. +S and -S terminals detect output voltage, and predetermined output voltage is supplied to the points (sensing points) where the terminals connect.

Note: Correcting major line drops can cause the voltage between output terminals to rise, and may lead to output being shut down by OVP.

For systems that do not use remote sensing (local sensing systems), due to cable resistors, the voltage in load terminals will drop below the predetermined output voltage values.

Example:
With an output current of 10 A and a cable resistance of 0.01 Ω, voltage decreases caused by cables will be 10 A × 0.01 Ω = 0.1 V, and load terminal voltage will be 5 V – 0.1 V = 4.9 V.

Fig. 18 Output voltage adjustment method (1)

Through remote sensing, the voltage of load terminals connected to +S and -S becomes the predetermined output voltage. The voltage between output terminals then exceeds the predetermined output voltage.

Example:
With an output current of 10 A and a cable resistance of 0.01 Ω, voltage decreases caused by cables will be 10 A × 0.01 Ω = 0.1 V, and load terminal voltage will be 5 V + 0.1 V = 5.1 V.

Fig 19. Output voltage adjustment method (2)

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Remote on/off control

Remote on/off control is a function that uses control signals to turn output on and off while power is still being input. Specifications differ depending on the product, and so it is important to check the handling instructions for each product.

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Parallel operation

Parallel operation is when multiple power supply units are used to increase output current.
In the case of a power supply breakdown, parallel operation is also used as a backup to maintain voltage supply to the load through a reserve power supply. Specifications differ depending on the product, and so it is important to check the handling instructions for each product.

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Series operation

Series operation is when multiple power supply units are used to obtain the necessary voltage.

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Instantaneous voltage drop protection

SEMI-F47

SEMI F47 is one standard for semiconductor processing equipment. SEMI F47 testing simulates when AC power supplies cause a sudden drop in voltage (several seconds from half a cycle) due to equipment failures or large load fluctuations in semiconductor factories, and assesses the durability of the equipment under test.

Fig. 20 Instantaneous voltage drop protection function

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Environment

Operating temperature (operating ambient temperature)

Operating temperature is the ambient temperature range of a power supply that can guarantee continuous operation while fulfilling standard specifications, and is expressed using the unit ℃.

In some cases, due to ambient temperature, power supplies must be used while derating output power to ensure internal components do not exceed the rated temperature. Please note that the output derating curve may differ depending on the mounting method and presence or absence of a cover.

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Storage temperature

Storage temperature is the ambient temperature range at which performance can be maintained even when the power supply is inactive, and is expressed using the unit ℃.

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Operating humidity (operating ambient humidity)

Operating humidity is the relative humidity range of a power supply that can guarantee continuous operation while fulfilling standard specifications.

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Storage humidity

Storage humidity is the relative humidity range at which electrical performance can be maintained even when the power supply is inactive, and is expressed using the unit %RH.
Continuous storage in highly humid conditions can lead to rust and other issues and so should be avoided.

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Vibration resistance

Vibration resistance ensures power supplies will not breakdown in regulated testing conditions (JIS-C-60068-2-6 Sinusoidal vibration test method).

Example:

(1) Vibration frequency range: 10 to 55 Hz

(2) Time frequency is changed: 10 to 55 Hz and 55 to 10 Hz are changed each minute.

(3) Speed: Constant 19.6 m/s².

(4) Vibration directions: Three directions X, Y, Z

(5) Testing time: One hour in each direction, total of three hours. The test is undertaken while the power supply is inactive. （9.8m/s²=1G）

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Impact resistance

Impact resistance ensures power supplies will not breakdown in regulated testing conditions (IEC 60068-2-6 (2007) Impact test method). The test is undertaken while the power supply is inactive.

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Cooling method

Cooling is a way to release the heat generated from the power supply.

Natural air cooling

Heat release through natural convection

Fig. 21 Heat release through natural convection

Forced air cooling

Heat release through use of a fan

Fig. 22 Heat release through use of a fan

Conduction cooling

Heat release through thermal conduction

Fig. 23 Heat release through thermal conduction

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Insulation

Withstand voltage

Withstand voltage is the applied voltage that can be withstood between specific terminals. Withstand voltage checks for insulation faults in primary-secondary circuits and primary-FG circuits.

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Insulation resistance

Insulation resistance is the resistance value when a regulated direct current voltage is applied between specific terminals, and is expressed using the unit MΩ. Insulation resistance measures the resistance value of insulation material and confirms whether the insulation has degraded.

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Reference: TDK Lambda material

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