Notes on choosing a switching power supply?


  1. To increase the reliability of the S.P.S., we suggest users choose a unit that has a rating of 30% more power than actual need. For example, if the system needs a 100W source, we suggest that users choose a S.P.S. with 130W of output power or more. By doing this, you can effectively boost the reliability of the S.P.S. in your system.
  2. We also need to consider about ambient temperature of the S.P.S. and whether there is additional device for dissipating the heat. If the S.P.S. is working in a high temperature environment, we need to make some derating to the output power. The derating curve of "ambient temperature" versus "output power" can be found on our spec sheets.
  3. Choosing functions based on your application:
              • Protection function: Over Voltage Protection (OVP), Over Temperature Protection (OVP), Over Load Protection (OLP), and etc.
              • Application function: Signaling Function (Power Good, Power Fail), Remote Control, Remote Sensing, and etc.
              • Special function: Power Factor Correction (PFC), Uninterruptible Power Supply (UPS) function.

     4.   Make sure that the model qualifies for the safety standards and EMC regulations you need.


Can MEAN WELL's power supply be used in the range of 45Hz ~ 440Hz? If YES, what will happen?


MEAN WELL's power supply can be used within this frequency range. But if the frequency is too low, the efficiency will also be lower. For example, when a SP-200-24 is operated under 230VAC and rated load, if the frequency of AC input is 60 Hz, the efficiency is around 84%; however, if the frequency of AC input reduces to 50 Hz, the efficiency will be around 83.8%. If the frequency is too high, the power factor of the S.P.S. with PFC (power factor correction) function will reduce and this also will cause higher leakage current. For example, when a SP-200-24 is operated under 230VAC and rated load, if the frequency of AC input is 60 Hz, the power factor is 0.93 and the leakage current is around 0.7mA; however, if the frequency of AC input increase to 440 Hz, the power factor will decrease to 0.75 and the leakage current will rise to around 4.3mA.


What is minimum load requirement and how can I read it from the spec?


There are some minimum-load requirements on MEAN WELL's multi-output power supplies. Please read the specification first before connecting to the load. In order to allow the power supply to work properly, a minimum load for each output is required, or else, the output voltage level will be unstable or outer tolerance range. Please refer to “Current range” in the specification as shown in the table below: Channel 1 requires a 2A minimum-load; channel 2 requires 0.5A; Channel 3 requires 0.1A ; Channel 4 does not need any minimum-load.


Why did the power supply shuts down during operation and after turning it off, I can restart the power supply again?


In general there are two circumstances that will cause the power supply to shut down. The first one is the activation of the over-load-protection (OLP). To deal with this situation, we suggest increasing the rating of the output power or modifying the OLP point. The second one is the activation of over-temperature protection (OTP) when the internal temperature reaches the pre-set value. All of these conditions will let the S.P.S. enter protection mode and shut down. After these conditions are removed, the S.P.S. will be back to normal.


What is the control mechanism for cooling fans?


Cooling fans have a relatively shorter lifetime (typical MTTF, Mean Time To Failure, of around 5000-100000 hours) compared with other components of power supply. As a result, changing operating method of the fans can extend the operation hours. The most common control schemes are shown as below:

  1. Temperature control: if the internal temperature of a power supply, detected by a temperature sensor, is over the threshold, the fan will start working at full speed, whereas, if the internal temperature is less than the set threshold, the fan will stop working or run at half speed. In addition, cooling fans in some power supplies are controlled by a non-linear control method whereby fan speed can be changed with different internal temperatures synchronously.
  2. Load control: if the loading of a power supply is over the threshold, the fan will start working at full speed, whereas, if the loading is less than the set threshold, the fan will stop working or run at half speed.


What is "Inrush Current"? What will we notice?


At input side, there will be (1/2 ~1 cycle, ex. 1/120 ~ 1/60 seconds for 60 Hz AC source) large pulse current (20~100A based on the design of S.P.S.) at the moment of power on and then back to normal rating. This "Inrush Current" will appear every time you turn on the power. Although it will not damage the power supply, we suggest not turning the power supply ON/OFF very quickly within a short time. Besides, if there are several power supplies turning on at the same time, the dispatching system of AC source may shut off and go into protection mode because of the huge inrush current. It is suggested that these power supplies start up one by one or use the remote control function of S.P.S. to turn them on/off.


What is PFC?


Power Factor Correction or PFC is to improve the ratio of apparent power to real power. The power factor is around 0.4~0.6 in non-PFC models. In models with PFC circuit, the power factor can reach above 0.95. The calculation formulas are as follows: Apparent Power=Input Voltage x Input Current (VA), Real Power= Input Voltage x Input Current x Power Factor (W).
From the point of view of environment friendly, the power plant needs to generate a power which is higher than apparent power in order to steadily provide electricity. The real usage of electricity is defined by real power. Assuming the power factor is 0.5, the power plant needs to produce more than 2WVA to satisfy 1W real power usage. On the contrary, if the power factor is 0.95, the power plant only needs to generate more than 1.06VA to provide 1W real power, It will be more effective in energy saving with PFC function.
Active PFC topologies can be divided into single-stage active PFC and two-stage active PFC, the difference is show as in the table below.

PFC topology Advantage Disadvantage Limitation
 active PFC
 Low cost
 Simple schematic
 High efficiency in 
 watt application
 Huge Ripple
 complex feedback 
 1.Zero “hold up time”. The output is
    affected by the AC input directly.
 2.Huge ripple current results in lower LED life
    cycle.(drive the LED directly)
 3.Low dynamic responds, easily affected by
 Two-stage active
 High efficiency
 Higher PF
 Easy feedback control
 High adoptive against 
 load condition
 Higher cost
 Complex schematic
 Suitable for all kinds use



What is the difference between -V and COM which are marked on the output side?


COM (COMMON) means common ground. Please see below:
Single output: Positive pole (+V), Negative pole (-V)
Multiple output (Common ground): Positive pole (+V1, +V2,.), Negative pole (COM)


In MEAN WELL's catalog, we see AC and DC at input, what is it all about?


Due to different circuit designs, MEAN WELL power supply's input consists of three types as below:
c.85~132VAC/176~264VAC by Switch; 250~370VDC

          • In a and b inputs models, power supply can work properly no matter under AC or DC input. Some models need correct connection of input poles, positive pole connects to AC/L; negative pole connects to AC/N. Others may require opposite connection, positive pole to AC/N; negative pole to AC/L. If customers make a wrong connection, the power supply will not be broken. You can just reverse the input poles and power supply will still work.
          • In c input models, please make sure that you switch the 115/230V input correctly. If the switch is on the 115V side and the real input is 230V, the power supply will be damaged.           


What is MTBF? Is it distinct from Life Cycle?


 MTBF (Mean Time Between Failure) and Life Cycle are both indicators of reliability. MTBF can be calculated by two different methodologies, which are “part count” and “stress analysis”. The regulations, MIL-HDBK-217F Notice 2 and TELCORDIA SR/TR-332(Bellcore) are commonly used to calculate MTBF. MIL-HDBK-217F is a United States military standard, and TELCORDIA SR/TR-332(Bellcore) is a commercial regulation. MEAN WELL utilize MIL-HDBK-217F(Stress Analysis) as the core of MTBF. The exact meaning of MTBF is, after continuously using the power supply for a certain amount of time, the average time that the probability of proper operation is down to 36.8%(e-1=0.368). Currently MEAN WELL is adopting MIL-HDBK-217F, forecasting the expected reliability through Stress Analysis (excluding fans); this MTBF means the probability of the product can continue the normal work after working continuously up to the calculated MTBF time is 36.8% (e-1=0.368). If the power supply is continuously used at double the MTBF time, the probability of proper operation becomes 13.5%(e-2=0.135). Life Cycle is found by using the temperature rise of electrolytic capacitors under maximum operating temperature to estimate the approximate life of the power supply. For example, RSP-750-12 MTBF=109.1K hours(25°C); electrolytic capacitor C110 Life Cycle=213K hours (Ta=50℃)
DMTBF(Demonstration Mean Time Between Failure) is a way of evaluate MTBF。Please refer to the following equation for MTBF calculation.


MTBF:Life time determine by specification.

X2:Can be found in chi-square distribution

N:Number of sampling

AF:Acceleration factor, which can be derived from acceleration factor equation.


K(Boltzmann Constant)=(eV/k)

T1:Rated temperature of specification. Note: Kelvin will be the unit use for calculation

T2:The temperature that is used in the meaning of acceleration, and the chosen temperature could not result in physical change in materials. Note: Kelvin will be the unit use for  calculation.  


What is power good and power fail signals and how can use it?


Some power supplies provide a "Power Good" signal when they are turned on, and send out a " Power Fail" signal when they are turned off. This is usually used for monitoring and controlling purpose.
Power Good: after the output of a power supply reaches 90% rated voltage, an TTL signal (about 5V) will be sent out within the next 10-500ms.
Power Fail: before the output of a power supply is less than 90% rated voltage, the power-good signal will be turned off at least 1ms in advance.


I have an TN-1500 inverter. Why the LED indication of AC IN is not lit after mains input has been applied?


Cooling fans have a relatively shorter lifetime (typical MTTF, Mean Time To Failure, of around 5000-10000 hours) as compared with other components of power supplies. As a result, changing operating method of cooling fans can extend the operation hours of the fans. The most common control methods are shown below:

  1. Temperature control: if the internal temperature of a power supply detected by a temperature sensor is over the set threshold, the fan will start working at full speed, whereas, if the internal temperature is less than the set threshold, the fan will stop working or at half speed. In addition, cooling fans in some power supplies are controlled by a linear control method whereby fan speed can be changed with different internal temperatures synchronously.
  2. Load control: if the loading of a power supply is over the set threshold, the fan will start working at fullspeed, whereas, if the loading is less than the set threshold, the fan will stop working or at half speed.


What are the protection forms of overload/overcurrent?


When current drawn exceeds the rating of the PSU, the protection circuit will be triggered to protect the unit against overload/overcurrent. 
Protections of overload/overcurrent can be divided into several forms:
Output current decreases about 20% of rated current, shown as curve (a) in the figure below. 
Output current remains at a constant level and within the specified range while the output voltage drops to a lower level, shown as curve (b) in the figure below. 
Output power remains constant. As output load increases, output voltage decreases in proportion, shown as curve (c) in the figure below.  
Output voltage and current keep pulsing ON and OFF repeatedly when protection is activated. The unit automatically recovers when faulty condition is removed.
 Output voltage and current are cut off when output load reaches protection range. 
NOTE: Protection mode of some of the products combines with different types of the forms mentioned, such as constant current limiting + shut down.

Recover method:
(1)Auto Recovery: PSU recovers automatically after faulty condition is removed.
(2)Re-power on: PSU restarts by manual AC re-power on after faulty condition is removed.
Note:Please do not operate PSU in overcurrent or short-circuit condition for a long period of time to prevent a shorten lifespan or damaging the PSU.


What is Ripple & Noise? How to measure it?


It is the small unwanted residual periodic variation of the direct current (DC) output of a power supply which has been derived from an alternating current (AC) source. The wave form is shown as figure below.

There are two AC contents, also known as Ripple and Noise (R&N), on the DC output. The first one, coming from sine wave rectification, is at a low frequency which is 2 times of the input frequency; the second one is at high frequency which is from the switching frequency. For measuring high frequency noise, configurations of an oscilloscope with a bandwidth of 20MHz, a scope probe with shortest ground wire possible, and add 0.1uF and 47uF capacitors in parallel with test point for filtering out noise interference are requires to be made.


What is Withstand Voltage?How to measure it?


Indicates Hi-Pot Test or Electric Strength Test. The input should be shorted together as well as the output before test. The test will proceed under particular loop, such as I/P-O/P, I/P-FG and O/P-FG with certain high voltage value for 1 minute. (The typical leakage current is 25mA when testing with AC)

  1.  Hi-Pot Test is a way to ensure if the isolation between primary to secondary is done properly, preventing damaging to S.P.S. when facing high voltage between input and output. The test voltage should be gradually increased from 0V to preset level and remains at preset level for 60 seconds with raise time greater than 1 second. In mass production, the test period could be reduced to 1 second. If the leakage current flowing through the isolation material increases rapidly when applying test voltage, it indicates ineffectiveness of isolation (dielectricbreakdown). Corona effect/discharge or transient electrical arc is not considered as failure.
  2. When AC test voltage is applied, Y capacitors are the main cause of leakage current. A 4.7nF capacitor can cause leakage current of 5mA. According to regulations of UL-554, the Y capacitors should be removed for Hi-Pot test, which is not practical for mass production. The only solution is to increase the leakage current setting, typically 25mA, of test instrument. Presently, the criteria of leakage current are not defined in safety regulations.
  3. According to regulations of IEC60950-1, DC test voltage can be substituted when there are bridging capacitors coupled between primary and econdary circuits, so as to solve the problem of leakage current.


How to choose a suitable power supply for a charging application?


MEAN WELL has launched ENC, HEP-600C, GC, PA, PB, RPB and RCB series for battery charge applications (30~360W). However, if these models still cannot fulfill customer’s requirements, there is an alternative for the purpose. Power supplies with constant current limiting as overload protection are suggested. Charge current varies in battery percentage (full or flat), there is high possibility to trigger overload protection when battery is low, those with overload protection as hiccup or shutdown will stop charging the battery in low battery condition. Yet, using a power supply as charging purpose is considered as over load usage, modification is required. Please contact MEAN WELL for the request.


what are the applications for PSUs connected in series?


there are two types of the applications, one is to create positive and negative voltages, one is to build higher output voltage. Connection methods are shown as below:
(1)Positive and negative voltage

(2)Increase the output voltage (current does not change). If there is no reverse blocking diode in the power supply, we should add an external blocking diode to prevent the damage of power supply while starting up. The voltage rating of the external diode should be greater than V1+V2. Transit current rating of diodes shall be greater than rate output current of S.P.S.



Requirements on parallel operation?


When power supplies are connected in parallel, it is able to increase the output current or use them as the redundant (back-up) function. Be sure that the difference in
output voltage and wiring impedance should be small when operating in parallel.
1. Connect P(LP/CS) terminals together such as the PSP models (Please refer to the parallel function of specification). Input and output should be connected in parallel before connecting to the AC source and loads. Shown
as in picture below (some S.P.S. require a minimum load after paralleling).

2. Output voltage difference between S.P.S. units should be as small as possible, normally < 0.2V.
3. The power supplies should be paralleled with short and large diameter wiring together first, and then connected to the load.
4. After paralleling, the maximum usage of total power should be around 90% of the rated total power.
5. When power supplies are paralleled, if the load is lower than 10% of rated load of individual S.P.S. The LED indicator or signals (Power Good、Pok、Alarm Signal) may malfunction.
6. To ensure that the load current is effectively shared in parallel operation, in general, it is recommended not to use more than 4-6 power supplies at one time.
7. In some models, the +S, -S terminals should be used to reduce unstable pulsation of output voltage.


Why I can not turn on the power supply smoothly when the loads are motors, light bulbs or capacitive loads?


If you connect the S.P.S. to motors, light bulbs, or high capacitive loads, you will have a high output surge current when you turn on the S.P.S. and this high surge current will cause failure of start up. We suggest using S.P.S. with constant current limiting protection to deal with these loads.


The output ground (GND) and frame ground (FG) is the same point in my system, can MEAN WELL's power supplies be used in such system?


Yes. Since our products are designed based on isolation concept, it will be no problem that the output ground (GND) and frame ground (FG) is the same point in your system. But, EMI may be affect by this connection.


During the operation of your power supply, there is some leakage current on the case. Is this normal? Will this leakage current hurt human body?


Due to the requirement of EMI, there will be some Y capacitors between line and neutral to the FG (case) to improve EMC. These Y capacitors will cause some leakage current flow from line or neutral to the case (normally case will be connected to earth ground). For example, IEC-60950-1 requires that this current should be less than 3.5mA for IT equipment, so basically the leakage current you find on the case will not hurt human body. Proper connection to Earth ground will solve the leakage current problem.


For certain need, is it possible to reduce the noise of fan?


Noise is directly related to the fan which is build into the power supply. Lowering the airflow of the fan means reducing the heat dissipation ability. It will also influence the reliability of the products. Furthermore, minimum airflow of fans is defined by Safety Organization and a safety appendage will be needed if using a new fan. Generally, when we choose a suitable power supply, fan is not necessary if wattage is under 150W. Between 150~500W, both fan and fanless products are available. Above 500W, a fan is needed.


What should be noticed when installing a power supply in vertical and horizontal directions?


Most small wattage and fanless power supplies are mainly installed in the horizontal position. If you have to install it vertically because of mechanical limitation, you should consider the output derating due to the heat concern. The temperature derating curve can be found on the spec sheet. Regarding the power supplies with built-in fan or the application has forced cooling system, vertical and horizontal installations have less difference. Ex. In SP-150 derating curve, the ambient temperature difference in application is 5 Celsius from vertical to horizontal. The output wattage in forced cooling can be 20% higher than air cooling convection.


How to select a suitable proper circuit breaker or fuse for MEAN WELL SPS?


At input side, there will be a large transient inrush current during the power on of an SPS. The inrush current is usually of 20~100A based on the design of SPS, which will last for 1/2~1cycle of AC input (ex. 1/120~1/60 seconds for 60Hz AC source). Every breaker or fuse will actually have its own time-current (V-I) characteristic curve. Users can choose a breaker or fuse which has a rated current just over input current of SPS and check whether the V-I curve of the breaker or fuse can withstand the high inrush current from the SPS with a maximum duration of 1 cycle of AC input (e.g. 1/60 second).


Can MEAN WELL charger use on Lithium battery or other type of battery?


All MW chargers are designed for lead-acid batteries. Every Lithium battery or other type of battery will have its own charging/discharging characteristic. Modification will be required for MW's charger in order to match the profile of each battery or users' requirements. Please contact your local distributor if you would like to have a modification for your own battery.


Points on connecting wires used in power supplies?


To install a power supply into the system, you would need wires for the connection both to the loads and to the energy source. There are a couple of points that should be taken into consideration when choosing wires, one is current rating, it may cause high heat on the wires or burnt out in the worst case, if the rating is not enough. The other is voltage drop, there would be voltage reduction at load side as current moves through the wires owing to the internal resistance. If too much voltage drop in line, there could be no sufficient voltage to drive the loads. You can find the right wires for use by referring to the table below on the basis of your system design.


How to select adaptors with the correct AC plug for use in different countries?


An adaptor may need connection of a power cord to receive energy needed from the utility. You can refer to the specification of the adaptor for the connector (AC inlet) at the adaptor end of the power cord; Different countries/regions vary in type of AC socket and voltage, please look at the table below for the information of the AC plug you need.


A few things you should know before using the Remote Sensing function


First, use twisted wires, connect +S to the positive end of the output, then -S to the negative end of the output, as shown in the illustration. Also, keep the wires off the AC and output cables to prevent noise interference.
Add capacitors at output end where the Remote Sensing wires are connected to if dynamic load (frequency above 1K Hz) is used. The purpose is to reduce noise as Remote Sensing is a sensitive function. A suitable capacitor requires two factors:
a. Rated Ripple Current is 0.2 times greater than the output current
b. Rated Voltage is higher than the output voltage


Is it possible to build your own charging curve on the smart chargers when the pre-defined charging curves do not satisfy battery charging requirements?(Video tutorial Inside)


Yes, charging curves of smart chargers including ENC series, RPB series and RCB series can be set and adjusted through SBP-001, the charging programmer.
SBP-001 utilizes the software with the connection between the charger and itself to allow users to programme charging curves.
Adjustable functions are:
Charging parameter adjustment: Values of constant current (CC), constant voltage (CV), float voltage (FV) and tapper current (TC) can be set and adjusted.
Battery temperature compensation: Various charging voltage compensation is provided for battery at different temperature conditions.
Timeout setting: Fully programmable timeout during stages enables to be set to shutdown the charger to prevent battery over-charge.
Please refer to the user’s manual as link below for detailed information:
Following video is an example of demonstrating the ENC-120.


Why the input voltage marked on the spec. sheet is 88~264 VAC while the label on the power supply says that it is 100~240VAC?


During safety verification process, the agency will use a stricter standard -- ?0%(IEC60950 uses +6%, -10%) of the input voltage range labeled on the power supply to conduct the test. So, operating at the wider input voltage range as specified on the spec. sheet should be fine. The narrower range of input voltage labeled on the power supply is to fulfill the test standard of safety regulation and make sure that users insert input voltage correctly.


Will MEAN WELL's products with CE marking meet the EMC requirements after assembling into my system?


We cannot guarantee 100% that the final system can still meet the EMC requirements. The location, wiring and grounding of the switching power supply in the system may influence its EMC characteristics. In different environment or applications, the same switching power supply may have different outcomes. Our test results are based on setup shown in the EMC report.


What is different between information (EN60950-1) and medical (EN60601-1) safety standard?


According to safety standard, the leakage current in EN60950-1 Class I cannot exceed 3.5mA; in EN60601-1 cannot exceed 0.3mA. Others criteria like safe distance and numbers of fuse are also different. Please consult the diagram below:


What is class 2, class II and LPS? What is the difference between class I and class II?


Class I: Equipment where protection against electric shock is achieved by using basic insulation and also providing a means of connecting to the protective earth conductor in the building where by routing those conductive parts that are otherwise capable of assuming hazardous voltages to earth ground if the basic insulation fails. This means a class I SPS will provide a terminal/pin for earth ground connection.
Class II: Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions, such as double insulation or reinforced insulation are provided, there being no reliance on either protective earth or installation conditions. This means a class II SPS does NOT have a terminal/pin for earth ground connection.
LPS: When an electronic circuit is powered by a limit power source (LPS), its output current and power are under the limitation shown in IEC60950-1 Table 2B, and the risk of fire can be reduced significantly. So, the safety distances and flammability rating of components can be much lower. Therefore, the plastic enclosure of these power supplies could use HB flammability rating to reduce cost. This definition comes from ITE product (IEC/EN/UL60950-1).

Class 2: When an electronic circuit is powered by class 2, its output current and power are under the limitation shown in UL1310 Table 30.1, and the risk of fire can be reduced significantly. So, the safety distances and flammability rating of components can be much lower. Therefore, the plastic enclosure of these power supplies could use HB flammability rating to reduce cost. This definition comes from UL class 2 power unit (UL1310).