(a)EMI Filter: The built-in filter reduces the EMI generated from the S.P.S
(b)Inrush Current Suppressor: Inrush current occurs at the time of power-on of the S.P.S. If the inrushcurrent is too large, the S.P.S. might be damaged, and the power distribution system could be adversely influenced. Thus, an inrush

Current suppressor is installed inside each S.P.S. to limit the high current.

(c)Input Rectifier: It rectifies the AC input voltage to a DC level.

(d)Power Factor Correction (PFC): The active/passive PFC circuit upgrades the

Power factor (PF) figure and approximates the waveform of input current to a

Sine wave, so as to be compliant to the harmonic current regulation and reduce

Reactive power.

(e)Converter: It converts the DC voltage to high-frequency signal, and the signal is increased or decreased by a high-frequency transformer.

(f)Output Rectifier: It rectifies the DC pulses to a steady DC level.

 (g)Feedback Control: It regulates the output voltage to a steady level to respond to variation of loads and AC input voltage.

(h)Protection Circuit: When in abnormal situations, the S.P.S. will shut down or go into other protection modes by activating protection circuits. The protection

Circuits consist of over voltage, over temperature and over current (overload)

Protections and etc.

Comparison between Switching Power Supply and Linear Power Supply

Compared to linear power supplies, switching power supplies possess the

Advantages of high efficiency and light weight.

The advantages correspond to the

Contemporary demands of being light and handy, and energy saving.

Thus S.P.S. has gained much popularity in field applications.

Applications of Switching Power Supply

Switching power supplies have gradually taken the place of linear power supplies and been widely used in every kind of electronic equipments such as the source for module Application, driving circuit, uninterruptible power supply (UPS), and battery charger, etc. Applications of switching power supplies can be divided into industrial and commercial usage,

Circuit Principle of Switching Power Supply

Among all DC-to-DC converters, S.P.S. is to be divided into 3 basic circuits

Topologies based on input voltage, output voltage and polarity:

(a)Step-down or buck converter: Used for output voltage lower than input voltage.

(b)Step-up or boost converter: Used for output voltage higher than input voltage.

(c)Inverter or buck-boost converter: Used when the output polarity is reversed from the input one. It can also be used in either step-up or step-down application.

If the input side needs to be isolated from the output side, the above 3 basic

Converters would be inappropriate. Instead, the converter topologies, which are

Forward, Fly back, Half-bridge, Push-pull, or Full-bridge, derived from the 3 basic

Ones would be the right ones to use. There are various methods of generating

Switching signal, one is to use self-oscillation whose oscillation frequency depends on input voltage and output loading, and the other is to use PWM IC whose oscillation frequency depends on the control IC.

Buck Regulator

When the switch (MOSFET) is ON, the energy is transferred to the load (RL)

Through L1 and stored in L1 at the same time; when the switch is OFF, the energy stored in L1 supplies the load through D1 and L1.

Boost Regulator

When the switch (MOSFET) is ON, the energy is stored in L1; when the switch is

OFF, the energy stored in L1 is transferred to the load (RL) through L1 and D1.

Output voltage can be higher than the input due to the summation of the voltage

Pre-stored in L1 and Vcc. The topology is popular in PFC application.

Buck-Boost Regulator

When the switch (MOSFET) is ON, the energy is stored in L1.When the switch is

OFF, the energy stored in L1 is transferred to the load (RL) through L1 and D1.

Isolated Type

Fly back Converter

When the switch (MOSFET) is ON, the energy is stored in the transformer; when

The switch (MOSFET) is OFF, the energy in the transformer is transferred to the

Load (RL) through D1.

Forward Converter

When the switch (MOSFET) is ON, the energy is transferred to the load (RL) and

Stored in L1 through D1 and transformer. When the switch is OFF, the energy

Stored in L1 is transferred to the load through D2.The topology of using two

MOSFETs (Double-end) can reduce the voltage stress on the switches.

Push-Pull Converter

When switch 1 (MOSFET 1) is ON and switch 2 (MOSFET 2) is OFF, the energy is transferred to the load (RL)through transformer and D2;when switch 2 is ON and switch 1 is OFF, the energy is transferred to the load through transformer and D1.

Half-Bridge Converter

When switch 1 (MOSFET 1) is ON and switch 2 is OFF, the energy is transferred to the load (RL)through transformer,C2,and D1;when switch 2 is ON and switch 1 is OFF, the energy is transferred to the load through transformer,C1,and D2.The

Asymmetric Half-Bridge Converter (commonly used in S.P.S. with PFC)derived

From it can achieve higher efficiency.

Full-Bridge Converter

When switch 1, 4 (MOSFET 1, 4) are ON and switch 2, 3 (MOSFET 2, 3) are OFF, The energy is transferred to the load (RL) through transformer and D2; when Switch 2, 3 are ON and switch 1, 4 are OFF, the energy is transferred to the load Through transformer and D1.The collocation of this topology and phase-shift control is commonly used to achieve high-efficiency Zero Voltage/Current

Transition (ZVT/ZCT).

The Characteristics and Applications of Hard Switching Converters in S.P.S.

Existing varieties of topology in S.P.S. can be used to meet all types of demand. In theory, S.P.S. only has conduction loss and is without switching loss since it operates only at the ON/OFF period of the switch (MOSFET).However, in reality, the leakage inductance and the capacitance effect in components lead to the existence of switching loss, which is an obstacle to improving efficiency. The converters with switching loss are of the hard switching topology.

The Characteristics and Applications of Soft Switching Converters in S.P.S.

Soft switching technique is to reduce the switching loss coming from hard switching converters. By adding auxiliary and resonant circuits, switching transitions only take

Place either under zero-voltage or zero-current status. With significant decrease in switching loss, not only efficiency can be upgraded, but the temperature rise on switches can be reduced. Thus, plus the downsized heat sink, the power supply of new Generation with minimized size and high power density is realized.

Inrush Current Limiting -RTH1 (thermistor) suppresses the inrush current during cold start. Thermistor is a resistor whose resistance changes in indirect proportion to temperature, which is why it is called a NTC (Negative Temperature Coefficient)Thermistor. At cold start, the high resistance suppresses inrush current; after starting up for a while, the resistance decreases to reduce power consumption in normal operation.

S.P.S. of middle/high power rating uses a circuit comprising TRIAC and relay to have lower conduction loss. Please refer to Section 2.3 for detailed information.

(2)Anti-Lightning Surge – ZNR1 (transient/surge absorber) is a resistive component. Part number 471, commonly used in our products, it possesses high resistance at normal

Condition. As the voltage across it increases, its conductivity also increases; the

Resistance decreases greatly when the voltage across it reaches 470VDC or 332VAC, and the conductive current is around 1mA.With that characteristic, the transient/surge absorber is able to bypass the abnormal surge voltage so as to effectively inhibit the surge voltage from entering the S.P.S.

EMI filter is commonly used at the input side of our products to filter out

Noises interference. Basically, it consists of three types of components, which are X capacitor, Y capacitor, and common-mode choke. The functionality of each component

Input Rectification – BD1 (bridge rectifier) rectifies AC input voltage to DC level with 120Hz ripple voltage.

PFC (Boost) Circuit -Block consists of Q1,D1,L1,and U1:1 (PFC

Controller) boosts the voltage to 380VDC.The main function of the block is for power factor correction, to approximate the waveform of input current to sine

Wave, and to reduce harmonic current to correspond to the demand of CE regulations. Either active or passive PFC circuits can be used. Passive PFC circuit uses a low-frequency transformer comprising a core made of silicon steel sheet to upgrade power factor figure

From 0.5 to 0.7; active PFC circuit upgrades it to 0.95.

PWM Control at Primary Circuit – Block consists of U1:2 (PWM controller)and Q2

(Main power transistor) converts DC voltage from PFC circuit to high-frequency pulse train signal, so as the high-frequency transformer can increase/decrease it to get lower conversion of output voltage.

Isolation Transformer – Its main functions are isolation and decreasing voltage level.

Temperature rise and the ability to meet withstand voltage rating of safety regulations are the main considerations when designing the transformer.

Output Rectification – Block consisting of D10, L2, and C12~C14, rectifies and

Filters the high-frequency DC pulses to a steady DC level.

Feedback Control Loop – The loop, consisting of R74, PC1, etc., senses the divided output voltage to compare with the reference voltage of SHR1,and feedback the signal to PWM control IC (U1:2) through PC1 to regulate the duty cycle of PWM to achieve the

Purpose of output voltage control.

Over Voltage Protection Circuit – It consists of ZD7, PC2, etc. When the output voltage exceeds the specified value (ZD7), PC2 conducts to activate SCR1 to pull the reference voltage of PWM IC (U1:2)to ground to cease operation of PWM IC, so as to protect the S.P.S or users ’system from damage.

Snubbed Circuit: A and CA, located in primary side of T1 and secondary rectifiers

(D10), are mainly to reduce the high-frequency spikes and oscillation coming from switching transitions of switches or rectifiers, such as Q2 and D10.It also improves EMI performance and decreases withstand voltage on components.

Input Voltage /Frequency

S.P.S. has been used in various field applications all over the world. Depending on type of product, input source of either alternating current (AC)or direct current (DC)is acceptable. Prior to operation of the power supply, users should confirm the following:

the type of input voltage (AC or DC),the range of input voltage, and other conditions written on specification sheet. Input voltage exceeding the operating range would cause damage to the power supply. Also, input voltage with distorted waveform, even within the operating range, could also lead to malfunction of the power supply.

If input voltage is of DC or square wave, its maximum value should be equal to 1.4 (√2)times of the peak value of sine wave. For example, input voltage of

85~264VAC can be replaced with DC voltage of 120~370V.

Input source of DC or square wave is not acceptable for S.P.S. with function of input voltage auto-selection.

Input Frequency:

The frequency of AC mains for S.P.S. is generally 50Hz or 60Hz.Taking into

Account deviation of ±3Hz from power plants, S.P.S. is designed to accept input

Frequency within the range of 47~63Hz.For special applications, such as for boats or ships, the frequency of 440Hz is acceptable, but users should be aware of the increased leakage current.

The input range for products with safety approvals is available on model labels of

The casings.

Input Current/Power Factor

It is common in S.P.S. designs to connect a bulk capacitor right after the bridge rectifier.

This leads to charging of the filtering capacitor at the peaks of the input sine wave.

Apparent power becomes high due to high RMS current and this leads to low PF.

Current meters with “True RMS” function are required for measurements of input

Current. PF for typical S.P.S. falls within the range of 0.4~0.6; for S.P.S. with PFC Function, PF can be above 0.95.The relation among input current, output power, input voltage, power factor, and efficiency is as follows:

Input Current = Efficiency Factor Power Voltage Input

Power Output

Inrush Current

When AC is initially powered ON, a peak current is required to charge the empty bulk capacitor. This momentary peak is also called the inrush current.

The magnitude of the inrush current varies according to the type of current limiting circuit used. S.P.S. commonly uses a thermistor to limit inrush current. The high resistance of the thermistor limits inrush current at cold start of S.P.S. the low resistance of the thermistor reduces power consumption after power-on. Thus, in order not to accumulate inrush current do not turn the power supply ON and OFF rapidly.

Generally, after turning-off, a delay of few seconds before turning-on is suggested.

Compared to the input current in steady state, the inrush current could be several to tens of times higher. Be aware that application with multiple S.P.S. leads to accumulate inrush current. In this case, the ability of input wiring, external fuses, and switches to withstand transient current should be taken into consideration.

Line Regulation

The variation in output voltage, corresponding to change in input voltage, is called the line regulation (measurement done with full load at output).Although the effect of line change, within specified range, on output voltage would not be significant, minimizing length of input wiring and limitation on the amounts of units connected to the same AC source is recommended.

Leakage Current

Leakage current is the current flowing from the protective earth (PE) conductor, such as metal enclosure, of equipments to frame ground (FG).Due to EMI requirements in S.P.S., there are Y capacitors (C2 and C3 in Figure 2.2) connected in between AC L/N and PE conductor. A low current will flow through the Y-caps to FG. In reality, leakage current should be regulated to comply with safety standards. In regulations of IEC60950-1 for IT products, leakage current should be less than 3.5mA for portable Class I equipment,0.75mA for hand-held Class I equipment, and 0.25mA for Class II equipment.

Output Voltage/Tolerance/Adjustment Range

The measurement of output voltage is defined to be at the output terminal of S.P.S. If voltage is measured at the load end, the measurement would deviate due to voltage drop on wiring. The voltage tolerance is the biggest voltage variation measured during simultaneous line and load change. In general, the tolerance accounts for line regulation, load regulation, cross regulation and setup tolerance. For compensating voltage drop, output voltage is factory set a little bit higher. Most products have a built-in variable resistor for output voltage adjustment. However, there are some important requirements to be followed:

(1)Rated power should not be exceeded. If output voltage is set higher, output current must be decreased accordingly.

(2)Rapidly decrease in output voltage with no load or light load would lead to

Momentary malfunctions of S.P.S.

(3)Output voltage would be unable to be set higher with low input voltage.

(4)Increasing the main output voltage of a multi-output model could have adverse

Effects on output voltage of auxiliary outputs such as extra power loss on auxiliary Outputs (for auxiliary outputs adopting linear regulator or step down regulator).

(5)In addition to the built-in VR, high end models might also have the voltage trimming function using external resistors or voltage signal.

Maximum Output Current/Power

Maximum output power equals to Vout × Iout should be decreased to keep output

Power within rated value when Vout is increased. For instance, a model with rated voltage of 5V and maximum output current of 10A, the maximum allowed output current would be 9A while output voltage is set at 5.5V.Please note that the maximum allowed output current would remain at 10A even though output voltage is set at 4.5V.

Some channels of multi-output models would be allowed to deliver at the maximum value of output current range, but the total output power should not exceed rated value.

For example, take D-60A with outputs of 5V@6A &12V@4A,and maximum output power of 58W.If we use 5V output at 6A, and 12V output at 4A, the total output power would be 78W which exceeds the maximum value.

Consequently, either output current of 5V or 12V should be decreased to keep output power equivalent to or less than 58W.

Ripple Noise

An AC waveform can be identified on the DC output of a S.P.S., which is shown in There are two AC components, 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 adding 0.1uF and 47uF capacitors in parallel with test point for filtering out noise interference are required to be made.

Load Regulation

The variation in output voltage, corresponding to change in output load between

Minimum value and full value, is called load regulation. In general, high output current would lead to slight voltage drop at the output terminal.

Cross Regulation

For power supplies with two or more channels, cross regulation stands for the

Variation in output voltage of the channel under test which is loaded with 60%of its rated load, while the load of other channels varies between 20%and 100%.

Efficiency

The ratio of output power to real input power in terms of percentage.

Set Up, Rise, Hold Up Time

Set up time (ton): The time from power on to 90%of rated output voltage,or from AC powered on to operation of S.P.S.

Rise time (tr): The time it takes for output voltage to rise from 10%to 90%of rated

Value. It is usually within 50ms.Longer period than that might lead to

Malfunction of system.

Hold up time (th): The time from power off to 90%of rated output voltage.The general requirement is at least 16ms to allow sufficient time for UPS to take

Over.

Overshoot, Undershoot

The deviation of output voltage to tolerance value prior to steady state after AC is

Powered on.

Transient Recovery Time

The time required for output voltage to settle within specified tolerance value while output load is in drastic change. The general requirement is 500us at most.

Temperature Coefficient

It stands for the variation in output voltage due to change in ambient temperature. The unit is in terms of %/°C. Measurements are usually made after burn-in of half an hour.

Over Current Protection (O.C.P.)/Overload Protection (O.L.P.)

When the output power or current reaches the range of O.L.P./O.C.P.(typically

105%~150%of rated power/current), the S.P.S. would be protected by decreasing or cutoff of output power. The protection modes for over current/overload are categorized into the following types:

Protection Types:

(1)Fold back Current Limiting

Output current is folded back to 20%or less of rated load current

(2)Constant Current Limiting

Outputs current remains constant and within the specified range while the output

Voltage drops to a low level

(3)Over Power Limiting

Output power remains constant. As output load increases, output voltage

Decreases in proportion

(4)Hiccup Current Limiting

Output voltage and current keep pulsing ON and OFF repeatedly when protection is activated. The unit automatically recovers when fault condition is cleared.

(5)Shut Off

Output voltage and current are cut off when output load reaches protection range.

Recovery Methods:

(1)Auto Recovery: S.P.S. recovers automatically when over current/overload condition is removed or over.

(2)Re-power on: S.P.S. restarts by manual AC re-power on after over

Current/overload condition is removed or over.

Over Voltage Protection (O.V.P.)

When output voltage functions abnormality (e.g. lost of regulation),it could exceed the rated value. In consequence of that, O.V.P. is triggered to protect end equipments from damage. The protection modes for over voltage are categorized into the following types:

Protection Types:

(1)Shut Off

S.P.S. restarts by manual AC re-power on of after fault condition is removed or

Over.

(2)Hiccup Voltage Limiting

Output voltage keeps pulsing ON and OFF repeatedly when protection is activated.

The unit automatically recovers when fault condition is cleared

Basically, there are two kinds of O.V.P. detection:

(1)Secondary detection circuit feeds back signal to disable PWM IC from operation.

(2)Short circuiting output by using a Crowbar circuit. This would in turn trigger OLP.

Over Temperature Protection (O.T.P.)

This protection is to prevent internal components of S.P.S. from overheating and

Damage, or to prevent decrease in lifetime due to high ambient temperature,

Overloading, or malfunction of S.P.S.(e.g. damaged cooling fan).The fault conditions needs to be cleared then S.P.S. would recover automatically or restarts by manual re-power on after internal temperature drops below activating temperature. In general, it would take a few to tens of minutes.

Vibration Test

The test simulates the usage of S.P.S. in high vibration environment, and verifies the S.P.S.’s capability of operating under this situation. It is conducted with a single unit or an entire carton in the X,Y, and Z axes, and with sine wave acceleration of specified amplitude, frequency in each of the three axes at specified period of time.

The relation between amplitude and frequency to constant G is as follows: 

Hi-Pot Test

(1)Hi-pot testing is made on behalf of safety regulation, and is one way to verify the effectiveness of primary-to-secondary isolation of S.P.S. so as to confirm that the SPS can withstand high voltage without breakdown. The test voltage should be gradually increased from 0V to preset level and remains at preset level for 60

Seconds. In mass production, the test period could be reduced to 1 second. If the

Leakage current flowing through the isolation material increases rapidly after

Applying test voltage, it indicates ineffectiveness of isolation (dielectric breakdown).

Corona effect/discharge or transient electrical arc is not considered as a 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 20mA,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 secondary circuits, so as to solve the problem of leakage current. 

Isolation Resistance

It is to determine the dielectric strength of insulation materials by applying DC voltage.

The unit is expressed in M Ω (mega-ohms).The tests are to verify the isolation

Resistance of transformers, PCB, and etc. The criteria of isolation resistance are not defined in IEC60950-1.

Power Good (P.G.)/(Pok)and Power Fail (P.F.)Signal

When S.P.S. is powered on or off., PG/Pok, or PF signals are sent out for status

Monitoring. P.G.:A TTL (+5V)signal will be sent out with a delay of 10~500ms after output voltage Reaches 90%of rated value.

P.F. :The TTL signal will be turned off at least 1ms before the output voltage drops to 90%of rated value.

Pok: A TTL signal is sent out in synchronization with output voltage. It is without

Functionality in time sequence.

Alarm

The two contacts provided by S.P.S. are short-circuited when output voltage reaches

Specified voltage level. On the contrary, the contacts are open-circuited.

(1)The contacts of this alarm functions typically derives from solid-state

Semiconductor components. In addition to limitations on withstand voltage and

Current, polarity needs to be aware of.

(2)Some alarm functions use relays as the contacts. Since relays are just mechanically open/close contacts, it is free of polarity problem.

(3)Alarm functions could malfunction when the total output load is below 10%of

Rated current in parallel application of S.P.S.

Auxiliary Power

External power source may be required for S.P.S. with remote control or alarm

Function for status monitoring. For convenience of application, the power source is designed into the S.P.S.

(1)The auxiliary power source is designed based on isolation concept, and

Independent of other power sources in the unit.

(2)The auxiliary power source is only for applications of control functions in S.P.S. It should not be used as power source for other end equipments  

Introduction of EMC

EMC (Electromagnetic Compatibility)is divided into two sections, which are EMI

(Electromagnetic Interference)and EMS (Electromagnetic Susceptibility).

By definition, EMI refers to emissions of electromagnetic energy from a device or

System that interferes with the normal operation of another device or system. On the other hand, EMS refers to the ability of a device or system to function without

error in its intended electromagnetic environment.

Regulations and Explanations of EMI

(1)If the functionality of an electrical device is interrupted by external voltage/current

Sources, and the device cannot function as intended. Then, we can conclude this

Device is being interfered by electromagnetic wave. For example, the picture on

TV may distort when a nearby PC is starting up; a radio may send out noises if a

Motorcycle is starting while the radio is operating. These phenomenons's are

Considered as electromagnetic interference. There are two ways that

Un-intentional external voltage/current sources causes interference: The first one

is Conducted Emission – this source of noise interferes with other systems through power cord, plug, and dispatching system; the second one is Radiated Emission - this source of noise interferes with other systems through the radiation of its electromagnetic field. That is, radiates unwanted electromagnetic wave through free space.

Definition of Harmonic Current and Related Regulations

(1)For a traditional AC/DC converter, there is a bridge rectifier and bulk capacitors which charges/discharges in a very short period of time. Hence, a high current pulse

Is introduced at AC input line and generates harmonic current noises through the

Power line. The un-intentional harmonic current noise could interfere with other

Electrical devices and also reduce the Power Factor In order to

Prevent interference with other equipments, PFC (Power Factor Correction) circuit should be used on the switching power supply that makes the current waveform more sinusoidal and improves the quality and efficiency of the AC power source. 

Explanation of EMS

EMS (Electro-Magnetic Susceptibility) refers to the correct operation of electrical

Equipments in the presence of electromagnetic disturbances. The most common

Electromagnetic interferences are listed on the following table.

Introduction to EMC Regulation:

(1)Currently, most countries implement their own EMC regulations. In general, all the regulations are based on the standards issued by international committees, such as IEC (International Electro technical Commission), and CISPR (Committee International Special des Perturbations Radioelectraque).Individual countries adapt these international standards, and modify it to include the actual need for that particular environment.

(2)Following are the EMC standards issued by European Union for each product

Category:

Reliability

Curve of Lifetime and Failure Rate

High reliability of switching power supply has been proved from its extensive use in household appliance and industrial equipment. The industrial sector holds SPS in high regards. Figure 6.1 is a typical curve of product lifetime and failure rate

Initial Failure Period

To prevent “initial failure”, all manufacturers have sifted the defective components out by making burn-in tests. Normally when customers receive the S.P.S., it has already entered the “random failure period”.

 Random Failure Period

The stability of the S.P.S is related to its reliability (MTBF: Mean Time between

Failure) and generally speaking, the failure rate is very low. But,the random failure Rate has a lot to do with user’s installation and operating conditions (ambient Temperature, derating, ventilation, and vibration).

Fatigue Failure Period

After the “random failure period”, S.P.S is going to have higher failure rate and

Enters the “fatigue failure period’. At this time most of the components h ave aged (such as fans, aluminum electrolytic capacitors)and hence induce failure.

Ambient Temperature and Components' Lifetime

(1)High frequency and high efficiency switching technology, improvement of

Components, and integration with up to date technology can reduce the size of the S.P.S. But, components must also populate the PCB at higher density. The lifetime of components on a S.P.S. will vary significantly due to differences in ambient temperature. When an electrolytic capacitor is used for filtering, its internal chemical reaction will raise the surface temperature. Lifetime of the electrolytic capacitors is sensitive to ambient temperature.

(2)In general, the lifetime of the electrolytic capacitors will be reduced by half for

Every 10 °C increase in the ambient temperature. On the contrary, when

Temperature goes down 10 °C, its life time will be doubled.

If the S.P.S. is continuously operated in high temperature environment, the electrolytic capacitors will enter the “fatigue failure period" earlier while in the mean time the other components are still at the “random failure period”. To extend the lifetime of this S.P.S., we need to replace all the electrolytic capacitors.

Ambient Temperature and Frequency of Return for Repair

Electronic technology has been developing at an astounding rate leading to more S.P.S. being used in all types of system. Although the lifetime of S.P.S. is pretty high, it will not last forever. So,regular maintenance is very important to guarantee the S.P.S.’s reliability. How often maintenance should be done depends on the operating condition and temperature. The operating temperature will affect the frequency of return for repair,

Reliability and Operating Environment

The operating environments of the S.P.S.will affect its reliability.

Vapor, Moisture

If the S.P.S. is used in vaporous or moisture rich environment, there’s the

Possibility that the S.P.S. may become out of order after a certain period of

Operation. Therefore don't use the S.P.S. in moisture rich environment. If it is

Necessary, please contact the supplier for more information about special

Treatment against the moisture.

Dust

If the S.P.S. is used in a dusty environment, the dust may collect on the fan and

PCB causing circuit failure because of decay in the insulation. Protective measure should be taken to reduce dust settlement (e.g. dust filter).

Vibration and Impact

If the S.P.S. is used in a vibrating environment or subjected to impact, the power

Supply may become out of order.

Electromagnetic Field

If S.P.S is used in an environment with high electrical field, magnetic field, or strong electromagnetic wave, the power supply may become out of order.

MTBF （Mean Time between Failure ）

Mean Time between Failures is the expected value for reliability prediction. Estimating the reliability of the products under development or in mass production can help decide whether customers' needs are satisfied.

MTBF is the mean operating time until the reliability of the product goes down to 36.8%. For instance, if the product’s MTBF is 20,000 hours, it doesn't mean that this product can be used for 20,000 hours, but rather that after being used for 20,000 hours, the probability for proper operation is 36.8%(e MTBF can be calculated by two different methods, which are “Part count" and “Stress

Analysis”. The regulations, MIL -HDBK-217F Notice 2 and TELCORDIA

SR/TR-332(Bell Code)are commonly used to calculate MTBF.MIL-HDBK-217F is a United States military standard, and TELCORDIA SR/TR-332(Bell Code)is a

Commercial regulation. Currently, Mean well adapts MIL-HDBK-217F Notice 2 as the main calculation model.

Parts Count Method

Use MIL-HDBK-217F to get the failure rate for all components in the product and

Combining all the failure rates to get the reliability value.

Stress Analysis Method

Operating a product at normal condition and within rated specification, a list of

Components' stress data can be gathered. Using the stress data in conjunction with the MIL-HDBK-217F standard, we can estimate reliability by the stress analysis method.

Technical Support Q &A

Notes on choosing a switching power supply?

Ans :(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 an 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:

A. Protection function: Over Voltage Protection (OVP), Over Temperature

Protection (OVP), Over Load Protection (OLP), and etc.

B. Application function: Signaling Function (Power Good, Power Fail), Remote

Control, Remote Sensing, and etc.

C. 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.

How do we choose a power supply to charge a battery?

Ans: MEAN WELL has ESC,SC,PA,PB series for battery charger purpose (120~360W).If

These series does not meet customers ’demands and they like to choose a power supply as a charger, we advice to pick up one which over load protection (OLP) mode is constant current limiting. The models in this mode provide constant current even when the protection circuit is triggered.

The second choice is fold-back current limiting or constant wattage model. In this

Model, when a battery is running low, the output current of the power supply will gently increase. The level of increase depends on battery’s capacity and degree of exhaustion. Hiccup or shut down model are not recommended because it will stop to generate current when OLP happens.

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

Ans: 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.

If we need a 24V output power supply, but MEANWELL does not have this model, can we use two 12V power supplies connecting in series instead of one 24V power supply?

Ans: YES, basically you can do this to get the right output voltage, but be careful that the rated output current of the series system should be the rating of the minimum one in these series connected power supplies. Furthermore, we like you to parallel a diode at the output of power supply to prevent possible damage of internal capacitors.

If we need a 600W output, can we use two units of S-320 connected in parallel?

Ans: No, you can not do this connection because S-320 is not equipped with the parallel function. When two power supplies are connected in parallel, the one with higher output voltage will share more loading and deli ver more (even “all”) power to the load and

cause these two power supplies to be unbalanced. We suggest using PSP-300 or PSP-500 because it is equipped with the current sharing function.

When testing a dual output power supply, the +5V channel is correct, but the +12V channel is over the specification. What is going on?

Ans: There are some minimum-load requirements on MEAN Well's multi -output power supplies, please refer to the specification first before connecting the load. When the load condition is 5V/4A,12V/0A,the output voltage of the 12V channel will be around12.8V that is over the ±6%of voltage deviation (12.72V) specified in the spec. sheet. If we add in the minimum load 0.2A to the 12V channel, the 12V channel will reduce to around 12.3V which is within the specification.

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

Ans: 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.

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

Ans: 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.

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?

Ans: 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 MEAN WELL power supply, there is some leakage

Current on the case. Is this normal? Will this leakage current hurt human body?

Ans: 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?

Ans: 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 fan less products are available. Above 500W,a fan is needed.

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

Horizontal directions?

Ans: Most small wattage and fan less 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.

Why the fan is not running after turning on certain models?

Ans: Some models control the fan based on internal temperature to extend its life time. For example, the Fan ON temperature for the S-240 series is ≧40C (RT1).If the internal temperature does not reach the preset value,the fan will remain still until receiving the start up signal.

What is “Inrush Current”? What will we notice?

Ans: At input side, there will be (1/2 ~1 cycle, ex.1/120 ~1/60 seconds for 60 Hz AC source) large pulse current (20~60A 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?

Ans: PFC stands for Power Factor Correction. The purpose of PFC is to improve the ratio of apparent power and real power. The power factor is only 0.4~0.6 in non-PFC models. In PFC models, the power factor can reach above 0.95.The calculation formulas are as below:

Apparent Power=Input Voltage x Input Current (VA)

Real Power=Input Voltage x Input Current x Power Factor (W)

From the environment friendly point, the electric power plant needs to generate a

power which is higher than apparent power in order to steadily provide electricity to the market. The real usage of electricity should be defined by real power. Assuming the power factor is 0.5,the power plant needs to produce more than 2VA to satisfy 1W real power. 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 need. It will be more effective.

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

Ans: 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)

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?

Ans: During safety verification process, the agency will use a stricter standard --±10% (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?

Ans: 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?

Ans: 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.

What is MTBF? Is it distinct from Life Cycle?

Ans: MTBF and Life Cycle are both indicators of reliability. MEAN WELL

Uses MIL-HDBK-217F as the core of MTBF. An expected reliability is forecasted

Through accounting component’s number (exclude fans).The exact meaning of MTBF is after continuously using of power supply in a certain time, the probability of operating properly is 36.8% If power supply is continuously used at

Double the MTBF time, the probability of operating properly becomes 13.5%

Life Cycle is found through using the temperature rise of electrolytic capacitor to

estimate the approximate life of power supply. For example,SP-750-12

MTBF=769.3K hours; electrolytic capacitor C108 Life Cycle=202.2K hours (Ta=45°C).