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AC Input Voltage – The nominal range on the power supply label as approved by the safety agency is always 10% less than the usable input voltage range.  For example, a switching power supply with a nominal 100 to 240VAC has an actual range of 90 to 264VAC. Our datasheet shows the usable voltage range; the label will show the safety agency rating.

AC Input Frequency – The nominal range of 47 to 63 Hz is important in linear power supplies, but most switchers are insensitive to input frequency and can operate from DC to 400Hz. This is especially important in countries where the power may be of poor quality.

AC Input Current – The current drawn under the maximum DC load. Safety agencies require this be marked on the label.

AC Inrush Current – The instantaneous current drawn when the power supply is turned on cold. Larger power supplies generally incorporate a thermistor to limit this amount. It is most important in considering an AC switch rating.

Cross Regulation – On a multiple output power supply, the effect on some outputs while varying the load on other outputs. Our standard is to hold the constant loads at 60% of full load, while varying the measured output from 20 to 100% (of rated load). NOTE: The effect may be extreme if the highest current output is fully loaded, and a quasi-regulated output is lightly or not loaded. A preload may be required if you have this condition in your system.

DC Output Voltage – The nominal voltage setting.  In adaptors, this voltage is preset. With open frame power supplies, a voltage adjust potentiometer is standard. The adjustable range is usually 5%.

Efficiency – Output power divided by Input Power. This will vary depending on the load and AC  input voltage. Values shown are usually nominal at rated voltage and load.

Emissions – Electromagnetic radiation emitted into the atmosphere from the power supply. Conducted is that energy sent down the AC line cord; radiated is sent into the air. Adaptors must meet both but many open frame switchers are only rated for conducted (the assumption being that they will be placed into the customer’s box).

Hipot (Dielectric Withstand) – The test voltage between the input and output, and output to ground.  See our safety compliance section.

Hold-up Time – Determined by the storage factor within the power supply.  In linear supplies, the time output fails when AC input fails, is almost immediate. In switching power supplies, energy is stored in the bulk (input electrolytic) capacitor providing a useable hold up time to protect against transient power outages. Since this is highly proportional to AC voltage and DC load, it is given under specified conditions.  High AC voltage and low DC loads provide the best conditions.

Immunity – Testing done to determine immunity from electromagnetic or electro static discharge (ESD). The European IEC 801 or 1000 or EN61000 are test specifications and procedures.  Dash numbers indicates the levels.  Generally, the higher the number, the better the power supply is able to withstand the test condition.

Leakage Current – See our safety compliance section.

Line Regulation – The change in DC output voltage when the AC input voltage is varied through out its range while the DC load is held constant.  In switchers, this can be 90 to 264 volts AC unless otherwise specified.  In linears, it may be as limited to 108 to 132 volts AC.

Load Regulation – The change in DC output voltage when the DC load is varied.  Unless otherwise specified, the range is minimum load to full load.  The AC voltage is held at nominal during this test.

Line and Load Regulation – The combined effect of varying the DC load and AC input voltage; the worse case condition.

Minimum Load – Some switching power supplies require a minimum load to stay within their specifications. However, operating them unloaded does no damage.

Maximum Load – The load specified for continuous operation. This will be the load used for safety applications.

MTBF – Mean time between failures was a term adopted by the military and elaborated on in MIL-HDBK-217, which provides values for capacitors, resistors, etc.  An alternate standard was developed by Bell Labs (Bellcore).

Over current Protection – Modern power supplies are almost always provided with some form of short circuit protection. 

Over voltage Protection – More important in linears, where failure of a pass transistor could easily put excess voltage on IC’s. With switchers, most failures cause no output at all.

Operation Temperature – The ambient temperature conditions under which the supply was designed to operate at full load. Most power supplies can operate at higher ambient if the output load is properly derated.

Peak Loading – For transient conditions only (such as disk drive start up).  The power supply may not remain within specified regulation during the peak condition.  The amount of time this load can be applied (duty cycle) varies greatly from design to design. It’s best to consult the factory with your load profile.

Ripple – The AC component superimposed over the DC output voltage. In linear power supplies, this is the traditional “hum” at 60 or 120 hertz. In switching power supplies, it is a complex waveform.  Although traditionally spec’d at 1%, it is often much lower. Ripple can increase at maximum loading and minimum input voltage.

Switching Frequency – Term used by switchers only. Many switching power supplies operate at a fixed frequency; some use variable frequency designs. 


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