1、How Does Altitude Affect AC-DC Power Supplies?Most AC-DC power supplies that meet the safety sta ndards per UL/EN 60950-1 for ITE (In formati on Tech no logy Equipme nt) applicati ons are desig ned to operate at typical office and factory altitudes, which can vary from slightly above sea level to as

2、 high as 2,000 meters (6,562 feet). And, many power supply man ufacturers provide units that are designed and rated for operation at higher altitudes,up to 3,000meters (9,843 feet) so their supplies can be used in major cities located at higher elevati ons (e.g.,Den ver , Santa Fe, Mexico City, Bogo

3、ta). Manybroadcast in g/commu ni cati ons stati on s/towers are located at altitudes up to 3,000 meters or higher in order to maximize their ran ge.Altitude affects the design of power supplies since air is used as an electric insulating medium (aka, dielectric) in the construction of power supplies

4、, as well as most electro nic devices. The den sity and dielectric stre ngth (in sulat ing property) of air is very good at sea level, but at higher altitudes, the thinner air loses some of its dielectric stre ngth, which n eeds to be compe nsated for. Switchmode power suppliesoperate off of high vo

5、ltages (in puts of 90 to 265Vac) and intern ally gen erate eve n higher voltages (400Vdc or more),which n eed to be in sulated and contained toprevent high voltage arcingor breakdown within the supply, and to protecttheen d-equipme nt and operat ing pers onn el.The drawing below shows a cross sectio

6、n of a typical printed circuit board (PCB), which is comprised of copper electric con ducti on paths that our chemically etched on an in sulated (dielectric) fiber board material (e.g., FR4, wove n fiberglass cloth with epoxy resin), plus electr onic comp onents that are not show n in this draw ing.

7、 As can be seen, the fiber board and air, combinedwith the distances between theetched conductive traces are the primary insulation mediums for the circuit board.Con ductorsDrawi ng Credits: Mamma no B,InsulatingClearanceSurfaceCreepageSafety Con siderati ons in Power Supply Desig n,Un derwriters La

8、boratory / TI* The term Clearanee refers to shortage path between the two conductive parts (circuit traces, comp onen ts, etc.), measured through air.? The term Creepage refers to the shortage path between two conductive parts measured along the surfaceof the in sulati on (PCB, in sulat ing material

9、s/barriers, etc.).What does this have to do with altitude? Since air gets thinner (reduced barometricpressure) at higher altitudes and becomes less of an insulator,the PCB andcomp onent layouts have to be desig ned with sufficie nt safety spac ing dista nces to preve nt high voltage arcs or breakdow

10、 nsbetwee n con ductorsan d/or electro niccomp onen ts.For example, typical power supply desig n practice may allow 8 mm spac ing dista nee betwee n primary and sec on dary circuits and 4 mm spac ing dista neebetwee nprimary and grou nd. These spac ing dista nces will vary depe nding upon the voltag

11、e levels betwee n con ductors andcomp onents and the expected humidity,temperatures, polluti on levels, and attitudes.For those power products that must be approved per the Chin ese CCC orga ni zati on (required to export supplies in to Chi na), the new Chin ese SafetyStan dard GB4943.1-2011, which

12、is similar to UL/EN 60950, requires strict specs for creepage and clearanee distances. As of December 1, 2012, the primary-to-secondary cleara nces must in crease by a factor of 1.48 to qualify the supply for operati on up to5,000 meters, since many regions in China are located at high altitudes. Th

13、e alter native for CCC certified power supplies is that they must clearly marked with a warning label that states that the power supply must be used below 2,000 meters (see table below).The base design altitude for ITE power supplies is 2,000 meters. However, as men ti oned before, as the altitude i

14、n creases, the air becomes a poorer in sulator and the spac ing dista nces have to be in creased per the follow ing table (assum ing an 8 mm cleara nee at 2000m).AltitudeBarometricMultiplicati on FactorResult ing(meters)Pressure (kPa)for Cleara neeCleara nee (mm)200080.01.008.00300070.01,149.1240006

15、2.01.2910.32500054.01.4811.84As can be see n from this table, if a power supply is to be operated at 5,000 meters, its con ductor/comp onentscleara nces must be in creased by 48% compared to asupply desig ned for 2,000 meters.The other major effect of high altitudes on power supplies is that the les

16、s dense air does not con duct heat as well. To compe nsate for higher altitudes, power supplies n eed to be derated, or employ larger heat sin ks, or have in creased forced air flow, or a comb in ati on of these to in sure proper cooli ng. In additi on, the power supply must be desig ned with the proper con ductor and comp onentcleara ncesdiscussed above.asIn summary, whe n