Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!newstop!jaytee!vergil!gsteckel From: gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Software) Newsgroups: sci.electronics Subject: Re: Switched power supplies, how do they actually work ? Summary: peak current draw by cap input has side effects Keywords: switcher harmonics Message-ID: <3011@jaytee.East.Sun.COM> Date: 22 Oct 90 00:28:54 GMT References: <17660122@hpfcdj.HP.COM> <1990Oct20.222618.2212@zoo.toronto.edu> <1990Oct21.173113.10031@wsrcc.uucp> Sender: news@East.Sun.COM Reply-To: gsteckel@east.sun.com (Geoff Steckel - Sun BOS Software) Organization: Omnivore Technology, Newton, Mass. (617)969-3448 Lines: 70 A couple of notes about the input current nonlinearities of switching power supplies: The "power factor" for a linear (but reactive) load is the ratio of resistive power drawn to total volts * amps drawn (Watts / VA). The UL (Underwriters Laboratories, the de facto US safety agency) and the writers of the National Electrical Code (other countries: CSA, VDE, etc.) require that devices connected to a wall outlet draw no more than the rated current, even if this is reactive (out-of-phase) current and thus not useful for driving the load. This limits the normal switcher to about 1000-1200 W when connected to a 15 A/110V outlet which can deliver 1650 VA (or 1650 W to a resistive load). Most switching power supplies generate about 350VDC from the AC input to run a high power/high frequency switch (transistor/FET/SCR mechanism), which drives inductors or transformers to generate the requisite voltage AC, which is rectified and filtered at the output. The high frequency of the switch allows the magnetic and capacitive components to be made (relatively) much smaller than the corresponding 60 HZ or 120 HZ parts used in linear supplies, since energy needs to be stored for much shorter times. Energy storage is mostly in the input capacitors, at high voltage. The normal input topology is a transformerless voltage doubler for 110V, full wave for 220V. `Wide range input (90-250V)' usually use the doubler and derate the heck out of the input storage caps/switch transistors. Unless fitted with a power factor/waveform corrector, the line input can only recharge the input capacitors when Vin is greater than VCapacitor. This means that the entire input current is drawn during the peaks of the input voltage wave. This highly nonlinear current draw effectively generates harmonics (measurable up to 11th or higher) of the input wave. A 1000W supply can draw several amperes of 120 and 180 HZ current from its source. One non-obvious side effect is the interaction of this with the building wiring codes (remember the safety agencies? (:-)). This gets a little fancy: Most wiring systems for large buildings use 3-phase power (usually 3 wire into the area, and 4 wire inside), and distribute loads across the three phases (0, 120, 240 degrees). Properly done, the return currents balance, so the net current in the neutral (4th) wire is 0. The safety agencies assume this, and allow the common neutral and safety ground wires to be the same size as the hot wires. This only works if the loads are linear. The 3'rd harmonic does not cancel in the neutral, but adds. According to an article in Electronics Products, (written by an employee of a power factor corrector manufacturer, caveat emptor) there have been (small) fires started by overheated neutral runs in massive office cubicle sets with many computers in the cubes. In any case, the European safety agencies and the US DOD have become concerned enough to require that soon all supplies over (100? W / Europe? and 300?W US sold to Gov't) to reduce the input harmonic current to a very small quantity. There are a number of circuit topologies to do this; the Electronics Products article discussed a number. Some involved inductors or other resonant elements, and some involved active input circuitry. One side effect of some of the topologies is the ability to work over a very wide range of input voltages. A previous poster noted the negative input impedance of a switching power supply! There may be other side effects of the nonlinear current drain involving power substation transformers or other more exotic equipment. Any info out there? geoff steckel (gwes@wjh12.harvard.EDU) (...!husc6!wjh12!omnivore!gws) Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line. This posting is entirely the author's responsibility.