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Not sure what I need please advise

Posted by: Mark Ross on
Finally they have accepted we can't do linear power supplies anymore where I work. We need a product that will accept a wide input voltage range. There are two thoughts here. If we do AC only, the range would be 70-240 volts (AC Mains, single phase). It would be nicer to do a universal input for our product that would go 9-240V in either AC or DC but lets take it one step at a time. Our output needs to be 24VDC at about 0.5Amps. Here is the kicker, since the product will go through Underwriters Laboratories, and the product has field wiring terminations, we need 100% guarantee that any failure mode will not put the AC mains on any field terminations (I'm thinking flyback). That is all I have on this, I just found the site and am poking around, but if anyone could steer me in the direction I need to go towards which product or products I would GREATLY appreciate any help. Thanks.

Comments

Submitted by PI-Tucker on 10/02/2008

Pretty much all power supplies are routinely designed such that the AC mains never appears on its output. Cellphone chargers and other "wall-wart" type power supplies, for example, for obvious reasons. It it true also for any power supply wherein the system circuit can be touched by the user. For example, a DVD player's rear panel terminals (audio, video) are connected to the PSU. If the AC mains gets connected to the system board, the rear panel terminals become live and present a shock hazard.

Most power supplies (both switchmode and linear), are routinely designed to have what is called "safety isolation" between the "primary" (circuitry connected to the mains), and "secondary" (circuitry connected to the load / user). The main elements separating the 2 are the transformer, the PCB, any "Y-caps" (EMI suppresion capacitors in certain positions), and the optocouplers. So the transformer will have to be designed for safety isolation (in fact UL as part of their safety testing will examine your transformer design), and use Y-caps and optocouplers that are "safety rated" for your application.

Aside from being designed to meet Safety guidelines, the individual PSU's need to pass a leakage current test in production. This measures the current flowing from primary to secondary (which are supposed to be isolated), when a high voltage is applied between them. This ensures that the product won't present a shock hazard.

Pls. see our appnote "Power Supply Design Techniques for EMI & Safety" for more:
http://www.powerint.com/sites/default/files/product-docs/an15.pdf

Submitted by prasun on 10/08/2008
mail me at prasunkuls@hotmail.com