Electronic Design

Fractional-Turn Transformer Helps Switcher Handle Any Output Range

Need a custom ac-dc switching supply for small production runs? Look at XP Power's RCL175 (see the figure). Designers can specify any voltage level up to the maximum rating of each output while maintaining full agency approvals for industrial, IT, and medical applications. It also suits Class I and Class II use (i.e., with or without a ground connection).

Supplies come with one to four outputs: 3 to 60 V on output one, 5 to 60 V on output two, and 5 to 30 V on outputs three and four. Outputs three and four are isolated, permitting parallel or series connection. Output one is user-adjustable by ±10%. The other outputs track this by the same percentage. XP's secret for getting this kind of output is the supply's fractional-turn transformer.

Fractional-turn transformers aren't new, but they rarely have been used in ac-dc power supplies. They're very handy for multiple outputs. Consider a simple 5/12-V supply. Ideally, it would be nice to use a single winding for the 5-V output. But the 12-V winding then becomes two-and-a-half turns, which can't be manufactured. Doubling the number of turns works, but that increases transformer size. Generating a higher voltage and regulating it down for the lower voltage lowers efficiency, eats up board space, and adds cost.

Enter the fractional-turn transformer, which despite its name does not involve partial or tapped turns. Rather, it's a small additional transformer with a number of windings that connect to the secondary of the power transformer. In a design that requires 5- and 3.3-V outputs, the main power transformer would be designed so a single-turn secondary delivers 4.5 V.

The separate fractional-turn transformer adds 0.5 V to deliver a 5-V output and subtracts 1.2 V to produce the 3.3-V output. This additional transformer can be very small because it handles very little low power due to the low voltage present across each winding. If a 12-V output were needed, the same main transformer could use a three-turn secondary to produce 13.5 V at its output. A small fractional-turn transformer could be used to reduce this to 12 V.

The RCL175 is rated at up to 120 W with convection cooling and 175 W with 12 CFM of forced-air cooling. It also has a peak rating of 200 W. Efficiencies range from 84% to 90% depending on output configuration. There are 18 standard models that can be quickly modified, both electrically and mechanically, for individual applications. Open-frame models measure 5.51 by 3.71 by 1.28 in. (140 by 94.1 by 32.6 mm) and weigh 1.1 lb (500 g).

Pricing starts at $155 each in quantities of 100 units or more.

XP Power

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