September 1, 1997

Isolated telecomm converter handles 25W

Ron Young, Maxim Integrated Products, Sunnyvale, CA

Many telecomm applications in the central office require an isolated 5V supply derived from ­48V. The circuit in Figure 1 inserts an isolated, high-bandwidth feedback circuit into the loop of a transformer-flyback switching converter, IC₁, configured to generate 5V at 5A from an input voltage of ­36 to ­72V. Primary transformer T₁ provides isolation in the forward direction. An isolated transformer driver, IC₂, in conjunction with a surface-mount transformer, T₂, converts the isolated 5V output to a voltage that's referred to the primary and proportional to 5V.

The diode bridge comprising D₃, D₄, D₇, and D₈ converts the transformer output to dc. A diode-resistor network (D₂, R₄, and R₅) compensates for the temperature coefficient of the diode bridge. The result is a voltage with zero temperature coefficient, slightly lower than half the fed-back isolated 5V. IC₂ operates with inputs from 3 to 6V. For 5V output, the feedback voltage at Pin 3 of IC₁ is 2.404V. At 100 kHz (much greater than the loop bandwidth), the delay through the feedback circuit equates to a 9° phase shift. Supply current for the feedback circuit is approximately 6 mA, including the load of the temperature-compensation network.

To accommodate the isolated feedback circuit, the only modification you need to make on a nonisolated converter is to reduce the value of R₂ such that the R₂-R₃ voltage divider matches the 1.5V reference voltage internal to IC₁. You should trim R₂ in production to compensate for turns-ratio variations in T₂, unless an initial tolerance of 5 to 10% is acceptable. For 1% initial tolerance, trimming is mandatory regardless of the feedback technique. The transformer provides isolation to 500V rms; ratings to 1500V rms are also available. The converter delivers an isolated 5V at 5A with efficiency greater than 80%. For load currents of 0 to 5A, load regulation is approximately 2%. For input voltages of ­30 to ­65V, the line regulation is better than 1%. (DI #2082)

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