This document introduces a low cost DC-DC isolated converter system, which uses off-the-shelf switch power IC and external components, the conversion efficiency is high, the input and output voltage range is wide
This design note presents a high‑efficiency isolated DC‑DC converter intended for supplying 3.3 V or 5 V peripheral functions. The circuit replaces the MAX253 driver, using the same transformer and a pair of ZETEX ZHCS750 Schottky diodes, with FMMT617 or ZTX618 transistors to generate a 240 kHz square‑wave that drives the transformer.
The converter achieves over 80 % overall efficiency, with the transformer itself rated at about 85 % efficiency. It can deliver up to 1 W (for the 5 V‑5 V version) or 500 mA (for the 3.3 V‑5 V version) into a load, making it suitable for RS‑485 and RS‑232 transceiver applications.
| Oscillation Frequency | 240 kHz |
|---|---|
| Transformer Efficiency | ≈ 85 % |
| Circuit Efficiency | > 80 % |
| Maximum Load Rating | 1 W (5V‑5V) / 500 mA (3.3V‑5V) |
The circuit oscillates at 240 kHz, which provides the highest efficiency.
Two ZETEX ZHCS750 Schottky diodes are employed as rectifiers.
Typical applications include RS‑485 and RS‑232 data‑interface transceiver circuits.