Isolated flyback converters usually evoke thoughts-or bitter memories-of custom transformers, slipped delivery schedules, and agency-approval problems. Off-the-shelf flyback transformers carry isolation ratings of only 300 to 500 V and rarely of as much as 1 kV. Gate-drive transformers are readily available from stock with high isolation ratings and low cost, but they are wound on ungapped cores, have high inductance (500 µH to 2 mH), and quickly saturate in a normal flyback-converter circuit. Thus, high isolation calls for an abnormal flyback converter (Figure 1).
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| Figure 1. | The second “coil” of this unusual flyback converter is not a coil but rather an off-the-shelf gate-drive transformer. This component offers 3750V rms isolation and full VDE approval. |
Based on the uncoupled SEPIC (single-ended primary-inductance-converter) topology, the converter operates from a 12 V battery-backed input supply and outputs 24 V at 200 mA. The key feature is the second “coil,” which is not a coil but rather an off-the-shelf gate-drive transformer, T1. This component offers 3750 V rms isolation and full VDE approval; it functions flawlessly in SEPIC service. The output is completely isolated from the input.
The converter derives feedback from the primary winding through D1. The transformer winding is 1-to-1. C1 peak detects a voltage roughly equal to the output. A minimum load of 3.6 kΩ prevents the output from rising uncontrollably at zero load (Figure 1).