This design note introduces a push-pull inverter for backlighting LCD displays. It has the following features: compact size, high efficiency, wide operating voltage range, and adjustable contrast.
This design note explains a push‑pull inverter for LCD display fluorescent backlighting. The inverter operates at an inaudible 30 kHz and must generate up to 1000 V peak‑to‑peak to start the tube and maintain about 500 V peak‑to‑peak during operation. A compact RM series transformer (RMB FX3440, 0.1 mm gap) is used for the 24 V supply.
The circuit uses Matrix geometry Super E‑Line (TO‑92) transistors (ZTX690B/692B) with BVCEV ratings of 45 V and 70 V, providing low VCE(sat) values of 50 mV and 80 mV at 500 mA. The note also discusses voltage rating considerations and suggests alternative high‑performance FMMT series devices for lower power requirements.
| Inverter start voltage | 1000 V peak‑to‑peak |
|---|---|
| Inverter running voltage | 500 V peak‑to‑peak |
| Inverter operating frequency | 30 kHz |
| Transformer model | RMB FX3440, 0.1 mm gap |
| Transistor BVCEV rating | 45 V (ZTX690B) / 70 V (ZTX692B) |
| VCE(sat) at 500 mA | 50 mV (ZTX690B) / 80 mV (ZTX692B) |
The inverter must generate about 1000 V peak‑to‑peak to initiate conduction.
The inverter operates at an inaudible 30 kHz.
A compact RM series transformer, model RMB FX3440 with a 0.1 mm gap, is used.
The ZTX690B has a BVCEV rating of 45 V and the ZTX692B has a rating of 70 V.