Design Ideas February 17, 1997

Dual monostable drives quasiresonant inverter

Christophe Basso, Sinard, France

A zero-voltage-switching (ZVS) controller usually integrates a one-shot circuit, embodied in a VCO system. An error amplifier monitors the output voltage and adjusts the VCO's off-time to keep the output value at a constant level. Each on-time period commences as soon as the primary voltage drops to zero, thus eliminating on/off commutation losses associated with the switching element. The controller also incorporates other convenient features, such as MOSFET drivers, a voltage reference, and overvoltage and undervoltage lockouts. In low-cost circuits, such a complex architecture can lead to a prohibitive cost, especially if you don't need the cited features (in open-loop systems, for instance). Figure 1 shows an 8W, ZVS fluorescent-lamp converter made from two low-cost ICs, a CD4538 and an LM393.

The first 4538 part, IC_1A, generates the fixed on-time value; its output directly drives MOSFET Q₁. The Q output feeds back to the blanking generator (IC_1B), thereby preventing an off-time that is too short. The pullup network of R₃ and D₂ makes the oscillator run free as long as a single low-going pulse exists at IC_1A's input at start-up time. The zero-voltage-detector (ZVD) section comprises IC_2B, whose output goes high and initiates a new on-time period each time Q₁'s drain voltage is near 0V. At power-on, IC_2B's output is low, thus preventing the oscillator from starting. IC_2A momentarily disables the ZVD function at start-up and kicks the free-running oscillator until IC_2B starts to function.

The resonating network uses the natural leakage inductance of a simple E-core transformer associated with a two-section coil former (primary and secondary windings physically separated), whose description appears in Figure 1. An E shape is better suited for the network than is an RM core (in which a large leakage inductance is difficult to obtain), for instance, because of the intrinsically low magnetic leakage. C₃ not only sets the natural resonating frequency but also limits the voltage rise on Q₁'s drain to a safe value. You can adapt this quasiresonant converter for various fluorescent tubes by adjusting the on-time value with R₁ or C₄. (DI #1990)

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