Emergency strobe flasher generates 250V

Robert Sheehan, Linear Technology Corp, Milpitas, Corp -- EDN, 8/5/1999

Figure 1 shows a complete circuit for an emergency lamp that operates from a 12V automotive battery. The xenon flash tube requires a 250V-dc anode voltage and a 4-kV trigger pulse. To generate the 250V dc, IC₁, a switching regulator controller, and T₁, a standard Versa-PAC transformer, operate in the discontinuous-flyback mode. With this configuration, circuit efficiency is typically 75 to 80%. R₁ and IC₁'s internal-sense-threshold voltage limit the peak primary current to 1.6A. The R₂/R₃ divider and IC₁'s internal 1.25V reference at the VFB Pin determine the maximum-voltage setpoint. To generate the 4-kV trigger pulse, a standard cold-cathode- fluorescent-lamp (CCFL) backlight transformer, T₂, operates in the forward mode. IC₂, a dual MOSFET driver, functions as a 1-Hz oscillator and a one-shot for the trigger pulse for Q₂. Additionally, Q₃ blanks out the operation of the 250V supply during this time. This feature is important because the circuit must reduce the flash tube's anode current to a low level, allowing the tube to reset and wait for the next cycle. Otherwise, the Xenon flash tube may burn out.

The selected flash tube has a maximum flash energy of 4W/sec (joules) and a maximum frequency of 60 flashes/min. Using a 68-µF capacitor for C₁, the available flash energy, ½CV², is 2 joules, and a flash frequency greater than 1 Hz is possible.

If input voltages below 8V are desirable-for example, when you use a 6V lantern battery-it is important to limit the flash frequency to less than 1 Hz. This limit allows sufficient time for C₁ to charge. The peak primary current, converter switching frequency, and transformer primary inductance determine the charge time: t=(½CV²)/(½LI²f). You can omit Q₄, D₁, and R₄ and use V_IN to power IC₂. (DI #2393)

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