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Emergency strobe flasher generates 250V

Robert Sheehan, Linear Technology Corp, Milpitas, Corp -August 05, 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.

Figure 1 A complete circuit for an emergency lamp operates from a 12V automotive battery and generates a 250V-dc anode voltage and a 4-kV trigger pulse for the Xenon flash tube.

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 VIN to power IC2. (DI #2393)

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