Flybacks Charge Xenon Flash Capacitors

Mar 1, 2007 12:00 PM
By Rayleigh Lan, Field Applications Engineering Director, and Hunter Chen, Field Applications Engine

Input Voltage Versus Charging Time

Assume that the transformer primary inductance is 5 H, the secondary-to-primary turns ratio is 15 to 1, the peak current limit is 1.6 A and the output voltage ranges from 30 V to 300 V. Ignore power losses. Using Eq. 6, it is possible to simulate the curve for input voltage versus charge time. The simulated ideal (lossless) case is shown on the left side of Fig. 6.

This analysis considers the ideal case, which ignores power loss. Actual charging time is longer than the calculated values shown in Fig. 7. To simulate power loss, simply add a voltage drop to the input voltage to obtain a calculated charging time that should be close to that actually measured (Fig. 8).

Compared with other switching architectures, the MAX8622 peak current limit switching can optimize a design for faster charging time while suppressing the inrush current. The formulas derived for simulation give results very close to the actual measurements.

The MAX8622 also provides input-undervoltage detection, which extends battery life, and a high level of charging accuracy by monitoring the output voltage with an external resistor divider. The analysis and explanation presented in this article help design engineers choose the proper external components, and thereby design a photoflash charger that meets all specified requirements.

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© 2007 Penton Media, Inc.