Flybacks Charge Xenon Flash Capacitors

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

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As digital still cameras (DSCs) and mobile phone cameras become more popular, users are demanding higher-quality photos under lower-light conditions. The popular Xenon photoflash is excited by high voltage (200 V to 300 V) normally obtained from a circuit powered by a nickel-cadmium, a nickel-metal-hydride or a Li-ion rechargeable battery, or an alkaline nonrechargeable battery.

The input voltage for a DSC can range from 1.8 V when operating by battery to 5 V when operating by ac adapter. Given such low-level input voltages, a flyback converter offers a popular way to derive the required high voltage. Such converters store the energy in a high-voltage capacitor and then transfer it to the Xenon photoflash lamp. Flyback converters are available in several architectures, including peak current limit switching, fixed-frequency switching and constant on-time switching. A brief overview of each architecture will show that constant on time is the optimum method for design flexibility, performance and safety.

Fixed-frequency switching

Efficiency and performance is poor because the output-voltage range is large and the switching frequency must be adjusted while the output voltage is increasing.

Constant on-time switching

The on time of the MOSFET is fixed, and the peak transformer current passing through it is determined by the transformer primary inductance. Thus, the primary current can be too low or, if the primary inductance is low, too high, which raises safety issues. This control technique also limits flexibility in choosing the transformer.

Peak current limit switching

This approach offers high efficiency and safety advantages. Because it limits the peak current, it can be implemented safely with a wide variety of transformers. And because the peak current limit is adjustable, it allows an optimal balance between safety and fast charging.

Photoflash Charge Circuit

DSC photoflash circuits require small size, a minimum component count, high efficiency and a fast charging time. However, traditional charging circuits include many discrete components. An example of a discrete charging circuit is shown in Fig. 1.

In this circuit, Q1 is the main power switch. The circuit is self-oscillating and a PWM control signal applied to the Charge EN terminal controls the charge rate. Zener diode D3 regulates the voltage on capacitor C6 to approximately 300 V. When Flash TRG is enabled, a pulse exceeding 2 kV is applied to the body of the flash tube through T2, causing ignition.

Discrete circuits have low efficiency and occupy considerable pc-board space. As a result, the traditional discrete approach has become less popular. An integrated circuit can simplify photoflash applications by minimizing the external component count and pc-board space (Fig. 2). Such circuits exhibit one of the three control techniques described earlier. For example, the MAX8622 IC uses the peak current limit method.

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