Photoflash High-voltage Power Supply

Photoflash and strobe devices operate by discharging a high- voltage capacitor into a bulb. Operation of these circuits from a battery or other low-voltage source requires a Step-up dc- dc converter to boost up to a high voltage, typically 300V. One way to generate this voltage is by using a flyback converter topology. This circuit provides a simple and reliable way of charging a High-Voltage capacitor. It provides safety isolation via the flyback transformer. The circuit uses a minimum number of components to reduce board space and increase reliability. Theory The flyback converter performs two functions (Figure 1). The converter must boost the low-voltage input as well as provide isolation between input (battery) and output (high voltage). The main circuit components are the power transformer, output Diode output capacitor and MIC3172 controller chip. The MIC3172 packages the switching Transistor voltage regulation, and control Logic into one IC, which simplifies the circuit. Energy is stored in the transformer when the internal transis- tor of the MIC3172 turns on, allowing current to flow through the transformer primary. When the Transistor turns off, energy stored in the transformer flows through the output rectifying Diode into the capacitor. The voltage across the capacitor is increased with each switching cycle until it reaches the preset voltage. This preset voltage is determined by the resistive divider (R1, R2, and R3) and internal 1.24V reference of the MIC3172. The output voltage set point is: V=V 1 R1 R2 R3 OUT REF + +       Once this voltage is reached, the MIC3172 stops switching. Since energy leakage in the output components causes the capacitor to discharge over time, the MIC3172 provides occasional pulses of energy to keep the capacitor fully charged. When the output capacitor is discharged into the bulb the charging process is repeated. Circuit Description and Analysis The MIC3172 simplified block diagram is shown in Figure 2. It contains a switching Transistor current-limit circuit, enable input, and error Amplifier for voltage regulation. The switching frequency of the MIC3172 is 100kHz. When the output voltage is less than the preset voltage the error Amplifier output will be high, causing the controller to run the flyback converter at maximum power. The current-limit circuit in the MIC3172 and the input voltage limits the output power. As the
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Photoflash High-voltage Power Supply application circuits
Photoflash High-voltage Power Supply application circuits



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