The principle of camera flash includes circuit diagram.

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abstract

The next generation of mobile phones will have high-quality camera functions. With the introduction of improved image sensors and optical accessories, people gradually pay attention to high-quality "flash" lighting. Flash lighting is the key factor to obtain high-quality photographic performance, so it needs to be focused and carefully considered.

Flash solution

At present, there are two main solutions for flash lighting-LED and flash. The advantages of LED are continuous working ability and low density support circuit. The important feature of flash is that it can achieve high quality photography. Its light output brightness of linear light source is several hundred times that of LED point light source, and it can easily spread dense light in a wide area. In addition, the color temperature of the flash is between 5500oK and 6000oK, which is very close to natural light, thus eliminating the color correction process necessary for the blue peak output of white LED.

Basic knowledge of Flash

Xenon is filled in the cylindrical glass tube of the flash lamp, and the anode and cathode electrodes are in direct contact with the gas; While the trigger electrodes distributed on the outer surface of the flash lamp are not in contact with gas. The potential range of gas breakdown is several thousand volts. Once it is broken down, the flashlamp impedance drops to ≤ 1ω. When gas breaks down, the high current will produce strong visible light. In fact, the required large current requires the flash lamp to be in a low impedance state before emitting light. The trigger electrode is responsible for this function. It transmits high voltage pulse in glass tube and ionizes xenon in lamp tube. The ionization process breaks through the gas and makes it in a low impedance state. The low impedance enables a large amount of current to pass between the anode and the cathode, and generates strong light. The energy contained is so high that the current and output light should be limited within the pulse operation range. Continuous operation will quickly produce extreme temperature and even destroy the flash. When the current pulse decays, the voltage of the flash lamp drops to a low point, and the flash lamp returns to its high impedance state, so another trigger is needed to start conduction.

Support circuit

Figure 1, the principle of flash circuit includes charging circuit, energy storage capacitor, trigger and lamp. The trigger command ionizes the gas in the lamp to discharge the capacitor through the flash lamp. The capacitor must be charged before the trigger can make the flash flash again.

Figure 1 is the working principle diagram of the flash operation support circuit. The flash lamp is operated by a trigger circuit and a storage capacitor that generates a high transient current. The typical voltage of the flash capacitor is 300V when it works. At first, because the flash lamp is in a high impedance state, the capacitor cannot be discharged. The trigger circuit command can generate thousands of volts of trigger pulses in an instant. After the flashlamp is punctured, the capacitor can discharge 1. The impedance of capacitors, wires and lamps is usually only a few ohms, and the instantaneous current generated is within 100A. Strong current pulses will produce strong flashes. The most important limitation of the flash repetition rate is whether the flash can emit heat safely, and the second is the time required for the charging circuit to fully charge the flash capacitor. The large capacitor charged to a high voltage, together with the limited output impedance of the charging circuit, will limit the charging speed. According to the provided input power, capacitance value and charging circuit characteristics, the charging time is limited to 1 to 5 seconds.

Fig. 2: On the basis of fig. 1, a driver/power switch is added to allow the capacitor to partially discharge, thus controlling light emission. Allow low brightness light pulses before the main flash, which can minimize the phenomenon of "red eye".

The figure shows the discharge process of the capacitor after receiving the trigger command. Sometimes it is necessary to select partial discharge in order to produce less intense flash. This operation can reduce "red eye", that is, one or more flashes with reduced intensity will immediately lead the main flash 2. Figure 2 shows this mode of operation. It adds a driver and a high current switch on the basis of figure 1. These components can stop the discharge of the flash capacitor by opening the conduction path of the flash lamp. This layout allows the "trigger/flash command" to control the pulse width to set the current flow time and flash energy. The partial discharge of low-energy capacitor can allow fast charging, and it can flash several times in low brightness before the main flash without damaging the flash.

As shown in fig. 3, the flash capacitor charger circuit includes an IC regulator, a step-up transformer, a rectifier and a capacitor. The regulator controls the capacitor voltage by monitoring the T 1 flyback pulse, which eliminates the path loss of the traditional feedback resistor divider. The control pin includes charging instruction and "done" display.

Thoughts on charging circuit of flash capacitor

The flash capacitor charger (Figure 3) is basically a transformer coupled with a boost converter with special functions. When the "charging" control line becomes high, the regulator switches the power supply regularly, so that the step-up transformer T 1 generates high-voltage pulses. These pulses are rectified and filtered to produce a DC output voltage of 300 volts. The conversion efficiency is about 80%. When the required voltage is reached, the circuit will be adjusted by stopping driving the power switch. It can also lower the "done" line to show that the capacitor is fully charged. The leakage loss of all capacitors can be compensated by intermittent power switching period. Typically, feedback is provided by the resistive voltage division of the output voltage. Generally, this method is not adopted, because it requires an extra switching period to offset the constant power leakage of the feedback resistor. This way, regulation can be maintained, and it will additionally leak power from the main power supply (which can be assumed as a battery). On the contrary, the regulation is realized by monitoring the flyback pulse characteristics of T 1, reflecting the secondary amplitude of T 1. The output voltage is set by the turns ratio of T 1. This function allows accurate capacitor voltage adjustment, which is a necessary condition to ensure that the flash intensity does not exceed the lamp energy or the rated voltage of the capacitor. Similarly, the flash energy can be easily set by the capacitance value without changing other circuits.

Detailed circuit discussion

Before further discussion, readers must realize that they should be very careful when building, testing and adopting this kind of circuit. High voltage and fatal dangerous factors lurk in this circuit. So be very careful when using and connecting the circuit. Reiterate: this circuit contains danger and high voltage hidden danger, so be careful.

Fig. 4 shows a complete flash circuit, which includes a capacitor charging component (the left side of the figure), a flash capacitor C 1, a trigger (R 1 of T2 C2), a Q 1-Q2 driver, a Q3 power switch and a flash. The trigger command biases Q3 and the ionization flash simultaneously through T2. C 1 light is generated by lamp discharge.

Fig. 4 is a complete flash circuit based on the previous discussion. The capacitor charging circuit shown in the upper left corner is similar to Figure 3. A D2 is added to safely clamp the reverse transient voltage generated by T 1. Q 1 and Q2 drive the high current switch Q3. T 2 step-up transformer generates high-voltage trigger pulse. Assuming that C 1 is fully charged, when Q 1-Q2 turns on Q3, C2 accumulates current to T2 primary side, and then T2 secondary side transmits a high-voltage trigger pulse to the flash lamp for ionization and conduction. C 1 discharges in the lamp and forms a flash.

In fig. 5, the capacitor charging waveform includes charging input (trace a), C 1 (trace b), finished output (trace c) and trigger input (trace d). The capacitance and output impedance of the charging circuit determine the charging time of C 1. For clarity, the widened trigger input can appear at any time after DONE goes low.

Fig. 5 detail that charging sequence of the capacitor. Track A (i.e. "Charge" input) goes high. This leads to the transition of T 1 and the diagonal rise of C 1 (trajectory b). When C 1 reaches the adjustment point, the switch stops, and the "finish" line pulled up by the resistor drops (track c), showing the charging state of C 1. When "DONE" becomes low level, the "TRIGGER" command (track D) that can discharge C 1 through the lamp-to-Q3 path can be issued at any time (about 600ms in this example). Please note that the trigger command in the figure is extended for the clarity of the photo; The full discharge time of C 1 is usually 500μs to 1000μs ... Low brightness flash (such as reducing "red eye") uses short-time trigger input instructions.

Fig. 6, high-speed details of the trigger pulse (trace a) and the generated flash current (trace b). After the pulse ionization flash lamp is triggered, the current reaches 100A.

Fig. 6 reflects the high-speed details of the high-voltage trigger pulse (trace A) and the generated flash current (trace B). It takes some time for the flash to enter ionization and start conduction after triggering. Here, after the 8kVP-P trigger pulse, the flash current of 10μs starts to rise to close to 100A. The current rises steadily within 5μs, and begins to decrease after reaching the defined peak. The generated light (Figure 7) rises slowly, reaches a peak at about 25μs, and then enters attenuation. Oscilloscope scanning deceleration can capture complete current and light activity.

Fig. 7 shows that the light output of the flash lamp steadily rises to the peak within 25 μ s..

Lighting, layout, RFI and related issues

Consideration of lamp

Several problems related to flash lamp. We must fully understand and master the requirements of light triggering, otherwise it will cause incomplete or even no flicker. Most problems related to triggers lie in the selection, driving and physical location between the trigger transformer and the lamp. Some flash manufacturers provide a single integrated component consisting of trigger transformer, lamp and light diffuser. This means that the trigger transformer is certified by flash supplier and has good driving performance. In other cases, the flash is triggered by the transformer and driver selected by the user, which requires the lamp supplier to be certified before mass production.

The anode and cathode of the lamp pass through the main discharge path of the lamp. Attention must be paid to the polarity of the electrode, otherwise the service life of the lamp will be seriously shortened. Similarly, the energy dispersion limit of the lamp should also be considered, otherwise the service life will be damaged. Excessive lamp energy consumption will cause the lamp to burst or break. By selecting the capacitance value and charging voltage and limiting the flash repetition rate, the energy can be easily and reliably controlled. Considering the trigger problem, the flash condition of the circuit set by the user needs to be certified by the lamp manufacturer before mass production.

Assuming that the trigger and flash energy are appropriate, the expected service life of the lamp is about 5000 flashes. Although the service time of all lamps is stipulated by the supplier, the actual service time may be different due to the difference of specific models of lamps. The typical measurement of service life is that the brightness of the lamp drops to 80% of the original value.

general layout

High voltage and current management layout. Returning to fig. 4, the discharge path of C 1 is through the lamp Q3 and back to the ground. A peak current of about 100A means that the discharge path must maintain a low impedance. The conduction circuit between C 1, lamp and Q3 should be short-circuited and lower than 1ω. In addition, the emitter of Q3 and the negative terminal of C 1 should be directly connected to form a compact and highly conductive loop between the positive terminal of C 1, the lamp and Q3 returning to C 1. Because large current will cause conduit erosion in local high resistance area, sudden interruption of trajectory and pilot hole should be avoided. If pilot holes must be used, they must be filled and verified with low impedance, or multiple pilot holes must be used. The inevitable capacitance ESR, lamp and Q3 resistance are usually between 1ω and 2.5ω. Therefore, a total trajectory impedance of 0.5 Ω or less is sufficient. Similarly, the relatively slow rise time of high current (see Figure 6) means that it is not necessary to control the trace inductance particularly strictly.

C 1 is the largest component in the circuit; Considering the space, further installation may be required. Under the premise of keeping the interconnection resistance within the limit range, it can be realized by long traces or wires.

The IC layout of the capacitor charger is similar to the traditional switching regulator. The circuit consisting of VIN pin of IC, bypass capacitor, transformer main terminal and switch pin must be short and highly conductive. The grounding pin of IC should be directly connected to the flat ground with low resistance. The 300V output voltage of the transformer needs to exceed the minimum spacing requirements of all high voltage nodes to meet the circuit board breakdown requirements. Verify the breakdown parameters of the board to ensure that the board is not damaged by conduction during cleaning. The trigger winding of several kilovolts T2 must be directly connected to the trigger electrode of the lamp, and the conductor is preferably less than 1/4 inch. Sufficient high-pressure space must be ensured. In short, try not to let the conductor touch the circuit board. The extra T2 output length will lead to trigger pulse drop or radio frequency interference (RFI). From this point of view, the flash triggered transformer module assembly is the best choice.