White LED drivers provide a cost-effective option for driving power-limited xenon flashes to illuminate the scene. This article describes the basic concepts of xenon lamps and details a typical low-power, low-cost flash driver circuit design based on ON Semiconductor's NCP5007 chip.

Design of low power flash drive circuit based on NCP5007 chip

PCBfans.cn suggested that the picture name is 1: (a) breakdown voltage of xenon lamp flash lamp.

(b) Basic xenon flash.

Digital cameras and camera-equipped mobile phones have gone from new to mainstream products, with digital image sensors based on CCD or CMOS technology inside these products. In order to support the image sensor in low light conditions, flash circuits have become a necessary solution for digital cameras and are increasingly common in camera phones. The main light sources to meet the flash requirements are xenon bulbs and now emerging high-brightness white LEDs.

The main advantage of xenon lamps is that the flash pulse produces high power light output, while the advantages of LED solutions are that they are relatively small and thin, and can be turned on for longer periods of time in video capture, although their light output is only moderate compared to xenon lamps. In particular, xenon lamps provide very short flash pulses that enable the camera to capture photos as still images. They require special drivers to generate high voltages and large storage capacitors to store electrical energy.

We have found that white LED drivers provide a cost-effective option for driving power-limited xenon flashes to illuminate the scene. This article describes the basic concepts of xenon lamps and details a typical low power/low cost flash lamp application.

Xenon lamp concept

A xenon lamp has a glass envelope with an electrode at each end, which is filled with a low-pressure noble gas mixture. In steady state, as shown in Figure 1, the voltage value on the electrode is set below the trigger voltage. At this point no current flows and the system remains stable until a trigger voltage is applied to the third electrode. For the low power lamp considered, this high voltage pulse is in the range of 1kV and it comes from a transformer with a small magnetic core, triggered by a sudden discharge of capacitor C2 (see Figure 2).

Design of low power flash drive circuit based on NCP5007 chip

PCBfans.cn suggested that the picture name is 2: Schematic diagram of the demo board. (click to enlarge)

When the gas mixture is excited, the plasma produces flashes of light. For consumer applications, the typical duration of the flash is 2ms. Depending on the flash type concerned, the energy stored in capacitor C1 can be as low as 1 joule (for small cameras) or as high as several thousand joules (for professional applications). The energy consumed by the flash is shown in Equation 1:

Design of low power flash drive circuit based on NCP5007 chip

Depending on the type of xenon lamp used, the capacitor charging voltage is basically 160V to 600V. Low-power handheld cameras require no more than a few joules of energy and output voltages ranging from 160 to 250V . Higher voltages are typically seen in professional studio equipment.

In addition to the DC voltage, a high-voltage pulse must be supplied to excite the plasma in the bulb, creating a high-intensity arc between the two electrodes. The pulse amplitude depends on the type of bulbs used in the system, ranging from as low as 1.6kV to more than 10kV. The miniature xenon lamp is triggered by a 1.6kV/5μs pulse applied to the third external electrode of the lamp tube. This pulse is generated by connecting the dedicated pulse transformer shown in Figure 2 with a high voltage capacitor. The capacitor is charged to the flash DC voltage (200V) and suddenly discharges to the primary side of the pulse transformer after the button S1 is triggered. A high voltage from the secondary side is applied to the bulb (on the outer surface) and the flash is fired.

The main advantages of this concept are the high light output and short pulse duration, which allows for a quick shot to freeze a moving subject at a certain point. The disadvantages of this design are the large size of the storage capacitor, the need for higher voltages on the board, and the need to recharge the capacitor during shooting (5 to 10 seconds for consumer applications).

Depending on the imager sensitivity and lens aperture, relatively small capacitors are sufficient for bright pictures in the typical environment of consumer applications. Therefore, a simple converter can be used to boost the battery voltage to the 200V required by low-voltage xenon lamps.

Design of low power flash drive circuit based on NCP5007 chip

PCBfans.cn hints that the picture is named 3: Primary and secondary output voltage waveforms.

low power flash converter

This article introduces the low power flash converter based on ON Semiconductor's NCP5007 chip. The chip was originally developed to drive series-connected white LEDs. In this application, the main consideration is the voltage limitation associated with silicon breakdown up to 28V. To overcome this challenge, use an external transistor that can hold a minimum of 250V, or use a transformer with a primary-to-secondary ratio of 1:10.

Assuming that the storage capacitor is 50μF, the operating voltage of the small xenon lamp is 200V, and the flash energy is:

Design of low power flash drive circuit based on NCP5007 chip

This energy is transferred from the battery to the storage capacitor by using a DC/DC converter to form a boost converter. Although the NCP5007 structure is based on the flyback mode, it cannot be used directly because the chip operates in the pulse frequency mode (PFM), and the time parameters are variable Ton and constant at 300nsToff maximum. Therefore, if a traditional flyback topology is used, the secondary inductance will not be fully discharged within the Toff time, and the core will saturate quickly, resulting in very small inductance and low energy transfer on the primary side.

Design of low power flash drive circuit based on NCP5007 chip

Table 1: Xenon Flash Demo Board Parts List.

To overcome this limitation, a combined approach is to combine flyback and forward mode to increase the output voltage capability. This combination is obtained by encapsulating four diodes in a bridge configuration, as shown in Figure 3. Two diodes assembled in a SOT-23 package carry the output current during the switching cycle.

During the Ton time, the transformer T1 pin 12 is low, so the battery voltage appears on the secondary side pin 1, which is the forward mode of operation. The storage capacitor is charged by the current flowing through diode D1. After the Toff cycle begins, the primary voltage recovers, and the storage capacitor is charged by the current flowing through the diode D2, which is the flyback operating mode.

This concept is demonstrated by the demo board in Figure 3, which is powered by two standard alkaline AA dry cells. The system is powered by switch S1, the converter is controlled by switch S2 connected to the start pin, and the third switch S3 is the manual trigger flash button.

The waveform in Figure 4 shows the voltage during recharging of the storage capacitor. The signals correspond to U1 pin 4 (top curve), D1 pin 3 (middle curve) and D2 pin 3 (bottom curve). Forward mode occurs when the U1 switch is turned on, and the flyback cycle begins when the switch is turned off.