Development of a new type of pulse xenon lamp ignition and pre-ignition power supply

1 Introduction

With the development of computer, mechanical and electronic technology, road inspection vehicles have made it possible to obtain road use information on a large scale, quickly and accurately. The camera system on the inspection vehicle is divided into ordinary cameras, high-speed cameras and digital cameras according to the camera speed. Among them, high-speed cameras are mainly used to collect images of cracks, potholes and other damage conditions on the road surface. However, when using high-speed cameras, most inspection vehicles use a continuous light source. Due to the low intensity of the light source, it is often necessary to increase the exposure time in actual use to achieve high-quality image data. Therefore, increasing the intensity of the irradiated light source can improve the quality of the image data [1].

The advantage of pulse xenon lamp is that it can solve the contradiction between brightness and heat. It emits strong light when it discharges, but the flash duration is very short, so the heat impact is small. Due to the large instantaneous light energy, the image layering is better restored. In order to extend the life of the pulse xenon lamp and improve the photoelectric conversion efficiency, it is generally necessary to add a pre-ignition current before the pulse high current discharge when the repetition rate is low [2]. If the traditional industrial frequency transformer pre-ignition circuit is used, it is necessary to increase the filter capacitor and high-power current limiting resistor, which increases the volume of the circuit and is easily mistaken for the release of pre-ignition due to interference [3]. In addition, when the pulse xenon lamp is working, the arc discharge time is long, and the energy of the discharge capacitor is not fully released and accumulated, which may cause the discharge capacitor to fail to discharge normally. Based on the working principle of the pulse xenon lamp, this paper proposes a power supply structure for the starting and pre-ignition of the pulse xenon lamp, and develops the pre-ignition power supply of the pulse xenon lamp. The power supply adopts PWM technology control, and the starting and pre-ignition stages share a power supply, which is a voltage source during starting and a constant current source during pre-ignition. The test results show that the power supply has high efficiency, reliable operation, stable operation, and effectively solves the abnormal discharge phenomenon caused by the residual energy accumulation of the discharge capacitor.

2 Working principle of pulse xenon lamp

The operation of a pulse xenon lamp is divided into three stages: ignition, pre-ignition and high-voltage discharge [4], as shown in Figure 1. Its working process is relatively complex and is a non-steady-state gas discharge. In the ignition stage, the discharge first generates an ionization channel near the trigger wire on the inner wall of the quartz tube. The gas is heated by the collision with electrons, and the xenon gas in the lamp is rapidly ionized, resulting in glow discharge. The pulse transformer T, capacitor C2, thyristor VT2 and resistor R2 form the ignition circuit. When VT2 is turned off, the voltage U1 charges the capacitor C2 through the resistor R2, and the energy is stored on the capacitor C2. Usually, U1 is about 1kV, and the charging time is very short. When VT2 is turned on, capacitor C2 and the inductor of pulse transformer T resonate and discharge, generating a starting voltage of about 5kV at the secondary end of transformer T. Under the strong axial electric field and the triggering high-voltage pulse, the gas of the pulse xenon lamp is broken down to form a discharge channel; in the pre-combustion stage, when the input energy is large enough, the electrode is heated to have a certain thermal emission ability, and the gas in the lamp tube transitions from glow discharge to arc discharge. At this time, the pulse xenon lamp can be approximated as a resistor, and the voltage U2 is added to the two ends of the pulse xenon lamp through the resistor R1 and the diode D to form a pre-combustion circuit; in the high-voltage discharge stage, the pulse xenon lamp is arc discharge. When VT1 is turned off, the voltage U3 charges the capacitor C1. When VT1 is turned on, the capacitor C1 discharges to the pulse xenon lamp, so that the pulse xenon lamp has arc stroboscopic phenomenon. In the high-voltage discharge stage, the pre-combustion circuit always provides the pulse xenon lamp with a maintenance current (about 100mA).

Figure 1 Schematic diagram of the working principle of pulsed xenon lamp

In the traditional pulse xenon lamp ignition and pre-combustion system, voltage sources are required in the ignition stage and the pre-combustion stage respectively, as shown in Figure 1, U1 is the ignition voltage, and U2 is the pre-combustion voltage, which increases the complexity of the power supply design. The new pulse xenon lamp ignition and pre-combustion power supply adopts PWM technology control, and the ignition and pre-combustion stages share a power supply, which is a voltage source during ignition and a constant current source during pre-combustion. In the ignition stage, the maximum duty cycle outputs the highest voltage, and the high-voltage ignition voltage is obtained through series resonance; in the pre-combustion stage, the duty cycle is adjusted to output a constant current to maintain the current (about 100mA).

3 Design of pre-combustion power supply

The pre-ignition power supply consists of two parts: the high-voltage trigger of the pulse xenon lamp and the pre-ignition current maintenance, as shown in Figure 1. U1 and U2 of the traditional pre-ignition circuit are usually obtained by boosting the power frequency step-up transformer and rectifying with a diode. This circuit has the following main disadvantages: the power frequency step-up transformer is large and heavy; the power consumed by the current limiting resistor R1 is relatively large, generally between 100W and 300W; a high-voltage trigger circuit is required; and the pre-ignition current of the output pulse xenon lamp cannot be adjusted [5].

The block diagram of the new ignition pre-ignition power supply system is shown in Figure 2. The power supply consists of a high-frequency push-pull converter, a high-frequency transformer, a high-voltage ignition circuit, a control protection circuit, and a pre-ignition detection circuit. It has the characteristics of high conversion efficiency and small output current ripple. The AC 220V input voltage is isolated by a transformer, rectified and filtered as the input of the push-pull converter. The push-pull converter converts the input voltage into a high-frequency AC pulse voltage, and completes the voltage matching and high-frequency isolation functions through a high-frequency transformer. The pre-ignition voltage is then output by the output rectification and filtering link. At the same time, the high-voltage ignition circuit boosts the output high-voltage ignition voltage, eliminating the bulky and heavy industrial frequency output transformer and reducing audio noise [6]. The control protection circuit consists of a UC3825 device and peripheral circuits. According to the current signal fed back by the main circuit, it provides a PWM drive signal for the switch device. The pre-ignition detection circuit compares the detected current signal with the reference power supply and outputs a pre-ignition success signal, and at the same time turns off the high-voltage ignition.

Figure 2 Block diagram of the ignition pre-ignition power supply system

Research shows that pulse xenon lamps exhibit resistance characteristics when working at high frequencies. Before lighting, its equivalent resistance is very large, equivalent to an open-circuit load. At this time, the control chip UC3825 outputs a PWM drive signal with the maximum duty cycle, and the push-pull converter outputs a high-voltage ignition voltage to ionize and conduct the gas in the pulse xenon lamp; after ignition, the high-voltage ignition circuit stops working, and its equivalent resistance decreases sharply, equivalent to a short-circuit load. At this time, the control chip UC3825 detects the current peak of the main circuit and adjusts the duty cycle of the output PWM. The system enters closed-loop control, and the push-pull converter outputs the maintenance current of the pulse xenon lamp during pre-ignition operation; thereafter, the equivalent resistance of the pulse xenon lamp gradually reaches a steady state and remains constant.

3.1 Ignition pre-ignition main circuit

The main circuit of the new pre-ignition power supply is shown in Figure 3. The power switching devices Q1 and Q2 form a push-pull converter; the high-frequency transformer T1 forms a boost link, as shown in Figure 3 (a); the high-frequency coupled boost transformer composed of inductor L2, capacitor C6 and inductors L3 and L4 forms a high-voltage ignition link, as shown in Figure 3 (b); diodes D3~D6 form input and output rectification and filtering links.

The AC 220V voltage is 125V after transformer isolation, rectification and filtering, which is used as the DC voltage input to the push-pull converter. The push-pull converter uses the power MOSFET device IRF460. Capacitor C1, resistor R1 and diode D1 form a peak absorption circuit. When the device is turned off instantly, it absorbs the peak voltage on the switching device and the line, reducing the voltage stress when the power MOS tube is turned off. C1=0.01μF, R1=3.6kΩ, and the withstand voltage of diode D1 is greater than 500V. UF4007 is selected.

(a)

(b)

Figure 3 Ignition pre-ignition circuit: (a) Pre-ignition main circuit; (b) Ignition main circuit

3.2 High-frequency transformer design

The core of the high-frequency transformer is made of soft ferrite, and the magnetic flux density is usually Bm=0.2T. The input voltage of the primary end of the transformer is Ui=125V, and the working duty cycle is 0.25. The maximum output voltage of the secondary end is 1000V, the pre-ignition voltage is 250V, and the pre-ignition maintenance current is I=0.1A. The turns ratio of the primary and secondary ends is n=N1/N2=125/1000=1/8. The effective area of ​​the core S is 2cm2. According to formula (1)

(1)

Calculation shows: ; N2=176.

Take the transformer winding current density j=3A/mm2; output power P0=U0I0=250×0.1=25W; input power Pi=P0/0.9=28W; primary end working current ; required winding wire cross-sectional area =0.3mm2.

The current at the primary end is relatively large. If a single-strand winding is selected, the effective area diameter D=0.62mm. The wire diameter is large and the skin effect is serious. Therefore, multiple strands of wire are wound in parallel. The primary end uses 5 strands of wire in parallel, and the cross-sectional area of ​​each strand of wire is S=0.3/5=0.06mm2, and the wire diameter of each strand is D=0.28mm. The secondary end uses a single-strand winding, and the required wire current cross-sectional area is S=0.1/3mm2, and the wire diameter is D=0.22mm.

3.3 Output Filter

The maximum voltage of the rectifier bridge at the secondary end of the transformer is 1000V. The withstand voltage of the diode should be greater than 1000V, and 36MB160A is selected. The capacitor in the rectifier circuit generally uses an electrolytic capacitor to smooth and filter and reduce the AC component in the DC voltage. If the selected filter capacitor is too small, the DC voltage pulsation after filtering will be large. In order to obtain the required output voltage, a larger duty cycle adjustment range and too high closed-loop gain are required. The minimum value of the DC voltage after filtering is also relatively small, requiring the primary and secondary turns ratio of the high-frequency transformer to be larger, and the current in the switch tube to increase, and the reverse voltage of the output rectifier diode to increase; if the selected filter capacitor is too large, the charging current pulse width becomes narrower, the amplitude increases, and EMI increases. Since the output voltage ripple frequency of the high-frequency transformer is relatively high, a smaller filter capacitor value can be selected. Resistor R3 and capacitor C3 are connected in series to form an absorption circuit, and inductor L1 and capacitor C4 play a filtering role. After calculation, R3 = 4.7kΩ/2W, C3 = 1000pF/3kV, C4 = 0.01μF/3kV, L1 = 2mH.

3.4 High Voltage Ignition Circuit

The pulse xenon lamp uses the high voltage breakdown gas provided by the high voltage ignition circuit to break down the gas in the xenon lamp. When the normally closed contact relay S is closed, the voltage regulator tube D10 is short-circuited, and the capacitor C6 is charged to 600V. After the charging is completed, the relay S is disconnected, and the voltage regulator tube D10 charges the capacitor C5 until the bidirectional trigger tube D11 is turned on, thereby turning on the thyristor VT1, and the capacitor C6 and the coupling inductor L3 are in series resonance, so that a high ignition voltage is generated at both ends of the coupling inductor L4, and the gas in the pulse xenon lamp is broken down.

The voltage regulation value of D10 is 50V. The trigger voltage of the bidirectional trigger diode D11 is 35V. Resistors R7=510Ω and R8=4.7kΩ are connected in series to divide the voltage. R5 acts as a voltage divider. Resistor R9 limits the charging current of capacitor C6. The withstand voltage of C6 should be greater than 1kV, and C6=1μF/1200V is selected. Resistors R6=3.6kΩ and R10=22MΩ are the discharge resistors of the capacitor. L2 is the current limiting inductor, with a value of 1μH. L3=48μH, L4=6.5mH. Diodes D12 and D13 provide a resonant circuit.

3.5 Control Circuit Design

The control chip UC3825 is highly integrated, and the circuit design is mainly focused on the peripheral circuit parameters. The peripheral circuit includes voltage error comparison compensation network, oscillation input, current limiting protection, and output peak voltage absorption circuit. UC3825 adopts peak current control mode, and the voltage error feedback is used as the given current peak value, which is compared with the sampled current to generate a PWM drive signal.

The control circuit is shown in Figure 4. This circuit is designed according to the maximum output duty cycle. Therefore, the chip's in-phase input terminal 2 is connected to the reference voltage output pin 16 through the current limiting resistor R11, and the inverting input terminal 1 is connected to the ground. The timing resistor R14 of pin 5 and the timing capacitor C12 of pin 6 determine the frequency f and the maximum duty cycle DMAX of the output signal. Output pins 11 and 14 output PWM drive signals with complementary dead time respectively. The current limiting resistor R17=R18=100Ω, the resistor R19=1kΩ, and the voltage regulator diode D18 are 18V, forming a peak voltage absorption circuit.

Figure 4 Control circuit

(1) Oscillation parameter selection

The oscillator of UC3825 is a sawtooth waveform. The current value determined by the resistor value connected to the RT pin and the capacitor value connected to the CT pin determines the rising edge time of the output sawtooth wave. The falling edge time of the sawtooth wave determines the output dead time [7]. The maximum output duty cycle DMAX is determined by the value of R14. The value of C12 is calculated based on the PWM output frequency f=50kHz and the maximum duty cycle DMAX=0.92. Calculated by the formula:

(2) Current limiting protection design

The current limiting protection signal samples the instantaneous current on the power switch devices Q1 and Q2 of the main circuit. The voltage on the Zener diode D20 is 3.6V, and the voltage on the sampling resistor R21=0.5Ω/2W charges the capacitors C16 and C17. When the charging voltage is greater than the internal current limiting voltage of 1V, the chip UC3825 reduces the duty cycle of the PWM pulse to limit the current peak of the main circuit; when the charging voltage is greater than the internal protection voltage of 1.2V, the chip UC3825 turns off the PWM pulse output and the device self-locks. The current limiting resistor R16=1kΩ, C16=1000pF, C17=330pF.

3.6 Pre-ignition detection circuit

The detection circuit constructed around the voltage comparator LM311 is shown in Figure 5. Before pre-ignition, the pre-ignition feedback voltage uf is close to 0 V; after pre-ignition, the pre-ignition feedback voltage uf is greater than 0.2V. The reference voltage uref is 5.1V, and 0.2V is obtained by the voltage division of resistors R23 and R24, taking R23 = 24kΩ, R24 = 1kΩ. Resistor R22 = 47kΩ and capacitor C19 = 1μF form a low-pass filter. Capacitor C20 filters out interference caused by wiring and device placement.

Figure 5 Pre-ignition detection circuit

The output of the voltage comparator is connected to the pull-up resistor R26 = 10kΩ. When the voltage uf is less than 0.2V, the pulse xenon lamp is not pre-ignited, the comparator output is low level, and the normally closed relay S is disconnected; when the voltage uf is greater than 0.2V, the pre-ignition is successful, the comparator output is high level, and the relay S is closed. R27 = 4.7kΩ acts as a current limiter, and the capacitor C21 = 0.1μF is a power supply filter capacitor.

4 Test results and analysis

4.1 Test results and analysis of the ignition stage

The driving waveforms of the power switching devices Q1 and Q2 are shown in Figure 6(a). After power is turned on, the equivalent resistance of the pulse xenon lamp is very large, which is equivalent to an open load. The circuit is in a non-working state, and the secondary power of the transformer is zero. The primary current is very small, about 0.1A. The control chip UC3825 outputs a complementary PWM drive signal with a maximum duty cycle and dead time. At this time, the primary voltage of the high-frequency transformer is very high, as shown in Figure 6(b). The resonant voltage waveform across the inductor L3 is shown in Figure 6(c), and the first pulse voltage is greater than 600V. After the thyristor VT1 is turned on, the charging capacitor C6 and the coupled inductor L3 resonate in series, and the resonant voltage is suddenly added to the two ends of the inductor L3 until the power is consumed. At this time, the gas in the pulse xenon lamp is ionized and turned on.

(a) (b)

(c)

Figure 6 Experimental waveforms in the starting stage: (a) driving waveform of the power switching device; (b) transformer primary terminal voltage waveform; (c) resonant voltage waveform across inductor L3.

4.2 Pre-combustion stage test results and analysis

After ignition is completed, the pulse xenon lamp is turned on and the high-voltage ignition circuit stops working. Its equivalent resistance decreases sharply, which is equivalent to a load short circuit. The control chip UC3825 adjusts the output PWM drive signal and the system enters a closed-loop control, as shown in Figure 7(a). The voltage at the primary end of the transformer drops, as shown in Figure 7(b). The push-pull converter outputs a pre-ignition voltage to maintain the holding current when the pulse xenon lamp is working. After that, the equivalent resistance of the lamp gradually reaches a steady state and remains constant. The actual picture after ignition and pre-ignition is shown in Figure 8, and the four parallel pulse xenon lamps work simultaneously.

(a) (b)

Figure 7 Pre-ignition stage test waveforms: (a) driving waveform of the switching device; (b) transformer primary terminal voltage waveform

Figure 8 Pulse xenon lamp pre-combustion actual picture

5 Conclusion

In order to meet the needs of pulse xenon lamps on road inspection vehicles, this paper designs and develops a new type of pulse xenon lamp ignition and pre-ignition power supply. According to the characteristics of gas discharge during the ignition and pre-ignition of pulse xenon lamps, a push-pull converter controlled by the UC3825 control chip and a high-frequency transformer are used to form a pre-ignition circuit, and the series resonance principle is used to achieve high-voltage ignition. For the four parallel pulse xenon lamps of Ф7×120mm we use, the pre-ignition current of a single pulse xenon lamp is stable at 100mA, and the pre-ignition voltage at both ends of the pulse xenon lamp is 250V. The power supply has a simple structure, stable output, small size, light weight and high efficiency. The test results show that the power supply is reliable and stable in operation, and has strong practical value.