Development of the latest type of pre ignition pow

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

Introduction

with the development of computer, machinery and electronic technology, road detection vehicles make it possible to obtain road use information on a large scale, quickly and accurately. The camera system on the detection vehicle is divided into ordinary camera, high-speed camera and digital camera according to the camera speed. The high-speed camera is mainly used to collect images of cracks, pits and other damage conditions on the road. However, when using high-speed cameras, most detection vehicles use a continuous light source. Due to the low intensity of the light source, it is often necessary to increase the exposure time to achieve high-quality image data in actual use. Therefore, improving the intensity of the illuminated light source can improve the quality of image data [1]

the advantage of pulse xenon lamp is that it can solve the contradiction between brightness and accompanying heat. It emits strong light when discharging, but the flash duration is very short, so the heat effect is small. Due to the large instantaneous energy, the hierarchical restoration of the image is better. In order to prolong the service life of pulse xenon lamp and improve the photoelectric conversion efficiency, in the case of low repetition rate, it is generally necessary to add pre ignition current before pulse high current discharge [2]. If the traditional power frequency transformer type precombustion circuit is adopted, the filter capacitance and high-power current limiting resistance must be increased, which increases the volume of the circuit, and the circuit is easy to be mistaken for the removal of precombustion due to interference [3]. In addition, when the pulse xenon lamp is working, the arc discharge time is long, and the energy release of the discharge capacitor is not sufficient, which may lead to the phenomenon that the discharge capacitor cannot discharge normally. According to the working principle of pulse xenon lamp, this paper presents a power supply structure of pulse xenon lamp ignition and precombustion, and develops the precombustion power supply of pulse xenon lamp. The power supply is controlled by PWM technology. The ignition and pre ignition phases share a common power supply. It is a voltage source during ignition and a constant current source during pre ignition. The test results show that the power supply has high efficiency, reliable operation and 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 pulse xenon lamp is divided into three stages: ignition, precombustion and high voltage discharge [4], as shown in Figure 1. Its working process is complex, which is a kind of unsteady gas discharge. In the lighting stage, the discharge first generates an ionization channel near the trigger wire on the inner wall of the quartz tube. The gas is heated due to collision with electrons, and the xenon in the lamp is rapidly ionized, resulting in glow discharge. Pulse transformer T, capacitor C2, thyristor vt2 and resistor R2 form a lighting circuit. When vt2 is turned off, voltage U1 charges capacitor C2 through resistor R2 and stores energy on capacitor C2. Generally, U1 is about 1kV and the charging time is very short. When vt2 is turned on, the capacitor C2 and the inductive resonance discharge of the pulse transformer t produce an igniting voltage of about 5kV at the secondary end of the transformer T. under the action of a strong axial electric field and a 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 capacity, and the gas in the lamp transits from glow discharge to arc discharge. At this time, the pulse xenon lamp can be approximately a resistance, and the voltage U2 is added to both ends of the pulse xenon lamp through resistance R1 and 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, voltage U3 charges capacitor C1. When VT1 is turned on, capacitor C1 discharges to the pulse xenon lamp, so the pulse xenon lamp appears arc stroboscopic phenomenon. In the high-voltage discharge stage, the precombustion circuit always provides the maintenance current (about 100mA) to the pulse xenon lamp

in the traditional pulse xenon lamp ignition and pre ignition system, voltage sources are required in the ignition stage and pre ignition stage respectively. As shown in Figure 1, U1 is the ignition voltage and U2 is the pre ignition voltage, which increases the complexity of power supply design. The new pulse xenon lamp ignition and pre ignition power supply is controlled by PWM technology. The ignition and pre ignition can also be used in the experimental stage of compressive strength of other non-metallic materials. They share a power supply. The voltage source is used for ignition and the constant current source is used for pre ignition. In the starting stage, the maximum duty cycle outputs the highest voltage, and the high voltage starting voltage is obtained through series resonance; In the pre combustion stage, the constant current output maintains the current (about 100mA) by adjusting the duty cycle

Figure 1 Schematic diagram of working principle of pulse xenon lamp

3 design of precombustion power supply

precombustion power supply is composed of two parts: high voltage triggering and precombustion current maintenance of pulse xenon lamp, as shown in Figure 1. U1 and U2 of traditional pre combustion circuit are usually obtained by power frequency step-up transformer and diode rectification. This circuit mainly has the following disadvantages: power frequency step-up transformer is large and bulky; The current limiting resistor R1 consumes more power, generally between 100W and 300W; There must be a high-voltage trigger circuit; The precombustion current of the output pulse xenon lamp is not adjustable [5]

the block diagram of the new ignition precombustion power supply system is shown in Figure 2. The power supply is composed of high-frequency push-pull converter, high-frequency transformer, high-voltage lighting circuit, control and protection circuit and precombustion detection circuit. It has the characteristics of high conversion efficiency and small output current ripple. The AC 220V input voltage is isolated by the transformer, and then it is used as the input of the push-pull converter after rectification and filtering. The push-pull converter converts the input voltage into high-frequency AC pulse voltage. The voltage matching and high-frequency isolation functions are completed through the high-frequency transformer, and then the pre ignition voltage is output by the output rectification and filtering link. At the same time, the high-voltage lighting circuit boosts and outputs the high-voltage lighting voltage, eliminating the bulky and bulky power frequency output transformer, Reduced audio noise [6]. The control and protection circuit is composed of uc3825 devices and peripheral circuits. According to the current signal fed back by the main circuit, it provides PWM driving signals for the switching devices. The pre ignition detection circuit compares the detected current signal with the reference power supply, outputs the pre ignition success signal, and turns off the high-voltage ignition at the same time

Figure 2 block diagram of ignition pre ignition power supply system

the study shows that the pulsed xenon lamp shows resistance characteristics when working at high frequency. Before lighting, its equivalent resistance is very large, which is equivalent to an open load. At this time, the control chip uc3825 outputs the PWM driving signal with the maximum duty cycle, and the push-pull converter outputs the high-voltage starting voltage to ionize and conduct the gas in the pulse xenon lamp; After igniting, the high-voltage igniting circuit stops working, and its equivalent resistance decreases sharply, which is equivalent to a load short circuit. 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 the closed-loop control, and the push-pull converter outputs the maintenance current when the pulse xenon lamp precombustion works; Thereafter, the equivalent resistance of the pulsed xenon lamp gradually reaches a steady state and remains constant

3.1 main circuit of ignition pre ignition

the main circuit of the new pre ignition power supply is shown in Figure 3. Power switching devices Q1 and Q2 form a push-pull converter; High frequency transformer T1 forms a step-up link, as shown in Figure 3 (a); The high-frequency coupling step-up transformer composed of inductor L2, capacitor C6 and inductors L3 and L4 forms a high-voltage lighting link, as shown in Figure 3 (b); Diodes d3~d6 form the input and output rectification and filtering link

the AC 220V voltage is 125V after being isolated, rectified and filtered by the transformer, which is used as the input DC voltage of the push-pull converter. The push-pull converter adopts power MOSFET device irf460. Capacitor C1, resistor R1 and diode D1 form a peak absorption circuit, which absorbs the peak voltage on the switching device and the line when the device is turned off instantly, and reduces the voltage stress when the power MOS tube is turned off. C1=0.01 F, r1=3.6k, and the withstand voltage of diode D1 is greater than 500V. Select UF4007

(a)

(b)

Figure 3 glow pre ignition circuit: (a) pre ignition main circuit; (b) Lighting main circuit 3.2 design of high-frequency transformer

the magnetic core of high-frequency transformer is soft ferrite, and the magnetic flux density is usually bm=0.2t. The input voltage at the primary end of the transformer ui=125v, and the working duty cycle is 0.25. The maximum output voltage of the secondary terminal is 1000V, the precombustion voltage is 250V, and the precombustion 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 s of the core is 2cm2, which is calculated according to formula (1)

(1)

; N2=176。

take the winding current density of transformer j=3a/mm2; Output power p0=u0i0=250 0.1=25w; Input power pi=p0/0.9=28w; Primary working current; The required cross-sectional area of winding conductor is =0.3mm2

the primary end current is large. If single strand winding is selected, the effective area diameter d=0.62mm, the wire diameter is large, and the skin effect is serious. Therefore, multi strand conductors are used for parallel winding. The original end adopts 5-Strand parallel winding, the sectional area of each conductor is s=0.3/5=0.06mm2, and the wire diameter of each share is d=0.28mm. The secondary end adopts single strand winding, and the required conductor 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 diode withstand voltage should be greater than 1000V, and 36mb160a is selected. Electrolytic capacitors are generally used in rectifier circuits to smooth filtering and reduce AC components in DC voltage. If the selected filter capacitor is too small, the DC voltage ripple after filtering will be large. In order to obtain the required output voltage, a large duty cycle adjustment range and too high closed-loop gain are required. The minimum value of the filtered DC voltage is also relatively small. It is required that the primary and secondary side turn ratio of the high-frequency transformer should be larger, and the current in the switch tube should be increased, and the reverse voltage of the output rectifier diode should be increased; If the selected filter capacitor is too large, the charging current pulse width will narrow and the amplitude will increase, resulting in increased EMI. Because the output voltage ripple frequency of the high-frequency transformer is high, a smaller filter capacitance value can be selected. Resistor R3 and capacitor C3 are connected in series to form an absorption circuit, and inductance L1 and capacitor C4 play a filtering role. After calculation, r3=4.7k/2w, C3 =1000pf/3kv, c4=0.01 f/3kv, L1 =2mh

3.4 high voltage igniting circuit

the pulse xenon lamp breaks down the gas in the xenon lamp through the high voltage breakdown gas provided by the high voltage igniting circuit. When the normally closed contact relay s is closed, the regulator tube D10 is short circuited, and the capacitor C6 is charged to 600V. After charging, the relay electronic universal testing machine is a kind of material testing machine. S is disconnected, and the regulator tube D10 charges the capacitor C5 until the bidirectional trigger tube D11 is connected, so that the thyristor VT1 is connected, and the capacitor C6 and the coupling inductor L3 are in series resonance, so that a high starting voltage is generated at both ends of the coupling inductor L4, and the gas in the pulse xenon lamp is punctured

d10 is 50V. The trigger voltage of bidirectional trigger diode D11 is 35V. Resistance r7=510 and r8=4.7k series partial voltage. R5 acts as a partial pressure. Resistance R9 limits the charging current of capacitor C6. The withstand voltage value of C6 should be greater than 1kV, taking c6=1 f/1200v. Resistance r6=3.6k, r10=22m is the discharge resistance of the capacitor. L2 current limiting inductance, value 1 h. L3=48 H，L4=6.5mH。 Diodes D12 and D13 provide resonant circuits

3.5 control circuit design

the control chip uc3825 is highly integrated, and the circuit design is mainly aimed at the peripheral circuit parameters. The peripheral circuit includes voltage error comparison compensation circuit, oscillation input, current limiting protection and output peak voltage absorption circuit. Uc3825 adopts the peak current control mode, and the voltage error feedback is used as the given peak current, which is compared with the sampling current to generate PWM drive signal

the control circuit is shown in Figure 4. This circuit is designed according to the maximum output duty cycle. Therefore, the in-phase input 2 of the chip is connected to the reference voltage output pin 16 through the current limiting resistor R11, and the inverting input 1 is connected to ground. The time base resistor R14 at pin 5 and the time base capacitor C12 at pin 6 determine the frequency f and the maximum duty cycle Dmax of the output signal. Output pins 11 and 14 are dead respectively