Due to the impact of the COVID-19 epidemic, UVC germicidal lamps have gained public attention and are expected to maintain good market growth for a long time to come. There is a wide range of UVC germicidal lamps on the market today, but overall the performance is uneven. Many products either use the immediate start, lamp life is not guaranteed; either the control part is very simple or even no, the user’s safety is not guaranteed. The appearance of a lighting product can be varied, but to determine the performance of its products is always the design of the electronic circuit. Good electronic circuit design can ensure better performance, more stable work, better security. UVC germicidal lamp electronic circuit and the general lighting circuit are not very different, but still have some of their own characteristics. So, how to design a germicidal lamp electronic circuit to meet the application requirements? What do you need to pay attention to? This article will be from the UVC germicidal lamp load, driveling, control line, and other aspects to do a specific introduction
Let’s dive right in now, and you can click on the question that interest you,
1 UVC germicidal lamp load
Currently, there are two main types of UVC germicidal lamp light source loads, gas discharge light sources and solid-state light sources, of which mercury lamps occupy the majority of the market share of UVC lamps. There are only two reasons for this: technology and cost. The current LED UVC conversion rate due to technical limitations compared to mercury lamps there is still a small gap; in addition, the price of UV LED chips is still in a very high position. These two reasons, making the LED program either small power, or high price, only in some small power occasions have used. In the use of germicidal UVC lamps for home use in large areas, the energy required is relatively large, the general power to 30 ~ 35 W, mercury lamps have become the second choice. Mercury lamp UVC lamp luminous principle and fluorescent lamps, the use of amalgam excitation inside the lamp, so that it produces 254 nm and 185 nm C band ultraviolet radiation (UV – C). 254 nm UVC germicidal principle is simply the use of high energy UVC destruction of microbial cells deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) molecular structure 185 nm radiation can combine with air to produce ozone, which is a strong oxidizing agent and has a sterilizing effect. UVC lamps on the market are divided into ozone-type and non-ozone type, but in fact, non-ozone type more or less ozone will still be generated.
Although the germicidal effect of UVC germicidal lamps is good, there is damage to the human body, will be harmful to the eyes, inducing conjunctivitis, and even affect vision; on the skin is also harmful, the lighter redness, itching, heavy skin cancer. So in the use of ultraviolet germicidal lamps, personnel, pets, etc. must be protected. Engineers debugging, in order to facilitate the lamp can be set on a plastic bottle of drinks, but also can play a protective role. Of course, you also need to do a good job of ventilation, because the ozone produced by ultraviolet light will make people dizzy, have nausea and other symptoms, and even cause respiratory disease
2 UVC lamp driver circuit
1) Inverter line of the discrete half-bridge
As mentioned earlier, the UVC lamp works on the same principle as the compact fluorescent lamp, so the driveline can naturally be used for the compact fluorescent lamp, see Figure 1.
Fig. 1 Split half bridge inverter circui
The lamp is in a high resistance state before ionization and needs a very high voltage to ionize the mercury vapor inside the lamp to produce ultraviolet light. After the lamp glows, in normal operation, the equivalent of a changing resistance, that is, with the increase in lamp current, the equivalent resistance will decline, so the circuit required to drive it has a high output impedance, in order to limit the lamp current, so that it will not be damaged because of fluctuations in the external voltage caused by excessive tube flow. The role of the line in Figure 1 is to provide a sufficiently high start-up voltage for the lamp to start glowing, and to provide a suitably limited lamp current during normal operation. In the figure, C1 and L1 form the EMI filter, which is used to filter out the electromagnetic interference signal generated by the half-bridge inverter circuit. D1 ~ D4 bridge rectifier circuit converts the AC voltage into DC pulsating voltage on electrolytic capacitor E. Electrolytic capacitor E plays the role of smoothing filter. Once this voltage reaches and exceeds the turning voltage of the trigger diode DB3, the diode turns on, and current flows into the base of V2, making V2 conductive. After V2 conducts, ic2, ib2 and vb2 all increase at the beginning, and vb2 reaches a peak at a certain point. Later, as the ring permeability decreases, vb2 will decrease with the rise of ic2 and V2 is in saturation. As the magnetic permeability of the ring decreases, the voltage vN2 on the magnetic surround group will be lower than vb2, so that the base current reverses, V2 from the saturation state and soon becomes cut off, and at the same time, the voltage on the magnetic surround group connected to V1 base changes polarity, V1 quickly from cutoff to on, the above process repeats itself over and over again, producing an alternating square wave voltage. This square wave voltage is resonated by the filament capacitor C5 and inductor L2 in series, which becomes close to a sine wave and produces a high voltage at both ends of C5 to the lamp, thus lighting up the lamp. After that, L2 acts as a ballast.
Products with power greater than 25 W require a high power factor (PF) and low harmonics. A passive power factor correction line as shown in Figure 2 can be added to increase PF and reduce THD. In this circuit, the AC voltage is rectified by a bridge and charged in series through D8 and D9 to electrolytic capacitors E1 and E2, which are charged up to half of the peak AC input voltage VM. They are always charged when the input AC sinusoidal voltage transient value is higher than 1 /2Vm and their charging time is longer. Because E1 is discharged to the load through D7 and E2 through D10 simultaneously, their voltage drop rate is faster than that of the AC supply voltage, and the capacitors stop charging only when the AC supply voltage falls below the voltage on E1 and E2. Therefore, in this circuit, the time of capacitor charging, i.e., the time of input current, is elongated, while the time of capacitor-discharge, i.e., the time of input current to zero, is shortened, and the power factor can be increased to about 0. 9.
The advantages of the discrete half-bridge inverter line are simplicity, low cost, stability, and reliability, but in terms of lamp start-up, to preheat time and preheat state are not well controlled, even with the addition of a preheat thermistor PTC. The starting state of the lamp determines the life of the lamp and the number of times it is switched on and off. The start state is not good, even if the life and switch are barely meet the requirements, but it is easy to lead to the early blackout of the lamp. But it is easy to cause the lamp early blackhead. So the high-end UV killing Germicidal lamps usually do not use a discrete half-bridge inverter line for the lamp drive line.
Fig. 2 Passive power factor correction circuit
2) Integrated Circuit Driven Half-Bridge Inverter Circuit
To solve these problems, a half-bridge inverter circuit driven by an integrated circuit (IC) with more precise control can be used. Such an IC provides two very clean signals that can drive the top and bottom switching tubes of the half-bridge inverter circuit. It also contains an RC oscillator, whose oscillation frequency can be adjusted by an external RC. The representative ICs are: IR215X from IR; L6569 from ST; UBA2024 from PHILIPS, and L6574 from ST with dimming function is introduced here. The pin layout is shown in Figure 3, and the typical application line is shown in Figure 4.
Fig. 3 Pins of L6574
The L6574 starts the lamp in three stages: filament preheat, ignition glow and normal lighting. In the preheat phase, the frequency is up to fmax and the duration is Tpre; in the ignition glow phase, the frequency drops from fmax to fmin and the duration is Tsh. Figure 5 shows the start-up timing diagram.
Tpre = 1.5s/μF-Cpre ( 1)
Tsh = 0.1Tpre ( 2)
fmin = 1.41 /( Rign-Cf ) ( 3)
fmax = ( 1.41( Rpre + Rign ) ) /( Rpre-Rign-Cf ) ( 4)
The preheat time Tpre, preheat frequency fmax, and operating frequency fmin can be set by equations ( 1) to ( 4). generally, we set the operating frequency first (usually set fmin≥40 kHz), and then set the appropriate preheat time and preheat frequency with the LC resonant frequency. If the preheat frequency is too far from the LC resonant frequency, the preheat is not sufficient; if the preheat frequency is too close to the LC resonant frequency, it will become an immediate start, so we need to debug to an optimal point.
Fig． 4 Typical application＇s connection of L6574
ig． 5 Start － up time diagram of L6574
L6574 also has two protection enable pins EN1 and EN2, the protection line, as shown in Figure 6 and Figure 7. EN1 is triggered to turn off the IC, usually used to protect the lamp is not connected, or the filament is broken, EN2 is triggered to re-enter the start program, usually used to protect against the failure to ignite the glow due to deactivation, of course, the two enable pins can also be used according to the actual needs of flexible adaptations. These two enable pins when the signal is greater than 0.6 V, duration greater than 200 ns will trigger, relatively sensitive, so the debugging should take into account the use of the lamp to the late will be difficult to start, the tube voltage becomes high, the design should leave a margin, otherwise, the germicidal lamp may be used for a period of time after the boot exception, a boot on the protection, can not be used.
Fig. 6 Open load safety circu
The preheat function and the protection function are two important reasons for choosing an IC-driven half-bridge inverter circuit. UVC germicidal lamps do not need a dimming function, why choose the L6574 with dimming function? When the power grid fluctuations, the input voltage becomes high, the germicidal lamp will become too large because of the entire lamp power, the temperature is too high and burned, so under certain voltage fluctuations, the germicidal lamp constant power design is necessary. The dimming function of the L6574 can be used to make a constant power design.
As shown in Figure 8, the half-bridge circuit under the tube current on R25 voltage is added to the IC operational amplifier inverting input 6 pins, while the IC’s 2 pin is a fixed voltage of 2 V, after dividing the voltage as a reference to the IC operational amplifier in-phase input 7 pins, the two compared, 5 pin output. When setting pin 6 and pin 7 are similar so that the output of pin 5 is slightly less than 5 V. Then when the input voltage becomes higher, the voltage of pin 6 becomes higher and the output voltage of pin 5 becomes smaller, causing the voltage change of pin 4 through D3 and R18. This change can adjust the IC internal voltage-controlled oscillator VCO frequency, thus adjusting the tube flow change to achieve the purpose of constant power.
For power factor correction of products with power greater than 25 W, of course, the previously mentioned passive power factor correction line can still be used. However, it is recommended to upgrade to a boosted constant voltage output active power factor correction line for better performance and for wide voltage input to meet different The performance is better and can be applied to wide voltage input to meet different market demands. There are many step-up constant voltage output lines, an example is given in Figure 9.
Fig.8 Current feedback loop of L6574
Fig.9 Booster type constant voltage output active power factor correction circuit
3 UVC germicidal lamp control line
Because UV is harmful to people, so it is necessary to add some protection lines to prevent some unexpected situations. For example, when the UV germicidal lamp work when someone mistakenly into the sterilization environment, requires an intelligent body sensing function to let the germicidal lamp to suspend work while there is an alarm function to urge people to leave. In addition, it should also contain a child lock to prevent misuse, show the germicidal lamp work status indicator, timing functions, dumping does not work and other functions, these functions can be controlled by an MCU (Microcontroller Unit).
1) Intelligent body induction
Intelligent body induction generally has microwave and infrared, here it is recommended to use the microwave. The infrared angle is small and easily disturbed by various light sources and heat sources. When the ambient temperature and human body temperature are close to, the sensitivity will be reduced or even fail. When using a microwave, pay attention to its power supply must be stable, especially to ensure that the microwave power supply is not affected when the lamp is started. Once the power supply is unstable, microwave induction becomes insensitive and easy to misfire, showing a self-excited phenomenon.
Microwave induction to control the germicidal lamp to pause, resume work, they need to use the control switch. When people enter the induction range, the microwave output a signal to the control switch, the control switch to turn off the lamp drive line power supply; when people leave the induction range, the microwave output signal to the control switch, the control switch to restore the lamp drive line power supply. As a control switch, mechanical relays have two obvious disadvantages, one is its reverse electric potential and mechanical action is easy to produce interference with the microwave; the other is that it has its own two-thousandths of a percent failure rate, the switch is normally open or normally closed, so that the germicidal lamp is not controlled, which is a very serious safety hazard. Optocoupler relay can solve the above problems.
2) Touch switch
Germicidal light switch choose touch switch, you can put the indicator light in the place of the touch button than the mechanical switch is better looking, more high-end, more in line with the positioning of home products. The focus of the touch switch is its sensitivity, it is recommended that the touch button using the spring, the spring and the panel can better fit, thus ensuring sensitivity, while the location of the spring can just place the indicator. In addition, in order to enhance the experience, you can add a buzzer, when the hand touches the switch, there is sound feedback, a better experience. Figure 10 is a touch switch circuit diagram, there are four touch buttons, a start button, a child lock button, two-timing keys, you can add or remove buttons according to actual needs.
Fig. 10 Touch switch circuit
3) Buzzer
The role of the buzzer is that when the hand to touch the button to give feedback and when the germicidal lamp work, people mistakenly into the germicidal environment to give a reminder, alarm. The buzzer is also a source of microwave interference, the placement needs to be considered. Figure 11 is a schematic diagram of the buzzer circuit.
4 ) Anti-dumping function
UVC germicidal lamps are fragile products, in order to ensure the safety of product use, it is necessary to increase the anti-tilt function so that the germicidal lamp can not work when it is tipped. The implementation of this function requires the use of a gravity sensing chip, that is, the gravity sensor, which uses elastic sensitive components made of cantilevered displacements, and the use of elastic sensitive components made of energy storage springs to drive the electrical contacts, to complete the conversion from gravity changes to electrical signals. This function of the line is not complex, the focus is on the control of the process. This gravity-sensing IC is generally small and easy to connect to solder, even with the solder paste process. When drawing the IC package, the pins should be properly lengthened so that the pads will pull tin when reflow soldering, making it less likely to be soldered together.
Fig.11 Buzzer circuit
4 UV germicidal lamp circuit board cloth
In order to meet the electromagnetic compatibility (EMC) requirements, the germicidal lamp printed circuit board layout (PCB Layout) need to pay attention to the following points. The strong part and weak part into two boards, that is, the lamp driver and control part of the power supply aboard, the control part of aboard.
This can prevent the control part from interference. All peripheral lines of ICs are as close as possible to the ICs, and the shorter the lines, the better. The ground of the lamp driver L6574 and the ground of the MOS tube are separately wired to the front electrolytic ground; then this ground and the ground of the control section are coupled with a Y-capacitor.
The whole control board needs to be shielded by a ground network to prevent interference.
TouchPad Sensor Pad alignment as thin and short as possible, IC placed in the middle of all the touchpads, all touchpads to the IC alignment length as consistent as possible, the greater the spacing between the lines the better, and in the middle of the ground network. Touchpad before the spacing is greater than 2.5 mm above and between the laying of ground network isolation, touchpad and ground network spacing, high sensitivity, but large interference; spacing small, low sensitivity, small interference, generally taken 0.5 ~ 2 mm. In addition, the touchpad back to lay the ground network with the thickness of the panel, mostly 4 mm above the laying of the ground network.
5 UVC germicidal lamp design verification
After an electronic circuit is constructed, it must be verified by experimentation before the design can be considered complete. UVC germicidal lamp electronic circuit contains drive lines, control lines, control lines, and contains many small functional lines, you can each part of the first assessment of normal before doing the whole lamp verification, so you can first exclude the interference between each other, saving verification time. Germicidal lamps and general lighting products are not too different from the experimental verification, mainly from the performance, reliability, and safety of the three aspects, the following describes the main items.
Input and output: verify that the power, power factor, harmonics are in line with the design requirements, tube voltage, tube flow is in line with the lamp indicators, and dispersion is
whether the dispersion is reasonable.
Temperature rise: The temperature rise can be used to initially determine whether each part is working properly and whether the device selection is reasonable.
Design margin assessment: By extending the working temperature and working voltage, i.e. high and low temperature, high and low voltage with a quick assessment of the design margin.
Switching test: The lamp needs to focus on the assessment of a test, used to verify the preheat state debugging OK.
Life acceleration test: Through the cycle of temperature, humidity, power to quickly verify the life of the whole lamp, but also test the PCB electromigration situation.
Electromagnetic interference (EMI): EMI, including conduction and radiation, refers to the entire lamp work on access to the grid and the surrounding electrical electromagnetic interference, is a must assessment of the project. the layout has a great impact on EMI, the preceding has been introduced to the UVC germicidal lamp Layout considerations, after the bridge to add a double π filter on the EMI also has a great help.
Surge: The main assessment of the entire lamp resistance to lightning strikes, the ability to surge.
In addition, there are many verification items such as leakage current, ESD, electrolytic life, device stress, etc., all of which need to be assessed. The more comprehensive verification is possible, the better.
6 Conclusion
The purpose of this article is to introduce how to build a UVC germicidal lamp electronic circuit ideas, rather than to introduce specific programs, the lines mentioned in the text, the program is only to facilitate the explanation, the actual design is more than the lines in the text, the program, but also more than the functions mentioned in the text, we can change the lines, add or subtract features according to the actual situation, design a more consistent with the market demand for UVC germicidal lamps for the benefit of the world, to help everyone overcome the epidemic
References
[1] UV hazards and protection[J]. Zhang Ling. Teaching Technical Physics.
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