Five principles of LED design
Release date: 2017-10-30 Source: Foreign station sharing: 
Chip fever
This is primarily for high voltage driver chips with built-in power modulators. If the current consumed by the chip is 2 mA, the voltage of 300 V is applied to the chip, and the power consumption of the chip is 0.6 W, which of course causes the chip to generate heat. The maximum current of the driving chip comes from the consumption of the driving power MOS tube. The simple calculation formula is I=cvf (considering the resistance benefit of charging, the actual I=2cvf, where c is the cgs capacitance of the power MOS tube, and v is the power tube conduction When the gate voltage, so in order to reduce the power consumption of the chip, we must find ways to reduce c, v and f. If c, v and f can not be changed, then please find a way to divide the power consumption of the chip into the off-chip device, be careful not to Introduce additional power consumption. A simpler one is to consider better heat dissipation.
Power tube heating
Regarding this issue, I have also seen people posting in the forum. The power consumption of the power tube is divided into two parts, switching loss and conduction loss. It should be noted that in most occasions, especially for LED mains drive applications, the switching damage is much greater than the conduction loss. The switching loss is related to the cgd and cgs of the power tube and the driving capability and operating frequency of the chip. Therefore, to solve the heat generation of the power tube, it can be solved from the following aspects: A, the MOS power tube cannot be selected according to the size of the on-resistance because The smaller the internal resistance, the larger the cgs and cgd capacitance. For example, the cgs of 1N60 is about 250pF, the cgs of 2N60 is about 350pF, and the cgs of 5N60 is about 1200pF. The difference is too big. When the power tube is selected, it is enough. B. The rest is the frequency and chip drive capability. Here we only talk about the frequency effect. The frequency is also proportional to the conduction loss. Therefore, when the power tube is hot, the first thing to think about is whether the frequency selection is a bit high. Find ways to reduce the frequency! However, it should be noted that when the frequency is reduced, in order to obtain the same load capacity, the peak current must be increased or the inductance is also increased, which may cause the inductor to enter the saturation region. If the inductor saturation current is large enough, consider changing the CCM (continuous current mode) to DCM (discontinuous current mode), which requires an additional load capacitor.
Operating frequency down frequency
This is also a common phenomenon in the debugging process of users. The frequency reduction is mainly caused by two aspects. The ratio of input voltage to load voltage is small and system interference is large. For the former, be careful not to set the load voltage too high, although the load voltage is high, the efficiency will be high. For the latter, you can try the following aspects: a, the minimum point of the minimum current setting; b, clean wiring, especially the critical path of sense; c, the small point of the inductor selection or the inductance of the closed magnetic circuit ; d, add RC low-pass filter, this effect is a bit bad, C's consistency is not good, the deviation is a bit large, but for lighting should be enough. No matter how bad the frequency is, there is no advantage, only the bad, so we must solve it.
Inductor or transformer selection
I finally talked about the point. I haven't gotten started yet. I can only talk about the impact of saturation. Many users react, the same drive circuit, the inductance produced by a is no problem, the inductor current produced by b becomes smaller. In this case, look at the inductor current waveform. Some engineers did not notice this phenomenon, directly adjust the sense resistance or the operating frequency to reach the required current, which may seriously affect the service life of the LED. Therefore, before the design, reasonable calculation is necessary. If the theoretical calculation parameters and the debugging parameters are a little far away, consider whether the frequency reduction and the transformer are saturated. When the transformer is saturated, L will become smaller, causing the peak current increase caused by the transmission delay to rise sharply, and the peak current of the LED will also increase. Under the premise that the average current is constant, only the light fades.
LED current size
Everyone knows that if the LEDripple is too large, the LED life will be affected, how big the impact is, and no experts have ever seen it. I have asked the LED factory this data before, they said that it is acceptable within 30%, but it has not been verified. It is recommended to try to control the small points. If the heat solution is not good, the LED must be derated. I also hope that some experts can give a specific indicator, or else affect the promotion of LED.
LED drive power common problems
In order to save energy and save electricity, LED has been greatly promoted, but LEDs need to have a power driver, its quality will directly affect the LED life, so how to do a LED driver power is the focus of LED power painters weight. This article analyzes some of the challenges of LED drive power and expects to be able to provide some assistance to engineers.
1, the drive circuit directly affects the LED life
The LED driver we refer to includes digital drive and analog drive. Digital drive refers to digital circuit drive, including digital dimming control, RGB full color change. Analog driving refers to analog circuit driving, including AC constant current switching power supply, DC constant current control circuit. The driving circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life, and any failure of the device may cause the failure of the entire circuit or some functions may be invalid. The service life of LEDs is 50,000-100,000 hours. According to 50,000 hours, they are lit up successively and have a life span of nearly 6 years. The life of the switching power supply is difficult to reach for 6 years. The warranty period of the switching power supply sold in the market is usually 2-3 years. The power supply that reaches the 6-year warranty is military-grade. The price is 4-6 times that of the normal power supply. Usually, the price is usually 4-6 times. The lighting factory is hard to bear. Therefore, the faults of LED lamps are mostly driven circuit faults.
2, heat dissipation problem
The LED is a cold light source, and the operating junction temperature cannot exceed the limit, and a certain margin must be left when drawing. The whole luminaire should be considered in terms of beautiful appearance, convenient installation, light distribution, heat dissipation and many other problems. It is necessary to find a balance among many elements, so that all the lamps are the best. LED lamps are not developed for a long time. There are not many experiences, and many depictions are constantly improving. Some LED luminaire manufacturers use the power supply for the outsourcing or outsourcing. The luminaire painter knows little about the power supply, and the heat dissipation space for the LED is large, and the heat dissipation space for the power supply is small. It is usually necessary to draw a suitable power supply to match the lighting, so that it will bring certain difficulties to the power supply. It is often encountered because the internal space of the luminaire is small or the internal temperature is high, and the cost control is low, and it is not suitable for a suitable power source. Some LED luminaire factories have the ability to develop power. At the beginning, the luminaires are evaluated in the early stage, and the power supply is drawn simultaneously, which can handle the above problems. In the depiction, it is necessary to summarize the heat dissipation of the LED and the heat dissipation of the power supply, and the overall temperature rise of the control lamp can be used to describe a better lamp.
3. Problems in power supply drawing
a, power depiction. Despite the high luminous efficiency of the LED, there is still 80-85% of the thermal energy loss, resulting in a temperature rise of 20-30 degrees inside the lamp. If the room temperature is 25 degrees, the inside of the lamp has 45-55 degrees, and the power supply is always in a high temperature environment. In order to ensure the longevity, it is necessary to increase the power headroom, usually leaving a margin of 1.5-2 times.
b, component selection. Under the condition that the internal temperature of the lamp is 45-55 degrees, the internal temperature rise of the power supply is about 20 degrees, and the temperature adjacent to the component should reach 65-75 degrees. Some components will drift in the high temperature condition, and some life will be shortened, so the equipment should be selected for use at higher temperatures for a long time, paying special attention to electrolytic capacitors and wires.
C, electrical function depiction. The switching power supply is aligned with the parameters of the LED, mainly the constant current parameter. The current is determined by the brightness of the LED. If the batch current error is large, the brightness of the whole batch of lamps is not uniform. And temperature changes can also cause the power supply output current to shift. Usually the batch error control is within +/- 5%, to ensure that the brightness of the lamp is common. The forward voltage drop of the LED has an error, and the constant current voltage scale drawn by the power supply should include the voltage scale of the LED. When multiple LEDs are connected in series, the minimum voltage drop multiplied by the number of series is the lower limit voltage, and the maximum voltage drop multiplied by the number of series is the upper limit voltage. The constant current voltage scale of the power supply is slightly wider than this scale, usually upper and lower limits. Leave 1-2V margin.
d, PCB layout. LED lamps are left to the power supply with a small scale (unless it is external to the power supply), so there is a high demand for PCB drawing, and there are many factors to consider. The safety interval should be sufficient. The power supply for input and output blocking is required. The primary side circuit and the secondary side circuit require a withstand voltage of 1500-2000VAC, and at least 3MM intervals should be left on the PCB. In the case of a metal-enclosed luminaire, the layout of the entire power supply should also be considered as a safe separation between the high voltage and the outer casing. If there is no space to ensure the safety interval, other methods must be used to ensure insulation, such as punching holes in the PCB, adding insulation paper, potting and insulating rubber. Other layouts should also consider heat balance, heating elements should be evenly distributed, can not be placed, to prevent part of the temperature rise. Electrolytic capacitors are kept away from heat sources, slowing down aging, and extending the lifespan.
e, the certification problem. At present, there is no specification for LED lamps in China. The relevant parts of the country are being researched and formulated. The lamps and lanterns sold in China are based on the specifications of lighting fixtures. The export is done with CE or UL certification, and some are based on foreign LED lamp specifications. . Therefore, in order to align this situation, it is more difficult to describe the switching power supply together with the above specifications. We can only align with different requirements and satisfy different certifications.
4. Use parameters.
The selection of purchased power sources mainly depends on the voltage scale of constant current and constant current. The constant current value is chosen to be the standard current of the LED. The selection of the voltage scale should be moderate, try not to choose a larger scale to prevent the dross of power.
Three protection methods for LED drive power circuit
1. Protect the LED circuit with a fuse (tube)
Since the fuse is disposable and the reaction speed is slow, the effect is poor, and the use is troublesome, the fuse is not suitable for use in the finished LED lamp, because the LED lamp is now mainly in the glory engineering and lighting engineering of the city. It requires the LED protection circuit to be very demanding: the protection can be started immediately when the current is exceeded, the LED power supply path is disconnected, the LED and the power supply can be protected, and the power can be automatically restored after the entire lamp is normal. Does not affect the LED work. The circuit should not be too complicated, the volume should not be too large, and the cost should be low. Therefore, it is very difficult to implement the fuse.
2. Use transient voltage suppression diode (TVS for short)
The transient voltage suppression diode is a high efficiency protection device in the form of a diode. When its two poles are subjected to reverse transient high energy impact, the high resistance between the two poles can be immediately reduced to low resistance at a very short time of 10 minus 12 power seconds, absorbing up to several kilowatts of surge power. The voltage clamp between the two poles is at a predetermined voltage value, which effectively protects the precision components in the electronic circuit. The transient voltage suppression diode has the advantages of fast response time, large transient power, low leakage current, good uniformity of breakdown voltage, easy control of clamp voltage, no damage limit, and small volume.
However, in actual use, it has been found that it is not easy to find a TVS device that satisfies the required voltage value . The damage of the LED beads is mainly caused by overheating inside the chip due to excessive current. TVS can only detect overvoltages and cannot detect overcurrents. It is difficult to master the proper voltage protection point, and the device cannot be produced and is difficult to use in practice.
3. Select self-recovery fuse
The self-recovering fuse is also called a polymer polymer positive temperature thermistor PTC, and is composed of a polymer and conductive particles. After special processing, the conductive particles form a chain-like conductive path in the polymer. When the normal operating current passes (or the component is at normal ambient temperature), the PTC resettable fuse is in a low-resistance state; when there is an abnormal overcurrent in the circuit (or the ambient temperature rises), the large current (or the ambient temperature rises) The generated heat causes the polymer to expand rapidly, which cuts off the conductive path formed by the conductive particles. The PTC self-recovery fuse is in a high-resistance state; when the overcurrent (over-temperature state) in the circuit disappears, the polymer cools the LED driving power source. , the volume returns to normal, in which the conductive particles reconstitute the conductive path, and the PTC resettable fuse is in an initial low resistance state. In the normal working state, the self-recovery fuse has little heat, and in the abnormal working state, its heat is very high, which limits the current passing through it, thereby protecting. In the specific circuit, you can choose:
1 branch protection. The general LED light is divided into a number of serial branches. We can add a PTC component in front of each branch to protect them separately. The benefits of this approach are high accuracy and good reliability of protection.
2 overall protection. A PTC element is added in front of all the beads to protect the entire lamp. The benefit of this approach is simple and does not account for size. For civilian products, the result of this protection in actual use is still satisfactory.
Tags: Five principles of LED design
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