Five Principles for Simplifying LED Designand common problems with LED drive power supplies

  Chip Heat

  This is mainly for the built-in power modulator high-voltage driver chip. If the chip consumes 2mA of current, 300V voltage added to the chip above, the chip's power consumption of 0.6W, of course, will cause the chip to heat up. The maximum current to drive the chip comes from driving the power MOS tube consumption, a simple formula for I = cvf (considering the resistance benefit of charging, the actual I = 2cvf, where c is the cgs capacitance of the power MOS tube, v is the gate voltage of the power tube conduction, so in order to reduce the power consumption of the chip, you must find a way to reduce the c, v and f. If the c, v and f can not be changed, then please Find a way to split the power consumption of the chip to devices outside the chip, being careful not to introduce additional power consumption. To make it even simpler, think about better heat dissipation.

  Power Tube Heat

  I've seen people posting about this issue in the forums. Power tube power consumption is divided into two parts, switching losses and conduction losses. Note that in most cases, especially for LED mains drive applications, the switching damage is much greater than the conduction loss. Switching losses and power tube cgd and cgs and chip drive capability and operating frequency, so to solve the power tube heat can be solved from the following aspects: A, can not be one-sided according to the size of the on-resistance to select the MOS power tube, because the smaller the internal resistance, cgs and cgd capacitance is greater. Such as 1N60 cgs for 250pF or so, 2N60 cgs for 350pF or so, 5N60 cgs for 1200pF or so, the difference is too big, when choosing power tubes, enough can be B. The rest is the frequency and chip driving ability, here only talk about the impact of frequency. Frequency and conduction loss is also proportional to the power tube heat, the first thing to think about is not a little high frequency selection. Think of ways to reduce the frequency! However, note that when the frequency is reduced, in order to get the same load capacity, the peak current is bound to become larger or inductor is also larger, which may lead to the inductor into the saturation region. If the inductor saturation current is high enough, consider changing the CCM (Continuous Current Mode) to DCM (Discontinuous Current Mode), which will require an additional load capacitor.

  Operating Frequency Downconversion

  This is also a relatively common phenomenon in the debugging process of the user, the frequency drop is mainly caused by two aspects. The ratio of input voltage and load voltage is small, and the system interference is large. For the former, pay attention not to set the load voltage too high, although the load voltage is high, the efficiency will be higher. For the latter, you can try the following: a, set the minimum current and then smaller; b, wiring clean point, especially sense this critical path; c, the inductor selection of a small point or the choice of closed magnetic circuit inductor; d, add RC low-pass filtering it, the impact of this a little bit of a bad, the consistency of the C is not good, the deviation of a little bit big, but for the lighting should be enough. In any case, there is no benefit to downconversion, only bad, so it must be resolved.

  Inductor or transformer selection

  Finally got to the point, I have not yet started, I can only blindly talk about the effect of saturation. Many users react, the same drive circuit, with a production of inductors no problem, with a production of inductors b current becomes smaller. Encountered this situation, to see the inductor current waveform. Some engineers did not notice this phenomenon, directly adjust the sense resistor or operating frequency to achieve the required current, which may seriously affect the service life of the LED. Therefore, before the design, a reasonable calculation is necessary, if the theoretical calculation of parameters and debugging parameters are a little far, to consider whether the frequency reduction and transformer saturation. When the transformer is saturated, L will become smaller, resulting in a sharp increase in the peak current increment caused by the transmission delay, then the peak current of the LED is also followed by an increase. Under the premise of the average current remains unchanged, you can only watch the light decay.

  LED current size

  We all know that if the LED ripple is too large, the LED life will be affected, how big is the impact, I have not seen any expert said. Previously asked the LED factory this data, they said 30% or less is acceptable, but then not verified. It is recommended to try to control the smallest point. If the thermal solution is not good, the LED must be used to reduce the amount. I also hope that some experts can give a specific indicator, otherwise it will affect the promotion of LED.

 LED driver power supply common problems

  In order to save energy and electricity, LED has been a great implementation, but LED need a power supply driver, its good or bad will directly affect the life of the LED, so how to do a good job of a LED driver power supply is the LED power portrayal of the top priority. This article analyzes some of the LED driver power supply problems, expect to be able to provide engineers with a little assistance.

  1, the driver circuit directly affects the LED life

  What we say LED driver including digital drive and push-like drive two categories, digital drive refers to the digital circuit drive, including digital dimming control, RGB full-color change. Extrapolation drive refers to the analog circuit drive, including AC constant current switching power supply, DC constant current control circuit. Driver circuit consists of electronic components, including semiconductor components, resistors, capacitors, inductors, etc., these components have a life expectancy, the failure of any one device will lead to the failure of the entire circuit or some of the functional failure. LED life is 5-10 million hours, according to 50,000 hours, continuous lighting, nearly 6 years of life. Switching power supply life is very difficult to reach 6 years, the switching power supply warranty period on the market is usually 2-3 years, up to 6 years warranty power supply is military grade, the price is usually 4-6 times the power supply, the usual lamps and lanterns factory is very difficult to afford. So LED lamps and lanterns are mostly faulty driver circuit.

  2, heat dissipation problems

  LED for cold light source, the operating temperature can not exceed the limit value, but also to leave a certain margin when depicting. The depiction of the entire lamps and lanterns to think about the appearance of beautiful, easy to install, light distribution, heat dissipation and many other aspects of the problem, to seek a balance point in many elements, so that all the lamps and lanterns is the best. LED lamps and lanterns is not a long time to carry out the development of not many experiences to learn, many of the depictions are constantly being perfected. Some LED lamps and lanterns manufacturers use power supply for outsourcing or outsourcing, lamps and lanterns depictor does not know much about the power supply, to the LED heat dissipation space is larger, to the power supply heat dissipation space is smaller. Usually depicts a good lamps and lanterns and then look for the appropriate power supply to supporting, so that the power supply to bring certain difficulties in supporting. Often encountered due to the lamps and lanterns internal space is small or high internal temperature, and cost control is low, can not match to the appropriate power supply. Some LED lamps and lanterns factory has power supply development ability, in the beginning of the portrayal of lamps and lanterns pre-evaluation, power supply portrayal synchronization, can deal with the above problems. In the portrayal of the LED heat dissipation and power supply to think about the heat dissipation, the whole control lamps and lanterns of the temperature rise, so as to portray a better lamps and lanterns.

  3, the power supply depiction of the problem

  a, power description. Despite the high luminous efficacy of LED, but there are 80-85% of the heat loss, resulting in the lamps and lanterns inside the temperature rise of 20-30 degrees, if the room temperature of 25 degrees, the lamps and lanterns inside the 45-55 degrees, the power supply for a long time in the high-temperature environment, to ensure that the life of the power must be increased to the margin, usually left to the margin of 1.5-2 times.

  b, component selection. Lamps and lanterns in the internal temperature of 45-55 degrees, the power supply internal temperature rise of about 20 degrees, the temperature of the neighboring components to reach 65-75 degrees. Some components in the high temperature condition parameters will drift, but also some life will be shortened, so the equipment should be selected to be able to use at higher temperatures for a long time, pay special attention to the electrolytic capacitors and wires.

  C, electrical function depiction. Switching power supply aligned with the parameters of the LED depiction, mainly constant current parameters, the current resolution of the brightness of the LED, if the batch current error is large, the brightness of the whole batch of lights is not uniform. And the temperature change can also cause the power supply output current offset. Usually the batch error is manipulated within +/-5%, to ensure that the brightness of the lights together. LED forward voltage drop error, the power supply depicts the constant current voltage scale to include the voltage scale of the LED. Multiple LED series use, the minimum voltage drop multiplied by the number of series for the lower limit voltage, the maximum voltage drop multiplied by the number of series for the upper limit voltage, power supply constant current voltage scale should be slightly wider than this scale, usually upper and lower limits of 1-2V margin.

  LED lamps and lanterns left to the power supply scale is small (unless the power supply is external), so the demand for PCB depiction is higher, to think about the elements are also more. Safety interval to stay enough, the need for input and output isolation of the power supply, the primary side circuit and the secondary side circuit needs to withstand voltage 1500-2000VAC, in the PCB at least to stay enough 3MM interval. If it is a metal shell of the lamps, the whole power supply board should also think about the high voltage and the shell of the safety interval. If there is no space to ensure safe spacing conditions should be used in other ways to ensure insulation, such as punching holes in the PCB, plus insulating paper, potting insulating adhesive and so on. Other cloth board should also think about the heat balance, heat components should be evenly distributed, can not be placed collectively to prevent part of the temperature rise. Electrolytic capacitors away from heat sources, slowing down the aging, extending the service life.

  
  4, the use of parameters.

  Outsourcing power supply in the selection of the main constant current and constant current voltage scale. Constant current value selected for the LED specification current under. Voltage scale selection should be moderate, try not to pick a larger scale, to prevent the waste of power.

LED drive power supply circuit of the 3 kinds of protection methods

  1. Protect the LED circuit with a fuse (tube)

  Due to the fuse is disposable, and slow reaction speed, poor effect, use trouble, so the fuse is not suitable for LED lights in the finished product, because the LED lights are now mainly in the city's colorful project and brightening project. It requires LED protection circuit to be very harsh: in excess of the normal use of current can immediately start the protection, so that the LED power supply path is disconnected, so that the LED and the power supply can be protected, in the whole light is normal and can automatically restore the power supply, does not affect the work of the LED. The circuit can not be too complex volume can not be too large, the cost should also be low. So it is difficult to realize the way of using fuse.

  2. The use of transient voltage suppression diode (referred to as TVS)

  Transient voltage suppression diode is a diode form of high-performance protection devices. When its poles by the reverse transient high-energy impact, can be 10 negative 12 times the speed of a very short period of time, so that their own high resistance between the two poles immediately reduced to low resistance, absorbing up to a few kilowatts of surge power, the voltage between the two poles of the clamping element in a predetermined voltage value, effective protection of electronic circuits in the precision components. Transient voltage suppression diode has the advantages of fast response time, transient power, low leakage current, good consistency of breakdown voltage deviation, clamping element voltage is easier to control, no damage limit, small size.

  However, in practice, it is found that it is not easy to find TVS devices to meet the required voltage value. LED bead damage is mainly due to excessive current caused by overheating inside the chip. TVS can only detect over-voltage can not detect over-current. To choose the right voltage protection point is difficult to grasp, this device can not be produced is also difficult to use in practice.

  3. Selection of self-recovery fuses

  Resettable fuses, also known as polymer positive temperature thermistor PTC, is composed of polymer and conductive particles. After special processing, conductive particles in the polymer to form a chain-like conductive pathway. When the normal operating current through (or components in the normal ambient temperature), PTC self-recovery fuse is a low-resistance state; when the circuit has an abnormal overcurrent through (or ambient temperature rise), the large current (or ambient temperature rise) the heat generated by the polymer rapid expansion, which also cuts off the conductive particles constitute the conductive path, PTC self-recovery fuse is a high-resistance state; when the circuit overcurrent (or over-temperature state) disappears, the polymer is a high resistance state; when the circuit (or over-temperature state), the polymer is a high resistance state. When the circuit overcurrent (overtemperature state) disappears, the polymer cooling LED driver power supply, the volume returns to normal, in which the conductive particles and re-constitute a conductive path, PTC self-recovery fuse is the initial low-resistance state. In the normal operating state of self-recovery fuse heat is very small, in the abnormal operating state of its heat is very high resistance is very large, it also limits the current through it, thus playing a protective role. In the specific circuit, you can choose:

  ① Split protection. General LED lights are divided into a lot of series connection branch. We can add a PTC element in front of each branch to protect them separately. The advantage of this approach is high precision and good reliability of protection.

  ② Overall protection. Add a PTC element in front of all light beads to protect the whole lamp. The advantage of this way is simple, does not occupy the volume. For civilian products, the results of this protection in actual use are still satisfactory.

.For more LED industrial lighting issues, please contact recolux.

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