Five principles of simplifying the design of LED driver

Five principles of simplifying the design of LED driver

Principle of chip heating
This is mainly for high-voltage drive chip with built-in power modulator. If the current consumed by the chip is 2mA and the voltage of 300V is added to the chip, the power consumption of the chip is 0.6W, which of course will cause the heat of the chip. The maximum current of the driver chip comes from the consumption of the power MOS tube. The simple formula is I=cvf (considering the resistance benefit of the charging, the actual I=2cvf, of which C is the CGS capacitance of the power MOS tube, and the V is the gate voltage when the power tube is conducting. So in order to reduce the power of the chip, it is necessary to reduce C, V and F. if C F can not be changed, then try to divide the power consumption of chips into devices outside the chip, and pay attention not to introduce extra power consumption. This is to consider better heat dissipation.

Mining lamp led driver
Heating principle of led power tube
The power consumption of the power transistor is divided into two parts, switching loss and conduction loss. It should be noted that in most occasions, especially it’s the LED switch power,the damage is greater than the conduction loss. The switching loss is related to the CGD and CGS of the power pipe and the driving ability and working frequency of the chip, so to solve the heat of the power pipe can be solved from the following aspects:
A  The MOS power tube can not be selected unilaterally according to the size of the on resistance, because the smaller the internal resistance, the larger the capacitance of CGS and CGD. For example, the CGS of 1N60 is about 250pF, the CGS of 2N60 is about 350pF, CGS of 5N60 is about 1200pF, and the difference is too great.
B  Here we talk about the effect of frequency. The frequency is directly proportional to the conduction loss. So when the power tube is heating, we should first consider whether the frequency selection is a bit high. Try to reduce the frequency. However, it is important to note that when the frequency is reduced, in order to get the same load capacity, the peak current must be larger or the inductance becomes larger, which may lead to the inductor into the saturation area. If the inductance saturation current is large enough, the CCM (continuous current mode) can be changed into DCM (discontinuous current mode), which requires an increase of a load capacitance.
Principle of reducing working frequency
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, we can try the following aspects:
A, set the minimum current ;
B, clean wire, especially the sense critical path;
C, select the small inductance or choose the inductance of the closed magnetic circuit.
D, plus RC low pass filter, this effect is a bit bad, C's consistency is not good, the deviation is a little big, but for lighting, it should be enough.
Selection of inductors or transformers
Many users respond, the same drive circuit, the inductance produced by a is no problem, and the inductance current produced by B is smaller. In this case, look at the inductance current waveform. Some engineers have not noticed this phenomenon, directly regulating the sense resistance or working frequency to reach the required current, which may seriously affect the life of the LED. Therefore, before the design, the reasonable calculation is necessary. If the theoretical calculation is a little far from the debugging parameters, the frequency reduction and the saturation of the transformer should be considered. When the transformer is saturated, the L will decrease, resulting in a sharp increase in the peak current increment caused by delay transmission, and the peak current of LED will also increase. On the premise that the average current is constant, we can only look at the decay of light.
Common problems of LED drive power supply
In order to save energy and save electricity, LED has been greatly promoted, but LED needs a power driver, which will directly affect the LED's life, so how to do a LED driver is the most important questions. This paper analyzes some problems of LED driver power supply, hoping to provide some help for engineers.
 Mining lamp led driver
1. The driving circuit has a direct impact on the LED life
What we call the LED driver includes two types: digital driving and analogical driving. Digital driving refers to digital circuit driving, including digital dimming manipulation, RGB full color variation and so on. Analogical driving refers to analogical circuit drive, including AC constant current switching power supply and DC constant current control circuit. The driving circuit consists of electronic components, including semiconductor components, resistors, capacitors, inductors and so on. All these components have application life, and any failure of the equipment will cause the failure of the whole circuit or some function failure.
 
The life of LED is 50-100 thousand hours. With 50 thousand hours, it has been lit for nearly 6 years. The life of the switching power supply is very difficult to reach 6 years. The warranty period for the switching power supply on the market is usually 2-3 years. The power supply to the 6 year warranty is the military grade, the price is 4-6 times the usual power supply, and the usual Lighting Factory is difficult to bear. Therefore, the defects of LED lamps are mostly the driving circuit problems.
2. Heat dissipation problem
LED is a cold light source. The junction temperature should not exceed the limit and there must be some allowance in the description. The depiction of the whole luminaire should consider many problems such as beautiful appearance, convenient device, light matching, heat dissipation and many other problems. It is necessary to find a balance point among many elements, so that all the lamps and lanterns are the best. Some LED lamps and lanterns manufacturers use power for outsourcing, the luminaire painter knows not much about the power supply, the heat dissipation space for LED is larger, and the heat dissipation space of the power supply is small.
 
Usually it is to describe the good lamps and then find the suitable power supply to match them, which will bring difficulty to the matching of the power supply. Often encountered because the interior space of the luminaire is small or the internal temperature is high, and the cost control is low, it can not be equipped with suitable power supply. Some LED lamps and lanterns factory has the ability of developing the power source. In the early stage, the evaluation of the lamps and lanterns, and the simultaneous description of the power supply, can solve the above problems. In the description, we should sum up the heat dissipation of LED and the heat dissipation of power supply, so that we can control the temperature rise of lamps and lanterns, so as to depict better lamps and lanterns.
3. Problems in power supply depiction
A.Ppower depiction. Although LED light efficiency is high, but the heat energy loss of 80-85%, the luminaire inside has 20-30 degrees of temperature rise, if the room temperature 25 degrees, the interior of the lamp is 45-55 degrees, the power long time in the high temperature environment, to ensure the life of the power to increase the power allowance, usually left to 1.5-2 times the allowance.
 
B. Component selection. When the interior temperature of the luminaire is 45-55 degrees, the temperature inside the power supply has a temperature of about 20 degrees, and the temperature near the element will 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 to be used at high temperature and long time, especially the electrolytic capacitor and wire.
 
C. The electrical function depicts. The switch power is designed to describe the parameters of LED, which is mainly constant current parameters, the size of the current is determined by the brightness of LED. If the bulk current errors are large, the brightness of the whole batch of lights is not uniform. And the temperature change can also cause the power output current to shift. The bulk error is usually controlled within +/-5% to ensure that there is an error in the forward voltage drop of the light luminance common.LED, and the constant current voltage scale described in the power supply includes the voltage scale of the LED. Multiple LED tandem use time division, the minimum pressure drop multiplied by the number of lower limit voltage, the maximum pressure drop multiplied by the number of connections as the upper limit voltage, the power supply constant current voltage scale is slightly wider than the size, usually the upper and lower limit of each remaining 1-2V allowance.
 
D. PCB board depiction. LED lamps leave a smaller scale for power supply (unless they are outside the power supply), so PCB needs higher demand and more elements to think. The safety interval should be kept enough, the power supply that needs input and output barrier, the primary side circuit and the two side circuit need the voltage 1500-2000VAC, and at least the interval of 3MM should be left on the PCB. If it is a metal shell luminaire, the whole power supply panel should also consider the safety interval between the high voltage and the outer casing. If there is no space to ensure safety margin, other ways should be used to ensure insulation, such as punching on PCB, adding insulation paper, sealing insulating adhesive, etc. Other cloth boards should also consider the balance of heat. The heating elements should be evenly distributed. They should not be put together to prevent part of the temperature rising. The electrolytic capacitor is far away from the heat source, slowing down aging and extending the use of life.
 
E. Certificate problem. At present, there is no standard for LED lamps and lanterns at home. The relevant parts of the state are being discussed and worked out. The domestic sale of lamps and lanterns is the standard of lighting fixtures. The export lighting is CE or UL. So in this situation, it is difficult to describe these specifications together with the description of the switching power supply. We can only satisfy different requirements for different requirements.
4. Use the parameters.
The selection of external power supply mainly depends on the voltage scale of constant current and constant current. The selection of voltage scale should be moderate. Try not to choose a larger scale to prevent power failure.
Protection methods for the circuit of led driver
1. Because the fuse it’s reaction speed is slow, the effect is poor, the use of trouble, so the fuse is not suitable for the LED lamp, because the LED lamp is now mainly in the brilliance project.It requires the LED protection circuit to be very harsh: when it exceeds the normal use current, it can start the protection immediately, let the power supply path of the LED be disconnected, the LED and the power supply can be protected, and the power supply can be automatically restored after the whole lamp is normal, without affecting the work of the LED. The circuit can not be too complicated, the volume can not be too big, and the cost is even lower. So it is very difficult to use fuse.
 
2. Using transient voltage suppression dipole (TVS)
Transient voltage suppression diode is a high efficiency protection device in the form of diodes. When its two poles are subjected to the inverse transient high energy shock, it can reduce the high resistance between the two poles to a low resistance at a speed of 10 negative 12 seconds, and absorb the surge power of up to several kilowatts, and the voltage clamp bits between the poles in a predetermined voltage value, effectively protecting the precision element in the electronic circuit. Device. The transient voltage suppression bipole has many advantages, such as fast response time, large transient power, low leakage current, good breakdown voltage deviation consistency, easy control of clamp potential voltage, no damage limit, small volume and so on.
But in practical use, it is difficult to find TVS devices that meet the voltage requirements. The damage to the LED bead is mainly due to the overheating of the current inside the chip. TVS can only detect over-voltage and can not detect overcurrent. It is difficult to choose suitable voltage protection points, and it is difficult to use this device in practice.
 
3. Select self recovery fuse
Self recovery fuse, also known as polymer thermistor PTC, is composed of polymer and conductive particles. After special processing, conductive particles form chain like conductive pathways in polymers. When the normal working current passes (or the element is in normal ambient temperature), the PTC self recovery fuse shows a low resistance state. When the circuit has an abnormal overcurrent passing through (or increasing the ambient temperature), the heat generated by the high current (or the ambient temperature) causes the polymer to expand rapidly and cut off the conductive particles. After the overcurrent (supertemperature state) disappears in the circuit, the polymer cooling LED drives the power supply and the volume is restored to normal, in which the conductive particles re form the conductive path, and the PTC self recovery fuse presents the initial low resistance state, when the overcurrent (overtemperature state) in the circuit is disappearing.
 
In the normal working state of self recovery insurance pipe is very small fever, in the abnormal working state of its high fever is very high, it also restricts the current through it, thus playing a protective role. In a specific circuit, you can choose:
 
(1) Separate protection. The general LED lamp is divided into many strings.  We can add one PTC element to the front of each branch for protection. The advantages of this method are high accuracy and good reliability of protection.
 
(2) Overall protection. A PTC element is added to the front of all beads to protect the whole lamp. The advantage of this approach is simple, not volume.
                                                                                                                                                                                                             
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