It's from the breadboard community dismantling! Electronic engineers are coming
I. Appearance analysis
As shown above in figure 19, a car bought in 19 years with a front-lighted led was broken in about 3 years。
The led-light source and aluminium-type plastics are partly designed to fix the car lamp。
There's also a heat-spread fan behind the dispersed aluminium
Driver power for metal shell
Power interface
In the led lightboard section, the surface of the led bead has been clearly damaged。
The led beads use 1919 bells for csp seals, which refer to chip size package or chip scale package, a chip-level seal and, in the led industry, a reverse chip。
The traditional led containment process requires welding wires, bottom fillings, support racks, etc., while the csp sealing directly connects the led chip to the encapsulation base, saves the middle process cost and allows the csp process to have better led containment heat and smaller size。
If the size of the figure is 1. 9* 1. 9 mm, with reference to seoul 1919, the maximum voltage is 2. 86-3. 25 v, with a maximum current of 1. 5 a and a maximum power of 3. 25*1. 5 = 4. 875 w。
Ii. Structure
The fan in the back was removed, and the wire was connected to the lightboard, which was a little strange. The fan line was also broken as a result of violence。
The engine shell is aluminum, and when the covers on both sides are removed, it is found to be geled。
In addition to being dustproof and waterproof, the irrigation function provides thermal heat-dispersion capability to transfer the heat from the pcb element to the aluminium shell outside. The encapsulation + aluminum shell design has a very good conductive heat dispersion effect。
A layer of insulation paper between the power source pcb and the aluminium shell avoids short circuits caused by exposure of the element to the metal shell。
We finally disintegrate the drive of the pcb and clean up the alcohol. Finally
The power pcb uses a two-panel process, with the background covered with a driver of ic and a patch of electrical resistance, with power senses and electrolytic caps for plugins, diodes and tvs for stickers, both of which are tin ointment processes。
It has to be said that the smt process at the bottom of the pcb is a little spicy and electrical resistance is twisted。
The driving ic model is lc2740, with a constant pressure + constant current relief of ic, with a maximum of 3a current output。
Circuit analysis
Based on the decomposition of pcb plates and ic data, we've drawn the corresponding theory that the entire circuit is a relatively simple constant-pressure constant-flow circuit for the buck。
One, the ic input end has a tvs tube, the main function of which is to absorb transient surges in the input voltage and to avoid high peak voltage hitting the driving ic. Under the negative pole there are two ss56 shortki diodes, which use a single diode, which act as a counter-connection and avoid the risk of ic burning as a result of a positive and negative charge. Electrolytic c4 low-frequency filtering and storage function, with a pv c9 high-frequency filter. L1 is buck power。
R7,r8 is a constant current sample resistance, with reference to ic specifications, with a reference voltage of 50 mv between cps-vsn, which results in an output current i=u/r= 0. 05 (0. 047 and 0. 036) = 0. 05/0. 0204 = 2. 451a designed
3. High frequency beheadings after depressure, which are converted to stable voltage through c5 electrolytic capacitors, and c2 is hf filtering。
4 r5, r6 for the design of a subvoltage resistance for the output voltage, the reference vfb reference voltage for the reference ic specification is 1v. The output voltage for the design of this drive can be calculated as follows:
V=i*r=1/10*(10+100)=11v.
This leads to a maximum output power of p=ui=11*2. 451=29. 96w.
For example, as a map of the connection of the upper-heated fan to the power of the driver, the dispersing fan is connected to the led light, which was actually tested and the fan has a relatively stable power of 9v-11v, but only about 0. 6w。
In fact, the power source was good, i. E. The led light was broken, and it was assumed that the led headlights were damaged by too high ambient temperatures as they approached the motor。
That's the problem
One, why do 11vs have to be so weird, not 9vs
The actual voltage of leds with 3v is generally in the 2. 8v-3. 2v range, and when calculated at 3. 2v: 3. 2*3 = 9. 6 v, combined with the internal resistance of the output line, the bulk current of 2. 45a will also generate a voltage reduction on the output route, with the maximum voltage estimated to be in the range of 10-10. 5 v, so as to ensure that the led works normally and leaves a surplus, and the output voltage is designed to be 11 v。
Is there a contradiction between pressure and flow
First of all, we need to understand why constant currents are needed, and the positive-temperature relationship of leds is negative, that is, as their temperature rises, their positive voltage vf is lower, so when the led light is activated to light for a certain period of time, its led pressure is lower, and if power is powered with a constant voltage, the output voltage is constant, the led voltage is lower, the output current is slowly larger, and the currents are burned if they exceed the led maximum nominal current. To ensure that leds do not exceed the rated values, we usually drive leds by constant flow。
The constant pressure is designed to ensure that the power provided meets the normal load work and to avoid excessive output voltage breaking the load at short notice, so a suitable stable output voltage needs to be set。
Summary
There is a large gap between led car lighting lamps and general lighting lamps, mainly in the form of input voltage fluctuations, changes in vibration intensity and frequency during use, as well as changes in heat in engine compartments and other surroundings, changes in temperature and humidity, solar radiation, salt fog erosion, dust deposition, etc. The large light source of the dismantled led car used the csp to contain led+ dispersible aluminium + heat fans with better dispersive heat, drive circuits and tvs as wave protection and drive the use of aluminium shell emulsion as protection, designed to meet the environmental requirements of car led lighting and improve reliability。
Unfortunately, the product had been broken in only three years, and the reliability design still needed to be improved。
