Source: Ofweek Date: 2017/5/31 16:01:04 Views: 12 Share to:
LED is popular in the market because of its high efficiency, low energy consumption characteristics. So it is widely used in display lighting and signal indicators and other fields. However, the application of the wide range of the environment is also a challenge to the LED itself, in the outdoor work, led driving circuit is very susceptible to over-voltage and over-current impact caused by the failure or damage to property and personnel caused losses. Therefore, in the production of a LED power supply must consider the complexity of its working environment, and make protective design, reduce the incidence of failure. This paper makes a simple discussion on how to protect the driving circuit of LED.
Familiar with the LED circuit of friends must know, led driving circuit is generally composed of several parts of the following components, including AC input, rectification, DC/DC conversion, and other modules. The protection measures need to be adjusted according to the different modules, such as each module and current in the case of surge, the need for different effective protection measures.
1, led driving Circuit surge protection application
In the AC input terminal surge Protection scheme, a pressure sensitive resistor (MOV) or a gas discharge tube (GDT/SPG) combination can be used to design. In the case of grounding, can be used as fig. 1 common mode at the same time protection concept, in the L-n shunt varistor (MOV), can effectively suppress the differential mode generated by the surge Overvoltage, up to the protection of the rear-level circuit, respectively, the l/n-pe between the Mov or MOV+GDT/SPG to the ground of the circuit connection can effectively reduce the common-mode surge energy to the earth, to prevent the surge into the back-level circuit caused damage; if there is no grounding line in the power supply, In Fig. 2, the differential mode protection can be carried out directly between the L-n line and the Varistor.
Fig. 1 schematic diagram of AC input Terminal common-mode/differential-mode protection circuit
schematic diagram of AC input-end differential-mode protection
In order to avoid the protection of MOV protective elements after the failure of protection, the possibility of short circuit failure ignition combustion, you can use Tmov or Pmov to protect. For the above MOV ac withstand voltage selection at least above the line maximum AC operating voltage 1.2 ~ 1.4 times times, in order to avoid Mian misoperation, at the same time when using discharge tube GDT/SPG, the lower limit value of the discharge tube breakdown voltage must be at least higher than the maximum peak voltage of the circuit, and the withstand current must choose different current level according to the demand of the surge grade to meet the requirements of the surge Test standard.
2, AC/DC protection circuit schematic diagram
Fig. 3 schematic diagram of AC-DC rear protection circuit
After the AC has been rectified, the chip in the back-end DC circuit is very sensitive to over-voltage and over-current, and the chip is susceptible to damage, as shown in Fig. 3, the parallel transient suppression diode TVs after rectification, in the case of over-voltage generation, TVs will be clamped in a safe range with a picosecond reaction speed action to protect the back-end chip from pressure-free impact. Abnormal current can be designed by the circuit in the recovery fuse PPTC protection, PPTC in the over-current generation impedance can be rapidly larger, so as to effectively block the abnormal current, until the fault elimination PPTC can continue to restore the low resistance state, so that the circuit can continue to return to normal working conditions.
TVs choose when the cut-off voltage is typically 1.2 ~ 1.4 times times the peak of the normal operating voltage, and the TVs power size is based on the power of the overpressure to select the appropriate grade. PPTC choice to combine circuit work current and voltage reference and ambient temperature is also the impact of PPTC selection of an important key indicators, PPTC the holding current will be reduced with the application of ambient temperature. The position of PPTC in the circuit is generally connected in the TVs front end, so that PPTC not only can protect the circuit chip effectively, but also can play a certain protective role to the TVs tube, can greatly improve the service life of the TVs tube.
3, LED direct drive circuit protection schematic diagram
Fig. 4 protection diagram of LED DC drive circuit
The brightness of LEDs is controlled by the current size of the LED unstable current and extremely easy to burn the LED, such as Figure 4 in the DC/AC module can be in the circuit in series constant-current diode to obtain a stable current, so not only can the led to achieve a stable brightness, and not due to current instability and burn led. Low-Power LED lamp working current is generally 10mA to 30mA, high-power LED lamp working current from 200mA to 1400mA, can be based on the required work current selection model suitable constant current diode. Due to the LED light is also susceptible to electrostatic discharge over pressure interference damage, so the back-end of DC circuit LED lights also need to do some effective overpressure protection, generally using TVs tube can.
4, LED lamp string protection schematic
Fig. 5 Schematic of LED bulb protection
When multiple LED lights are connected in series, as shown in Figure 5, when an LED lamp fails to open, the entire LED lights will be affected by the failure of other LED lights work properly, in order to solve this problem, each LED light on the parallel to a circuit-proof LED protection device TX, so as to fully improve the efficiency of each LED, when a single led failure open circuit failure, In parallel with the LED Open Circuit protection device TX will immediately guide, so that the sustainable maintenance is in the State, thus ensuring that the other series in the circuit LED is not due to a single led open circuit fault and extinguished, but the cost of this protection measures relatively high.
To sum up, led driving circuit generally by the AC input, rectification,, DC conversion, and other modules, so that an LED driving circuit roughly the overall protection scheme can be referenced as shown in Figure 6:
Fig. 6 Schematic of overall protection of LED power supply
When in practical application, there are many factors that affect the selection of surge protection elements, such as lightning surge Test level, chip parameters, work voltage, working environment, and so on. Therefore, in order to protect the LED power supply design, we need to consider a number of factors, only the comprehensive situation carefully considered, can have a purposeful design can fully play a role in the protection program.