Thursday, 28 July 2011

LED Street Lamp Design constant and current module selection strategy

We know all the led must adopt constant current source power supply, but now plenty of street lamp manufacturers are mostly in led module has design lovely cases again come looking for the right constant flow module. Little imagine this design process is of the issue, at least, makes this design is not optimal. May still require redesigned LED module.

Your constant current driver modules can take much power? This issue is actually cannot answer. Because constant-current module can take much power is related with plenty of factors. From the constant flow module itself, it is primarily & cooling requirements & radiation condition about, definitely also & drive chip current drivers intelligence. Yet even these are already finalized SLM2842, for example, have selected this constant flow module, so his drive ability seems ought to determined! Actually otherwise! Although its limit drive ability is can know, such as its the largest switch current, its maximum input current, its maximum input voltage & maximum output voltage, etc. However, a constant flow module driver power except & its related cooling capacity outside, the main also & its efficiency relevant. & its efficiency is & plenty of outside factors, about users. So in design or select to think about LED module constant current source requirements & characteristics.

LED connection architecture
  1. The series LED several shall be less than ten, because as a safety LED lamps and lanterns, on request. According to eu IEC used 2-13 (5/2006) standard, in view of the dc or ac powered LED module requirement in the voltage LED lamps and lanterns, shall not exceed the maximum safety low voltage (SELV), namely the work output voltage Vrms (over 25 35.3 Vdc), so the total series LED shall not exceed ten.
  2. As for the repair of lights, and simple to change usually adopts LED module, each module design usually grow square, constant flow module is designed in them. Recommend each module power less than 30W has. So, in the street lamps use 100W around 3-4 LED module, and 150WLED street lamp, used the 5-6 LED module. Then high-power can use a 7-8 module.
  3. In each LED module, usually adopts 1W leds, its connection mode for LED series-parallel way. If LED diode, so can a protection after first string if no protection and diode, so will first and string, in case a damaged after a string of all don't light.
  4. With a few strings of concrete, will further understand a few and constant current module performance and characteristics.
 Constant flow module performance and characteristics

Ordinary manostat is when the load changes, the output voltage unchanged. And constant flow module is a kind when the input voltage changes to keep output current constant a power supply. Before use, must understand its various characteristics:
  1. The current Settings: usually constant flow module output current can be in a large range according to user's requirements to fine-tune, basically modify the output current setting resistance. 
  2. Type: constant flow module has pressurization type, step-down type and lift pressed three kinds. Pick which sort of entirely from the necessary input voltage and the relationship between the output voltages and pick. But if from the viewpoint of get the maximum efficiency, then they ought to pick step-down type. Step-down voltage type is input is higher. So the input current is tiny, so the loss caused by cu losses is comparatively tiny, and step-down type constant-current module usually has higher efficiency. So, also must load in tandem LED number to reduce as far as feasible, in order to be able to adopt step-down type of constant current driver modules.
  3. Input voltage & output voltage relationship: Whether pressurization type or step-down type, the constant flow module than or step-down pressor than are more close to one higher efficiency. Of work also cannot equal one, & must permit 2-3V to constant flow module consumption. & this is basically the general lamp design by streetlights decision. So the general design of the street lamp when they must think about the factors to select the power supply voltage & load series LED number. than design lovely again request constant-current module must provide the high efficiency. 
  4. Dimmer ability: constant flow module usually have dimmer ability, & the dimmer ability is not basically adjust its output current, but using a call pulse width modulation (PWM) dimming technique, it used the fast switch ability & LED eye vision residual, make look brightness changes. Avoid a adjust current generated by the because gc changes. The light of the street lamps design ability is important, because use the performance can make the street lamp brightness according to the traffic flow to the alter, thus further realized the purpose of saving energy.

    Below they points solar lamps & utility LED street Light situations to speak about it.
Solar street lamps

Solar LED street lamp that the largest characteristic of it is usually composed of battery-powered, and batteries have some features is necessary to think about:
  1. Several provisions of the battery only three.1 voltage, most often used is the 12V automobile battery, because it is the standard voltage. 24V will need to make use of series. 36V is rarer. Therefore, often in solar LED street lamps need to adopt pressurization type of constant current driver modules. This will influence the efficiency of the constant module. 
  2. In solar street lamps of another important query to think about is the input voltage change.

For constant-current source for the output power and efficiency, to get the maximum efficiency will keep the ratio of the input and output voltage approximates to one, the better. But if the input voltage instability. It would be impossible to maintain the highest efficiency in the best state. Solar LED street lamp process usually adopts battery as the energy stored unit. And the battery in filled with electricity and speedy put over when the voltage change a lot. Usually this change over 30% above. For example, 12V battery for its output voltage can change to ten.8 V recent V. Of work for constant flow module for, such change is entirely can afford. Is also in such giant changes can keep its output current within the scope of the constant. But this comes at a cost, that is cannot work in the best state. The best condition is refers to the highest efficiency state. Or the highest state power output.

For constant flow module for, whether pressurization type or step-down type, only when the output voltage closest to the input voltage, the efficiency is highest. Usually output voltage is decided by the load, is seldom changes. So when the input voltage in a scope change, it is efficiency also follow changes. In order to guarantee in the worst cases can also work, cannot work in the best condition!

Now cite A specific examples: if solar LED street lamps use 12V battery, the load is three and 8 string is 1W leds at the output, then LED the load voltage is roughly 26.4 V, the current is one.05 A, power output for 27.7 W. And if the 12V battery, by the highest in voltage is than recent V, boost, when not is one.78 QFN encapsulation for heat when shell temperature is about 60 degrees. Still can accept, but when the battery voltage reduced to ten.8 V, than becomes two.44 times pressor, this time shell temperature will increase to 70 degrees above, but if use is TSSOP encapsulation, then shell temperature more raised about 85 degrees, is not allowed!

Besides ought to think about shell temperature beyond, think about maximum input current. Because the maximum input current as SLM2842 triple-a. In the calculation of maximum input current, need to think about the efficiency of the constant flow module itself. Now assume is 90%, and assume the output power is divided by 0.9 32W is averaging 35.5 W, and when the input voltage, for when the input current recent V two.4 for A, not beyond allowed 3A; But when battery voltage reduced to ten.8 V, to three.28 input current, it exceeded A triple-a, chip will automatically cease working.

So if not think about battery voltage change is dangerous design.

Lift pressed constant flow module characteristic is no matter how change input voltage, make its below or above output voltage, its output current are unchanged. That is it can automatically according to the input voltage & output voltage from the relationship between a step-down pressor automatically. For example, SLM2842SJ fluctuation pressure constant flow module in output current 1A, output voltage, the input voltage for 13V 25V from 7.5 V has been to can keep output current for constant 1A. So are all the case as far as feasible to adopt lift pressed? It is not, because lifting pressed the output power of smaller, & its efficiency is low, the cost is dear, not necessary conditions use less as far as feasible.

It can use batteries, & adopts step-down type series of constant flow module? In theory, this time can can also receive a higher efficiency, but because series may use battery much capacity (Ann), & increased costs, increased the chance when maintaining replacement battery. In general are not worthwhile.

Utility LED street lamp

In the circumstances, definitely captures best is using AC - DC direct constant-current output system.

But this plan most can only provide small Power output (< 40 w) only American Power with the company's TOP250YN not only provide 75W Power output system. But its circuit is very complex, expensive, & flexibility is low. Bosoar company domestic metallocene although also provides direct constant-current output power, but usually only a single output. In the event you need to personalize (must be multiplexed points & every road current). On the hand raised costs, & future will be any changes will be troubling.

So in the case had better captures with constant voltage power supply voltage stabilizer & then use several constant flow module constant flow. The largest advantage is that do so in various LED connection framework get the highest efficiency. Because can select any constant pressure source output voltage & accomplish the best matching. & high flexibility, simple to change its portfolio.

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