For multi-string high-voltage and low-current LED driving, the RC solution has inherent natural advantages over switching power supply in terms of efficiency, reliability and cost. But there is also a fatal flaw, which is the various surge threats brought by the step-down capacitor, which may seriously damage the service life of the LED, which becomes the biggest obstacle to the application of the resistor-reducing voltage drop in LED.
Generally, the practice of string current limiting resistors only reduces the surge damage, but it cannot be eradicated, and the effect of selecting the resistor is limited. If the resistance is too large, the power supply efficiency is lowered and the heat is severe.
Large-capacity filter capacitors can provide a good buffer for surges. However, in the case of frequent switching and accidental contact of LED strings, this capacitor is not only resistant to surges, but also becomes more than 300V. "High-pressure killer" instantly destroys the LED string behind!
All of this makes many people discouraged from blocking the pressure and even regard it as a garbage line!
However, the following simple circuit with only a few parts can solve all these problems perfectly: with a simple high-voltage constant current source, the surge current can be limited to a safe value for any reason. , neither overcurrent nor overpressure, thus completely eradicating the damage of the surge!
Since the current limiting resistor that is indispensable for the common RC is eliminated, the bleeder resistance of the filter capacitor is also the same as that of the R2. The power consumption of all components except the LED string in the figure below is only about 0.1W, and the total power efficiency is Close to 95%! The efficiency of killing all switching power supply drivers, the cost is only about 1 cent of a high-pressure pipe than the conventional resistance-capacitor step-down.
For different LED quantity and working current, it is necessary to adjust C1, R2 and R3. After modifying the parameters, it can generally be applied to 10~60 strings and up to tens of mA LED drive. Value principle: C1 is stepped down according to common capacitance; R2 provides base current to the triode, this current should be enough to ensure the triode is effectively saturated under normal conditions (typically the triode CE voltage is less than 1V, long-term work hands touch the triode without Temperature); R3 determines the maximum current limit value in the surge state (I limit = (UB-Vbe) / R3), this value must be slightly larger than the value of the step-down current of C1, otherwise the triode will not be effectively saturated under normal conditions, resulting in Fever or even burnt.
With the current quality of wound film capacitors, the thickness of the base film and the plating layer is getting thinner and thinner (see the volume), and the resistance to large currents is generally poor, except for operating currents close to 100 mA or more. In addition to C2, C1 is also best to use low-loss high-quality safety capacitors or multi-capacitor parallel shunt to improve reliability, and a 33D-7 NTC on the front end to reduce the impact of the startup surge on C1.
For large power LED lamps with too many LEDs, it is recommended to consider the 220V constant current direct drive scheme (sold post).
Supplementary plan:
If the triode is replaced with a 13000 series low-β high-pressure tube, the resistance of R2 will be forced to be greatly reduced, and the loss on R2 will increase several times, affecting the total power supply efficiency.
But if you don't care about this loss, you can change R2 to 1/4W, 100K (only for the 38-series scheme above), and the triode with 1301 of β>15, the other is unchanged - even so, the overall efficiency is still It can reach about 90%, still higher than the common switching power supply driver.
Add it again:
1. This line is not suitable for low-voltage and high-current LED driving, such as the common 1W lamp bead. In addition to the special switching power supply drive, this lamp can also be driven by ordinary low-voltage DC power supply + self-made constant current.
2. The yellow LED only passes current during the surge high voltage. During normal operation, the current will be short-circuited by the triode be and R3, which is basically not bright or very bright.
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