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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Voltage converter for LED lamp. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters Without a doubt, LEDs are by far the most economical and durable light sources. New devices of this class that have appeared in recent years have made a kind of revolution in the field of lighting and illumination. LED lamps have become widespread in everyday life, which came along with compact fluorescent lamps (CFLs) to replace uneconomical and short-lived incandescent lamps, and today they are increasingly replacing CFLs. Unfortunately, despite manufacturers' assurances about durability, estimated at many tens of thousands of hours, LED lamps sometimes fail, and much ahead of schedule. And the reason is often not in the quality of the LEDs, but, most likely, in the stinginess of the manufacturers: in order to save on the cost of lamps, the LEDs in them are forced to work in extreme conditions, at current values close to the maximum allowable, which has a noticeable effect on the rate of crystal degradation and phosphors, as well as the reliability of the lamp. And if we consider that due to the small dimensions of the lamps, unsatisfactory cooling conditions for LEDs are added to the above, it is not surprising that sometimes such lamps fail after a few hours of operation. An analysis of the failures of burned out lamps shows that in 90% of cases one of the LEDs fails, while the driver, as a rule, remains serviceable. Repair of such lamps is not difficult, but without taking measures to reduce the current through the remaining LEDs, it is often useless: after a while, the lamp fails again. Let's consider the possibility of restoring a 7W Elektrostandard lamp. Its appearance and the view of the driver board from the side of the printed conductors are shown in Fig. 1. First, you should find the burned-out LED in any way and close it with a jumper. Next, you need to reduce the current through the LEDs. To control the current, a sensor is used, consisting of two SMD resistors connected in parallel (circled in Fig. 1 with a red circle). To reduce the current, they need to be unsoldered and a new one with a resistance of 2 ohms should be soldered in place of any of them. After such a repair, the power and light output of the lamp will decrease somewhat, but it will be able to work for a long time. The foregoing is fully applicable to similar lamps with a power of 15 W (Fig. 2). On their board, to reduce the current through the LEDs, you need to solder one of the 5,6 ohm resistors (also circled in red).
But sometimes it is impossible to restore the lamp due to the failure of the controller. In this case, the LEDs can be powered from another source. Below is an option for connecting a board of LED lamps with a power of 5 or 7 W to a twelve-volt source (for example, a car battery). Depending on the rated power, 12 or 16 LEDs are installed in these lamps, respectively. Such a lamp can be useful for an emergency or car lamp. Since the LEDs are connected in series on the board, and we didn’t want to change the connection scheme by cutting the printed conductors and installing wire jumpers, it was decided to make a converter that increases the battery voltage to the level necessary for the LEDs to glow with normal brightness (in this case, respectively, up to 35 or 48 V ). A diagram of a simple converter, assembled from widely used and inexpensive parts, is shown in fig. 3. A master oscillator operating at a frequency of about 1.1 kHz is built on a Schmitt trigger DD25 according to a typical scheme. Elements DD1.2-DD1.6 connected in parallel invert the signal of the generator and increase its load capacity, providing fast charging and discharging of the capacitance of the field effect transistor VT2. The microcircuit is powered by a lamp power source through a linear voltage regulator DA1, connected according to a typical circuit. The current sensor is resistor R5.
The stabilization circuit works as follows. If the current through the LEDs becomes more than required, the transistor VT1 opens, shunting the input of the Schmitt trigger DD1 with the resistor R1.1. In this case, the duration of the control pulses applied to the gate of the field-effect transistor VT2 decreases, and the duration of the pauses between them, on the contrary, increases. As a result, the current through the LEDs decreases. Current stabilization is carried out in the input voltage range from 9 to 15 V, which is quite enough for a battery and car lamp. Resistor R3 serves to discharge the capacitor C4 after turning off the converter (without it, a weak glow of the LEDs would have been observed for a long time after turning off the power). All parts of the device are placed on a printed circuit board (Fig. 4), made of fiberglass laminated on one side. The VT2 transistor does not need a heat sink, but if during operation its case heats up noticeably, it is possible, in addition to the contact pad on the board used as a heat sink, to which the output of its drain is soldered, to provide it with a small U-shaped heat sink made of a flattened piece of copper wires with a cross section of 2,5 mm2 and 20 mm long. You can solder it both to the indicated area on the board (next to the transistor), and to the heat-removing flange of the transistor itself. The appearance of the finished node is shown in fig. 5. An additional heat sink is made of aluminum alloy sheet for the LED panel, and its appearance is also shown in this figure.
A few words about the details. In addition to that indicated in the diagram, any low-power npn transistor for surface mounting can be used as VT1. Field-effect transistor (VT2) - any with a drain current of at least 2 A and a drain-source voltage of at least 80 V, designed to control logic levels. A possible replacement for the 74NST14 (DD1) microcircuit is from the 74NS14 or 74AC14 series. Instead of the RGP10J (VD1) diode, you can use 1N4007, but it will noticeably heat up and the efficiency will decrease. Diodes of the KD226 series work practically without heating. Throttle L1 - industrial production in a cylindrical body, its type is unknown, and the appearance is shown in fig. 5 (black cylinder in the lower left corner of the board). If it is not possible to find an integral stabilizer for 5 V of the SMD version, a parametric stabilizer on a zener diode can be built into the power supply circuit of the DD1 microcircuit. You can place it and a ballast resistor with a resistance of 1 kOhm on the seat of the microcircuit. The device, assembled from serviceable parts, practically does not require adjustment. When you first turn on the converter, it is advisable to power it from a laboratory unit with an adjustable output voltage, gradually increasing it, starting from 5 V. If the LEDs do not light, check the polarity of their connection, the serviceability of the parts. When using replacement microcircuits instead of those indicated in the diagram (DD1), it may be necessary to select capacitor C1 or inductor L1 for maximum efficiency. It may be necessary to select the resistor R5 to obtain a current through the LEDs of 100 mA. If the required resistor is not available among those available, you can install R5 with a slightly higher resistance and select an additional resistor R5 'connected in parallel to it (shown in dashed lines in the diagram), a place for it on the board is provided. Next, you should check the range of input voltage values at which current stabilization through the LEDs is carried out. You can try to increase the efficiency of the converter by selecting the inductance of the inductor L1. When adjusting, it should be remembered that an open circuit of the LEDs can lead to a breakdown of the field-effect transistor, so you need to be very careful. Finally, the converter board should be covered with two layers of XB-784 varnish, this will protect it from moisture. When operating such a lamp, remember that when connecting it to a power source, polarity must be observed. Author: E. Gerasimov
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