With battery driven seven high brightness white LED circuit design

To drive more than one high brightness white LED, design engineers need to select is LED series connection LED or parallel connection. Of course, each connection is bring design compromise. Parallel connection on both ends of each LED only at a low voltage, but need to use ballast resistor or a current source to ensure that each LED brightness. If the offset current flows through each LED size is different, their brightness is also different, which leads to the uneven lighting source. However, the use of ballast resistance or consistent current source to ensure that the brightness of the leds will shorten the service life of batteries. Adopts the series connection in essence can well ensure the consistency of the current but need to impose high voltage LED string. To achieve the appropriate brightness, general need 3 white LED. 6 v bias voltage and maximum offset current 20 ma. Figure 1 shows the seven white LED string can adjust the brightness of the booster circuit inductance type low cost. This circuit can be divided into two parts: composed of Q1 and Q2 booster circuit, and the control circuit is composed of Q3 and JFET1. Assuming that Q1 closing, when the battery voltage is slightly higher than the Q2 VVB, Q2 will base through the positive current ( iB=( The battery voltage VBE varies) / RJET1) 。 At this point, Q2 conduction, grounding inductance L1. As the current di/dt on L1 speed increases, the energy stored in the L1 magnetic field. As the current increases gradually, it also through the Q2 resistance RSAT ( SD1 and LED string in cut-off condition) 。 Q2 collector voltage is high enough, can make the Q1 conduction. Q1 base voltage through feedforward network composed of R1 and C1 connected to the collector of Q2. R1 is also used to limit Q1 base current. Q1, after conducting base earth Q2, hence Q2 closing, the energy of the L1 was released as the field wanes to the LED string. L1 quickly back to zero movement imposed on the LED string above 26 v positive bias voltage, making leds emit white light. Due to the high frequency blinking eyes feel LED, so the circuit can provide constant lighting brightness. When the L1 after discharge, Q1 returns to the blocking state. Normal work, the self oscillation action repeated until the battery voltage drops to less than the VBE varies with JFET1 pressure drop of Q2 ( About 1 v) The sum of the Q2 conduction no longer. L1, Q2 RSAT and switch characteristic of Q1 and Q2 will also affect the oscillation period and duty cycle.

the battery pack ( 4 alkaline batteries) Voltage has been increased to more than 26 v, so that to made up of seven series of white LED LED string provide forward biased. Small dc current flowing through the R4 ( Less than 20 ua) To offset of Q3, to adjust the channel resistance of JFET1, which regulates the battery leakage current to extend battery life. JFET1 grid voltage higher than battery voltage zero. About 9 v. The p - Jfets are used as exhaust device, when VGS is equal to zero, p - Jfets conduction. ET source connected battery terminals. Design engineers can by improving the grid voltage ( Than the battery voltage is higher) To shut off the channel. Grid voltage than the battery voltage is higher, the greater the channel resistance. Therefore, when the battery voltage decreased from 6 v to 3 v, the oscillation frequency fall ( JFET1 VGS will slightly change) 。 At this point, the LED brightness fell slightly. Ideally, the LED constant current control loop can be maintained. But the human eye to light sensitivity obey logarithmic relationship, so before the battery voltage drop to about 2 v, the brightness of the small amplitude of linear decline is not easy to detect. Another solution is to keep the battery power output ( Current and voltage of the product) The same. Due to the loss of battery internal resistance, although this can keep unchanged LED brightness, but it will shorten the battery life, in addition, the complexity of the circuit will be greatly improved. All in all, this simple circuit of LED brightness will changed little during the whole battery life. Can slightly adjust the brightness of the LED string, such as design engineers through the change value of R2 slightly adjusted in view of the triode and LED manufacturing deviation, so that light output ( Unit: lumen) Can be set to a fixed value. When the battery energy is about to run out, can a dim light LED string of short circuit, and connect an LED only, as long as the battery pack and 1 v of residual voltage at this moment, you can make the LED light. This way of single LED connection can take advantage of the waste batteries for emergency lighting at last. From the security into consideration, when using alkaline batteries all batteries must match. When battery pack is the least amount of energy in the battery energy completely exhausted, and the other battery has enough energy to run out of battery formation reverse bias, will cause the energy out of emulsified acid battery overheating and leakage, resulting in a safety issue. To realize the battery matching, shall ensure that use the same packaging of the new battery to replace all four batteries at the same time. 4 AA alkaline battery rated capacity of 4% 26; # 215; 1000 mah, this means that LED to about 61 hours of continuous lighting. Circuit prototype test results show that the lighting time for two consecutive days, 48 hours) A little more.
editor: li jie