“To drive more than one high-brightness white LED, the design engineer needs to choose whether to connect the LEDs in series or in parallel. Parallel connection only needs to apply a lower voltage at both ends of each LED, but it is necessary to use a ballast resistor or current source to ensure that the brightness of each LED is consistent. If the bias current flowing through each LED is different, their brightness is also different, resulting in uneven brightness of the entire light source. However, using ballast resistors or current sources to ensure the consistent brightness of the LEDs will shorten the life of the battery.
To drive more than one high-brightness white LED, the design engineer needs to choose whether to connect the LEDs in series or in parallel. Parallel connection only needs to apply a lower voltage at both ends of each LED, but it is necessary to use a ballast resistor or current source to ensure that the brightness of each LED is consistent. If the bias current flowing through each LED is different, their brightness is also different, resulting in uneven brightness of the entire light source. However, using ballast resistors or current sources to ensure the consistent brightness of the LEDs will shorten the life of the battery.
The use of series connection can essentially ensure the consistency of the current, but it is necessary to apply a higher voltage to the LED string. In order to achieve proper lighting brightness, ordinary white LEDs require a 3.6V bias voltage and a maximum bias current of 20mA. Figure 1 shows a low-cost inductive boost circuit that can adjust the brightness of seven white LED strings.
This circuit can be divided into two parts: the boost circuit composed of Q1 and Q2, and the control circuit composed of Q3 and JFET1. Assuming that Q1 is off, when the battery voltage is slightly higher than the VVB of Q2, the base of Q2 will flow a positive current (iB=(battery voltage VBE)/RJET1). At this time, Q2 is turned on, and the Inductor L1 is grounded.
Based on a design that uses a battery pack to drive a string of white light LEDs to achieve consistent brightness. As the current on L1 increases at a di/dt rate, the energy is stored in the L1 magnetic field. As the current gradually increases, it also flows through the resistance RSAT of Q2 (SD1 and the LED string are in an off state). The collector voltage of Q2 is high enough to turn on Q1. The base voltage of Q1 is connected to the collector of Q2 through a feedforward network composed of R1 and C1. R1 is also used to limit the base current of Q1.
After Q1 is turned on, the base that drives Q2 is grounded, so Q2 is turned off, and the energy of L1 is released into the LED string as the magnetic field weakens.
L1’s rapid zero return action applies a forward bias voltage higher than 26V on the LED string, causing the LED to emit white light. Since the human eye does not perceive the high-frequency flicker of the LED, the circuit can provide constant-brightness illumination. When L1 discharges, Q1 returns to the cut-off state.
During normal operation, this self-oscillation action is repeated until the battery voltage drops below the sum of Q2’s VBE and JFET1 voltage drop (approximately 1V), and then Q2 is no longer conducting. The RSAT of L1 and Q2 and the switching characteristics of Q1 and Q2 also affect the oscillation period and duty cycle.
The voltage of the battery pack (4 alkaline batteries) is increased above 26V to provide forward bias to the LED string consisting of 7 white LEDs connected in series.
The small DC current (less than 20uA) flowing through R4 biases Q3 to adjust the channel resistance of JFET1, thereby adjusting the battery leakage current to extend battery life. The gate voltage of JFET1 is about 0.9V higher than the battery pack voltage. Here p-JFET is used as a depletion-type device, when VGS is equal to zero, p-JFET is turned on.
The source of ET is connected to the battery terminal. Design engineers can turn off the channel by increasing the gate voltage (higher than the positive battery voltage). The higher the grid voltage is than the battery voltage, the greater the channel resistance.
Therefore, when the battery pack voltage drops from 6V to 3V, the oscillation frequency drops (the VGS of JFET1 will change slightly). At this time, the brightness of the LED drops slightly. Ideally, the control loop will keep the LED current constant. However, the sensitivity of the human eye to light obeys a quasi-logarithmic relationship, so a small linear decrease in brightness is not easy to be noticed before the battery voltage drops to about 2V.
Another solution is to keep the battery’s output power (the product of current and voltage) unchanged. Due to the loss of internal resistance of the battery, although this can keep the brightness of the LED unchanged, it will shorten the battery life, and the complexity of the circuit will also be greatly increased. In short, the LED brightness of this simple circuit will change very little during the entire battery life.
The brightness of the LED string can be slightly adjusted. For example, the design engineer can adjust the manufacturing deviation of the transistor and the LED by slightly changing the resistance of R2, so that the light output (unit: lumens) can be set to a fixed value.
When the battery pack is about to run out of energy, you can short-circuit the dim LED string and connect only one LED. At this time, as long as the battery pack has a remaining voltage of 1V, the LED can emit strong light. This single-LED connection method can use waste batteries to provide the final emergency lighting.
For safety reasons, all batteries must be matched when using alkaline batteries. When the energy of the battery with the least energy in the battery pack is completely exhausted, and the other batteries have enough energy to form a reverse bias against the exhausted battery, the exhausted battery will overheat and leak the milky acid, thereby producing Security Question.
In order to achieve battery matching, it should be ensured that all 4 batteries are replaced with new batteries in the same package at the same time. The rated capacity of 4 AA alkaline batteries is 4×1000mAh, which means that the LED can be continuously illuminated for approximately 61 hours. The test result of the circuit prototype shows that its continuous lighting time is a little more than two days (48 hours).