Flyback switching power supply can light HBLED lamp at 1.1V
This design example shows a method of driving a low power electronic circuit using a single 1.5V battery. It is based on a self-excited oscillator that drives a flyback transformer to generate a higher controllable voltage. This design can be used to power analog circuits, microcontrollers, and any other light load. The power circuit was designed, simulated, and finally built. It can operate reliably with power consumption of less than 50mW, even at voltages down to 1.1V. It has been tested that for a given circuit value, the regulated output produced by adjusting the value of a single resistor is between 6V and 24V. The output power is sufficient to drive the PIC microcontroller in low power mode (15μA@32kHz). The circuit can also be used as a flash to illuminate a string of LEDs or an LED power indicator at a rate of 0.1 Hz to 20 Hz without any modification. While this design may appear to be too many circuits for other "simple" tasks, the design is still valuable given the advantages of low supply voltage and adjustable output or flicker control. In order to obtain a regulated output at +VREGOUT as shown in Fig. 1, for example, 20.7V mentioned below, resistors R2 and R3 should be set to 680 Ω and 100 kΩ, respectively. The voltage across R11 is set to approximately 140 mV through the R5/R11 divider. The oscillator consisting of Q1 continues to operate with the application of power and magnetically couples energy from L2 to L3. Once the voltage across C1 rises above 2V, comparator U1 will be effectively powered. Initially, the voltage across R2 will gradually increase as the capacitor is charged until it exceeds the voltage across R11. In this connection mode, the open collector comparator U1 continuously compares this voltage with 140mV. The thyristor consisting of Q2/Q3 is not powered and is not triggered by the output voltage of U1. The capacitor continues to charge smoothly. Therefore, based on the combined value of R2 and R3, the U1 output voltage will rise in the following cases: T1 can use magnetic cores of different shapes and sizes. In order to reproduce the above results, it is recommended to use a magnetic core having an inductance factor of about 80 nH and a relative magnetic permeability (μ) of about 80. EPCOS's ETD core with order number B66361G0100X1, Ferroxcube's toroidal core TN33/20/11-2P80 or similar products are very suitable. For the component values ​​in Figure 1 (except R2 = 680Ω and R3 = 100kΩ), the thyristor will be triggered when VC1 = 20.7V. The output filter consists of L4 and C3. C1 plays two roles in the circuit: first, as a charge storage "bucket"; second, along with R10, provides some loop stability. Resistor R4 is used to pull up the open collector of U1, while D3 is used to prevent the output bias of U1 from being disturbed.
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