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Using the boost circuit in consumer electronics applications

Feature

By Lin Zhiqi, Zhang Wei and Huijie Li of Changchun University of Technology, China

In the past, due to technical constraints, the only way to change the voltage level was by using multiple resistors to divide it. However, this method is inefficient for two main reasons: one, it only reduces the voltage but does not increase it; and two, much of the energy dissipates in resistance during the change. Hence, the boost circuit was invented, which uses an inductance coil to store energy to increase DC voltage. This circuit is widely used in steady voltage power supplies and consumer electronic products. Here we present some interesting applications of the boost circuit.

Boost Circuit’s Working Principle
The boost circuit consists of inductors, capacitors, diodes and a commutation switch. When the switch is closed, the inductor converts electric energy into a magnetic field, and when the switch is disconnected, the inductor converts stored magnetic field energy into electricity. This energy is superimposed with the input voltage and then filtered by the diode and capacitor, obtaining a smooth DC voltage that can be used by a load. Because this voltage is formed by a superimposing the input power supply voltage and the inductor’s magnetic energy, the output voltage is higher than the input one, thus achieving the goal of increasing the voltage.

The diode mainly acts as an isolator. When the switch is closed, its positive voltage is lower than the negative one. At that point, the diode cuts off, and energy stored in the inductor doesn’t affect the output capacitor to load the power supply. When the switch is disconnected, the two superimposed energies supply power to the load through the diode. The smaller the diode forward voltage drop, the more energy it provides to the load.

The inductor’s charging process is shown in Figure 1, where the switch is closed and, hence, shown as a wire. The input voltage flows through the inductor. The diode prevents the capacitance from discharging to ground. Because the input is DC, the current through the inductor increases linearly by a certain ratio, which depends on the size of the inductor. As the inductor current increases, some energy is stored in it.

Figure 1: Inductor charging setup

The inductor’s discharge process is shown in Figure 2, which is the equivalent circuit diagram when the switch is off. The current flowing through the inductor does not go to zero at once, but gradually decreases from its value at the end of the charge to zero. The original circuit has been disconnected, so the inductor can only form a circuit with the other components, that is, it starts charging the capacitance. The voltage at both ends of the capacitor rises, making the voltage higher than the input voltage.

Figure 2: Inductor discharging setup

If the capacitance is large enough, a continuous current can be maintained during the energy discharge at the output. If the on-off process is repeated, a voltage higher than the input voltage can be obtained at the two ends of the capacitor.

At the Heart of a Consumer Electronic Device
To enable automatic opening and closing of the switch, an induction coil is used to raise the voltage; see Figure 3. The inductor’s self-inductance effect and the principle of automatic switching of the mechanical switch are used to replace the boost circuit mentioned in this article. This principle is very similar to the mechanical structure of a relay, so it can easily be implemented by relays, too.

Figure 3: Induction coil is used to raise the voltage for automatic opening and closing of the switch

As Figure 3 shows, the 12V power supply is connected in series with the relay’s normally-closed contact J. When current passes through the coil RE1, an electromagnetic suction is generated, it disconnects the normally-closed contact. Because of the inductance’s self-inductance effect, the normally-closed contact is turned off and on at a certain frequency. Higher voltage is generated at both ends of the coil when the contact is disconnected, typically between 150 and 300V.

Figure 3a: Circuit prototype

The prototype is shown in Figure 3a; the relay used in it is SRD-12VDC-SL-C.

A Long-Lasting LED Lamp
Nearly all LEDs in the home are powered by rechargeable batteries. The traditional LED lamp is very strict about voltage; as long as the power supply is lower than its turn-on voltage, the LED lamp will be off. With the battery, the LED lamp’s voltage will decrease slowly with increased discharge time, and after a certain period, it won’t be enough to light up the lamp. However, at this stage, there’s still a lot of energy left in the battery.

The LED lamp discussed in this article can consume less than 0.4V, showing excellent performance; see its circuit in Figure 4.

Figure 4: LED circuit

This is quite a simple circuit: When power is applied, current flows from the battery through the 1K resistor and one winding of the transformer into the base of the transistor, turning it on. This causes a larger current to flow through the transistor via the other transformer winding. This current cuts off the transistor’s base current, as the two windings are connected in opposite phase. When the transistor turns off, the current tries to flow through the transformer winding, but can’t pass through the transistor, leaving it nowhere to go. The voltage builds up, until there is enough to switch on the LED. The current in the transformer falls, and the whole cycle repeats.

Although simplified, this process shows how the circuit works. The circuit runs at up to 30-50,000 cycles per second, so the transformer needs to be made from a material suitable for those frequencies.

The prototype is shown in Figure 4a. It was tested using 0.4V, making the LED lamp shine very brightly. We can see that the boost effect of the circuit is particularly good.

Figure 4a: LED circuit breadboard

Figure 4b shows an LED flashlight that uses a single battery.

Figure 4b: A consumer product with an LED circuit

Creating an Electric Arc
Traditional ignition equipment includes matches, various gas and fuel lighters and resistance wire lighters. However, traditional lighters don’t work continuously, and their long-term service temperature will rise sharply, leading to fuel leakage. These are harmful gases, resulting in environmental pollution.

The arc lighter is a new type ignition device that’s more environmentally-friendly since it doesn’t use fuel. There’s no flame, making it safe to use. Its circuit is shown in Figure 5. T1 is a three-winding transformer, coil 1 is an energy storage coil, coil 2 is a feedback coil and coil 3 a booster coil with a variable ratio of 1:50. High pressure of 10kV is generated at both ends of coil 3 after a boost. It can break through the air to produce high-temperature arcs that ignite other objects; see Figures 5a, b and c.

Figure 5: Circuit of an arc lighter

A Highly-Efficient Boost Circuit
Adding a boost circuit to consumer electronic devices makes these products more energy-efficient and more environmentally-friendly.

Figure 5a: Arc lighter circuit breadboard
Figure 5b: Arc lighter circuit setup

The boost circuit proposed in this article has high conversion efficiency, and its structure is simple and cheap, yet reliable. It is also suitable for volume production necessitated by consumer electronics.

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