Simple and efficient 1.5v boost circuit _ boost circuit principle analysis
**Introduction to the Boost Circuit**
A bootstrap circuit, also known as a boost circuit, is an electronic circuit designed to increase the voltage level by using components such as a bootstrap diode and a capacitor. This circuit works by combining the voltage from the power supply with the voltage stored in the capacitor, effectively boosting the output voltage. In some applications, the output voltage can be several times higher than the input voltage.
**How the Boost Circuit Works**
Let’s take a simple example: imagine you have a 12V system, but you need a 15V drive voltage for a FET in the circuit. The solution? A bootstrap circuit. Typically, this involves a capacitor and a diode. The capacitor stores energy during one phase of the cycle, while the diode ensures that the current flows in only one direction, preventing reverse flow. When the frequency is high, the capacitor's voltage adds to the input voltage, acting like a voltage booster.
Although the term "bootstrap" is commonly used in practical applications, it isn't a formal theoretical concept. It is often applied in class A and B single-supply complementary amplifier circuits. While these circuits theoretically could produce an output voltage up to half of the supply voltage (Vcc), in practice, the actual output is usually much lower due to limitations in voltage swing. To overcome this, a bootstrap circuit is employed to raise the effective voltage level.
**Common Bootstrap Circuits**
According to Fairchild’s application note AN-6076, “Design and Use Guidelines for Bootstrap Circuits for High-Voltage Gate Driver ICs,†bootstrap circuits are widely used in high-voltage gate driver designs. These circuits are essential in ensuring that the gate drivers receive sufficient voltage to operate properly, especially in systems where the supply voltage may not be enough on its own.
**Working Principle of a Switching DC Boost Circuit**
Also known as a step-up or boost converter, this type of circuit is capable of producing an output voltage higher than the input voltage. The basic configuration includes a switch (such as a transistor or MOSFET), an inductor, a diode, and a capacitor. The operation is divided into two main phases: charging and discharging.
**Charging Phase**
During the charging phase, the switch is turned on, allowing current to flow through the inductor. The capacitor remains isolated from ground by the diode, so it doesn’t discharge. As the inductor charges, it stores energy in the form of a magnetic field. The rate at which the inductor charges depends on its inductance value and the input voltage.
**Discharging Phase**
When the switch turns off, the inductor resists the sudden change in current, causing a voltage spike across it. This spike is then rectified by the diode and used to charge the capacitor. As a result, the capacitor’s voltage rises above the input voltage, completing the boosting process.
This energy transfer mechanism continues as the switch cycles between on and off states, allowing the capacitor to maintain a stable output voltage that is higher than the input. This principle is fundamental in many modern power supply designs.
**Simple 1.5V to 9V Power Supply Circuit**
A common example of a boost circuit is a 1.5V to 9V power supply. This circuit uses a combination of transistors, inductors, and capacitors to generate the required higher voltage. BG1 acts as the main oscillator, while L1 and C1 help control the oscillation frequency. L2 serves as the energy storage component, and BG3 functions as an automatic switch that activates when the load is connected.
When the circuit is powered, BG3 turns on, allowing BG2 and BG1 to conduct. Energy is stored in L2, and when BG1 turns off, the inductor generates a higher voltage, which is rectified and used to charge the output capacitor. If the output voltage exceeds a certain threshold, BG2 limits the current, stabilizing the output.
**Component Selection and Testing**
BG1 should be an NPN transistor with low saturation voltage, such as a 9013 or 8050, with a collector current rating of at least 300mA and a current gain (β) of 200 or more. BG2 can be a PNP transistor like 9012 or 9015, and BG3 can be a 9014 NPN transistor. The inductors L1 and L2 can be wound on a high-frequency ferrite core, using 0.1mm enameled wire. L1 has 6 turns, and L2 has 36 turns.
This circuit was tested with a DT890A digital multimeter, showing a working current below 45mA in the buzzer mode and under 25mA in other modes. Even when the battery voltage dropped to 0.9V, the circuit still provided sufficient power for most functions, making it a reliable and cost-effective solution.
**Conclusion**
Using a boost circuit allows you to power devices that require higher voltages from a lower supply. This design is particularly useful in applications where space and simplicity are important. By using a bootstrap method, you can avoid the need for a separate power switch, making the circuit more efficient and user-friendly. Overall, this circuit is easy to build, stable, and highly functional for a variety of low-voltage to high-voltage conversion needs.
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