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 that uses components such as a bootstrap diode and a bootstrap capacitor to combine the voltage from the capacitor with the supply voltage, effectively increasing the output voltage. In some designs, the output voltage can be several times higher than the input voltage, making it useful in various applications where a higher voltage is required.
**Principle of the Boost Circuit**
To illustrate simply: imagine a 12V circuit that requires a 15V drive voltage for a FET. How can this additional 3V be achieved? The solution is to use a bootstrap circuit. Typically, a capacitor and a diode are used in this process. The capacitor stores energy, while the diode prevents reverse current flow. When the switching frequency is high, the voltage at the output becomes the sum of the input voltage and the capacitor's voltage, effectively acting as a voltage booster.
Although the term "bootstrap circuit" is commonly used in practice, it is not a formal concept in theoretical electronics. It is primarily applied in class A and B single-supply complementary symmetrical amplifier circuits. While these circuits can theoretically produce an output voltage up to half of the supply voltage (Vcc), in reality, the output is often much lower due to limitations in voltage swing. This is where the bootstrap circuit comes into play, helping to achieve a higher output voltage than Vcc.
**Common Bootstrap Circuits**
According to Fairchild’s application note AN-6076, "Design and Use Guidelines for Bootstrap Circuits for High-Voltage Gate Driver ICs," there are standard configurations for bootstrap circuits that are widely used in high-voltage gate driver applications.
**Switching DC Boost Circuit Principle**
A boost converter, or step-up converter, is a type of switching power supply that produces an output voltage higher than its input. The basic configuration includes an inductor, a switch (such as a transistor or MOSFET), a diode, and a capacitor. The operation involves 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 diode blocks the capacitor from discharging back to ground. As the inductor charges, it stores energy in its magnetic field. The rate at which the current increases depends on the 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. This spike is directed through the diode to charge the capacitor, resulting in a higher output voltage. This cycle repeats, maintaining a continuous current and gradually building up the output voltage above the input level.
**Simple 1.5V to 9V Power Supply Circuit**
This circuit is designed as a pulse oscillation boost circuit. Transistors BG1, along with inductors L1, L2, and capacitor C1, form the oscillator. BG1 acts as a switching transistor, operating in a pulsed mode. L1 and C1 provide feedback for oscillation, while L2 serves as an energy storage winding. An automatic electronic switch composed of BG3 ensures the circuit only operates when needed, eliminating the need for a separate power switch.
When the load is connected to points A and B, BG3 turns on, activating BG2, which in turn activates BG1. Energy is stored in L2 during the on-time, and when BG1 turns off, the inductor generates a higher voltage, which is rectified by D1 and supplied to the load. If the output voltage exceeds the threshold set by D2, BG2 limits the base current of BG1, reducing the oscillation and stabilizing the output.
**Component Selection and Production Tips**
BG1 should be an NPN silicon transistor with low saturation voltage, such as 9013 or 8050, with a collector current of at least 300mA and a beta of over 200. BG2 can be a PNP transistor like 9012 or 9015, and BG3 should be an NPN transistor like 9014. The smaller the leakage current, the better. L1 and L2 are wound on a 8mm high-frequency ferrite ring using 0.1mm enameled wire, with L1 having 6 turns and L2 having 36 turns.
This circuit was tested to power a DT890A digital multimeter. At 1.5V input, the working current was below 45mA in the buzzer mode and under 25mA in other modes. Even when the battery voltage dropped to 0.9V, the circuit remained functional except for the buzzer mode. It is simple, stable, and economical, requiring no adjustment once properly wired.
**Conclusion**
Using a 1.5V battery to power a 9V device typically requires a separate power switch, which can be inconvenient. However, this circuit automatically controls the power based on the presence of current in the multimeter, eliminating the need for an external switch. This makes it ideal for portable and low-power applications.
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