This article explores what the bootstrap circuit in the circuit does.

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This article explores what the bootstrap circuit in the circuit does. [Copy link]

The bootstrap circuit is also called a boost circuit. It uses electronic components such as bootstrap boost diodes and bootstrap boost capacitors to superimpose the capacitor discharge voltage and the power supply voltage, thereby increasing the voltage. The voltage increased by some circuits can reach several times the power supply voltage.

1. Principle of MOS tube bootstrap circuit
I have seen this simple example on the EDA365 electronic forum: There is a 12V circuit, and there is a field effect tube in the circuit that requires a 15V driving voltage. How to get this voltage? It is to use bootstrap. Usually a capacitor and a diode are used. The capacitor stores charge and the diode prevents current backflow. When the frequency is high, the voltage of the bootstrap circuit is the voltage of the circuit input plus the voltage on the capacitor, which plays a role in boosting. The

bootstrap circuit is just a name given in practice, and there is no such concept in theory. The bootstrap circuit is mainly used in Class A and Class B single-power supply complementary symmetrical circuits. In theory, the Class A and Class B single-power supply complementary symmetrical circuit can make the output voltage Vo reach half of Vcc, but in actual tests, the output voltage is far less than half of Vcc.

The important reason is that a voltage higher than Vcc is required. Therefore, a bootstrap circuit is used to boost the voltage. Commonly used bootstrap circuits (from fairchild, user manual AN-6076 "Design and Use Guidelines for Bootstrap Circuits for High Voltage Gate Driver ICs") The boost converter, or step-up converter, is a switching DC boost circuit that can output a higher voltage than the input voltage.

Assume that the switch (transistor or MOS tube) has been disconnected for a long time, all components are in an ideal state, and the capacitor voltage is equal to the input voltage.

The following will explain this circuit in two parts: charging and discharging.

2. Working principle of MOS tube bootstrap capacitor
The internal high-end MOS needs to obtain a voltage higher than the IC's VCC, and the voltage is obtained by boosting the bootstrap circuit to a voltage higher than VCC. Otherwise, the high-end MOS cannot be driven. Bootstrapping

refers to a boost circuit composed of a switching power supply MOS tube and a capacitor, and charging the capacitor through the power supply to make its voltage higher than VCC. The simplest bootstrap circuit consists of a capacitor. In order to prevent the boosted voltage from returning to the original input voltage, a diode is added. The

advantage of bootstrapping is that the voltage at both ends of the capacitor cannot change suddenly to increase the voltage.

For example, if the Drink voltage of MOS is 12V, the Source voltage is 0V, and the Gate driving voltage is also 12V, then when MOS is turned on, the Source voltage will increase to the Drink voltage reduction minus a very small conduction voltage drop, then the Vgs voltage will be close to 0V, and MOS will be turned off, turned on again, and turned off again after the turn-on moment.

In this way, a high-frequency pulse N times the operating frequency will pass between the Drink and Source of MOS for a long time. Such a pulse spike will generate excessive voltage stress on MOS, and the MOS tube will be damaged soon.

If a small capacitor is connected between the Gate and Source of MOS, the capacitor is charged when MOS is not turned on, and when MOS is turned on and the Source voltage increases, the Gate voltage is automatically increased, so that MOS can continue to be turned on.

The upper tube is turned off and the lower tube is turned on/the lower tube is turned off and the upper tube is on.

The working principle of the MOS bootstrap circuit.
The principle of the boost bootstrap circuit
The bootstrap circuit is also called a boost circuit. It uses electronic components such as bootstrap boost diodes and bootstrap boost capacitors to superimpose the capacitor discharge voltage and the power supply voltage, thereby increasing the voltage.

Some circuits can increase the voltage to several times the power supply voltage.

Principle of boost circuit The
principle of the switching DC boost circuit (the so-called boost or step-up circuit) The boost converter, or step-up converter, is a switching DC boost circuit that can output a higher voltage than the input voltage.

The basic circuit diagram is shown in Figure 1:

Charging process
During the charging process, the switch is closed (the transistor is turned on), and the equivalent circuit is shown in Figure 2, where the switch (transistor) is replaced by a wire.

At this time, the input voltage flows through the inductor, and the diode prevents the capacitor from discharging to the ground. Since the input is direct current, the current on the inductor increases linearly at a certain rate, which is related to the size of the inductor. As the inductor current increases, some energy is stored in the inductor.

Discharge process
As shown in the figure, this is the equivalent circuit when the switch is turned off (transistor is turned off). When the switch is turned off (transistor is turned off), due to the current retention characteristics of the inductor, the current flowing through the inductor will not immediately become 0, but slowly change from the value when charging is completed to 0. The

original circuit has been disconnected, so the inductor can only discharge through the new circuit, that is, the inductor starts to charge the capacitor, and the voltage across the capacitor increases. At this time, the voltage is higher than the input voltage, and the boost is completed.

The boost process is an energy transfer process of an inductor. When charging, the inductor absorbs energy, and when discharging, the inductor releases energy. If the capacitance is large enough, a continuous current can be maintained at the output end during the discharge process. If this on-off process is repeated continuously, a voltage higher than the input voltage can be obtained at both ends of the capacitor.

3. Commonly used boost circuits
P-channel high-side gate driver
Direct driver: Applicable when the maximum input voltage is less than the gate-source breakdown voltage of the device.
Open collector: The method is simple, but it is not suitable for directly driving MOSFETs in high-speed circuits.
Level conversion driver: Applicable to high-speed applications and can work seamlessly with common PWM controllers.

N-channel high-side gate driver
Direct driver: The simplest high-side application of MOSFET, directly driven by a PWM controller or a driver based on ground, but it must meet the following two conditions:
VCC<vgs,max< p="">
Vdc<vcc-vgs,miller< p="">

Floating power supply gate driver: The cost impact of independent power supply is very significant. Optocouplers are relatively expensive, have limited bandwidth, and are sensitive to noise.

Transformer coupled driver: The gate is fully controlled within an uncertain period, but to some extent, the switching performance is limited. However, this can be improved, but the circuit is more complicated.

Charge pump driver: For switching applications, the conduction time is often very long. Due to the low efficiency of the voltage multiplier circuit, more low-voltage stage pumps may be required.

Bootstrap driver: simple, cheap, but also has limitations; for example, the duty cycle and on-time are limited by refreshing the bootstrap capacitor.

Although the bootstrap circuit does not exist in theory, it is widely used in practice. Therefore, if you want to be a circuit expert, you must understand and master the knowledge points of the bootstrap circuit. Today's sharing ends here!

annysky2012

Very clear, I learned a lot

winway

It is important to understand bootstrapping