Does the “bootstrap circuit” really matter? Why is it in many circuits?

The bootstrap circuit is also called a boost circuit. It uses Electronic components such as a bootstrap boost diode and a bootstrap boost capacitor to superimpose the capacitor discharge voltage and the power supply voltage, thereby increasing the voltage. several times the supply voltage.

The bootstrap circuit is also called a boost circuit. It uses electronic components such as a bootstrap boost diode and a bootstrap boost capacitor to superimpose the capacitor discharge voltage and the power supply voltage, thereby increasing the voltage. several times the 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 FET in the circuit that needs a 15V driving voltage. How to get this voltage? Just use bootstrapping. Usually, a capacitor and a diode are used. The capacitor stores the charge, and the diode prevents the current from flowing backward. When the frequency is high, the voltage of the bootstrap circuit is the voltage input by the circuit plus the voltage on the capacitor, which plays a role in boosting.

The bootstrap circuit is only a name given in practice, and there is no such concept in theory. The bootstrap circuit is mainly used in class A and B single-supply complementary symmetrical circuits. The class A and B single-supply complementary symmetrical circuit can theoretically make the output voltage Vo reach half of Vcc, but in practical 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 (taken from fairchild, instruction manual AN-6076 “Guidelines for Design and Use of Bootstrap Circuits for High Voltage Gate Driver ICs”) The boost converter, or step-up converter, is a switching DC boost converter. voltage circuit, it can be that the output voltage is higher than the input voltage.

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

The circuit will be described in two parts: charging and discharging.

2. Working principle of MOS tube bootstrap capacitor

The bootstrap capacitor, the internal high-end MOS needs to obtain a voltage higher than the VCC of the IC, which is obtained by boosting the bootstrap circuit, and the voltage is higher than the VCC, otherwise, the high-end MOS cannot be driven.

Bootstrap refers to a boost circuit composed of a switching power supply MOS tube and a capacitor, and the capacitor is charged through the power supply so that its voltage is higher than VCC. The simplest bootstrap circuit consists of a capacitor, and a Diode is added to prevent the boosted voltage from flowing back to the original input voltage.

The advantage of bootstrapping is to increase the voltage by using the characteristic that the voltage across the capacitor cannot be abruptly changed.

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

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

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

Does the “bootstrap circuit” really matter? Why is it in many circuits?

Does the “bootstrap circuit” really matter? Why is it in many circuits?

Top tube closed, bottom tube open/down tube closed, top tube open

MOS tube bootstrap circuit working principle

Boost Bootstrap Circuit Principle

The bootstrap circuit is also called a boost circuit, which uses electronic components such as a bootstrap boost diode and a bootstrap boost capacitor to superimpose the capacitor discharge voltage and the power supply voltage, thereby increasing the voltage.

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

Boost circuit principle

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

The basic circuit diagram is shown in Figure 1:

Does the “bootstrap circuit” really matter? Why is it in many circuits?

charging process

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

At this point, the input voltage flows through the Inductor, and the diode prevents the capacitor from discharging to ground. Since the input is DC, the current in the inductor increases linearly at a ratio that is related to the size of the inductor. As the inductor current increases, some energy is stored in the inductor.

Does the “bootstrap circuit” really matter? Why is it in many circuits?

discharge process

As shown, this is the equivalent circuit when the switch is off (transistor is 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 will slowly change from the value at the time of charging to 0.

The original circuit has been disconnected, so the inductor can only be discharged through the new circuit, that is, the inductor begins to charge the capacitor, and the voltage across the capacitor rises. At this time, the voltage is higher than the input voltage, and the boost is completed.

Does the “bootstrap circuit” really matter? Why is it in many circuits?

In other words, the boosting process is an inductive energy transfer process. 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 during the discharge process. If this on-off process is repeated, a voltage higher than the input voltage can be obtained across the capacitor.

Does the “bootstrap circuit” really matter? Why is it in many circuits?

3. Commonly used boost circuit

P-Channel High Side Gate Driver

Direct Drive: Suitable for maximum input voltages less than the gate-source breakdown voltage of the device.
Open collector: Simple method, but not suitable for directly driving MOSFETs in high-speed circuits.
Level-Shifting Driver: For high-speed applications, it works seamlessly with common PWM controllers.

N-Channel High Side Gate Driver

Direct Drive: The simplest high-side application of a MOSFET, driven directly by a PWM controller or a ground-referenced driver, but it must meet the following two conditions:

VCC
Vdc

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

Transformer-coupled driver: Controls the gate sufficiently for an indeterminate period, but limits switching performance to some extent. However, this can be improved, it’s just that the circuit is more complicated.

Charge Pump Drivers: For switching applications, the on-time tends to be long. Due to the inefficiency of the voltage multiplier circuit, more low voltage stage pumps may be required.

Bootstrap drivers: Simple, cheap, and limited; for example, both duty cycle and on-time are limited by the refresh 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 master, you must understand and master the knowledge points of the bootstrap circuit. Today’s sharing will come here first. .

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