Compared to Iron Man, whose power source is the Arc Reactor, our device relies on the core principle of energy transfer. How do we initiate this process? By mastering the intricacies of driving knowledge, we can unlock the true potential within us.
You are the hero in your own story. Let’s explore the common methods for driving MOS tubes: non-isolated direct drive, bootstrap drive, and isolated transformer drive with optocoupler isolation. In popular switching power supply topologies like Buck converters, full bridges, and half bridges, NMOS transistors are often used as switches. Sometimes, the source (S) terminals of these transistors are not grounded, causing their potential to fluctuate depending on the conduction state of the MOS tube. For proper operation, the gate signal must account for the S-terminal's potential, and the turn-on voltage for the MOS transistor typically needs to be around 4.5V. To ensure full conduction, the driving voltage should exceed 10V, ideally reaching about 12-15V. Thus, to guarantee normal operation, the G-terminal voltage must adapt to the S-terminal’s potential, ensuring a stable 12V voltage differential across the GS junction.
If we use a direct drive method, the reference level would be tied to the circuit's ground. However, when the S-terminal's potential increases, the G-terminal's voltage remains constant, preventing the MOS transistor from functioning correctly. To solve this issue, a capacitor can be introduced, with its negative terminal connected to the S-terminal and its positive terminal connected to a stable 12V power supply. This setup allows the capacitor to charge the G-terminal, achieving a stable 12V GS voltage. This concept is akin to a water level analogy: the capacitor acts like a floating reservoir, with the S-terminal’s potential representing the river's water level and the circuit’s ground as sea level. Just as the river's water level fluctuates with the weather, the capacitor ensures a constant voltage difference between the river and sea levels. This driving technique is known as bootstrap driving, with the capacitor referred to as the bootstrap capacitor and the S-terminal potential termed "floating ground." As illustrated in Figure 1 below, C5 represents the bootstrap capacitor, typically ranging from a few microfarads to 100 microfarads.
However, the circuit shown in Figure 1 is incomplete. While it powers the front-end drive via the bootstrap capacitor, it lacks a mechanism to recharge the capacitor. Assuming the circuit operates normally, only two states are relatively stable: the input voltage at 30V and the output voltage at 12V. If the input power supply runs for an extended period, there exists an 18V voltage difference (30V - 12V). Using a large current-limiting resistor results in slow charging of the capacitor due to insufficient current, hindering the startup of the drive circuit. Conversely, selecting a smaller resistor leads to excessive power loss, yet when the circuit functions properly, it provides ample charging current to stabilize the bootstrap capacitor's voltage.
Can the output voltage be directly connected to the bootstrap capacitor? No, during operation, when the MOS transistor turns off, the diode's clamping effect reduces the S-terminal's potential to -0.7V, making the G-terminal's potential less than or equal to 11.3V. This allows the output to charge the bootstrap capacitor. When the MOS transistor is on, the S-terminal's potential approaches 30V, causing the G-terminal's potential to exceed the output voltage. Direct connection would result in the bootstrap capacitor discharging back into the output, quickly depleting its charge and failing to restart the drive in subsequent cycles. Therefore, the charging process requires a directional mechanism, necessitating a fast-recovery diode.
Analyzing the circuit's operational sequence: upon power-up, the circuit fails to start because the bootstrap capacitor lacks initial voltage, preventing any output without driving. Hence, when charging initially, the power supply must charge the bootstrap capacitor until it reaches 12V, then power the front-end drive circuit. Given the 18V voltage difference, to minimize losses, a current-limiting resistor of approximately 20kΩ can be chosen, allowing a small current of about 1mA to slowly charge the capacitor. Additionally, this approach enables soft-start functionality, reducing stress. However, the charging current is only 1mA, whereas the driving circuit demands at least tens of milliamperes. Direct charging reveals that the bootstrap capacitor remains undercharged, as the current is insufficient. To address this, a switch can be incorporated. During the first power-on, the 30V input powers the bootstrap capacitor with a 1mA current. At this stage, the switch is off, isolating the rear circuit. Once the bootstrap capacitor's voltage reaches 12V, the switch turns on, enabling the bootstrap capacitor to power the drive circuit. This creates an output that supplements the bootstrap capacitor with timely energy, ensuring normal operation. The switch's primary purpose is to charge the capacitor to 12V initially and then remain open.
Following this logic, the switch transistor is selected. Since it connects to the power supply terminal, a P-channel MOSFET is chosen here. To charge to about 12V, an 11V zener diode can be placed between the base and ground. A current-limiting resistor is added to restrict current flow. However, due to the zener diode, when the bootstrap capacitor's voltage drops slightly, say to 11V, the switch turns off, and the output voltage rises to 12V, taking time to reach a higher potential than the bootstrap capacitor. This prevents further charging and halts circuit operation. The issue lies with the 11V zener diode. If it shorts when the switch is on, this problem can be resolved. Therefore, a switch is necessary here, shorting to ground, and an N-channel MOSFET is selected. As shown in Figure 2, Q5 is a PNP transistor, Q4 is an NPN transistor. Upon power-on, Q5 is off, allowing 30V to charge C5 through R5 until reaching 12V. When Q5 turns on, its collector potential reaches 11.3V, using this potential to drive Q4 on, effectively shorting it to D4. Even if the bootstrap capacitor drops to 6V, Q5 remains on, maintaining the output current to sustain the drive circuit and ensuring overall operation.
At this point, the bootstrap driver circuit for the Buck converter is designed. Due to its simplicity, bootstrap drivers are widely used in many applications, particularly in Buck converters and bridge circuits. However, from a circuit simplicity and reliability standpoint, choosing an integrated chip is more critical. Many integrated chips exist, such as IR's IR21XX series, commonly including IR2104 and IR2110, as well as TI's offerings, as depicted in Figure 3.
According to the datasheet, select an appropriate integrated chip to finalize the circuit design. After learning, I feel my knowledge ascending to new heights.
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