"What exactly does driving knowledge fail to accomplish? And what happens when you lack the know-how? How do you ultimately harness the capabilities of a triode? What sort of enigma lies within MOS tube driver design? Too many complex questions can be daunting, right? Five questions—five solutions! Let’s break it down.
Turn down the volume.
The principle behind the Buck circuit revolves around the switching transistor Q1 turning on and off to chop the input voltage, thus achieving an output that is stepped down. The relationship can be expressed as Vout = Vin * D. Besides designing the inductor, another challenge in Buck circuits lies in the design of the drive circuit, typically implemented using discrete components. The main circuit includes a triangular wave generator, a level-adjustable circuit, a PWM generator, a MOS tube drive circuit, a bootstrap charging circuit, soft start circuitry, overcurrent protection circuit, and overvoltage protection circuit.
Firstly, how does one design a triangular wave generator? Here, we utilize the RC capacitor charging and discharging principle, selecting a hysteretic comparator to produce a triangular wave with an adjustable frequency.
As illustrated in Figure 2, in a typical comparator circuit, R9 and R10 resistors serve as a voltage divider to provide a reference level; given the comparator's internal OC output, the output requires a pull-up resistor R12; fundamentally, the triangular wave is the charging and discharging process of the capacitor. When the comparator output is high, the capacitor charges; when low, it discharges. However, analyzing Figure 2 reveals only one reference level at 6V on the positive input terminal. Once the capacitor charges beyond 6V, it outputs a low level, starting the discharge process. When the discharge voltage drops below 6V, it outputs a high level and begins charging the capacitor again, resulting in a triangular wave with a small amplitude that’s not ideal. Thus, the next step is making the reference level variable, enabling the generation of a larger amplitude triangular wave. Here, feedback plays a clever role, utilizing the high and low levels of the output to adjust the reference level at the input, as shown in Figure 3:
Taking the parameters in the figure as an example, when the output is low, R10 and R13 are connected in parallel, dividing the voltage with R9, yielding 4V. When the output is high, R12 and R13 are connected in series and then in parallel with R9, combined with R10, giving a partial voltage of 7.2V, effectively varying the positive input level with the output.
Analyzing the complete process: upon power-on, there is a DC level at the positive input terminal, with no initial voltage on the capacitor, leading to a high output, and the positive input level at 7.2V. The 12V power supply charges the capacitor via R12 and R11. When the capacitor voltage exceeds 7.2V, the negative input terminal voltage surpasses the positive input, triggering a low-level output. At this point, the positive input level shifts to 4V, and the capacitor begins discharging via R11. When the capacitor voltage falls below 4V, the output switches back to high, restarting the cycle. Adjusting the resistance of R11 allows for modulation of the triangular wave frequency within a certain range, while varying R9, R10, and R13 adjusts the triangular wave amplitude, thus realizing an output-adjustable triangular wave.
Next, consider the DC level adjustable circuit. This refers to a circuit that provides a reference level based on requirements, allowing subsequent adjustment of the duty cycle. This involves numerous timing challenges.
From a rough analysis, separate the stages of machine power-on, normal operation, shutdown, and discharge. During power-on, for circuit safety, the output duty cycle needs to be zero or less than the required duty cycle. Assuming the DC voltage level is directly applied via a resistor divider, it takes time to charge the capacitor, and the resistor is linear. To achieve a zero duty cycle, the triangular wave is connected to the positive input of the comparator, and the DC level is connected to the negative input terminal. In the normal operational phase, only the duty cycle remains within the required limit, ensuring safety. During shutdown, the DC level is zero, and the capacitor discharges, resulting in a 100% duty cycle, posing significant risks to circuit safety. Hence, the resistor divider method isn't preferred.
Using the circuit shown in Figure 4, the power supply slowly charges the capacitor through R14. Upon power-off, the voltage is quickly discharged via the freewheeling diode, achieving slow charge and rapid discharge. The entire sequence is illustrated in Figure 5. The DC level is connected to the positive input of the comparator, and the triangular wave is connected to the negative input. During startup, the triangular wave charging speed exceeds the DC level charging speed, keeping the duty cycle at zero; during shutdown, the DC level discharge speed surpasses the triangular wave discharge speed, maintaining the duty cycle at zero, aligning with the original design intent. However, the circuit in Figure 4 has a drawback: during normal operation, when the DC level decreases, the capacitor discharge is slow due to slip resistance, requiring improvement. The enhanced circuit is shown in Figure 6:
With the emitter follower circuit, the e-pole level changes with the b-pole voltage. When the b-pole voltage decreases, the voltage across the eb exceeds 0.7V, placing the transistor in an amplification state, rapidly reducing the voltage through the ec loop discharge. At this stage, the level adjustable circuit is complete.
Next, the DC level signal and the triangular wave signal are input into the positive and negative input terminals of the comparator, respectively, producing a stable PWM wave, as depicted in Figure 7.
The circuit in Figure 7 generates a small-current PWM wave. To enable the MOS to switch on and off quickly, a high-current driving circuit is necessary. The principle of controlling a large current with a small current from a transistor is employed, adopting a push-pull output mode, as shown in Figure 8:
When the PWM input is high, N-tube Q2 turns on, and P-tube Q3 has a 0.7V negative voltage to ensure cutoff. The 12V power supply provides a large current through the current-limiting resistor R3 to charge the MOS tube inter-electrode capacitance, facilitating rapid opening. The size of the charging current is determined by resistor R3, which generally does not exceed tens of ohms. Resistor R4 serves to protect the MOS tube. When the circuit is not operating, it provides a discharge path for the inter-electrode capacitance, with the resistance not being too small. When the PWM input is low, the P-tube turns on, and the N-tube turns off, quickly releasing the energy stored in the inter-electrode capacitance through R3 and Q3. Resistor R17 prevents the comparator from malfunctioning or providing insufficient voltage, offering another loop for the IB current of Q3, ensuring quick discharge of the MOS tube inter-electrode capacitance energy.
It is important to note that the 'ground' in the above circuit is not the ground of the entire Buck circuit but the 'floating ground' of the potential at the S terminal of the MOS transistor, varying with the S terminal's potential. Ground is relative rather than absolute."
This version adds depth and clarity, ensuring it reads naturally while meeting the character count requirement.
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