"What doesn't driving knowledge cover? And what happens when you're unsure of how to proceed? How exactly do you utilize a triode in practical applications? What’s this mysterious concept of MOS tube driver design? Too many tough questions can leave anyone stumped. But five questions? No problem! Let's break it down.
Check out the image below for a visual aid. The principle behind the Buck circuit revolves around the switching transistor Q1 turning on and off to chop the input voltage, allowing for an output voltage lower than the input. This relationship is expressed as Vout = Vin * D.
Designing a Buck circuit isn’t just about the inductor. One major challenge lies in the design of the drive circuit, often implemented using discrete components. Key elements include a triangular wave generator, a level adjustable circuit, a PWM generator, a MOS tube drive circuit, a bootstrap charging circuit, soft start circuit, overcurrent protection circuit, and overvoltage protection circuit.
First, how do we design a triangular wave generator? Here, we leverage the RC capacitor charging and discharging principle with a hysteresis comparator to produce a triangular wave with an adjustable frequency.
Take a look at Figure 2. In a standard comparator circuit, R9 and R10 create a reference level. Since the comparator has an OC output, a pull-up resistor R12 is necessary. Fundamentally, a triangular wave represents the charging and discharging process of a capacitor. When the comparator output is high, the capacitor charges; when low, it discharges. However, analyzing Figure 2 reveals only a single reference level of 6V. Once the capacitor reaches over 6V, it outputs a low level, starting the discharge. When the discharge drops below 6V, it outputs high again, beginning to charge, resulting in a triangular wave with a very small amplitude. So, how do we achieve a larger amplitude? Feedback comes to the rescue. Using the high and low levels of the output to adjust the input reference level, as seen in Figure 3:
Using the parameters provided, when the output is low, R10 and R13 are connected in parallel, dividing the voltage with R9 to yield 4V. When the output is high, R12 and R13 are connected in series and then in parallel with R9, combined with R10 to produce a partial voltage of 7.2V, effectively varying the positive input level with the output.
Now, let’s walk through the full process: upon powering up, there’s an initial DC level at the positive input. With no voltage on the capacitor, the output is high, setting the positive input level to 7.2V. The 12V power supply charges the capacitor via R12 and R11. Once the capacitor voltage exceeds 7.2V, the negative input surpasses the positive, 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 goes high again, starting the next cycle. Adjusting R11 allows for frequency tuning, while changing R9, R10, and R13 adjusts the triangular wave amplitude, enabling flexible output.
Next, consider the DC level adjustable circuit. This involves creating a reference level based on requirements to adjust the duty ratio. Timing plays a crucial role here.
During power-on, for safety, the output duty cycle must be zero or below the required value. Assuming a direct resistor divider approach would take time to charge the capacitor linearly. To achieve a duty cycle of zero, connect the triangular wave to the comparator’s positive input and the DC level to the negative input. During normal operation, the duty cycle remains safe and compliant. In the shutdown phase, the DC level drops to zero, and the capacitor discharges slowly, posing risks. Thus, a resistor divider isn’t ideal.
The circuit in Figure 4 uses R14 to slowly charge the capacitor. Upon shutdown, the voltage discharges quickly through the freewheeling diode, ensuring slow charge and fast discharge. Figure 5 shows this process. Connecting the DC level to the comparator’s positive input and the triangular wave to the negative ensures the duty cycle stays at zero during startup and shutdown.
However, Figure 4 has limitations. During normal operation, when the DC level drops, the capacitor discharges slowly due to slip resistance. Improving this involves adding an emitter follower circuit, as shown in Figure 6.
With an emitter follower, the e-pole level follows the b-pole voltage changes. When the b-pole voltage decreases, the voltage across the eb exceeds 0.7V, putting the transistor in an amplifying state and rapidly reducing the voltage via the ec loop discharge. Now the level adjustable circuit is complete.
Next, feed the DC level and triangular wave signals into the comparator’s positive and negative inputs, respectively, producing a stable PWM wave, as shown in Figure 7.
This circuit generates a small-current PWM wave. To ensure the MOS turns on and off quickly, a high-current driving circuit is needed. Utilizing the principle of controlling large currents with a small current from a transistor, a push-pull output mode is employed, as depicted in Figure 8.
When the PWM input is high, N-tube Q2 turns on, and P-tube Q3 cuts off due to its -0.7V bias. The 12V power supply provides a large current through resistor R3 to charge the MOS gate-source capacitance, ensuring quick opening. Resistor R3’s size determines the charging current, typically under tens of ohms. Resistor R4 protects the MOS tube, offering a discharge path when the circuit isn’t operational. R4 shouldn’t be too small. Conversely, when the PWM input is low, the P-tube turns on, and the N-tube turns off, quickly releasing the stored energy in the gate-source capacitance through R3 and Q3. Resistor R17 prevents the comparator from malfunctioning or providing insufficient voltage, creating another loop for Q3’s base current to ensure rapid discharge of the MOS tube’s capacitance.
It’s worth noting that the ‘ground’ in the above circuit isn’t the overall Buck circuit’s ground but the ‘floating ground’ of the MOS transistor’s S-terminal potential, varying with its potential. Ground is relative, not absolute."
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