Switch solenoids can be classified into several types. Firstly, there are direct-acting solenoids, which operate with a straightforward mechanism for simple and quick switching. Secondly, pull-type solenoids exert a pulling force to achieve the switching action. Another type is the push-type solenoids, which provide a pushing force for the switch. Additionally, there are latching solenoids that can maintain their position even when power is removed. Finally, rotary solenoids are designed to produce rotational movement for specific switching requirements.
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Basic knowledge of circuit design: modulation method of digital power supply
Digital power modulation techniques are primarily divided into two categories: Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM). These methods control the energy transfer in power systems by adjusting either the width or frequency of the switching pulses, depending on the application requirements.
In PWM, the switching frequency remains constant while the duty cycle is adjusted by varying the pulse width. This method is often referred to as "fixed-frequency control." On the other hand, PFM maintains a constant pulse width but changes the switching frequency to regulate the output. This is known as "fixed-width control." Both approaches have their own advantages and trade-offs in terms of efficiency, noise, and complexity.
PWM can be further categorized into fixed-frequency control and variable-frequency control. Variable-frequency control includes several subtypes such as constant hysteresis loop width control, fixed on-time control, and fixed off-time control. Each of these methods has different circuit implementations and performance characteristics.
Constant hysteresis loop width control uses a Schmitt trigger to switch the power transistor on and off based on the output voltage levels. When the voltage reaches a threshold, the output flips, turning the switch off until it drops below another threshold, at which point it turns back on. This creates a stable oscillation with a consistent hysteresis window.
Fixed on-time control relies on a monostable multivibrator to set the duration for which the switch remains on. After this time, the circuit automatically resets, turning the switch off. This approach ensures a fixed on-time, allowing for predictable behavior under varying loads.
Fixed off-time control works similarly, but instead of controlling the on-time, it sets the off-time using a monostable circuit. The switch turns on again after the predetermined off-time, ensuring consistent operation across different conditions.
These variable-frequency methods are simpler in design but can lead to challenges in managing electromagnetic interference (EMI) due to the fluctuating switching frequencies.
Fixed-frequency control, however, is the most widely used technique today. It offers several benefits, including easier filter and transformer design, and greater availability of high-performance PWM controller ICs. In fixed-frequency control, a clock signal determines the switching frequency, and an error amplifier compares the output voltage to a reference. A comparator then adjusts the duty cycle based on a ramping sawtooth waveform, ensuring precise control over the power delivery.
In contrast, Pulse Frequency Modulation (PFM) adjusts the switching frequency based on the load and input voltage. Instead of maintaining a constant frequency, PFM varies the switching rate to match the system’s needs. This makes it particularly efficient under light loads, as it reduces unnecessary switching activity.
PFM typically operates in three phases: inductor current rising, falling, and remaining at zero. Two main mechanisms are used: one where the frequency and duty cycle remain fixed during active periods, and another using a one-shot timer to control the on/off cycles. This allows the system to adapt dynamically to changing conditions.
PFM includes several variations, such as Clock-Simulated PFM, Adjustable Period PFM (also known as Skip Cycle PFM), and Current-Limited PFM. All of them operate on similar principles, using feedback to determine when to activate or deactivate the power switch.
For example, in Clock-Simulated PFM, a comparator monitors the output voltage. If the voltage drops below a reference level, the system enters an "active" state, where the power switch is turned on periodically to recharge the inductor. Once the voltage rises above the reference, the system switches to an "idle" state, where the switch remains off, and the output capacitor supplies power to the load. This cycle repeats, adjusting the switching frequency based on the load demand.
Overall, both PWM and PFM have their place in digital power systems, with PWM offering better control and EMI performance, while PFM excels in efficiency under low-load conditions. Understanding the differences between these techniques helps engineers choose the best modulation method for their specific application.