There are two primary types of DC regulated power supplies commonly used in modern electronics: linear and switching power supplies. Linear power supplies are known for their excellent stability, low output voltage ripple, and reliability. However, they typically require a large, heavy power frequency transformer and bulky filters. The adjustment transistor in a linear supply operates in a linear amplification mode, which means it must handle a significant voltage difference between the collector and emitter to maintain stable output. This leads to high power consumption and poor efficiency, usually around 45%. Additionally, because the adjustment transistor dissipates a lot of heat, high-power transistors and large heat sinks are necessary, making them unsuitable for compact or lightweight electronic designs.
In contrast, switching power supplies offer superior efficiency, smaller size, and reduced weight compared to linear power supplies. With the advancement of pulse width modulation (PWM) and resonant conversion technologies, high-frequency switching power supplies have become lighter, more efficient, and more reliable. As a result, they are now the most widely used type of power supply in modern electronics.
When designing a constant current source, the process involves a systematic approach. It begins with an overall system analysis, followed by the design of individual components. Then, the overall functionality and auxiliary circuits are developed, and finally, testing and optimization are performed to ensure performance and reliability.
A switching power supply consists of several key parts: an input rectification and filtering circuit, a high-frequency transformer, a rectification, freewheeling, and filtering stage, a protection circuit, a feedback loop, a control circuit, and a power switch. The input rectifier filters out noise from the grid and converts AC to DC. The high-frequency transformer provides isolation, voltage transformation, and energy storage. The output rectification and filtering circuit ensures a clean DC output, while the feedback circuit monitors and adjusts the output based on sampled voltage or current values. The controller uses this information to regulate the output and maintain stability.
Constant current and constant voltage sources are closely related. They complement each other and can be converted into one another. The sampling circuits of these sources respond differently depending on what is being monitored. A constant voltage source tracks and controls the output voltage in real time, ensuring it remains stable regardless of load changes. A constant current source, on the other hand, adjusts the output voltage based on load variations to keep the current constant. The current sampling circuit measures the output current, often converting it to a voltage signal through an I/V conversion before feeding it back to the control circuit.
Switching constant current sources operate through a series of steps:
1. AC power is first rectified and filtered into DC.
2. A high-frequency PWM signal controls the switching transistor, applying the DC voltage to the primary winding of the transformer.
3. The secondary winding of the transformer induces a high-frequency voltage, which is then rectified and filtered to supply the load.
4. The output current is sampled via a resistor, and any change in current is converted into a voltage change. This signal is fed back to the control chip, which adjusts the duty cycle to maintain a stable current. To reduce noise, an RC filter can be added at the current signal input. A constantan wire resistor is often used for current sensing due to its low temperature coefficient.
This detailed approach ensures that the power supply delivers consistent and reliable performance, meeting the demands of modern electronic systems.
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