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 a bulky filter. This is because the adjustment transistor operates in a linear amplification mode, which means it must handle a significant voltage difference between its collector and emitter to maintain stable output. As a result, the adjustment transistor consumes a lot of power, leading to low efficiency—usually around 45%. Additionally, due to the high power consumption, these power supplies need high-power transistors and large heat sinks, making them unsuitable for the compact and efficient designs required by today's electronic devices.
On the other hand, switching power supplies offer better efficiency, smaller size, and lighter weight compared to their linear counterparts. With the continuous advancement and maturation of pulse width modulation (PWM) and resonant conversion technologies, high-frequency switching power supplies have become increasingly lightweight, compact, and efficient, making them the most widely used type of power supply in modern applications.
Constant current source design involves a systematic approach. The overall system is considered first, followed by the design of individual components, then the integration of auxiliary functions, and finally testing and optimization. A switching power supply typically consists of several key parts: an input rectification and filtering circuit, a high-frequency transformer, an output rectification, freewheeling, and filtering circuit, a protection circuit, a feedback circuit, a control circuit, and a power switch.
The input rectification and filtering circuit helps eliminate noise from the power grid and provides the necessary DC voltage for the rest of the system. The high-frequency transformer plays a crucial role by providing electrical isolation, voltage transformation, energy storage, and controlling current or resistance within the circuit. The output rectification, freewheeling, and filtering circuits convert the high-frequency AC from the transformer into stable DC, while also removing unwanted noise.
The feedback circuit can be either voltage-based or current-based. It compares the sampled output values with reference values from the controller and adjusts the output accordingly. The controller uses this feedback to regulate the output current and voltage, ensuring stability. The power switch is controlled by the PWM signal from the controller, allowing precise regulation of the duty cycle and, thus, the output.
Constant current and constant voltage sources are closely related and can complement each other. Their responses differ based on what the sampling circuits detect. A constant voltage source must continuously monitor and adjust the output voltage to remain stable regardless of load changes, whereas a constant current source adjusts the output voltage based on load variations to maintain a consistent current. The current sampling circuit usually measures the voltage across a shunt resistor after I/V conversion to achieve this.
In a switching constant current source, the process begins with converting the AC input to DC through rectification and filtering. The DC voltage is then applied to the primary winding of the switching transformer via a high-frequency PWM signal. The secondary winding induces a high-frequency voltage, which is rectified and filtered before being supplied to the load. The output is monitored through a current sampling circuit, where the current is converted into a voltage signal using a sampling resistor. This signal is then fed back to the PWM control chip, which adjusts the duty cycle to maintain a stable output current. To prevent noise interference, an RC filter may be added to the current signal input.
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