Principles of Operation for DC Regulated Power Supplies
A DC stabilized power supply is a device that converts 220V AC power into a stabilized output DC voltage. It requires four stages of transformation, rectification, filtering, and stabilization to complete. The working principle of the four stages is as follows:
(1) Power transformer: It is a step-down transformer that converts the 220V AC voltage of the power grid into the required AC voltage and sends it to the rectifier circuit. The transformation ratio of the transformer is determined by the secondary voltage of the transformer.
(2) Rectification and filtering circuit: The rectification circuit converts the AC voltage Ui into a pulsating DC voltage. After filtering out larger ripple components through a filtering circuit, a DC voltage U1 with smaller ripple is output. Common rectification and filtering circuits include full wave rectification filtering, bridge rectification filtering, etc.
(3) Filter circuit: It can filter out most of the AC components in the output voltage of the rectifier circuit, thereby obtaining a relatively smooth DC voltage. Each filter capacitor C satisfies RL-C=(3-5) T/2, or T is the input AC signal period, and RL is the equivalent load resistance of the rectifier filter circuit.
(4) Voltage regulator circuit: The function of a voltage regulator circuit is to stabilize the output DC voltage, which does not vary with changes in AC grid voltage and load. The commonly used integrated voltage regulators include fixed three terminal voltage regulators and adjustable three terminal voltage regulators. The commonly used adjustable positive voltage integrated regulators include the CW317 (LM317) series, whose output voltage can be adjusted from 1.25V to 37 volts. The simplest external circuit components only require a fixed resistor and a potentiometer. The chip has transition, overheating, and safe working area protection, with a maximum output current of 1.5A. The typical circuit is shown in the figure below, where resistor R1 and potentiometer R2 form an output voltage regulator. The expression for the output voltage Uo is: Uo=1.25 (1+R2/R1), where R1 is generally taken as 120-240 ohms, and the voltage difference between the output terminal and the adjustment terminal is the reference voltage of the regulator (typical value is 1.25V).
