description:
The power supply of ATX (computer motherboard) usually adopts pulse width modulation type switching power supply. This power supply has the characteristics of low power consumption, high conversion efficiency, reliable operation, perfect protection and wide voltage regulation range. The circuit is mainly composed of input filter circuit. The inverter, the pulse width modulation circuit, the protection circuit and the output circuit are partially composed.
- (1) The input filter circuit includes an input filter, a rectifier, and a smoothing filter circuit. The input filter is located at the input end of the power supply circuit, and the filter capacitor and the inductor form a π-type filter circuit for filtering high-frequency clutter components in the AC input voltage, preventing high-frequency interference in the power grid from entering the power supply, and suppressing the switching power supply. The impact on the grid. The rectifier uses a bridge rectifier diode to rectify the input AC voltage into a DC voltage for DC/DC conversion by the inverter. The smoothing filter circuit converts the rectified pulsating DC voltage into a smooth DC voltage and suppresses high frequency interference.
- (2) The inverter is a self-excited feedback type oscillation circuit composed of a switching transistor, a transformer, a resistor, a capacitor, etc., and its function is to convert an unstable DC voltage into a high-frequency pulse voltage. In this circuit, when a DC voltage is applied to the switching transistor, the switching transistor is continuously turned on and off under the control of the PWM controller, and a high-frequency pulse is generated in the primary winding of the transformer, and is coupled to the output circuit via the transformer coupling.
- (3) The pulse width modulation circuit is a feedback circuit that controls the output pulse width by using an error voltage. The circuit generates an error voltage by detecting a change in the output voltage, and feeds the error voltage to the inverter to control the on-time of the switching transistor to change the width of the output pulse, thereby maintaining the stability of the output voltage. The pulse width modulation circuit usually consists of an optocoupler and a PWM controller. The optocoupler feeds back the error voltage taken out from the output circuit to the PWM controller, and the PWM controller controls the turn-on and turn-off of the switching transistor. When the output voltage rises, the current fed back to the PWM controller increases, and the PWM controller shortens the on-time of the switching transistor, that is, narrows the width of the output pulse, thereby causing the output voltage to drop and maintaining the output voltage. stable.
- (4) The protection circuit of the switching power supply consists of an overcurrent protection circuit, an overvoltage protection circuit, an overheat protection circuit and a surge absorbing circuit. The resistor connected in series to the emitter of the switching transistor constitutes an overcurrent protection circuit for preventing overcurrent of the switching transistor when the switching power supply is started. The resistor and capacitor connected in parallel on the primary winding of the transformer constitute a surge absorbing circuit for venting the energy accumulated on the leakage inductance of the transformer, and protecting the switch tube from being broken down.
- (5) The output circuit is mainly composed of a high frequency rectifier, a smoothing filter circuit and a voltage stabilizing circuit. In the output circuit, the high-frequency rectifier rectifies the high-frequency pulse voltage outputted by the inverter into a DC voltage, and after smoothing and voltage-stabilizing, respectively outputs the voltages required by the digital satellite receiver.
The ATX's power supply typically has 3.3 V, 5 V, 12 V, and -12 V 4 voltages. 3.3 V voltage is used to supply power to demultiplexers, decoders, demodulators, etc.; 5 V voltage is mainly used to supply audio D/A converters, audio amplifiers, tuners, etc.; 20 V voltage is passed twice After regulation, the 3.3 V/12 V voltage is output under the control of the main CPU to supply 12 V to the LNB for powering the AGC circuit in the tuner.
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