Switch transistor
As a contactless fast electronic switch in switching circuit, it decides whether the current flows or not according to the control signal, which plays an important role in active power factor correction circuit and primary circuit of power stage.
PCBA components of power board switch transistor
In the picture, the top is the common n MOSFET (n-type metal oxide semiconductor field effect transistor) in the power supply, and the bottom is the NPN BJT (NPN double junction transistor).
With the circuit composition of the switch module, different power levels can be formed, such as double crystal forward type, half bridge type, full bridge type, push-pull type, etc.
transformer
The reason why it is called isolated switching step-down power supply is that using transformer as high and low voltage separation and using magnetic energy for energy exchange can not only avoid the risk of leakage in high and low voltage circuit fault, but also simply produce a variety of voltage output. Because of its high operating frequency, the volume of transformer is smaller than that of general AC transformer.
Power board PCBA components transformer
Because the transformer is one of the power transmission paths, the current large output power supply has the design of using multiple transformers to avoid the saturation phenomenon of a single transformer and limit the power output. In the photo, the smaller transformer at the top is the pulse transformer for auxiliary power supply circuit and signal transmission, and the larger transformer at the bottom is the main power transformer and the ring secondary side adjustment transformer.
Taking the transformer as the isolation boundary, the output voltage of the secondary side is much lower than that of the primary side, but it needs rectification, adjustment, filtering and smoothing circuits to become the DC voltage required by computer parts.
diode
Inside the power supply, different types and specifications are used according to the circuit requirements and output size of each part. In addition to the general silicon diode, there are Schottky barrier diode (SBD), fast recovery diode (FRD), zener diode (ZD), etc.
PCBA components of power board diode
The picture shows the common packaging form of diode.
FRD is mainly used for active power factor correction and primary circuit of power stage; SBD is used in the secondary side of the power stage to rectify the output of the transformer; ZD is used as voltage reference.
Inductor
With the different structure of magnetic core, inductance value and installation position on the circuit, inductor can be used as energy storage component in switching circuit, voltage adjustment component of magnetic amplification circuit and output filter after secondary rectification, which is widely used in power supply.
Power board PCBA components inductor
The inductors in the picture are circular and cylindrical in shape. The number of turns and the thickness of enameled wire vary with the inductance and current bearing capacity.
capacitor
Like inductors, capacitors are also used as energy storage components and ripple smoothing. In order to withstand the rectified HVDC, the high voltage electrolytic capacitor is used in the primary circuit of the power supply; In order to reduce the loss caused by continuous charging and discharging of the output electrolytic capacitor, a large number of electrolytic capacitors with high temperature resistance, long life and low impedance are used in the secondary circuit.
PCBA components of power board capacitor
The larger one at the bottom of the picture is the high voltage electrolytic capacitor used in the primary side, while the lower one at the top is used in the secondary side and peripheral control circuit.
Due to the chemical substance (electrolyte) in the capacitor, the working temperature has a considerable impact on the service life of the electrolytic capacitor. Therefore, in addition to maintaining the good heat dissipation of the power supply, the brand and series of electrolytic capacitors used also determine the reliability and service life of the stable operation of the power supply.
resistor
The resistor is used to limit the current flowing through the circuit and release the charge stored in the capacitor after the power supply is turned off to avoid electric shock accident.
PCBA components of power board resistor
In the picture, the left side is a high-power cement resistor, which can withstand large power excess, while the right side is a common resistor, whose color code indicates its resistance value and error.
If the circuit composed of the above components is not matched with the control circuit, it can not play its function, and each output also needs to be monitored and managed at any time. When any abnormality occurs, it is necessary to cut off the output immediately to protect the safety of computer components.
Various control ICs
The control IC in the power supply can be divided into PFC circuit, power stage primary side PWM circuit, PFC / PWM integrated control, auxiliary power circuit integrated components, power monitoring and management IC, etc. according to its installation location and purpose.
PCBA components of power board - various control ICs
PFC circuit: as active power factor correction circuit control, so that the power supply can maintain a certain power factor, and reduce the generation of higher harmonics.
Power stage primary side PWM circuit: as power stage primary side switch crystal drive, PWM (pulse width modulation) signal generation, with the power output state of its duty cycle control. Generally, there are UC3842 / 3843 series PWM control IC.
PFC / PWM integrated control: the combination of the above two controllers in a single IC can simplify the circuit, reduce the number of components, reduce the volume and reduce the failure rate. For example, the common cm680x series is PFC / PWM integrated control IC.
Integrated components for auxiliary power supply circuit: since the auxiliary power supply circuit still needs continuous output after the power is turned off, it must form an independent system. Because its output wattage does not need to be too high, the industry's low-power integrated components are used as its core, such as the TOPSwitch series of PI.
Power monitoring and management IC: monitor and protect UVP (low voltage protection), OVP (over voltage protection), OCP (over current protection), SCP (short circuit protection), OTP (over temperature protection) of each output. When it exceeds its set value, it will close and lock the control circuit, stop the output of power supply, and restart after troubleshooting.
In addition to the above components, there are other ICs that manufacturers add on their own, such as fan control IC, etc.
Optical coupler
optical coupler is mainly used for signal transmission between high-voltage circuit and low-voltage circuit, and to maintain its circuit isolation, so as to avoid abnormal current flow between high-voltage and low-voltage circuits in case of failure, which will damage low-voltage components. The principle is to use light-emitting diodes and phototransistors to transmit signals by light, and because there is no circuit connection between them, the isolation of circuits at both ends can be maintained.
PCBA components of power board optical couple
Summary: the electronic components often involved in PCBA processing of power board are generally of the following types: AC input socket, cross line filter capacitor, line bypass capacitor, cross inductance, fuse, negative temperature coefficient resistance, metal oxide varistor, bridge rectifier, switching transistor, transformer, diode, optical coupler, etc
