1. Describe several forms of indicators that the input voltage affects the output voltage
(1) Voltage regulation coefficient
①Absolute voltage regulation coefficient K
It means the ratio of the output DC voltage change △Uo of the regulated power supply to the input grid voltage change △Ui when the load remains unchanged, that is, K=△Uo/△Ui.
② Relative voltage regulation coefficient S
It represents the ratio of the relative change △Uo/Uo of the output DC voltage Uo of the voltage stabilizer to the relative change △Ui/Ui of the input grid voltage Ui when the load remains unchanged, that is, S=△Uo/Uo/△Ui/Ui.
(2) Power grid adjustment rate
Indicates the relative change of the output voltage of the regulated power supply when the input grid voltage changes from the rated value by +/- 10%, sometimes expressed as an absolute value.
(3) Voltage stability
The load current is maintained at any value within the rated range, and the relative change △Uo/Uo (percentage value) of the output voltage caused by the change of the input voltage within the specified range is called the voltage stability of the voltage stabilizer.
2. Several index forms of the influence of the load on the output voltage
(1) Load regulation (also called current regulation)
Under the rated grid voltage, when the load current changes from zero to the maximum value, the maximum relative change of the output voltage is usually expressed as a percentage, and sometimes it is also expressed as an absolute change.
(2) Output resistance (also called equivalent internal resistance or internal resistance)
Under the rated grid voltage, the output voltage changes △Uo due to the change of load current △IL, then the output resistance is Ro=|△Uo/△IL|Ω.
3. Several index forms of ripple voltage
(1) Maximum ripple voltage
Under the rated output voltage and load current, the absolute value of the output voltage ripple (including noise), usually expressed in peak value or rms value.
(2) Ripple coefficient Y (%)
Under the rated load current, the ratio of the effective value Urms of the output ripple voltage to the output DC voltage Uo, that is, Y=Umrs/Uox100%.
(3) Ripple voltage rejection ratio
Under the specified ripple frequency (eg 50HZ), the ratio of the ripple voltage Ui～ in the input voltage to the ripple voltage Uo～ in the output voltage, namely: ripple voltage suppression ratio=Ui～/Uo～.
4. Electrical safety requirements
(1) Safety requirements for power supply structure
①Space requirements
UL, CSA, VDE safety regulations emphasize surface, space distance requirements between live parts and between live parts and non-live metal parts.
UL and CSA requirements: between high-voltage conductors with an inter-electrode voltage greater than or equal to 250VAC, and between high-voltage conductors and non-live metal parts (excluding wires here), no matter between surfaces or spaces, there should be 0.1 Wood ho; VDE requires 3mm creep or 2mm clearance between AC wires; IEC requirements: 3mm clearance between AC wires and 4mm clearance between AC wires and ground conductors. In addition, VDE and IEC require at least 8mm space between the output and input of the power supply.
②Dielectric experiment test method
High voltage: between input and output, input and ground, and input AC.
③Leakage current measurement
The leakage current is the current flowing through the ground wire of the input side, and in the switching power supply, it is mainly the leakage current through the bypass capacitor of the noise suppression filter. Both UL and CSA require that exposed uncharged metal parts should be connected to the ground. The leakage current is measured by connecting a 1.5kΩ resistor between these parts and the ground, and the leakage current should not be greater than 5mmA.
VDE allows a 1.5kΩ resistor to be connected in parallel with a 150nPF capacitor, and applies 1.06 times the rated operating voltage. For data processing equipment, the leakage current should not be greater than 3.5mA, generally about 1mA.
④Insulation resistance test
VDE requirements: There should be a 7MΩ resistance between the input and the low-voltage output circuit, and a 2MΩ resistance between the accessible metal part and the input or a 500V DC voltage for 1min.
⑤Printed circuit board
UL Listed 94V-2 material or better is required.
(2) Safety requirements for the structure of power transformers
①Insulation of transformer
The copper wire used in the winding of the transformer should be enameled wire, and other metal parts should be coated with insulating substances such as porcelain and paint.
②The dielectric strength of the transformer
Insulation cracking and arcing should not occur during the experiment.
③Insulation resistance of transformer
The insulation resistance between the windings of the transformer should be at least 10MΩ, and a DC voltage of 500 volts should be applied between the windings and the magnetic core, skeleton, and shielding layer for 1min, and no breakdown or arcing should occur.
④Transformer humidity resistance
The transformer must be tested for insulation resistance and dielectric strength immediately after being placed in a humid environment and meet the requirements. The humid environment is generally: the relative humidity is 92% (tolerance is 2%), the temperature is stable between 20°C and 30°C, and the error is allowed to be 1%. At this time, the temperature of the transformer itself should not be 4°C higher than that before entering the humid environment.
⑤ VDE requirements on the temperature characteristics of transformers.
⑥UL, CSA requirements for transformer temperature characteristics.
5. Electromagnetic compatibility test
Electromagnetic compatibility refers to the ability of a device or system to work normally in a common electromagnetic environment without causing unacceptable electromagnetic interference to anything in the environment.
There are generally two propagation paths for electromagnetic interference waves, which should be evaluated according to each path. One is to propagate to the power line with a longer wavelength band to interfere with the emission area, generally below 30MHz. Such a longer wavelength frequency is less than 1 wavelength within the length of the power cord attached to the electronic device, and the amount of radiation radiated into the space is also small. From this, the voltage occurring on the LED power cord can be grasped, and the voltage on the LED power cord can be grasped. Fully assess the magnitude of the interference, which is called conducted noise.
When the frequency reaches above 30MHz, the wavelength will also become shorter. At this time, if only the noise source voltage that occurs in the power line is evaluated, it does not match the actual interference. Therefore, a method of evaluating the magnitude of the noise by directly measuring the interference wave propagating into space is adopted, and the noise is called radiated noise.
There are two methods for measuring radiated noise: a method of directly measuring an interference wave propagating in space according to the strength of an electric field, and a method of measuring the power leaked to the power supply line.
The electromagnetic compatibility test includes the following test contents:
① Magnetic field sensitivity
(Immunity) The degree of undesired response of a device, subsystem or system to exposure to electromagnetic radiation. The lower the sensitivity level, the higher the sensitivity and the lower the noise immunity. Including fixed frequency, peak-to-peak magnetic field testing.
②Electrostatic discharge sensitivity
Charge transfer caused by the proximity or direct contact of objects with different electrostatic potentials. The 300PF capacitor is charged to 15000V and discharged through the 500Ω resistor. It can be out of tolerance, but it should be normal after it is finished. After the test, the data transmission and storage cannot be lost.
③LED Lamps power transient sensitivity
Including spike signal sensitivity (0.5μs, 10μs 2 times), voltage transient sensitivity (10% ~ 30%, 30S recovery), frequency transient sensitivity (5% ~ 10%, 30S recovery).
④Radiation sensitivity
A measure of radiated interference fields that degrade equipment. (14kHz～1GHz, electric field strength is 1V/M).
⑤Conduction sensitivity
When causing an undesired response of a device or causing its performance to degrade.
A measure of interfering signals or voltages on power, control or signal lines (30Hz to 50kHz/3V, 50kHz to 400MHz/1V).
⑥ Magnetic field interference in non-working state
The packing box is 4.6m, and the magnetic flux density is less than 0.525μT; 0.9m, 0.525μT.
⑦ Magnetic field interference in working state
The upper, lower, left and right AC magnetic flux density is less than 0.5mT.
⑧ Conducted interference The interference propagated along the conductor. 10kHz～30MHz, 60(48)dBμV.
⑨ Radiated interference: electromagnetic interference transmitted through space in the form of electromagnetic waves.
10kHz～1000MHz, 30 shielded room 60(54)μV/m.