1. Describe several indicator forms that the input voltage affects the output voltage

(1) Voltage stabilization coefficient

①Absolute voltage regulation coefficient K

When the load is constant, the ratio of the output DC voltage change △Uo of the regulated power supply to the input power grid voltage change △Ui, namely K=△Uo/△Ui.

②Relative voltage regulation coefficient S

Indicates the ratio of the relative change △Uo/Uo of the output DC voltage Uo of the regulator to the relative change △Ui/Ui of the input grid voltage Ui when the load is constant, that is, S=△Uo/Uo/△Ui/Ui.

(2) Grid adjustment rate

When the input grid voltage changes by +/-10% from the rated value, the relative change in the output voltage of the regulated power supply is 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 in output voltage △Uo/Uo (percentage) caused by the change of the input voltage within the specified range is called the voltage stability of the regulator.

2. Several indicators of the influence of load on output voltage

(1) Load regulation rate (also called current regulation rate)

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 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 load current change △IL, then the output resistance is Ro=|△Uo/△IL|Ω.

3. Several indicator 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) is usually expressed in terms of peak value or effective 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 (for example, 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, and VDE safety regulations emphasize the requirements for the surface and space distances between live parts and between live parts and non-charged metal parts.

UL and CSA requirements: between high-voltage conductors with an inter-electrode voltage greater than or equal to 250VAC, as well as between high-voltage conductors and non-charged metal parts (not including between conductors), whether between surfaces or spaces, there should be 0.1 Mu Ho; VDE requires 3mm creep or 2mm clearance between AC lines; IEC requires: 3mm clearance between AC lines and 4mm clearance between AC lines and grounding conductors. In addition, VDE and IEC require a space of at least 8mm between the output and input of the power supply.

②Dielectric test method

Hit high voltage: between input and output, input and ground, and input AC.

③Leakage current measurement

Leakage current is the current flowing through the ground wire on the input side. In the switching power supply, it is mainly the leakage current through the bypass capacitor of the noise filter. Both UL and CSA require that the exposed non-charged metal parts should be connected to the earth, and the leakage current measurement is by connecting a 1.5kΩ resistor between these parts and the earth, and the leakage current should not be greater than 5 millimA.

VDE allows 1.5kΩ resistance and 150nPF capacitor to be connected in parallel, and 1.06 times the rated voltage is applied. 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 resistance of 7MΩ between the input and the low-voltage output circuit, and between the accessible metal part and the input, there should be a resistance of 2MΩ or add 500V DC voltage for 1min.

⑤Printed circuit board

It is required to use UL-certified 94V-2 material or better.

(2) Safety requirements for power transformer structure

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 materials such as porcelain and lacquer.

②The dielectric strength of the transformer

In the experiment, there should be no insulation cracking and arcing.

③Insulation resistance of transformer

The insulation resistance between the transformer windings is at least 10MΩ. Apply a 500 volt DC voltage between the windings and the magnetic core, frame, and shielding layer for 1 min. There should be no breakdown or arcing.

④Transformer humidity resistance

After the transformer is placed in a humid environment, the insulation resistance and dielectric strength test must be carried out immediately and meet the requirements. The humidity 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 1%. The above experiment should be carried out immediately after being placed in the interior for at least 48 hours. At this time, the temperature of the transformer itself should not be 4°C higher than the test before entering the humid environment.

⑤VDE requirements on the temperature characteristics of transformers.

⑥ UL and CSA requirements on 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 and not cause unbearable electromagnetic interference to anything in the environment.

There are generally two ways of propagation of electromagnetic interference waves, which should be evaluated according to each way. One way is to propagate to the power line in a longer wavelength band to interfere with the emission area, generally below 30MHz. This kind of longer wavelength frequency is less than 1 wavelength within the length of the power cord attached to the electronic device, and the amount of its radiation into the space is also very small, so the voltage generated on the LED power cord can be grasped, and then Fully evaluate the size of the interference, this kind of noise is called conducted noise.

When the frequency reaches above 30MHz, the wavelength will also become shorter. At this time, if only the voltage of the noise source that occurs in the power line is evaluated, it does not match the actual interference. Therefore, a method of evaluating the magnitude of noise by directly measuring the interference waves propagating into the space is adopted, and the noise is called radiated noise.

Methods of measuring radiated noise include a method of directly measuring interference waves in a propagation space according to electric field strength and a method of measuring the power leaked to the power line.

The electromagnetic compatibility test includes the following test contents:

①Magnetic sensitivity

(Immunity) The degree of undesirable response of equipment, sub-systems or systems exposed to electromagnetic radiation. The lower the sensitivity level, the higher the sensitivity and the worse the immunity. Including fixed frequency, peak-to-peak magnetic field test.

② Electrostatic discharge sensitivity

Charge transfer caused by objects with different electrostatic potentials close to each other or in direct contact. The 300PF capacitor is charged to 15000V and discharged through a 500Ω resistor. It can be out of tolerance, but it should be normal after the release. After the test, the data cannot be lost during transmission and storage.

③ Transient sensitivity of LED power supply

Including spike 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 the radiated interference field that causes equipment degradation. (14kHz～1GHz, electric field intensity is 1V/M).

⑤ Conduction sensitivity

When causing an undesired response of the device or degrading its performance.

Measurement of interference signals or voltages on power, control or signal lines (30Hz～50kHz/3V, 50kHz～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 up, down, left and right AC magnetic flux density is less than 0.5mT.

⑧ Conducted interference The interference that propagates along the conductor. 10kHz～30MHz, 60(48)dBμV.

⑨Radiation interference: electromagnetic interference propagated in the form of electromagnetic waves through space.

10kHz～1000MHz, 30 shielded room 60 (54) μV/m.