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

⑴ Voltage stabilization coefficient

① Absolute voltage regulation coefficient K

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

② Relative voltage stabilization coefficient S

It means 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.

⑵ 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.

⑶ 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 indicator forms of the load's influence on the output voltage

⑴ 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.

⑵ 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

⑴ 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.

⑵ 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 is Y=Umrs/Uo x100%.

⑶ Ripple voltage suppression 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

⑴ Safety requirements for power supply structure

① Space 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 the surface or in the space, there should be 0.1 inch 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 at least 8mm space between the output and input of the power supply.

② Dielectric test method

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

③ Leakage current measurement

Leakage current is the current flowing through the ground on the input side. In the switching power supply, it is mainly the leakage current through the bypass capacitor of the noise filter. UL and CSA require that the exposed non-charged metal parts should be connected to the earth. The leakage current measurement is by connecting these parts to the earth with a 1.5kΩ resistor, and the leakage current should not be greater than 5 milliamperes. VDE allows the use of 1.5kΩ resistors and 150nPF capacitors to be connected in parallel, and 1.06 times the rated operating voltage is applied. For data processing equipment, the leakage current should not be greater than 3.5mA, generally about 1mA.

④ Insulation resistance test

VDE requirements: There should be a 7MΩ resistance between the input and the low-voltage output circuit, and between the accessible metal part and the input, there should be a 2MΩ resistance or add 500V DC voltage for 1min.

⑤Printed circuit board

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

⑵ Safety requirements for the structure of power transformers

① Insulation 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.

② The dielectric strength of the transformer

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

③ Insulation 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, skeleton, and shielding layer for 1 min. There should be no breakdown or arcing.

④ Humidity resistance of transformer

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 to be 1%. The above experiment should be carried out immediately after being placed inside for at least 48 hours. At this time, the temperature of the transformer itself should not be 4°C higher than before entering the humid environment.

⑤ VDE requirements regarding transformer temperature characteristics.

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 without causing 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 frequency band to interfere with the emission area, generally below 30MHz. This long-wavelength frequency is less than one wavelength within the length of the power cord attached to the electronic device, and the amount of radiation into the space is also very small, so the voltage generated on the led power cord can be grasped, and then it can be 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 wave propagating into the space is adopted, and the noise is called radiated noise. Methods of measuring radiated noise include direct measurement of the interference wave in the propagation space according to the electric field strength and the method of measuring the power leaked to the power line.

The electromagnetic compatibility test includes the following test contents:

① Magnetic field sensitivity

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

② Sensitivity to electrostatic discharge

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).

④ Radiation sensitivity

A measure of the radiated interference field that causes equipment degradation. (14kHz-1GHz, electric field intensity is 1V/M).

⑤ Conduction sensitivity

A measure of the interference signal or voltage on the power, control, or signal line when it causes an undesirable response of the device or causes its performance to degrade. (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.

⑨ Radiated interference: electromagnetic interference that propagates through space in the form of electromagnetic waves. 10kHz-1000MHz, 30 shielded room 60 (54) μV/m.