How to test the reliability of LED power supply

LED power supply test specifications

1 Several indicators describing the impact of input voltage on output voltage

⑴ Voltage stabilization coefficient

① Absolute voltage regulation coefficient K

It indicates the ratio of the change in the DC voltage output by the regulated power supply, △Uo, to the change in the input grid voltage, △Ui, when the load remains unchanged, that is, K=△Uo/△Ui.

② Relative voltage stability coefficient S

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

⑵ Grid regulation rate

It indicates the relative change in the output voltage of the regulated power supply when the input grid voltage changes by +/-10% from the rated value. It is sometimes also expressed as an absolute value.

⑶ Voltage stability

The relative change in output voltage △Uo/Uo (percentage) caused by the load current being maintained at any value within the rated range and the input voltage changing within the specified range is called the voltage stability of the voltage regulator.

2 Several indicators of the impact of load on output voltage

⑴ Load regulation (also called current regulation)

Under rated grid voltage, when the load current changes from zero to maximum value, the maximum relative change of output voltage is usually expressed as a percentage, and sometimes as an absolute change.

⑵ Output resistance (also called equivalent internal resistance or internal resistance)

Under rated grid voltage, the output voltage changes △Uo due to the load current change △IL, so the output resistance is Ro=|△Uo/△IL|Ω.

3 Several indicators of ripple voltage

⑴ Maximum ripple voltage

The absolute value of the output voltage ripple (including noise) at rated output voltage and load current, usually expressed as a peak value or effective value.

(2) Ripple coefficient Y(%)

Under 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/Uo x100%.

⑶ Ripple voltage suppression ratio

At 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, that is: 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 specifications emphasize the distance requirements on the surface and space between live parts and between live parts and non-live metal parts. UL and CSA require that there should be a distance of 0.1 inches between high-voltage conductors with an inter-pole voltage greater than or equal to 250VAC, and between high-voltage conductors and non-live metal parts (not including wires), both on the surface and in space; VDE requires a 3mm creep or 2mm clear space gap between AC lines; IEC requires a 3mm clear space gap between AC lines and a 4mm clear space gap between AC lines and grounding conductors. In addition, VDE and IEC require a minimum of 8mm of space between the output and input of the power supply.

② Dielectric experimental test method

High voltage: between input and output, input and ground, and between input AC levels.

③ Leakage current measurement

Leakage current is the current flowing through the input side ground wire. In the switching power supply, it is mainly the leakage current through the bypass capacitor of the noise filter. UL and CSA both require that the exposed non-charged 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. The leakage current should not be greater than 5 milli-mA. VDE allows the use of a 1.5kΩ resistor in parallel with a 150nPF capacitor and applying 1.06 times the rated operating voltage. For data processing equipment, the leakage current should not be greater than 3.5mA, generally around 1mA.

④ Insulation resistance test

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

⑤ Printed circuit board

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

(2) Safety requirements for power transformer structure

① Insulation of transformer

The copper wire used in the transformer winding should be enameled wire, and other metal parts should be coated with insulating materials such as porcelain and lacquer.

② Dielectric strength of transformer

There should be no insulation rupture and arcing during the experiment.

③ Insulation resistance of transformer

The insulation resistance between transformer windings shall be at least 10MΩ. A DC voltage of 500V shall be applied between the winding and the core, frame and shielding layer for 1 minute. No breakdown or arcing shall occur.

④ Transformer humidity resistance

The transformer must be placed in a humid environment and immediately undergo insulation resistance and dielectric strength tests, and meet the requirements. A humid environment is generally: relative humidity is 92% (tolerance is 2%), the temperature is stable between 20℃ and 30℃, and the error is allowed to be 1%. The above tests must be carried out immediately after being placed in the humid environment for at least 48 hours. At this time, the temperature of the transformer itself should not be 4℃ higher than the test before entering the humid environment.

⑤ VDE requirements on 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 that environment.

There are generally two propagation paths for electromagnetic interference waves, and each path should be evaluated. One is to propagate to the power line with a longer wavelength band, which interferes with the emission area, generally below 30MHz. This longer wavelength frequency is less than one wavelength within the length of the power line attached to the electronic device, and the amount of radiation into space is also very small. From this, the voltage occurring on the LED power line can be grasped, and the size of the interference can be fully evaluated. This noise is called conducted noise.

When the frequency reaches 30MHz or more, the wavelength will also become shorter. At this time, if only the noise source voltage generated in the power line is evaluated, it will not match the actual interference. Therefore, a method of evaluating the noise level by directly measuring the interference wave propagating into space is adopted. This noise is called radiation noise. There are two methods for measuring radiation noise: a method of directly measuring the interference wave propagating into space according to the electric field strength and a method of measuring the power leaked into the power line.

Electromagnetic compatibility test includes the following test contents:

① Magnetic field sensitivity

(Immunity) The degree of unwanted response of a device, subsystem or system when exposed to electromagnetic radiation. The lower the sensitivity level, the higher the sensitivity and the worse the immunity. Includes fixed frequency, peak-to-peak magnetic field testing.

② Electrostatic discharge sensitivity

The charge transfer caused by objects with different electrostatic potentials approaching or directly contacting each other. A 300PF capacitor is charged to -15000V and discharged through a 500Ω resistor. It can be out of tolerance, but it must be normal after discharge. After the test, data transmission and storage cannot be lost.

③LED power supply transient sensitivity

Including spike signal sensitivity (0.5μs, 10μs twice), 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 can cause equipment degradation. (14kHz-1GHz, electric field strength 1V/M).

⑤ Conducted sensitivity

A measure of the amount of interference signals or voltage on power, control or signal lines that cause an undesired response from the equipment or degrade its performance. (30Hz-50kHz/3V, 50kHz-400MHz/1V)

⑥ Magnetic field interference in non-working state

The packaging 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 is interference propagating along the conductor. 10kHz-30MHz, 60(48)dBμV.

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