Why do power supply people always work overtime? The key is: the basics are not good enough!
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The power supply is indispensable for product manufacturers. When a product has problems, the first thing to do is to check whether the power supply is normal. Today I will share with you what factors need to be considered to make a good power supply.
1. Describe the influence of input voltage on output voltage in several indicators
1. Voltage regulation coefficient
A. Voltage regulation coefficient: indicates the ratio of the output DC change △U0 of the voltage regulator to the input grid change △Ui when the load remains unchanged. That is: K=△U0/△Ui.
B. Relative voltage regulation coefficient: indicates the ratio of the relative change △Uo of the output DC voltage △Uo of the voltage regulator to the relative change Ui of the output grid Ui when the load remains unchanged. That is: S=△Uo/Uo/△Ui/Ui.
2. 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, and is sometimes expressed as
3. Voltage stability When
the load current is kept at any value within the rated range, the relative change in output voltage caused by the input voltage changing within the specified range (△Uo/Uo percentage) is called the voltage stability of the voltage regulator.
2. Several indicators of the impact of load on output voltage
1. Load regulation (also called current regulation) refers to
the relative change in output voltage when the load current changes from zero to rated grid voltage. It is usually expressed as a percentage, and sometimes as a change.
2. 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, then the output resistance is Ro=|△Uo/△IL| ohms.
3. Several indicators of ripple voltage
1. Ripple voltage
The magnitude of the output voltage ripple (including noise) at rated output voltage and load current, usually expressed as peak-to-peak value or effective value.
2. Ripple factor Y (%):
The ratio of the effective value of the output ripple voltage Urms to the output DC voltage Uo at rated load current, that is, y=Umrs/Uo x100%
3. Ripple voltage suppression ratio
At a specified ripple frequency (e.g. 50Hz), the ratio of the ripple voltage Ui~ in the output voltage to the ripple voltage Uo~ in the output voltage, i.e.: ripple voltage suppression ratio = Ui~/Uo~.
Here is a statement: noise is different from ripple. Ripple is a component that appears between the output terminals and is synchronized with the input frequency and the switching frequency. It is expressed as a peak-to-peak value and is generally less than 0.5% of the output voltage. Noise is a high-frequency component other than ripple that appears between the output terminals. It is also expressed as a peak-to-peak value and is generally around 1% of the output voltage. Ripple noise is a combination of the two, expressed as a peak-to-peak value, and is generally less than 2% of the output voltage.
Inrush current refers to the instantaneous current that flows before the input current reaches a stable state when the input voltage is connected or disconnected at a specified time interval.
It is generally 20A-30A.
5. Overcurrent protection
Overcurrent protection is a power supply load protection function to prevent damage to the power supply and load caused by excessive output current, including short circuit on the output terminal.
The given value of overcurrent is generally 110%-130% of the rated current.
6. Overvoltage protection
Overvoltage protection is a function that protects the load from excessive voltage between terminals.
It is generally stipulated as 130% - 150% of the output voltage.
7. Output undervoltage protection
When the output voltage is below the standard value, the output voltage drops or the power supply is stopped and an alarm signal is issued to protect the load and prevent misoperation.
It is usually around 80%-30% of the output voltage.
When an abnormality occurs inside the power supply or the power supply temperature rises beyond the standard due to improper use, the power supply will stop working and an alarm signal will be issued.
9. Temperature Drift and Temperature Coefficient
Temperature drift: Changes in ambient temperature affect the parameters of components, thus causing changes in the output voltage of the regulator. The temperature coefficient is commonly used to indicate the magnitude of temperature drift.
Temperature coefficient: The change in output voltage caused by a temperature change of 1 degree Celsius, △UoT, the unit is V/℃ or millivolt per degree Celsius.
Relative temperature coefficient: The relative change in output voltage caused by a temperature change of 1 degree Celsius is △UoT/Uo, the unit is V/℃.
When the input voltage, load current and ambient temperature of the voltage regulator remain constant, the stability of the component parameters will also cause changes in the output voltage. Slow changes are called drift, fast changes are called noise, and the changes in between are called fluctuations.
There are two ways to express drift:
The drift time can be set to 1 minute, 10 minutes, 1 hour, 8 hours or longer. Only in the regulator with higher accuracy, there are two indicators: temperature coefficient and temperature drift.
Response time refers to the adjustment time it takes for the output voltage of the regulator to reach a new stable value from the beginning of the change when the load current changes suddenly.
In a DC voltage regulator, this characteristic is represented by the output voltage waveform when the rectangular wave load current is applied, which is called the overcurrent characteristic.
Distortion is unique to AC voltage stabilizers. It means that the output waveform is not a positive waveform, resulting in waveform distortion, which is called distortion.
According to the audible frequency regulation of 30Hz-18kHZ, this is not a problem for the conversion frequency of the switching power supply, but it needs to be regulated according to needs for the power supply with a fan.
In order to keep the switching power supply working normally, the input noise index should be formulated according to the rated input conditions and the pulse voltage (0-peak) superimposed on the industrial frequency in addition to the allowable input.
Generally, the applied pulse width is 100-800us, and the applied voltage is 1000V.
This is to add a surge voltage to the input voltage at a specified number of times at intervals of more than 1 minute to avoid abnormal phenomena such as insulation damage, flashover, and arcing.
The value specified for communication equipment, etc. is several thousand volts, generally 1200V.
Refers to a repetitive pulse of static electricity that keeps the entire output circuit in normal working condition when applied to any part of the power supply frame under rated input conditions.
Generally it is guaranteed to be within 5-10KV.
Under the allowable use conditions, the relative change of output voltage is △Uo/Uo.
18. Electrical safety requirements (GB 4943-90)
1. Safety requirements for power supply structure
1) Spatial requirements
UL, CSA, and VDE safety specifications emphasize the distance requirements on surfaces and spaces 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 (excluding wires), both on the surface and in space; VDE requires a 3mm creep or 2mm clear gap between AC lines; IEC requires a 3mm clear gap between AC lines and a 4mm clear gap between AC lines and grounding conductors.
In addition, VDE and IEC require that there be at least 8mm of space between the output and input of the power supply.
2) Dielectric experimental test method (high voltage: input and output, input and ground, input AC between two 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 measurement is to connect a 1.5K ohm resistor between these parts and the ground. The leakage current should not be greater than 5 mA.
VDE allows: Use a 1.5K ohm resistor and a 150nP capacitor in parallel. Apply 1.06 times the rated voltage. For data processing equipment, the leakage current should not exceed 3.5 mA. Generally, it is about 1 mA.
4) Insulation resistance test
VDE requirements: There should be a 7M ohm resistance between the input and the low voltage output circuit, and a 2M ohm resistance or a 500V DC voltage for 1 minute between the accessible metal part and the input.
5) Printed circuit board requirements
: UL 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 cracking and arcing during the experiment.
3) Insulation resistance of transformer
The insulation resistance between transformer windings is at least 10M ohms. A 500V DC voltage is applied between the winding and the core, frame, and shielding layer for 1 minute. No breakdown or arcing should 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% (with a tolerance of 2%), the temperature is stable between 20 and 30 degrees Celsius, and the error is allowed to be 1%. The above test must be performed 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 degrees Celsius higher than the test before entering the humid environment.
5) VDE requirements for transformer temperature characteristics
6) UL and CSA requirements on transformer temperature characteristics.
Note:
①IEC:
International ElectrotechnICal Commission
②VDE:Verbandes
③Deutcher ElectrotechnICer
④UL:Underwriters' Laboratories
⑤CSA:CANadian Standards Association
⑥FCC:
Federal Communications Commission
19. Radio disturbance (tested in accordance with GB 9254-1998)
1. Power terminal disturbance voltage limit
2. Radiated disturbance limit
20. Electromagnetic compatibility test
ElectromagnetIC compatibility test (EMC)
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 long wavelength band, which interferes with the emission area, generally below 30MHz. This long wavelength frequency is less than 1 wavelength within the length range of the power line attached to the electronic equipment, and the amount of radiation into space is also very small. From this, the voltage occurring on the 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 the space is adopted. This noise is called radiation noise. The methods for measuring radiation noise include the above-mentioned method of directly measuring the interference wave propagating into the space according to the electric field strength and the method of measuring the power leaked to the power line.
Electromagnetic compatibility tests include the following tests:
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. Fixed frequency, peak-to-peak magnetic field
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 500 ohm resistor. It can be out of tolerance, but it must be normal after discharge. Data transmission and storage cannot be lost.
③ Power supply transient sensitivity: including spike signal sensitivity (0.5us 10us twice), voltage transient sensitivity (10%-30%, 30S recovery), frequency transient sensitivity (5%-10%, 30S recovery).
④ Radiated sensitivity: A measure of the radiated interference field that causes equipment degradation. (14K-1GHz, electric field strength is 1V/M)
⑤ Conducted sensitivity: A measure of interference signals or voltages on power, control or signal lines that cause unwanted responses or performance degradation in equipment. (30Hz-50KHZ 3V, 50K-400M 1V)
⑥ Magnetic field interference in non-working state: The magnetic flux density of the packaging box at 4.6m is less than 0.525uT, and at 0.9m it is 0.525Ut.
⑦ Magnetic field interference in working state: the AC magnetic flux density of up, down, left and right is less than 0.5mT.
⑧ Conducted interference: interference propagating along the conductor. 10KHz-30MHz 60 (48) dBuV.
⑨ Radiated interference: Electromagnetic interference propagating through space in the form of electromagnetic waves. 10KHz-1000MHz 30 Shielded room 60 (54) uV/m.
21. Environmental Experiment
Environmental testing is the process of exposing products or materials to natural or artificial environments to evaluate their performance under storage, transportation, and use conditions that they may actually encounter.
Including low temperature, high temperature, constant humidity and heat, alternating humidity and heat, collision (impact and collision), vibration, constant acceleration, storage, mildew, corrosive atmosphere (such as salt spray), sand and dust, air pressure (high or low pressure), temperature change, flammability, sealing, water, radiation (solar or nuclear), soldering, terminal strength, noise (micro-beating 65DB), etc.
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