1. Describe several index forms that the input voltage affects the output voltage 1. Absolute voltage stabilization coefficient
A. Absolute voltage stabilization coefficient: indicates the ratio of the output DC change △U0 of the regulated power supply to the input power grid change △Ui when the load remains unchanged. That is: K= U0/ Ui.
B. Relative voltage stabilization coefficient: It means the ratio of the relative change △Uo of the output DC voltage Uo of the stabilizer to the relative change △Ui of the output power grid Ui when the load remains unchanged. That is: S= Uo/Uo / Ui/Ui
2. Grid adjustment rate
It represents the relative change in the output voltage of the regulated power supply when the input grid voltage changes by ±10% from the rated value, sometimes expressed as an absolute value.
3. Voltage stability
The load current remains at any value within the rated range, and the relative change in output voltage △Uo/Uo (percentage) caused by the input voltage change 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, 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| ohm.
Three. Several indicator forms of ripple voltage 1. Maximum ripple voltage
Under the rated output voltage and load current, the magnitude of the absolute value of the output voltage ripple (including noise) is usually expressed in terms of peak-to-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, namely: y=Umrs/Uo x100%
3. Ripple voltage rejection ratio
Under the specified ripple frequency (for example, 50HZ), the ratio of the ripple voltage Ui~ in the output voltage to the ripple voltage Uo~ in the output voltage, namely: 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 by a peak to peak value and is generally below 0.5% of the output voltage; noise appears between the output terminals. A high-frequency component other than ripple is also expressed by a peak-to-peak value, which is generally about 1% of the output voltage. Ripple noise is a combination of the two, expressed as a peak to peak value, generally below 2% of the output voltage.
4. Impulse current Impulse current refers to the maximum instantaneous current that passes before the input current reaches a stable state when the input voltage is turned on or off at a specified time interval. Generally it is 20A～30A.
5. Overcurrent protection is a power load protection function to avoid damage to the power supply and load caused by overload output current including short circuits on the output terminals. The given value of overcurrent is generally 110% to 130% of the rated current.
6. Overvoltage protection is a function of load protection for excessive voltage between terminals. It is generally specified as 130%～150% of the output voltage.
7. Output under-voltage protection When the output voltage is below the standard value, it detects the output voltage drop or stops the power supply and sends out an alarm signal to protect the load and prevent misoperation, mostly about 80% to 30% of the output voltage.
8. The overheating protection stops the work of the power supply and sends out an alarm signal when an abnormality occurs inside the power supply or the temperature rise of the power supply exceeds the standard due to improper use.
9. Temperature drift and temperature coefficient temperature drift: The change of the ambient temperature affects the change of the parameters of the components, which causes the output voltage of the regulator to change. The commonly used temperature coefficient indicates the size of the temperature drift. Absolute temperature coefficient: A temperature change of 1°C causes a change in output voltage △UoT, in V/°C or millivolt per degree Celsius. Relative temperature coefficient: the relative change of output voltage △UoT/Uo caused by temperature change of 1℃, the unit is V/℃.
10. When the input voltage, load current and ambient temperature of the drift regulator are kept constant, the stability of the component parameters will also cause changes in the output voltage. Slow changes are called drift, and fast changes are called noise. Called ups and downs.
There are two ways to express drift:
1. The change in output voltage value △Uot within a specified time.
2. The relative change of output voltage △Uot/Uo within a specified time.
The time for investigating drift can be set to 1 minute, 10 minutes, 1 hour, 8 hours or longer. Only in high-precision regulators, there are two indicators of temperature coefficient and temperature drift.
11. Response time refers to a period of adjustment time from the beginning of the change to the new stable value when the output voltage of the regulator changes suddenly when the load current changes suddenly. In the DC voltage regulator, the output voltage waveform at the time of the rectangular wave load current is used to express this characteristic, which is called the excessive characteristic.
12. Distortion This 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.
13. The noise is regulated according to the audible frequency of 30HZ～18kHZ. This is not a problem for the switching frequency of the switching power supply, but the power supply with a fan should be regulated according to the needs.
14. Input noise In order to maintain the normal operation of the switching power supply, the input noise index should be formulated according to the pulse-shaped voltage (0～peak) superimposed on the industrial frequency and outside the allowable input according to the rated input conditions. Generally, the applied pulse width is 100～800us, and the applied voltage is 1000V.
15. Surge This is to add a surge voltage to the input voltage at an interval of more than 1 minute at a specified number of times to avoid abnormal phenomena such as insulation damage, flashover, and arcing. The prescribed value for communication equipment is several thousand volts, generally 1200V.
16. Electrostatic noise refers to a kind of repetitive pulse-like static electricity that can maintain the normal working state of the full output circuit when it is applied to any part of the power supply frame under the rated input condition. Generally guarantee within 5～10KV.
17. The maximum relative change of the output voltage is △Uo/Uo under the condition of the allowable stability.
18. Electrical safety requirements (GB 4943-90) 1. Safety requirements for power supply structure
1) Space requirements. The 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, CSA requirements: Between the high voltage conductors with a voltage greater than or equal to 250VAC, and between the high voltage conductors and non-charged metal parts (not including the conductors), there should be a distance of 0.1 inches between surfaces and spaces ; 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 conductor. In addition, VDE and IEC require a space of at least 8mm between the output and input of the power supply.
2) Dielectric experiment test method (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 wire 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 is measured by connecting a 1.5K ohm resistor between these parts and the earth, and the leakage current should not be greater than 5 mA. VDE allows: Use 1.5K ohm resistor and 150nP capacitor in parallel. And apply 1.06 times the rated voltage. For data processing equipment, the leakage current should not be greater than 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 between the accessible metal part and the input, there should be a 2M ohm resistance or add 500V DC voltage for 1 minute.
5) Printed circuit board requirements. The requirement is UL-certified 94V-2 material or a better material.
2. Safety requirements for power transformer structure
1) Insulation of transformers. 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 paint.
2) The dielectric strength of the transformer. In the experiment, there should be no cracking of the insulating layer and arcing.
3) The insulation resistance of the transformer. The insulation resistance between the windings of the transformer is at least 10M ohms. A 500 volt DC voltage is applied between the windings and the magnetic core, frame, and shielding layer for 1 minute. 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 and 30 degrees Celsius, the error is allowed 1%, and 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 degrees Celsius higher than the test before entering the humid environment.
5) VDE requirements regarding transformer temperature characteristics.
6) UL, CSA requirements on transformer temperature characteristics
