Detailed explanation of the definitions of various indicators in switching power supply design

1. Several indicators that describe how input voltage affects output voltage.

1. Absolute voltage regulation coefficient.

A. Absolute voltage regulation coefficient: It indicates the ratio of the DC change △U0 of the voltage-regulated power supply output to the change △Ui of the input power grid when the load remains unchanged. That is:

K= U0/ Ui.

B. Relative voltage regulation coefficient: It means the ratio of the relative change △Uo of the DC voltage Uo output by the voltage regulator to the relative change △Ui of the output power grid Ui when the load remains unchanged.

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 an absolute value.

3. 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.

1. Load regulation (also called current regulation).

Under rated grid voltage, when the load current changes from zero to maximum, the maximum relative change of output voltage is usually expressed as a percentage, and sometimes as an absolute 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, so the output resistance is Ro=|Uo/IL| ohms.

3. Several indicators of ripple voltage.

1. Maximum ripple voltage.

The absolute value 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(%).

Under rated load current, the ratio of the effective value of the output ripple voltage Urms to the output DC voltage Uo is y=Umrs/Uo x100%

3. Ripple voltage rejection ratio.

At 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 is:

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 the 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.

4. Inrush current.

Inrush current refers to the maximum 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 usually 20A-30A.

5. Overcurrent protection.

It is a power supply load protection function to avoid damage to the power supply and load caused by overload 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.

It is a function to protect the load from excessive voltage between terminals. It is generally specified as 130% - 150% of the output voltage.

7. Output undervoltage protection.

When the output voltage is below the standard value, the output voltage drop is detected or the power supply is stopped and an alarm signal is issued to protect the load and prevent misoperation. The value is usually around 80%-30% of the output voltage.

8. Overheat protection.

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: The change of ambient temperature affects the change of the parameters of the components, thus causing the change of the output voltage of the regulator. The temperature coefficient is often used to indicate the magnitude of temperature drift. Absolute temperature coefficient: The change of output voltage value caused by a temperature change of 1 degree Celsius is △UoT, the unit is V/℃ or millivolt per degree Celsius. Relative temperature coefficient: The relative change of output voltage caused by a temperature change of 1 degree Celsius is △UoT/Uo, the unit is V/℃.

10. Drift.

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:

1. The change in output voltage value △Uot within a specified time.

2. The relative change of output voltage within a specified time △Uot/Uo.

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.

11. Response Time.

It refers to the adjustment time from the beginning of the change to the new stable value of the output voltage of the voltage regulator when the load current changes suddenly. In DC voltage regulators, the output voltage waveform under rectangular wave load current is used to represent this characteristic, which is called transition characteristic.

12. Distortion.

This is unique to AC voltage stabilizers. It means that the output waveform is not a positive waveform, and the waveform is distorted, which is called distortion.

13. Noise.

According to the audible frequency regulation of 30HZ-18kHZ, this is not a problem for the switching frequency of the switching power supply, but it needs to be regulated according to needs for the power supply with a fan.

14. Input noise.

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 by the allowable input. Generally, the external pulse width is 100-800us and the external voltage is 1000V.

15. Surge.

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, arcing, etc. The value specified for communication equipment is several thousand volts, generally 1200V.

16. Static noise.

Refers to a kind of repetitive pulse static electricity that can keep the whole output circuit in normal working state when it is applied to any part of the power supply frame under rated input conditions. It is generally guaranteed to be within 5-10KV.

17. Stability.

Under the allowable conditions of use, the maximum relative change of output voltage is △Uo/Uo.

18. Electrical safety requirements (GB 4943-90).

1. 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: 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 here), there should be a distance of 0.1 inches both on the surface and in the space; VDE requires a creep of 3mm or a clear gap of 2mm between AC lines; IEC requires: a clear gap of 3mm between AC lines and a clear gap of 4mm between AC lines and grounding conductors. In addition, VDE and IEC require a space spacing of at least 8mm 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 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. And apply 1.06 times the rated operating 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 resistor between the input and the low voltage output circuit, and a 2M ohm resistor or a 500V DC voltage for 1 minute between the accessible metal part and the input.

5) Printed circuit board requirements: 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. No insulation rupture or arcing should occur during the experiment.

3) Insulation resistance of transformer. The insulation resistance between transformer windings shall be at least 10M ohms. A 500V DC voltage 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% (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 on transformer temperature characteristics.

6) UL and CSA requirements on transformer temperature characteristics.