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Full explanation of switching power supply

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.25
Learn more about switching power supplies, start here!

What is a switching power supply？

A switching power supply is a type of power supply that utilizes modern power electronics technology to control the time ratio of switching on and off, and maintain a stable output voltage. 

Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and MOSFETs. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. At present, switching power supplies are widely used in almost all electronic devices due to their small size, lightweight, and high efficiency, and are an indispensable power supply method for the rapid development of the electronic information industry today.

According to the input and output types of switching power supplies, they can be divided into AC-DC, DC-DC, DC-AC, etc.

The function of switching power supply, such as AC-DC power supply:

●  Rectification: Convert the input AC signal into a DC signal;

●  Voltage conversion: Output the required voltage amplitude;

●  Filtering: Smoothing the input rectified voltage;

●  Adjustment: Control the output voltage to maintain a constant amplitude;

●  Isolation: Electrical isolation of the input and output parts;

●  Protection: Prevent harmful voltages such as surges from reaching the output section and causing equipment damage.

Switching power supply products are widely used in industrial automation control, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instruments and meters, medical equipment, semiconductor refrigeration and heating, air purifiers, electronic refrigerators, LCD displays, LED lamps, communication equipment, audio-visual products, security monitoring, computer cases, digital products and instruments, and other fields.


Figure 1：Various types of switching power supplies

Typical structure of switching power supply



Figure 2：Typical structure of switching power supply

A typical AD-DC switching power supply structure is shown in Figure 2.

After the input AC power enters the power supply, it first goes through the front stage rectifier circuit for rectification, and after being rectified by the full bridge rectifier diode, the voltage becomes all positive phase voltage. However, there are still significant fluctuations in the voltage obtained at this time, so it is necessary to use a high-voltage filtering capacitor for initial voltage stabilization and correct the waveform to a waveform with smaller fluctuations.

Convert direct current into high-frequency pulsating direct current, which is accomplished by the control circuit. The output part is fed back to the control circuit through a certain circuit, which is used to adjust the switching time ratio of high-frequency switching components to achieve stable output voltage. The control circuit has been integrated and made into various integrated circuits for switching power supplies.

Send the obtained pulsating DC current to a high-frequency switching transformer for voltage reduction. A low-voltage filtering circuit composed of diodes and filtering capacitors is used for rectification and filtering to obtain pure static low-voltage direct current used on the equipment.

The ideal switching power supply should work in the same way as the design and modeling performance: provide a stable and smooth output voltage, no matter how the input, load or ambient temperature changes, and have 100% energy conversion efficiency. But in fact, all kinds of components are imperfect: the load may change, the power supply may distort, and environmental changes can affect performance. In addition, the need to improve performance, efficiency, shrink size, and reduce costs further increases the complexity of power supply design.

The current development direction of switching power supplies is high-frequency, high reliability, low consumption, low noise, anti-interference, and modularization. The key technology is high-frequency, which improves the loss of secondary rectification devices.

Testing and indicators of switching power supplies

For switching power supplies in work, the equipment load and demand may vary significantly between different times. Even commercial switching power supplies must be able to withstand sudden peak currents that far exceed the average operating current. Engineers designing power supplies or systems using power supplies need to understand the working status of the power supply under various conditions and conduct detailed testing and verification of switching power supplies, such as testing the quality of input power, testing the impact of input fluctuations on output, testing the impact of load changes on output, and overall energy efficiency testing and evaluation are all necessary.

The testing parameters of switching power supply mainly include input parameters, output parameters, and energy efficiency parameters.

Input parameter 

The input parameters of a switching power supply include the following main indicators:

●  Input voltage: The voltage amplitude of the switch power supply input;

●  Input current: The amplitude of the current input from the switching power supply;

●  Input power: The power value of the switching power supply input;

●  Input frequency: The voltage frequency of the switch power supply input;

●  Input current harmonics and voltage harmonics;

●  Harmonic distortion: The percentage of the root mean square value of the harmonic content in the AC signal to the fundamental component.


Figure 3:Example of Testing Input Parameters for Switching Power Supplies

Output parameters

The output parameters of a switching power supply include the following main indicators:

1. Indicators reflecting the impact of input voltage on output voltage

●  The input voltage affects several forms of output voltage indicators:

●  Absolute voltage stabilization coefficient S=△ Uo/Uo/△ Ui/Ui;

●  Power grid adjustment rate: When the input power grid voltage changes by+-10% from the rated value, the relative change in the output voltage of the stabilized power supply is △ Uo/Uo, sometimes expressed as an absolute value;

●  Voltage stability: The voltage stability of a voltage regulator is the relative change in output voltage caused by a change in input voltage within the specified range, Δ Uo/Uo (percentage value), when the load current is maintained at any value within the rated range.


Figure 4: The impact of input voltage fluctuations on output voltage

2. Indicators reflecting the impact of input voltage on output current

The input voltage affects several forms of output current indicators:

●  Absolute voltage stabilization coefficient S=△ Io/Io/△ Ui/Ui;
●  Power grid adjustment rate: When the input power grid voltage changes by+-10% from the rated value, the relative change in the output voltage of the stabilized power supply is △ Io/Io, sometimes expressed as an absolute value;
●  Current stability: The relative change in output current caused by a change in input voltage within the specified range, Δ Io/Io (percentage value), while the load current remains at any value within the rated range, is called the current stability of the voltage regulator.


Figure 5: The impact of input voltage fluctuations on output current

3. Indicators reflecting the impact of load on output voltage

Several indicator forms of the impact of load on output voltage:

●  Load adjustment rate (also known as current adjustment rate)

●  At the rated grid voltage, the maximum relative change in output voltage △ Uo/Uo when the load current changes from zero to maximum, often expressed as a percentage or sometimes expressed as an absolute change.

●  The maximum relative change △ Io/Io of the output current at rated load, no-load, or minimum load under rated grid voltage, commonly expressed as a percentage, sometimes also expressed as an absolute change.


Figure 6: The impact of load fluctuations on output voltage and current

4. Ripple voltage

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

Ripple coefficient Y (%);

The ratio of the effective value Urms of the output ripple voltage to the output DC voltage Uo at rated load current, i.e. y=Urms/Uo x 100%

Noise is different from ripple. Ripple is a component that appears between the output terminals and is synchronized with the input frequency and switching frequency. It is expressed as a peak to peak value, usually below 0.5% of the output voltage; Noise is a high-frequency component that occurs outside of the ripple between output terminals and is also represented by peak to peak values, typically around 1% of the output voltage. Ripple noise is a combination of the two, expressed as peak to peak values, typically below 2% of the output voltage.

5. Response time

Response time refers to the adjustment time during which the output voltage of a voltage regulator reaches a new stable value when the load current suddenly changes.

In DC voltage regulators, this characteristic is represented by the output voltage waveform when the rectangular wave load current is applied, which is called the transition characteristic.

6. Distortion

This is unique to AC voltage regulators. It refers to the waveform distortion caused by the output waveform not being a sine waveform, which is called distortion.

The distortion of the output waveform is mainly due to the presence of harmonics in the signal.

Harmonics can have an impact on devices connected to power lines.

7. Temperature drift and temperature coefficient

Temperature drift: Changes in environmental temperature affect the parameters of components, resulting in changes in the output voltage of the voltage regulator. The commonly used temperature coefficient represents the magnitude of temperature drift.

Absolute temperature coefficient: The change in output voltage value caused by a temperature change of 1 degree Celsius, Δ UoT, in units of V/℃ or mV/℃.

Relative temperature coefficient: A temperature change of 1 degree Celsius causes a relative change in output voltage △ UoT/Uo, measured in V/℃.

Only in high-precision voltage regulators, there are two indicators: temperature coefficient and temperature drift.

8. Drift

When the input voltage, load current, and ambient temperature of a voltage regulator are maintained at a certain level, the stability of component parameters can also cause changes in the output voltage. Slow changes are called drift, fast changes are called noise, and between the two are called fluctuations.

There are two methods to represent drift:

The change in output voltage value △ Uot within the specified time;

The relative change in output voltage △ Uot/Uo within a specified time;

The time for investigating drift can be set at 1 minute, 10 minutes, 1 hour, 8 hours, or longer.

Energy efficiency indicators

The energy efficiency index of switching power supplies is an important type of testing index and a necessary condition for market access of switching power supplies. Energy efficiency indicators include the following indicators:

Efficiency

Po is the output power, Pi is the input power

The output power value of multiple channels is the total power output of each channel

Efficiency is the ratio of output power to input power: η =  Po/Pi

Power factor

In AC circuits, the phase difference between voltage and current（ Φ) The cosine of is called the power factor, denoted by the symbol cos Φ In numerical terms, power factor is the ratio of active power to apparent power, i.e λ= P/S.

Standby power consumption

The input power of the power supply is connected to the power grid, and the output power is not connected to the load and does not consume the input active power in the state of power consumption.

Switching power supplies for different applications need to meet certain energy efficiency standards. The energy efficiency standard, also known as the energy utilization efficiency standard, is a standard that specifies the energy utilization efficiency level or energy consumption level of energy products over a certain period of time. Energy efficiency standards have high social and economic benefits. The current energy efficiency standards mainly include:

EU market

●  ErP Directive
●  Eco Label
●  Energy Efficiency Class Label

North American market

●  Energy Star
●  CEC

Australian market

●  MEPS
●  Energy Rating Label

Chinese market 

●  Energy efficiency level label

Summary

Switching power supplies play a very important role in modern electronic devices. Its high efficiency and stability make it a widely used power source choice in various fields. In the future development, switch mode power supplies will continue to improve efficiency and stability, better meeting the needs of various electronic devices.
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