With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. At present, switching power supply is widely used in almost all electronic equipment due to its small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of today’s electronic information industry.
There are two types of modern switching power supplies: one is DC switching power supply; the other is AC switching power supply.
What is mainly introduced here is only the DC switching power supply. Its function is to convert the original power supply (coarse power) with poor power quality, such as mains power supply or battery power supply, into a higher quality DC voltage that meets the requirements of the equipment. The core of the DC switching power supply is the DC/DC converter.
Therefore, the classification of DC switching power supplies relies on the classification of DC/DC converters. In other words, the classification of DC switching power supplies is basically the same as the classification of DC/DC converters. The classification of DC/DC converters is basically the classification of DC switching power supplies.
The switching power supply is roughly composed of four major parts: main circuit, control circuit, detection circuit, and auxiliary power supply.
1. Main circuit
Inrush current limiting: Limit the inrush current on the input side at the moment when the power is turned on.
Input filter: Its function is to filter the clutter existing in the power grid and prevent the clutter generated by the machine from being fed back to the power grid.
Rectification and filtering: directly rectify the grid AC power into smoother DC power.
Inverter: converting rectified DC power into high-frequency alternating current, which is the core part of high-frequency switching power supply.
Output rectification and filtering: Provide stable and reliable DC power supply according to load needs.
2. Control circuit
On the one hand, samples are taken from the output terminal, compared with the set value, and then the inverter is controlled to change its pulse width or pulse frequency to stabilize the output. On the other hand, based on the data provided by the test circuit and identified by the protection circuit, it provides The control circuit performs various protection measures on the power supply.
3. Detection circuit
Provide various running parameters and various instrument data in the protection circuit.
4. Auxiliary power supply
Realize the software (remote) startup of the power supply to provide power for the protection circuit and control circuit (PWM and other chips).
Here are some classic answers about switching power supplies:
1. If the switching power supply transformer uses copper tape instead of enameled wire, how to calculate the current allowed to pass through it? For example, for a copper strip with a thickness of 0.1mm, how to calculate the current allowed to pass?
Expert answer: If the switching power supply transformer uses copper strips instead of enameled wires, the eddy current losses of the copper strips (enameled wires) can be greatly reduced, and the operating frequency can be increased accordingly, but the DC losses will remain almost unchanged. The current density allowed to pass through the copper strips should generally not exceed 4.5A/mm². The current density is equal to the current divided by the cross-sectional area of the conductor, which is equal to the thickness (0.1mm) times the width (the width of the copper strip).
2. Are the AC circuit of the power switch and the AC circuit of the rectifier most likely to produce electromagnetic interference?
Expert answer: The most serious place where electromagnetic interference is generated by a switching power supply is the circuit composed of the primary and secondary coils of the switching transformer. However, its interference will cause radiation and conduction interference to other circuits through induction. The most serious place for conduction interference and radiation interference is It is the power cord, because the power cord can easily become a half-wave oscillator antenna of the radiation source. In addition, it is connected to external lines and can easily transmit interference signals to other devices. Therefore, the power line must be effectively isolated at the input end of the switching power supply.
3. What are the specific methods to reduce the temperature rise of the transformer?
Expert answer: One way to reduce the temperature rise of the transformer is to reduce the value of the maximum magnetic flux increment (Bm) of the transformer core, because the loss of the transformer core (hysteresis loss and eddy current loss) is proportional to the square of the magnetic flux density. Proportional; the other is to reduce the operating frequency of the switching power supply, because the loss of the transformer core (hysteresis loss and eddy current loss) is proportional to the operating frequency; the other is to reduce the loss of the coil, the loss of the coil (mainly eddy current loss), The eddy current loss and skin effect loss of the coil are also proportional to the operating frequency. To reduce the DC loss of the coil, the current density of the wire must be reduced. Generally, the current density of enameled wire cannot exceed 4.5A/square millimeter.
4. How does the duty cycle of the flyback switching power supply change?
Expert answer: The duty cycle of the flyback switching power supply is mainly determined by the input voltage and the withstand voltage of the switching power supply tube. When the input voltage changes, the duty cycle will also change. For example, when the input voltage is AC260V, if the withstand voltage of the power switch tube is 650V, the duty cycle is 0.306; when the input voltage is AC170V, the duty cycle is approximately 0.5; when the input voltage is lower than AC170V, the duty cycle The ratio is greater than 0.5. But no matter how the input voltage changes, the switching power supply will increase the value of the output voltage to stabilize (or change) by changing the duty cycle.
5. What is the main difference between forward and flyback?
Expert answer: Forward switching power supply means that when the power switch tube is turned on, the power supply provides power output to the responsible person, but there is no power output when it is turned off. The flyback switching power supply is just the opposite. When the power switch is turned on, it only stores energy in the transformer and does not provide power output to the load. It only provides output to the load when the power switch is turned off. The output voltage of the forward switching power supply is the average value of the rectified output voltage, and the output voltage of the flyback switching power supply is the average half-wave value of the rectified output voltage. The phases of the two voltage outputs are exactly opposite.
6. Can you talk about loop design in detail?
Expert answer: The gain of the feedback loop is neither bigger nor better, nor smaller is better. When the gain of the feedback loop is too high, the output voltage will track back and forth around the average value, and the output voltage will fluctuate greatly. The higher the gain, the greater the amplitude of the fluctuation. In severe cases, oscillation will occur; when the gain of the feedback loop When it is too low, the output voltage will be unstable because the voltage cannot track properly and there will be a hysteresis error.
In order to stabilize the output voltage without oscillation, the feedback loop is generally divided into three loops. One loop is used to determine the size of the differential gain, the other loop is used to determine the size of the integral gain, and the third loop is Determine the size of the DC gain. The purpose of this is that when the error signal is small, the loop gain is large, and when the error signal is large, the loop gain becomes small, that is, the gain of the error amplifier is dynamic. By carefully adjusting the gains of these three feedback loops, the switching power supply can be stable without oscillation.
7. How to minimize the flyback power switch MOS? Especially under hard switching conditions.
Expert answer: Reduce the duty cycle, but if the duty cycle is too low, the working efficiency of the power supply will be greatly reduced, and the voltage adjustment range will also be reduced.
8. What proportion of copper foil loss accounts for power loss?
Expert answer: Very small. If the copper foil loss is large, the temperature rise of the copper foil will be very high. If it exceeds 80 degrees, the paint on the copper foil will turn yellow. But it is only equivalent to the loss of a metal film resistor of about 1 to 3 watts at the same temperature rise.
9. What causes the problem of large and small driving waveforms?
I have a power supply, and the output driver is normal when the voltage is AC85-120V. When the voltage changes to 120-150V, the driver has small and large ripples, and the output current drops significantly. When the voltage increased to 150V-265V again, the frequency of the driving waveform was completely wrong, and the output was also wrong.
Expert answer: This situation will occur if your drive circuit uses a capacitor or transformer for output, because when the capacitor or transformer transmits a waveform (signal), the signal cannot contain a DC component. If it contains a DC component, the output waveform will be severely distorted. Only when the output waveform of the drive circuit has a duty cycle of 0.5, the output waveform will not be distorted. If the duty cycle is too large or too small, distortion will occur.
10. I would like to ask about the selection of rectifier bridge. What kind of rectifier bridge should I choose for different powers?
Another thing is that I made a 30W power supply and used a 3A700V rectifier bridge. I found that the rectifier bridge was very hot. The temperature reached over 60 degrees within a few minutes. What are the causes of the rectifier bridge getting hot?
Expert answer: The selection of the rectifier diode is mainly determined by the three parameters of the current flowing through the rectifier diode, the withstand voltage and the operating frequency. When designing circuit parameters, the current flowing through the rectifier diode can generally only take the nominal value ( one-third of the value at 25°C) because the operating temperature flowing through the rectifier diode may rise above 80°C. If the turn-on and turn-off speed of the rectifier diode is very low, it will still conduct for a period of time when the voltage is reversed, that is, the reverse current is very large, so the rectifier diode will also generate heat. Your rectifier bridge heating may fall into the latter situation.
11. How to start with feedback loop design and compensation? I hope the teacher will answer patiently.
Expert answer: The gain of the feedback loop is neither bigger nor better, nor smaller is better. When the gain of the feedback loop is too high, the output voltage will fluctuate around the average value. The higher the gain, the greater the amplitude of the fluctuation. In severe cases, oscillation will occur; when the gain of the feedback loop is too low, the output voltage will fluctuate. will be unstable. In order to stabilize the output voltage without oscillation, the feedback loop is generally divided into three loops. One loop is used to determine the size of the differential gain, the other loop is used to determine the size of the integral gain, and the third loop is Determine the size of the DC gain. By carefully adjusting the gains of these three feedback loops, the switching power supply can be stable without oscillation.
12. The efficiency of DC TO DC is a bit low recently. How to solve it?
Expert answer: Reduce the operating frequency, or replace the power switch tube with a high-speed switching tube. In addition, you can increase the size of the transformer and reduce the value of the maximum magnetic flux density (Bm), that is, change the primary coil of the switching transformer. The number of turns increases because the hysteresis losses and eddy current losses of the switching transformer are proportional to the operating frequency and proportional to the square of the maximum flux density increment.
13. Hello teacher, how do you calculate the minimum DC voltage? I looked at several versions but couldn’t find the one that suited me best.
Expert answer: I don’t quite understand what you mean by “minimum DC voltage” here? If it is the minimum input DC voltage of the switching power supply, it can generally be calculated based on the minimum input AC voltage. For example, if the minimum input AC voltage is AC100V (effective value), then the converted minimum DC input voltage is approximately 120V (average value). Because the maximum value after rectification and filtering is 140V, the minimum value is 100V, and the average value is 120V.
If the minimum DC voltage is the positive feedback voltage of a transistor self-excited switching power supply, then this voltage is best selected to be 2 times the operating voltage when the transistor is turned on, leaving 1 times as an adjustable margin. If the minimum DC voltage is the minimum operating voltage of the field effect transistor drive circuit, the minimum operating voltage cannot be less than 16V, because the driving voltage required for deep saturation of high-power field effect transistors is above 12V (preferably 20V).
14. Hello teacher, there are burrs on the output side of the flyback transformer power supply I made, and the frequency of the burrs is the same as the switching frequency of the primary side. How can I eliminate the burrs?
Expert answer: A small inductor is connected in series between the secondary rectifier and the filter capacitor, but the inductor cannot be saturated when DC flows through it. The magnetic circuit of this inductor cannot be closed, and a large air gap must be left.
15. Hello teacher! How to optimize the switching frequency of flyback power supply? How to optimize the setting of VOR flyback voltage, and under what circumstances is it most suitable? Thanks! How to optimize the turns ratio calculation? Thanks.
Expert answer: The selection of the operating frequency of the flyback switching power supply is mainly related to the working efficiency and size of the switching power supply, and the working efficiency of the switching power supply is mainly related to the losses (hysteresis loss and eddy current loss) of the switching power supply tube and switching transformer. , the losses of both are proportional to frequency. The loss of the switching power supply tube is mainly composed of turn-on loss (on-time loss) and turn-off loss (off-time loss). The longer the on-time and off-time of the switching power supply tube, the greater these two losses are.
Generally, the on-time and off-time of high-power switching power supply tubes are much longer than those of low-power switching power supply tubes, so the operating frequency of high-power switching power supplies is generally lower. When considering the working efficiency of the switching power supply, it is best to consider the size and cost of the switching power supply.
It is more appropriate to choose an operating efficiency of about 80%. At this time, the loss of the switching power supply tube accounts for about 50% of the total loss, the loss of the switching transformer accounts for about 30% of the total loss, and the loss of the remaining circuits accounts for about 50% of the total loss. 20%. The turns ratio of the switching transformer is related to the ratio of the input and output voltages, and to the duty cycle of the switching power supply.
16. Hello teacher! How to set the initial peak current IP and flyback voltage VOR as well as the optimal flyback power supply duty cycle, thank you!
Expert answer: The magnitude of the flyback voltage generated by the primary and secondary coils of the flyback switching power supply is related to the duty cycle of the switching power supply and the input voltage. When selecting the duty cycle of the switching power supply, it must be considered. The sum of the peak value of the flyback voltage generated by the primary and secondary coils and the operating voltage (input voltage) cannot exceed 0.7 times the withstand voltage Bvmax of the power switch tube. According to this condition (Bvmax), the maximum input voltage of the flyback switching power supply can be calculated The maximum duty cycle Dmax. For example, for a power switch with a Bvmax of 650V, when the input voltage is AC260V, its duty cycle can only be selected to be about 0.306.
17. Hello teacher! How to optimize the switching frequency of flyback power supply? How to optimize the setting of VOR flyback voltage, and under what circumstances is it most suitable? Thanks! How to optimize the turns ratio calculation?
Expert answer: The selection of the operating frequency of the flyback switching power supply is mainly related to the working efficiency of the switching power supply, and the working efficiency of the switching power supply is mainly related to the losses (hysteresis loss and eddy current loss) of the switching power supply tube and switching transformer. Both losses are proportional to frequency. The loss of the switching power supply tube is mainly composed of turn-on loss (on-time loss) and turn-off loss (off-time loss). The longer the on-time and off-time of the switching power supply tube, the greater these two losses are.
Generally, the on-time and off-time of high-power switching power supply tubes are much longer than those of low-power switching power supply tubes, so the working frequency of high-power switching power supplies is generally lower. When considering the working efficiency of the switching power supply, considering the size and cost of the switching power supply, it is better to choose an operating efficiency of about 80%. At this time, the loss of the switching power supply tube accounts for about 50% of the total loss. The loss of the switching transformer accounts for approximately 30% of the total loss, and the loss of the remaining circuits accounts for approximately 20% of the total loss. The turns ratio of the switching transformer is related to the ratio of the input and output voltages, and to the duty cycle of the switching power supply.
18. Hello teacher, there are burrs on the output side of the flyback transformer power supply I made, and the frequency of the burrs is the same as the switching frequency of the primary side. How can I eliminate the burrs?
Expert answer: A small inductor is connected in series between the secondary rectifier and the filter capacitor, but the inductor cannot be saturated when DC flows through it. The magnetic circuit of this inductor cannot be closed, and a large air gap must be left.
19. Hello teacher, how do you calculate the minimum DC voltage? I’ve looked at several versions but can’t find the most suitable one?
Expert answer: I don’t quite understand what you mean by “minimum DC voltage” here? If it is the minimum input DC voltage of the switching power supply, it can generally be calculated based on the minimum input AC voltage. For example, if the minimum input AC voltage is AC100V (effective value), then the converted minimum DC input voltage is approximately 120V (average value). Because the maximum value after rectification and filtering is 140V, the minimum value is 100V, and the average value is 120V.
If the minimum DC voltage is the positive feedback voltage of a transistor self-excited switching power supply, then this voltage is best selected to be 2 times the operating voltage when the transistor is turned on, leaving 1 times as an adjustable margin. If the minimum DC voltage is the minimum operating voltage of the field effect transistor drive circuit, the minimum operating voltage cannot be less than 16V, because the driving voltage required for deep saturation of high-power field effect transistors is above 12V (preferably 20V).
20. The efficiency of DC TO DC is a bit low recently. How to solve it?
Expert answer: Reduce the operating frequency, or replace the power switch tube with a high-speed switching tube. In addition, you can also increase the size of the transformer and reduce the value of the maximum magnetic flux density (Bm), that is, change the primary coil of the switching transformer. The number of turns increases because the hysteresis loss and eddy current loss of the switching transformer are proportional to the operating frequency and proportional to the square of the maximum flux density increment.
21. Question: How to start with feedback loop design and compensation? I hope the teacher will answer patiently.
Expert answer: The gain of the feedback loop is neither bigger nor better, nor smaller is better. When the gain of the feedback loop is too high, the output voltage will fluctuate around the average value. The higher the gain, the greater the amplitude of the fluctuation. In severe cases, oscillation will occur; when the gain of the feedback loop is too low, the output voltage will fluctuate. will be unstable. In order to stabilize the output voltage without oscillation, the feedback loop is generally divided into three loops. One loop is used to determine the size of the differential gain, the other loop is used to determine the size of the integral gain, and the third loop is Determine the size of the DC gain. By carefully adjusting the gains of these three feedback loops, the switching power supply can be stable without oscillation.
