Main circuit impulse current limiting
Limit the impact current on the input side of the power supply.
Input filter
The function is to filter the clutter existing in the power grid, which prevents the clutter feedback from returning to the power grid.
Rectification and filtering
The AC power supply of the power grid is directly rectified to a smoother DC current.
Inverter
The main part of high frequency switching power supply is to change the rectified DC into high frequency alternating current.
Output rectification and filtering
According to the load demand, provide stable and reliable DC power supply.
control circuit
Take samples from the output end, compare with the set value, then control the inverter, change its pulse width or pulse frequency to make the output stable. On the other hand, according to the data provided by the test circuit, the protection circuit can identify and provide various protection measures for the power supply by the control circuit.
Detection circuit
Provide various parameters and various instrument data in operation in the protection circuit.
Auxiliary circuit
The software (remote) of power supply is started, and the power supply is provided for the protection circuit and control circuit (PWM and other chips).
If copper tape is used in the transformer of switching power supply instead of enameled wire, the allowable current calculation
If copper tape is used in the transformer of switching power supply instead of paint clad wire, the eddy current loss of copper tape (enameled wire) can be large and small, and the working frequency can be increased correspondingly, but the DC loss is almost unchanged. The current density allowed to pass through copper tape shall not exceed 4.5a/ square millimeter generally. The current density is equal to the current division and the cross-sectional area of the conductor, and the cross-sectional area of the conductor is equal to the thickness (0.1mm) times the width (width of the copper strip).
The most serious electromagnetic interference is the circuit composed of primary and secondary coils of switch transformer. However, the interference of the circuit will generate radiation and conduction interference to other circuits through induction. The most serious place of conduction interference and radiation interference is the power line, because the power line is easy to become the half wave vibrator antenna of the radiation source, and in addition, the power line is easy to become the half wave oscillator antenna of the radiation source It is also connected with the external line, which is easy to transmit interference signal to other equipment. Therefore, the power supply line must be effectively isolated at the input end of the switch power supply.
One way to reduce the temperature rise of transformer is to reduce the maximum flux increment (BM) of transformer core, because the loss of transformer core (hysteresis loss and eddy current loss) is proportional to the square of flux density; the other is to reduce the working frequency of switching power supply, because the loss of transformer core (hysteresis loss and eddy current loss) is proportional to the working frequency; the other is that the loss of the transformer core (hysteresis loss and eddy current loss) is proportional to the working frequency; the other is that Reduce the loss of coil, the loss of coil (mainly eddy current loss), the eddy current loss of coil and skin effect loss are also proportional to the working frequency. To reduce the DC loss of the coil, the current density of the conductor must be reduced. Generally, the current density of the enameled wire shall not exceed 4.5a/ square millimeter.
The duty cycle of flyback switch power supply is mainly determined by input voltage and voltage withstand of switch power supply tube. The duty cycle will also change when the input voltage changes. For example, when the input voltage is ac260v, if the voltage of the power switch is 650V, the duty cycle is 0.306; when the input voltage is ac170v, the duty cycle is about 0.5; when the input voltage is lower than ac170v, the duty cycle is greater than 0.5. However, no matter the input voltage changes, the switching power supply will change the duty cycle to a stable (or change) output voltage value.
Forward excitation switch power supply is when the power switch tube is on, the power supply provides power output to the responsible person, but there is no power output when the switch is off. The flyback switch power supply is the opposite. When the power switch is on, it only stores energy to the transformer, does not provide power output to the load, and only when the power switch is off, it can provide output to the load. The output voltage of forward excitation switching power supply is the average value of rectifier output voltage, and the output voltage of flyback switch power supply is the half wave average value of rectifier output voltage, and the phase of the two voltage output is exactly the opposite.
The gain of feedback loop is neither the bigger the better nor the smaller the better. When the gain of feedback loop is too high, the output voltage will be tracked back and forth around the average value. The output voltage fluctuates very strongly, the higher the gain, the larger the amplitude of the fluctuation, and there will be oscillation when the gain of the feedback loop is too low, the output voltage will be unstable again, because the voltage tracking is not in place, there will be a lag error.
In order to make the output voltage stable, but there is no oscillation, the feedback loop is generally divided into three circuits. One loop is used to determine the size of differential gain, the other is used to determine the size of integral gain, and the other is to determine the size of DC gain. The purpose of this is that when the error signal is small, the loop gain is very large, and the loop gain will be smaller when the error is small, that is, the gain of the error amplifier is dynamic. By carefully adjusting the gain of these three feedback loops, the switching power supply can be stable without oscillation.
How to minimize the switching MOS of flyback power supply
The duty cycle is reduced, but the duty cycle is too low, the working efficiency of the power supply is greatly reduced, and the voltage adjustment range will also be reduced.
Copper foil loss in power loss ratio
Very small. If the copper foil is lost, the temperature rise of the copper foil will be very high, if it is more than 80 degrees, the paint of the copper foil will turn yellow. But it is also equivalent to the loss of a metal film resistance of about 1-3 Watts at the same temperature rise.
How to design feedback loop and how to compensate
The gain of feedback loop is neither the bigger the better nor the smaller the better. When the gain of feedback loop is too high, the output voltage will fluctuate around the average value. The higher the gain, the larger the amplitude of fluctuation will be, and there will be oscillation when the gain of feedback loop is too low, the output voltage will be unstable. In order to make the output voltage stable, but there is no oscillation, the feedback loop is generally divided into three circuits. One loop is used to determine the size of differential gain, the other is used to determine the size of integral gain, and the other is to determine the size of DC gain. By carefully adjusting the gain of these three feedback loops, the switching power supply can be stable without oscillation.
The output side of flyback transformer has burr, and the burr frequency is the same as the original switch frequency. How to eliminate burr
A small inductance is string between the secondary rectifier and the filter capacitance, but the inductance cannot be saturated when it flows through DC. The magnetic circuit of this inductance cannot be closed, and there must be a large air gap.
How to optimize the switching frequency of flyback power supply
The selection of the working frequency of flyback switching power supply is mainly related to the working efficiency and volume of the switching power supply, while the working efficiency of the switching power supply is mainly related to the loss of switch power supply pipe and switch transformer (hysteresis loss and eddy current loss), both of which are directly proportional to the frequency ratio. The loss of switch power supply pipe is mainly composed of opening loss (on-off time loss) and off loss (time loss of off). The longer the on-off time and closing time of the switch power supply pipe, the greater the two losses.
