How to suppress switching power supply ripple and EMI to improve the overall performance of the power supply
ST PWM IC MOSFET
The switching power supply is generally composed of a pulse width modulation PWM control IC and a MOSFET, which controls the time ratio of the switching tube to maintain a stable output voltage. Due to the reduction in the number of turns of the iron coil and the volume of the iron core, the loss of the switching power supply is very low, and the efficiency is generally high, generally reaching 90% power efficiency. Coupled with its small size and stable output, it has obvious advantages in many aspects.
But from another angle, its shortcomings are also obvious. Problems such as complex circuit, high power supply noise, insufficient transient response, complex output ripple, and easy electromagnetic interference are also in front of us. For some low-noise circuits, switching power supplies are often powerless. Where do these shortcomings arise and how can they be avoided?
Suppressing ripple and improving the overall performance of switching power supply
Ripple can cause various hazards to the circuit, and it is extremely deadly to the circuit. First of all, once the ripple is generated, it is easy to cause harmonics, which will cause harm to the circuit itself and reduce the efficiency of the power supply. Higher ripple can cause surges that directly destroy the circuit. Even if it does not directly destroy the circuit, it will greatly interfere with the logic relationship of the digital circuit and affect the operation.
The output ripple of the switching power supply mainly comes from the residual low-frequency output ripple, the high-frequency ripple of the same high frequency as the switching operation, the common-mode ripple noise caused by the parasitics, the ultra-high-frequency resonance noise of the power device switching, and the closed-loop adjustment control. ripple noise. The residual low-frequency output ripple is due to the unsatisfactory filter capacitor capacity of the output circuit. The way to suppress this type of ripple is relatively simple, by increasing the output capacitor or reducing the ESR in parallel. The ripple brought by the closed-loop regulation control can be directly improved on the regulator parameters, which is also easier to achieve.
The high-frequency ripple of the same high frequency as the switching operation occurs when the power device is used to perform high-frequency switching conversion, rectification and filtering of the input DC voltage, and then realize the regulated output, which is mainly related to the conversion frequency of the switching power supply, the structure and parameters of the output filter. related. Wanting to suppress also starts with the conversion frequency of the switching power supply, the structure and parameters of the output filter. Increasing the output high-frequency filter or using multi-stage filtering can better suppress the ripple. Starting from the operating frequency of the switching power supply, the ripple frequency can be increased by increasing the operating frequency of the switching power supply to reduce the current fluctuation in the inductor.
There are many places where parasitic capacitance and parasitic inductance exist. The parasitic capacitance between power devices and transformers, the parasitic inductance of wires, and the common mode ripple noise caused by parasitics need to be eliminated (suppressed) by specially designed EMI filters. Diodes with better reverse recovery performance are also useful.
The ultra-high frequency resonance noise generated by the power device during the switching process is more complex, not only related to the junction capacitance, but also closely related to the leakage inductance of the transformer and the distribution parameters of the switching power supply. A reasonable PCB layout can give the entire circuit system higher stability at any time, which is an important principle when solving such ripples.
Reducing distributed capacitance is a major direction to suppress ultra-high frequency resonance noise. Specifically, the method can reduce the distributed capacitance between the switch tube and the heat sink by using a shielded substrate, or reduce the winding as much as possible by improving the winding process and structure. distributed capacitance between. The choice of diode and switch is also very important. The junction capacitance of the switch will directly affect the noise level. It is best to choose a diode with soft recovery characteristics, which can reduce the ultra-high frequency resonance noise as much as possible.
In addition, temperature changes will change the parameters of the device, which will affect the ripple, which also needs attention.
EMI damage of switching power supply
EMI will exist in any electronic system. In the switching power supply, during the switching process of the transistor and the diode, the current changes greatly during the rise and fall times, and it is easy to generate radio frequency energy to form an interference source. On components such as switching tubes, diodes, and high-frequency transformers, interference sources are very easy to generate, and the EMI signal of switching power supplies occupies a wide frequency range and has a certain amplitude.
We always try to suppress EMI as much as possible, and there are many technologies for suppressing EMI. Filtering technology, shielding technology, sealing technology, and grounding technology that are commonly used for EMI control in switching power supplies are relatively common. During the switching process of the switch tube and diode, due to the leakage inductance of the transformer and the line inductance, it is easy to generate a peak voltage. Usually, the method used in this case is to use an RC/RCD absorption loop, which will significantly improve the switching waveform.
Reducing the du/dt when the power switch is on and off is an important part of suppressing the interference of the switching power supply. On the basis of the switching circuit, adding a small inductance, capacitance and other resonant elements can form a soft switching circuit. The soft switching circuit introduces resonance before and after the switching process, which can eliminate the phenomenon of overlapping voltage and current, and greatly reduce switching loss and interference.