Switching Power Supply Design skills--Moso power supply professional agent imported power supply
The characteristic of switching power supply is to generate strong electromagnetic noise. If it is not strictly controlled, it will generate great interference.
The technology described below is helpful to reduce the noise of switching power supply and can be used in high sensitivity analog circuits.
1 The selection of circuits and devices a key point is to keep dv/dt and di/dt at a lower level. There are many circuits to reduce radiation by reducing dv/dt and/or di/dt, this also reduces the pressure on the switch tubes. These circuits include ZVS (Zero voltage switch). ZCS (Zero current switch). Resonance mode. (A kind of ZCS). SEPIC (Single-ended primary inductor converter). CK (A set of magnetic structures, named after its inventor)Wait.
Reducing the switching time does not necessarily lead to an increase in efficiency, because the RF oscillation of the magnetic element requires a strong loss buffer, * the weakening return can be observed.
Although using soft switching technology will slightly reduce efficiency, it has greater benefits in saving cost and filtering/shielding space.
2 damping in order to protect the switch tube from the impact of the oscillating spike voltage caused by parasitic parameters and other factors, damping is often needed. The damper is connected to the coil with problems, which can also reduce the emission.
There are many types of dampers: from EMC's point of view, RC dampers are usually * Good at EMC, but they generate more heat than others.
Weighing the advantages and disadvantages of all aspects, inductive resistors should be carefully used in the buffer.
3 There is a 50pF capacitance between the radiator and the collector or the drain of the TO247 power device, so strong emission can be generated.
Simply connecting the heat sink directly to the chassis only leads the noise to the ground and probably cannot reduce the overall emission level.
It is better to connect them to an appropriate circuit node--
Primary Rectifier output, but pay attention to safety requirements.
Insulating isolation plates with shielding effect can be connected to the switch tubes, and their shielding inner layers are connected to the primary rectifier end, and the heat sink is either suspended or connected to the casing.
The heat sink can also be connected to a dangerous voltage line through a capacitor, and the inductance formed by the lead of the capacitor and the PCB track may 'resonate' with the capacitor ', this can be especially effective in solving problems of some special frequencies.
It should be tested many times on the prototype, * finally find the * good installation method of the heat sink.
4 The rectifier device is used for the rectifier and the secondary rectifier on the primary power supply, because its reverse current can cause a lot of noise, * it is good to use fast soft switching devices, 5 problems related to magnetic components and solutions special attention should be paid to the closing of magnetic circuits of inductors and transformers.
For example, a ring-shaped or seamless magnetic core is used, and the ring-shaped iron powder core is suitable for storing magnetic energy. If a seam is opened on the magnetic ring, A complete short circuit ring is needed to reduce parasitic leakage. The primary switching noise of magnetic constant will be injected into the secondary through the coil turn-to-turn capacitance of the isolation transformer, and common mode noise will be generated in the secondary, which is difficult to filter out, moreover, due to the long flow path, the phenomenon of * * shooting will occur.
A very effective technique is to connect the secondary ground to the primary power line with a small capacitor, thus providing a return path for these common-mode currents, but pay attention to safety, never exceed the total leakage current indicated by the safety standard. This capacitor also helps the secondary filter to work better. Coil interturn shielding (Inside the isolation transformer)
The primary switching noise induced on the secondary can be suppressed more effectively.
Although there have been more than five layers of shielding, three layers of shielding are more common.
The shield close to the primary coil is usually connected to the primary power line, and the shield close to the secondary coil is often connected to the common output ground (If any), The middle shield is generally connected to the chassis.
In the prototype stage * good repeated experiments to find the coil turn shielding * good connection.
The above two technologies can also reduce the secondary switching noise induced on the input.
The output inductance of appropriate size can change the secondary AC waveform into a half sine wave, so the noise between transformer windings can be significantly reduced (DC ripple)