4G Cell Phone Jammer Switching power supply first frequency AC to DC, and then as a high-frequency inverter, and finally through the rectifier filter circuit output, stable DC voltage, and therefore contains a lot of harmonic interference itself.
Meanwhile, due to the leakage inductance of the transformer and the output diode reverse recovery current spikes caused, have formed a potential electromagnetic interference. Switching power supply source of interference is mainly focused on the voltage, current changes large components, the most prominent on the switch, diode, high-frequency transformers. 4g cell phone jammer
① electromagnetic interference generated by the switching circuit
One of the main switching circuit is a switching power supply source of interference. Switching circuit is the core of the switching power supply, mainly by switching tube and high-frequency transformer. It produces du / dt has a more substantial pulse, wide bandwidth and rich harmonics. Main reason for this pulse interference is: switch load for high-frequency transformer primary coil is inductive loads. In an instant the switch is turned on, the primary coil produces a large inrush current and high surge voltage spikes occur at both ends of the primary coil; switch off at the moment, due to the leakage flux of the primary coil, causing part of the energy is not transmitted from the primary coil to the secondary coil, which stored in the inductor part of the energy in the collector circuit capacitance and the resistance is formed with an attenuation peak of the oscillation is superimposed on the turn-off voltage to form a turn-off voltage spike.
Interrupt is generated when the supply voltage is turned on with the same primary coil magnetizing inrush current transients, this is a transient conduction type electromagnetic interference, affecting both the transformer primary, but also to return to the distribution system conducted interference, resulting harmonic electromagnetic interference, which could affect the safe and economic operation of other devices.
② electromagnetic interference generated by the rectifier circuit
Rectifier circuit, when the output rectifier diode has a reverse current cut-off, it's time to return to zero and the junction capacitance and other factors. Among them, can quickly return to zero reverse current diode called a hard recovery characteristics diodes, diode under the influence of this transformer leakage inductance and other distribution parameters will produce a strong high-frequency interference, its frequency up to tens of MHz .
A high frequency rectifier diode rectifier circuit with a large forward current is flowing through the conducting time, in its turn by the reverse bias voltage is turned off, since the PN junction has a greater accumulation of carriers, and thus the carrier a period of time before the disappearance of the child, the reverse current will flow, resulting in the disappearance of carriers drastically reduced reverse recovery current and a large current change occurs (di / dt).
③ high-frequency transformer
High-frequency transformer primary winding, high-frequency switching current loop switch and filter capacitor may constitute a greater space radiation, the formation of radiation interference. If the capacitor filter capacity is insufficient or poor high-frequency characteristics, high frequency impedance capacitor causes a high-frequency current to be transmitted to differential mode conducted interference in the formation of the AC power supply.
Note that, in the electromagnetic interference generated in the diode rectifier circuit, the rectifying diode reverse recovery current di / dt than the free-wheeling diode reverse recovery current di / dt is much greater. As sources of electromagnetic interference to study the formation of the rectifier diode reverse recovery current interference intensity, frequency bandwidth. However, the voltage of the rectifier diode transition is far less than the power switch is turned on and the voltage generated when the jump off. Therefore, it may not count diode rectifier │dv generated / dt│ influence, to study the coupling channel is part of EMI as the rectifier circuit.
④ distributed capacitance caused by interference
In the high-frequency switching power supply status, and thus its distributed capacitance can not be ignored. On the one hand the contact area of the insulating sheet, the heat sink and the switching between the collector is large, and the thin insulating sheet, the distributed capacitance between the two and therefore at high frequencies can not be ignored. High-frequency current to flow through the distributed capacitance on the heat sink, and then flow to chassis ground, common mode interference; on the other hand, there is a distributed capacitance between the primary and secondary pulse transformer primary side voltage can be directly coupled to the secondary side on the secondary side DC output for the two power line common-mode interference.
⑤ stray parameters that affect the characteristics of the coupling channel
In the conduction band interference (<30MHz), the majority of the switching power supply coupled-channel interference can be used to describe a circuit network. However, the switching power supply of any actual components, such as resistors, capacitors, inductors and the switch, the diode contains stray parameters, and the wider the band of the study, the higher the order of the equivalent circuit. Therefore, including the various components of stray coupling between the parameters and components including switching power supply will be much more complex equivalent circuit. At high frequencies, the spurious coupling channel parameters on the characteristics of a large, distributed capacitance becomes channel presence of electromagnetic interference. 4g cell phone jammer
In addition, the power switch is large, the collector generally distributed capacitance required to add heat sink, heat sink and switch between at high frequencies can not be ignored, it can form oriented space radiation interference and power cord conduction common mode interference.
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