The circuit of the emergency charger, which is a single-tube DC conversion circuit, adopts the form of a single-ended flyback converter circuit. The so-called single-ended circuit in the circuit means that the magnetic core of the high-frequency converter works only on one side of the hysteresis loop.
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The so-called flyback means that when the switching tube VT1 is on, the induced voltage of the primary coil NP of the high-frequency transformer T1 is 1 + 2 negative, and the rectifier diode VD3 is in the cutoff state, storing energy in the primary coil. When the switching tube VT1 is off, the energy stored in the transformer T1 primary coil is output back to the load by VD3 rectification and capacitor C4 filtering.
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The triode VT1 is a switching power supply tube; It and T1, R1, R2, C2 and other self-excited oscillation circuit. After the input power is added, electricity flows through R1 to the base of VT1, making VT1 on, and R1 is called the starting resistor.
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Once VT1 is on, the transformer primary coil NP is added to the input voltage, the collector current IC increases linearly in NP, the feedback coil Nb produces 3 positive 4 negative induced voltage, so that VT1 gets a positive base, the emission is extremely negative positive feedback voltage, this voltage through C2, R2 into VT1 base current, so that the collector current of VT1 further increases. The positive feedback generates an avalanche process that saturates VT1. During VT1 saturation conduction, T1's primary coil NP stores magnetic energy.
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At the same time, the induced voltage charges C2. With the increase of C2 charging voltage, VT1 base potential gradually decreases. When the change of VT1 base current cannot satisfy its continued saturation, VT1 exits the saturation zone and enters the amplification zone. After VT1 enters the amplified state, its collector current drops, and 3 negative and 4 positive induced voltage is generated in the feedback coil Nb, which reduces the base current of VT1, and its collector current decreases accordingly. The positive feedback once again appears the avalanche process, and VT1 is quickly cut off. After VT1 is cut off, the energy stored by the transformer T1 is provided to the load, and the negative 2 positive reverse voltage generated by the primary coil NP is rectified and filtered by the diode VD3, and the 5.8V DC voltage is obtained in C4. Charge your phone through its special charging plug.
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When VT1 is cut off, the DC power supply input voltage and the 3-negative 4-positive voltage induced by Nb are reversely charged by R1 and R2, gradually increasing the VT1 base potential, making it re-on-turn, and again turning to reach the saturation state, and the circuit repeats the oscillation in this way. VD1, VD2, C3, etc. constitute a voltage regulator circuit; During the VT1 cutoff period, the 3-negative 4-positive voltage induced by Nb is charged to C3 via VD2, when the voltage on C3 (negative up and positive down) is greater than 6.2V. The voltage regulator diode VD1 starts to switch on and play a shunt role, reducing the base current of VT1, so that the collector current Ic of VT1 can be controlled to achieve a stable output voltage.
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It should be noted that: because the input DC voltage is low, it does not need to be isolated, and the input DC voltage and the output DC voltage are relatively close, so the high-frequency transformer does not have a secondary coil, and the energy of the load circuit is directly obtained from the primary coil.
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This has two advantages: one is to improve the conversion efficiency of the circuit, and the other is that VD3, C4, R4, etc. at the same time constitute a surge voltage absorption loop to absorb the reverse high voltage generated by the VT1 cut-off moment.