Inside the low cost mobile charger
Introduction: - Friends today I will discuss the working of a low cost mobile phone battery chargers available in market and its hidden aspects. These mobile chargers have poor voltage regulation and line regulation. The current rating is also not specified.
Mainly the mobile phone chargers are made on the topology called as RCC or ringing choke converter , popularly called as the self oscillating converter. This enables the low cost design for the low power application.This topology is not recommended for the higher wattage usage. The branded mobile phone charger coming with the branded mobile handset is more sophisticated and more reliable than the one discussed here. It contains more protections and safety features.
Circuit diagram: - Below is the circuit diagram of the low cost mobile battery charger.
Working: - This is a self oscillating converter or ringing choke converter, consisting two transistors for generating the PWM and controlling the output power. It works as, the input AC is rectified by the bridge rectifier consisting the diode D1-2, D4-5. The rectified AC is then filtered by the capacitor C3 and is fed to the transformer primary. The fuse resistance R2 reduces the inrush current and also blows in case of any fault.
The other part of transformer primary is connected to a high voltage switching transistor’s collector which through the emitter and the power limiting resistance R7 goes to ground. Resistance R3 is used for the initial start up of the transistor Q1. In the initial phase when the transistor is ON through R3, then the current rises in the primary and the primary inductance is charged, at the same time the feedback winding voltage also increases. This feedback voltage is rectified by the diode D9 and then filtered by the capacitor C4. When the feedback voltage is much sufficient to cross the zener voltage, a turn off pulse is fed to the base of transistor Q1 by making Q1 on. Now when the transistor Q1 is OFF, the primary side voltage polarity of the transformer is reversed, since the voltage can’t be changed in an inductor instantaneously, the RCD snubber formed by R1, C1, D6 prevents the transistor from breakdown. Since the polarity is now reversed at the input the output diode D3 is now forward biased and passes the voltage. The voltage is then filtered by the capacitor C2 and fed to output.
The components R8, C5 forms a RC oscillator and the frequency of oscillation can be set by setting the values.
R7 is used as power limiting resistance, which provides voltage samples of input primary current to the transistor Q2. Whenever the input voltage increases so that the voltage at the base of Q2 is more then .6V the transistor Q2 is ON and a turnoff signal is sent to the base of Q1.
Conclusion:- These kinds of chargers are readily available in the market that also at very low cost. But using them is not safe. Below points must be considered while using these types of chargers
Mainly the mobile phone chargers are made on the topology called as RCC or ringing choke converter , popularly called as the self oscillating converter. This enables the low cost design for the low power application.This topology is not recommended for the higher wattage usage. The branded mobile phone charger coming with the branded mobile handset is more sophisticated and more reliable than the one discussed here. It contains more protections and safety features.
Circuit diagram: - Below is the circuit diagram of the low cost mobile battery charger.
Working: - This is a self oscillating converter or ringing choke converter, consisting two transistors for generating the PWM and controlling the output power. It works as, the input AC is rectified by the bridge rectifier consisting the diode D1-2, D4-5. The rectified AC is then filtered by the capacitor C3 and is fed to the transformer primary. The fuse resistance R2 reduces the inrush current and also blows in case of any fault.
The other part of transformer primary is connected to a high voltage switching transistor’s collector which through the emitter and the power limiting resistance R7 goes to ground. Resistance R3 is used for the initial start up of the transistor Q1. In the initial phase when the transistor is ON through R3, then the current rises in the primary and the primary inductance is charged, at the same time the feedback winding voltage also increases. This feedback voltage is rectified by the diode D9 and then filtered by the capacitor C4. When the feedback voltage is much sufficient to cross the zener voltage, a turn off pulse is fed to the base of transistor Q1 by making Q1 on. Now when the transistor Q1 is OFF, the primary side voltage polarity of the transformer is reversed, since the voltage can’t be changed in an inductor instantaneously, the RCD snubber formed by R1, C1, D6 prevents the transistor from breakdown. Since the polarity is now reversed at the input the output diode D3 is now forward biased and passes the voltage. The voltage is then filtered by the capacitor C2 and fed to output.
The components R8, C5 forms a RC oscillator and the frequency of oscillation can be set by setting the values.
R7 is used as power limiting resistance, which provides voltage samples of input primary current to the transistor Q2. Whenever the input voltage increases so that the voltage at the base of Q2 is more then .6V the transistor Q2 is ON and a turnoff signal is sent to the base of Q1.
Conclusion:- These kinds of chargers are readily available in the market that also at very low cost. But using them is not safe. Below points must be considered while using these types of chargers
- Avoid using the mobile phone while charging , the mobile battery may explode
- Do not plug and leave the charger with mobile for a long time, this may damage the battery by overcharging
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