INSIDE A MOBILE PHONE CHARGER(FLYBACK CONVERTERS)

Due to the poor efficiency of linear Power supply which uses a bulkier transformer(due to the lower frequency of the mains) and linear regulators to step down and regulate the input voltage respectively; switch mode power supplies was invented.

• In a switched-mode power supply (SMPS), the AC mains input is directly rectified and then filtered to obtain a DC voltage.
• The resulting DC voltage is then switched on and off at a high frequency by electronic switching circuitry.
• thus producing a pulsating DC that will pass through a high-frequency transformer or inductor.

Switching occurs at a very high frequency (typically 10 kHz — 1 MHz), thereby enabling the use of transformers and filter capacitors that are much smaller, lighter, and less expensive than those found in linear power supplies operating at mains frequency.
After the transformer secondary, the high frequency pulsating DC is rectified and filtered to produce the DC output voltage.

(Basso, 2008) The flyback converter probably represents the most popular structure found on the market. The vast majority of consumer products make use of this converter: notebook adapters, DVD players, set-top boxes, satellite receivers, CRT TVs, LCD monitors, and so on. Three traits can justify this success: simplicity, ease of design, and low cost.

Figure 1: A typical Self oscillating flyback converter

In a flyback converter, flyback transformers are used . The flyback transformer(inductors coupled on the same core with different polarity) can be used to step up/down(depending on coupling factor) of the voltage aside serving as storage elements and provide isolation between primary and secondary side of the power supply. A typical example of flyback converter is a phone charger.

Figure 2: A phone Charger Circuit board 

FUSIBLE RESISTOR

The fusible resistor served as a fuse (protective device) which breaks when a high current flows through it.
It has low resistance.

RECTIFIER

The rectifier is a full wave bridge rectifier which uses four diodes to converts the AC from the mains directly to DC.
In the Switch Mode Power Supply (SMPS) topology the AC mains is directly converted unlike the linear supplies where the AC mains are step down first before rectifying.

INPUT FILTER CAPACITORS

The capacitors at the primary side are used to filter the DC after rectification to remove the ripples
after filtration the output voltage is around 300Vdc for 230V AC supply and around 160Vdc to 170Vdc for 115V AC supply

Figure 3: Schematic diagram of  A rectifier and filtration circuit

CONTROL CIRCUIT

The control circuit generate a square wave using a PWM with a fixed duty cycle.

the control circuit can be the self oscillating type where the auxiliary winding are used in the switching of the transistor. this topology is known as Ring Choke Oscillator  and its mostly used in phone chargers.
The output voltage of a flyback converter depends on the duty cycle and the transformation ratio of the flyback transformer. The transformer ratio once chosen is fixed so in other to change the output voltage we can only do that by changing the duty cycle of the control circuit.

OSCILLATOR IC
This IC uses Pulse Width Modulation(PWM) to generate a square wave with high frequency which is then supplied to the power transistor (driver circuit) which will then pulsate the DC voltage.

SNUBBER NETWORK

Snubber networks   are usually made up of dissipative resistor-capacitor diode networks as shown in figure 1 above; they are often fitted across high-voltage switching devices and rectifier diodes to reduce switching stress and EMI problems during turn-off or turn-on of a switching device.
In off-line flyback converters, this network is particularly important, as the flyback voltage can easily exceed 800 V when 137-V (ac) voltage-doubled input rectifier circuits or 250-V (ac) bridge rectifier circuits (dual input voltage circuits) are used.

HIGH FREQUENCY TRANSFORMER(Flyback Transformer)

The high frequency pulsating DC is now supplied to the high frequency transformer to step it down to a desired value.
during the ON time, energy is stored in the primary inductor, so the energy previously stored in the output capacitor will supplying the load at this time.
at the OFF time the energy stored in the primary inductor is transferred to the secondary side which supply the load and at the same time charge the output capacitor.
because of the higher switching frequency the transformer used here have smaller size.

OUTPUT RECTIFIER

After stepping the voltage down to a desirable Voltage; it is still Pulsating so a secondary rectifier is used (which is usually a schottky diode) to rectifier the voltage

OUTPUT FILTER CAPACITORS

These capacitors are used to smoothen the output voltage.

FEEDBACK CIRCUIT 

This circuit comprises of sensing circuit and an opto-isolator circuit. The sensing circuit monitors the output voltage and compared it to a reference voltage if the is a change the signal is sent to the control circuit via the opto-isolator. Opto-isolator is a device which uses light to control circuit at other side.

Figure 4: A phone charger circuit with feedback

A simple sensing circuit for a phone charger is shown at figure 5 below

Figure 5: A simple feedback circuit

In figure 5 the resistor R1 and Zener diode ZD1 form the sensing circuit. Initially the values of R1 and ZD1 are chosen so as to provide a fixed voltage drop across the R1 corresponding to the desired output voltage.

the Zener diode provide a nearly constant reference voltage

The sensing circuit monitors the output voltage source and when there is a change (Increases or decreases its level) there is also a change in the voltage drop across R1, this change in potential difference across R1 activates the Optoisolator IC feedback as needed to control the Duty cycle of the switching supply so that it will not deliver more or less power to the secondary side. 

References

Basso, C. P. (2008). In C. P. Basso, Switch Mode Power Supply, Spice Simulations and Practical Design (p. 579). The McGraw-Hill Companies.