To continue with the line charger circuits, I now publish the analysis of a higher power charger. My goal is to publish a circuit based mostly on discrete components rather than specialized ICs, so that we can get deeper into the guts and learn from how it works. Oddly enough on the web I didn't find anything functional 100% of the features I was looking for, so I think this schematic will bail out more than one.
With this circuit we can obtain a power of up to 10W with which we can even charge a tablet or iPad, that is, more power than the cell phone charger circuit I posted earlier.
What I found inside the loader that I disassembled, is basically what is known as switching mode power supply or switched source; but there are many types of switching sources and the one we have here is a self-oscillating source of the type flyback converter.
The source is relatively safe because it isolates the 110 / 220V electrical network from our computer, through a transformer and an optical circuit. The only thing that doesn't convince me is that capacitor C4 which I'm not sure what Wednesday they put it there for. I would take it out. Otherwise it seems to be a relatively safe source to use, except if you use poor quality components, which is not the case. Of course, it must be taken into account that there are high DC voltages in certain parts of the circuit and this will always be a risk.
To explain how this circuit works I am going to divide it into 4 stages as in the following figure.
Stage 1: Input voltage rectification and filtering
This part is easy to understand. We have a bridge rectifier made up of 4 diodes that rectify the negative part of the input sine wave and we have the capacitor C1 that smoothes or filters the voltage to have a DC voltage. The only important thing to note here is that the DC voltage is high (117 x 1.4142 = 165.5 Volts DC in my case); so we have to be very careful when handling it.
Stage 2: Oscillator and Primary Driver
The active components of this stage are 2 transistors. A small one, the S9014 and another with more power, the 13003 that is responsible for switching the primary of the transformer. Components C6, R3, and D3 only serve to form a current loop with the primary when transistor Q1 is off. Because the primary presents inertia to the current, the current will tend to continue to flow when Q1 turns off, but it will do so through C6, R3 and D3.
The transistor Q2 can also be said to be part of stage 3 of the circuit, since that is where the two feedback signals are mixed: the one that comes through the coil marked FBACK and the one that comes through the optical circuit. With the two signals, any voltage deviation that may exist is corrected.
Stage 3: Feedback loop
Any deviation in the output voltage is corrected by two signals, the first comes from the secondary winding marked FBACK and the second comes from an optocoupler. The two signals are taken in such a way that there is electrical isolation between the output voltage and the input voltage. The isolation is optical in the case of the optocoupler and magnetic in the case of the transformer.
The optocoupler uses a 4.3 volt zener diode at its input, which added to the 0.9 volt voltage drop from the device itself adds up to about 5.2 volts. If the output voltage rises more than 5.2 volts, the output of the optocoupler device decreases its apparent resistance and this modifies the duty cycle of the wave that feeds the transformer.
Stage 4: Adequacy of the output voltage
Here only the voltage is rectified and filtered to make it useful for consumption. Instead of a conventional rectifier diode, a Schottky diode is used to take advantage of its speed and low threshold voltage.
Finally, I have assembled the circuit on a bread-board to see if it works and the results were as expected. Just as an interesting note I took the waveform of the oscillation in the transformer secondary (before going through the Schottky diode). Here's a photo of the no-load output waveform (I took it with a portable oscilloscope I had on hand, not necessarily accurate). As you can see, the period of oscillation is approximately 800 uS.