SMPS Power Supply

This project is about designing SMPS system in PCB and explaining how this system functions. For charging electronic devices, Switch Mode Power Supply system is preferred over Linear Power Supply system. In Linear Power Supply system High voltage AC signal is transformed into low voltage AC signal using center tapped transformer and then rectified either using full wave bridge or center tapped rectifier. Also filters the pulsating DC voltage into constant DC voltage using capacitors. In this system the transformer that is used is quite big and heavy to be fitted for any small electronic device for charging. Also due to large number of coil turns, power loss by heat dissipation is very likely to happen. Overall this system is not recommended for charging applications.

According to Faraday’s law, Induced voltage in the coil is proportional to the frequency of the signal(f), number of turns(N) and core cross-sectional area(A)

E=kfNA

If the frequency of the signal is somehow increased, then for generating the same induced voltage, less number of turn and less core cross-sectional area coil will needed to be used, as this the transformer size and weight both drastically decreases and power efficiency also increases.

To increase the frequency of the AC signal, switch mode power supply system is introduced. In this system, Using a full wave bridge rectifier and filtering capacitor, high voltage AC signal is turned into pulsating high voltage DC signal. This pulsating DC signal is used to create high frequency oscillations using the switching mechanism of transistor, MOSFETs or other ICs. This oscillation creates high frequency square wave and passes to the primary coil of the transformer(Small and light weighted step down ferrite transformer is used in this system). In the secondary coil, high frequency low voltage square wave is generated, which is then filtered and constant low voltage DC signal is generated.

Working of SMPS:

At first, low frequency high voltage AC signal is passed through the full wave bridge rectifier, then C1 capacitor filters the signal and turns this into high voltage pulsating DC signal. Here, R1 resistor is in the mega ohm range, so very small current passes through it and then the current goes to the base of the T1 transistor. Thus, the collector-emitter path closes and current passes through the primary coil P1 and voltage induces in secondary coil S2, which charges capacitor C2 and decent amount current flows through the base of the T1 transistor, as this, high signal produces in both P1 and S1 coil, but the magnitude of the signal in S1 is less than P1 since it’s a step down transformer. Since, high current flows through T1 transistor’s collector to emitter, this high current also goes though base of the T2 transistor and acts as a closed switch for it’s collector and emitter. The collector terminal of T2 transistor is connected to the base of the T1 transistor. So, T2 pulls all the current through itself, and T1 base current is 0, as this T1 acts as an open switch, and signal is now low in P1 and also at S1 and S2. Since T1 is an open switch, now no current flows through it’s emitter which is connected to the base of the T2 and base current of T2 is also 0. So T2 transistor is also open switch now. That’s how first cycle of high frequency square wave is generated.  After that, very small current passes through the R1 resistor to the base of the T1, and acts as a closed switch, and the process repeats itself, and the high-low square wave oscillation continues.

To filter the high frequency low voltage signal in S1 terminal, a diode and capacitor C4 is used to turn the signal into constant 12V DC voltage. A Zener diode of 12V and optocoupler input terminal in series is used parallel to the capacitor C4. (Optocoupler is a component which has an infrared LED in the input, made out of GaAs. And there is a phototransistor in the output. Phototransistor passes photocurrent through it’s collector to emitter terminal when infrared wave comes out of the input of the IR LED) The optocoupler emitter is connected to the base of the T2 transistor. When output voltage is more than 12V, the Zener diode turns on, which turns on the optocoupler, and current flows through the base of the T2 transistor, which activates T2, and thus T1 acts as an open switch, and the Oscillation is instantly restarted after that.