Buck Regulator Based Phone Charger

This tutorial shows you how to make a phone charger which uses any DC source (AAA/AA/SLA battery) as input and is efficient in terms of power utilization.

Components Required:

1.LM2576 Switching regulator IC (5V version and not the adjustable version)

2.Inductor 100uH

3.Electrolytic Capacitors 100uF/25V, 1000uF/25V

4.1N5822 Diode

5.Small PCB

6.USB female connectors

7.Wires

Linear voltage regulators vs. switching voltage regulators:

We’ve been holding onto 7805 regulator IC for a very long time and hence a question may arise as to why we aren’t using the same this time. Well, earlier we didn’t care about the efficiency since it was either a low current application or ones which didn’t run completely on battery but now it is different. This time our aim is to create a device which runs on battery and needs to be utilizing the battery’s energy efficiently.

7805 or for that matter any linear voltage regulators are simple to use but quite inefficient. This is because they dissipate the voltage difference in form of heat. The amount of power wasted is given as

P = (Vin-Vout) X Iload

So for example if the input to the regulator is given from a 12V lead acid battery and the output voltage/current drawn is 5V/1A then the current drawn from the battery will also be 1 Amp so the power wasted in form of heat will be:

P = (12-5) X 1 = 7 Watts

Wasting 7 watts and that too through battery is pretty bad. And hence we need to think of an alternative. Well people have already thought about the alternative and that’s how switching regulators were born.

Switching regulators unlike the linear regulators regulate the output voltage by switching the storage elements (like capacitor or inductor) in different electrical configurations.

This is how a typical This is how a typical buck (step-down) regulator circuit looks like: looks like:

Fig. 1: Typical Circuit Diagram of Buck Regulator

Remember: Inductors do not like changing currents. They always try to oppose the change by creating a voltage drop across them.

When the switch is opened, no current passes through any components.

When the switch is closed, immediately, the current starts increasing in the circuit and the inductor opposes the change of current by developing a voltage drop. This voltage drop counteracts the voltage of source and hence the net voltage to the load decreases.

After a while, the rate of change of current decreases and eventually turns to zero (which means the current becomes constant) and hence the opposing voltage across the inductor also reduces. During this time the inductor stores the energy in form of magnetic field.

Fig. 2: Image showing flow of current in Buck Regulator Circuit when switch is closed

When the switch is opened, the current again starts decreasing (i.e. changing) and hence again a voltage is developed in the inductor but this time, it adds to the source voltage instead of opposing it. The energy stored in the form of magnetic field is now utilized to supply the current to the load. If the switch is closed again before the inductor’s energy is completely used, then the voltage across the load always stays above zero.

Fig. 3: Image showing flow of current in Buck Regulator Circuit when switch is open

Here are the waveforms:

Fig. 4: Image showing input and output waveforms of Buck Regulator Circuit

Vi is the input voltage and VL is the voltage across the inductor.

Basically, the output voltage regulation is done by the Pulse Width Modulation technique. Remember hearing it somewhere? Yes, you did read about it in my project “DIY Table lamp” project. There we used PWM to convert 12 Volts to 10 Volts for powering 3 LEDs in series without using a current limiting resistor. We also use the same principle to control the speed of DC motor.

But the only difference here is that we directly supplied the PWM signal to the LEDs in case of the lamp and here we would be smoothing out the signal (using the diode, inductor and capacitor) to get a fixed DC voltage.

So for example, if we want to step-down 15V to 5V then the regulator IC adjusts the Pulse Width (aka Duty Cycle) to 33%

Therefore, T_ON = 3 * T_OFF

Setup

– Test the circuit on breadboard first and if everything works fine, then proceed to soldering.

– Once you are done with soldering, power the circuit using a 9V battery or couple of AA/AAA batteries or a SLA battery. Just make sure that you provide more than 7 Volts in total.

– Test the USB’s output using a multi-meter set in voltmeter mode to see if +5V is coming or not. If it does, connect your phone to it.

The regulator can handle 3 Amperes of current and hence we can charge more than one phone simultaneously if we are able to provide enough input power.