This simple, enhanced, 5V zero drop PWM solar battery charger circuit can be used in conjunction with any solar panel for charging cellphones or cell phone batteries in multiple numbers quickly, basically the circuit is capable of charging any battery whether Li-ion or Lead acid which may be within the 5V range.
Using TL494 for the Buck Converter
The design is based on a SMPS buck converter topology using the IC TL 494 (I have become a big fan of this IC). Thanks to "Texas Instruments" for providing this wonderful IC to us.
You may want to learn more about this chip from this post which explains the complete datasheet of IC TL494
We know that a 5V solar charger circuit can be easily built using linear ICs such as LM 317 or LM 338, you can find more info on this by reading the following articles:
However the biggest drawback with these linear battery chargers is the emission of heat through their body or through case dissipation, which results in wastage of precious power. Due to this issue these IC are unable to produce a zero drop voltage output for the load and always require at least 3V higher inputs than the specified outputs.
The circuit of the 5V charger explained here is completely free from all these hassles, let's learn how an efficient working is achieved from the proposed circuit.
Referring to the above 5V PWM solar battery charger circuit, the IC TL494 forms the heart of the entire application.
The IC is a specialized PWM processor IC, which is used here for controlling a buck converter stage, responsible for converting the high input voltage into a preferred lower level output.
The input to the circuit can be anywhere between 10 and 40V, which becomes the ideal range for the solar panels.
The key features of the IC includes:
Generating Precise PWM output
In order to generate accurate PWMs, the IC includes a precise 5V reference made by using bandgap concept which makes it thermally immune. This 5V reference which is achieved at pin#14 of the IC becomes the base voltage for all the crucial triggers involved within the IC and responsible for the PWM processing.
The IC consists of a pair of outputs which can be either configured to oscillate alternately in a totem pole configuration, or both at a time like a single ended oscillating output. The first option becomes suitable for push-pull type of applications such as in inverters etc.
However for the present application a single ended oscillating output becomes more favorable and this is achieved by grounding pin#13 of the IC, alternatively for achieving a push pull output pin#13 could be hooked up with pin#14, we have discussed this in our previous article already.
The outputs of the IC has a very useful and an interesting set up internally. The outputs are terminated via two transistors inside the IC. These transistors are arranged with an open emitter/collector across the pin9/10 and pins 8/11 respectively.
For applications which require a positive output, the emitters can be used as the outputs, which are available from pins9/10. For such applications normally an NPN BJT or an Nmosfet would be configured externally for accepting the positive frequency across the pin9/10 of the IC.
In the present design since a PNP is used with the IC outputs, a negative sinking voltage becomes the right choice, and therefore instead of pin9/10, we have linked pin8/11 with the output stage consisting of the PNP/NPN hybrid stage. These outputs provide sufficient sinking current for powering the output stage and for driving the high current buck converter configuration.
The PWM implementation, which becomes the crucial aspect for the circuit is achieved by feeding a sample feedback signal to the internal error amplifier of the IC through its non-inverting input pin#1.
This PWM input can be seen hooked up with the output from the buck converter via the potential divider R8/R9, and this feedback loop inputs the required data to the IC so that the IC is able to generate controlled PWMs across the outputs in order to keep the output voltage consistently at 5V.
Other output voltage can be fixed by simply altering the values of R8/R9 as per ones own application needs.
The IC has two error amplifiers set internally for controlling the PWM in response to external feedback signals. One of the error amp is used for controlling the 5V outputs as discussed above, the second error amp is employed for controlling the output current.
R13 forms the current sensing resistor, the potential developed across it is fed to one of inputs pin#16 of the second error amp which is compared by the reference at pin#15 set on the other input of the opamp.
In the proposed design it is set at 10amp through R1/R2, meaning in case the output current tends to increase above 10amps, the pin16 can be expected to go higher than the reference pin15 initiating the required PWM contraction until the current is restricted back to the specified levels.
Buck Power Converter
The power stage shown in the design is a standard power buck converter stage, using a hybrid Darlington pair transistors NTE153/NTE331.
This hybrid Darlington stage responds to the PWM controlled frequency from pin8/11 of the IC and operate the buck converter stage consisting of a high current inductor and a high speed switching diode NTE6013.
The above stage produces a precise 5v output ensuring minimum dissipation and a prefect zero drop output.
The coil or the inductor can be wound over any ferrite core using a three parallel strands of super enameled copper wire each with a diameter of 1mm, the inductance value can be anywhere near 140uH for the proposed design.
Thus this 5V solar battery charger circuit can be considered as an ideal and extremely efficient solar charger circuit for all types of solar battery charging applications.
For Higher Voltages
For solar panels with higher voltages, such as 60 V solar panels, the design can upgraded by adding zener diode regulator at pin12 of the TL494, as shown below: