ADJUSTABLE VOLTAGE BOOSTER CONVERTER USING A 555TIMER

 

ADJUSTABLE VOLTAGE BOOSTER CONVERTER USING A 555TIMER

Hi friends and lover of electronics, today we are going to build  a universal booster converter based on 555timer IC is made in PWM mode.

This booster can boost from a 12volt to 300vdc it lets for you to adjust the output using the variable resistor to maintain you desired output, It can handle a 3Amps load without no stress.

CIRCUIT DIAGRAM:

DESCRIPTION:

We used 555 timer to generate PWM control signal from the pin3 of the IC to switching transistor.

The PWM control part is formed by this few components C2,D2,D4,RV-1 where the RV-1 and C2 values determine the switching frequency that are used to drive the L1. The output of the timer is driving the switch transistor through R1 and R2 is current sensing resistor which is 0.22ohms, if a voltage drop on it exceeds about 0.7V on the transistor side then the  transistor voltage will drop, the Q2 conducts and pulls the timer output low through pin4 of the IC. 

With the 0.22 Ohm resistor shown the current through switching transistor Q1 will be limited to about 3.2A. If your input voltage source is not able to supply destructive levels of current then you may want to omit R6,R3 and Q2 then connect the pin 4 to pin 8 of the IC.

The U2 is a linear voltage regulator for limiting the input supply voltage for 555 timer, the output voltage to the timer IC will not exceed 15V that is typically specified maximum supply voltage for the timer chip, so s to protect the timer from over-volt supply, but once you are sure that you are using a 12V input supply, then you can omit U2 and supply the timer directly from input voltage.

The critical parts of the circuit are inductor which is the L1, the switching transistor, and switching frequency. You want a good beefy inductor if your load current is more than few milliamps. 

CATTION:

On the theoretically aspect you would be able to use much less inductance at higher frequencies, but higher frequencies make the switching element driving even more critical than it already is. I experimented quite a bit, and found that 1mH inductor and switching frequency around 30kHz works very well in the circuit shown. You do not want to make the switching frequency too high, then the next cycle would start before the voltage rise on the inductor, causing excessive losses without further rise in output voltage.

The ability to boost the output is also heavily dependent on switching speed of the power transistor. While it looks tempting to use a power MOSFET for switching element, high voltage high power ones usually have gate capacitance that requires more drive current for efficient operation than 555 timer is capable of. The MJE13005  switch transistor is readily available from the market, and dirt cheap. You can also often scavange these from old PC power supplies. MJE13005 has pathetic amplification ratio (typically the beta number is around 25), thats why you also need to drive it with significant base current, but it is still easier load than the MOSFET gate.

IMPORTANT: 

You may need a small heatsink for the power switch Q1, and with high input voltage you may need to use higher power 7812 or perhaps 7815 voltage regulator because at higher input voltage the power loss on the linear regulator will grow.

If you need higher output current then you will need to use higher current inductor and higher current D1. The C4,C5 is electrolytic capacitors types, with appropriate voltage rating for the input and output voltage. If you are using the circuit for load of 100V rating would be sufficient.

TESTING:

The output voltage of the booster is dependent on its input voltage, PWM fill ratio of the switching signal, and output load current. While this circuit can easily boost 12V to 70V, it is much easier to boost 24V to 70V with lower PWM fill ratio. Unless the voltage rating on C6 is very high you do not want to run the booster without any load on output. When testing, use a lowish input voltage and mark the position of lowest PWM fill factor on the Rv-1 trim-pot, and leave it adjusted there. Then connect the booster to its intended load, supply it with the input voltage that you intent to use, and adjust the RV-1 for required output voltage of your choice.

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