The post explains an universal IC 555 based buck-boost circuit which can be used for various different applications involving efficient power processing requirements.

Using IC 555 for the Buck-Boost


This highly efficient and effective buck-boost circuit using the work horse IC 555 would allow you  to convert an input source voltage to any required degree, either bucked or boosted, as desired.

We have already learned the concept comprehensively through one of my previous articles where we discussed the versatility of this buck-boost type of topology.

As shown in the circuit diagram below (click to enlarge) the configuration is basically a combination of two distinct stages, viz the upper buck-boost converter stage and the lower IC 555 PWM controller stage.

The buck-boost stage consists of a mosfet which acts like a switch, the inductor which is the main power converting component, the diode which just like the mosfet forms a complementary switch, and the capacitor quite like the inductor forms a complementary power converter device.

The mosfet needs to operate through pulsed triggering so that it alternately switches the input voltage ON and OFF across the inductor in response to its gate voltage.

Therefore the gate voltage should also be in a pulsed form which is accomplished through a IC555 PWM generator stage.

Simulation and Working


The associated IC555 PWM generator is integrated to the mosfet for accomplishing the above discussed operation.

During the ON time of the mosfet, the input voltage is allowed to pass through the mosfet and is applied right across the inductor.

The inductor owing to its inherent property tries to counter this sudden infliction of current by absorbing and storing the power in it.

During the subsequent OFF period of the mosfet, the input voltage is shut off by the mosfet, the inductor now experiences a sudden change in current from peak to zero. In response, the inductor counters this by reversing its stored power across the output terminals via the diode which now acts in the forward biased condition.

The above power from the inductor appears with opposite polarity across the output where the intended load is connected.

The capacitor is positioned to store a portion of the power in it, so that it can be used by the load during the ON time of the mosfet when the diode is reverse biased and power cut off across the load.

This heps to maintain a steady and stable voltage across the load during both ON and OFF cycles of the mosfet.

Using PWM as the ontroller


The level of voltage, whether it's a boosted voltage or bucked voltage depends upon how the mosfet is controlled by the PWM generator.

If the mosfet is optimized with higher ON time than the OFF time then the output would generate boosted voltage and vice versa.

However there may be a limit to this, care must be taken not to exceed the ON time beyond the full saturation time of the inductor, and the OFF time must not be below the minimum saturation time of the inductor.

For instance suppose it takes 3ms for the inductor to become fully saturated, the ON time in this case can be set within 0 - 3ms, and not beyond that, This will result in a boost from minimum to maximum depending upon the value of the chosen inductor.

The associated pot wit the IC555 PWM generator can be effectively tweaked for acquiring any desired buck-boost voltage at the output.

The inductor value is a matter of trial and error, try to incorporate as many winding as possible for acquiring better and efficient results and diverse range.

 Circuit Diagram


buck-boost control circuit



The above design can be suitably upgraded for implementing an automatic output voltage correction with the help of the following modifications:



The 1K preset may be appropriately set initially for determining the desired control point.

IC 555 Pinouts

IC 555 pinout

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