or Step Up

http://schmidt-walter.fbe.fh-darmstadt.de/snt/snt_eng/snte_pdf.html

http://alpha400.ee.unsw.edu.au/elec4240/Lecture_13.pdf

The boost circuit takes advantage of reverse polarity generated across the inductor during the OFF period, and places it in series with the input voltage. This allows the output voltage to exceed input voltage. The output is determined by the switch duty cycle.

Initial State (simplified circuit)

Step 1 – Close the switch.

When the switch closes:

• The inductor starts to store energy.
• The diode does not conduct.

The inductor current during the ON period is:

and reaches a maximum of:

Step 2 – Open the switch.

When the switch opens:

• The voltage across the inductor reverses polarity.
• The diode starts to conduct.

The inductor current during the OFF period is:

The slope of the falling inductor current is:

The output voltage rises to:

The inductor discharges its energy to the capacitor and load.

Step 3 – Close the switch.

When the switch closes:

• The voltage across the inductor reverses polarity.
• The capacitor discharges into the load.
• The process repeats.

Principle Waveforms

The capacitor current can be approximated by using Kirchoff’s current law: the sum of all currents in a node is zero.

By increasing the ON time, this circuit can create an output several times the input voltage.

Notice that the stored energy in the inductor is transferred to the load when the switch is OFF. This creates very high surge currents through the diode. It also creates very high voltages across the switch.

Deriving the Minimum Inductor Size

From the falling inductor current (assuming it falls to zero at the end of the cycle) we obtain:

From this we can determine the inductor size: