The main difference between a step-up and step-down chopper is that average value of output voltage of former is greater than its input DC voltage whereas it is less than the DC input voltage for latter. This is the basic but major functional difference between the two.
Step-down and step-up choppers are static devices which coverts constant DC input into variable DC output. But the purpose of both these types are different. Due to functional difference between the two types, their circuit diagram also differs. Figure below differentiates their circuit diagram.
From the above circuit diagram, it is clear that chopper (CH) is connected in series with the load for step-down type whereas it is connected in parallel with the load in step-up chopper.
Some of the major differences between step-up and step-down chopper are tabulated below:
|Step-up Chopper||Step-down Chopper|
|The average value of DC output voltage is more than the input source voltage.||The average value of DC output voltage is less than the input source voltage.|
|The average output voltage is given by formula V0 = [Vs / (1-α)], where α is duty cycle of chopper.||The average output voltage is given by formula V0 = αVs|
|Ideally output voltage control from zero to infinite may be achieved. However, the duty cycle is changed in such a way to achieve desired output voltage.||Voltage control from zero to source voltage (Vs) may be achieved.|
|Power may flow from load to source if load contains a DC source.||The direction of power flow is always from source to load in this type of chopper.|
|Step-up chopper is a class-B type chopper.||Step-up chopper is a class-A type chopper.|
|A high value of inductance is required in series with the load. This is required as the energy stored in this inductor during chopper ON period is delivered back to the load during chopper OFF time. In this way, the energy fed to load is from source as well as from the inductor during chopper OFF time.||Inductor is not required in step-down chopper.|
|This type of chopper is used for battery charging, voltage booster, regenerative breaking etc.||This chopper is mostly used for motor speed control.|