Step Up/Down Chopper

In this article, we will be going through what is a chopper, we will look at its classification. Then we will go into detail about Step up/down chopper. We will look at its working with working formula and waveform. At last, we will be looking at its Advantages, Disadvantages, and Applications, as well as a solved Example.

What is Chopper?

A chopper is a power electronic device that is used to control the amount of power delivered to a load by adjusting the average voltage or current. Choppers are primarily employed in DC-DC conversion applications, where they regulate the output voltage by rapidly switching the input power. The basic operation involves the periodic interruption or chopping of the input power, followed by smoothing or filtering of the chopped waveform to obtain the desired output. This process is achieved by using semiconductor devices such as thyristors, transistors, or integrated circuits. The chopper operates by turning on and off at a high frequency and by adjusting the duty cycle the effective voltage delivered to the load can be controlled. This capability makes choppers highly valuable in applications where precise voltage regulation is essential, such as in battery management systems, motor drives, and renewable energy systems. Choppers find extensive use in applications requiring variable-speed motor control such as in industrial drives or electrical vehicles. By employing chopper circuits, it becomes possible to control the speed and torque of electric motors with high efficiency and accuracy. There are different types of choppers, such as step-up choppers that increase the output voltage, step-down choppers that decrease the output voltage, and voltage inverter choppers that can provide both step-up and step-down operations. The selection of the appropriate type depends on the specific requirements of the application. The key advantage of choppers is the ability to convert DC power with minimal energy loss making it a versatile and energy-efficient solution.

Classification of Choppers

Choppers are commonly categorized according to their operational modes, including:

• Step – up chopper
• step – down chopper
• Step up/down chopper

Step Up/Down Chopper (Buck-Boost Chopper)

A step up/down chopper also known as a buck-boost converter is a power electronics device designed to control and modify the voltage level supplied to a load . This chopper operates by employing pulse width modulation techniques to control average output voltage making it a versatile tool for both increasing and decreasing the DC voltage. It belongs to the family of chopper circuits ,which are used for DC-DC voltage conversion .The primary function of a step-up/down chopper is to either increase or decrease the input DC voltage to achieve the desired output voltage .The basic configuration of a step up/down chopper consists of a semiconductor switch , a freewheeling diode , an inductor and a load .The chopper operates periodically turning the switch on and off at a high frequency . During the on state , energy is stored in an inductor and during the off state this stored energy is transferred to the load through the freewheeling diode.

Construction and Components of Step Up/Down Chopper

It is an power electronic device used for voltage regulation by either increasing or decreasing the input voltage . It consists of a power semiconductor switch , typically a transistor and an energy storage element such as an inductor .In step-up mode the chopper increases the input voltage by controlling the switch’s duty cycle , storing energy in the inductor during the on state and releasing it during the off state . To step down the voltage the switch is turned on for a shorter duration , resulting in a lower output voltage. Control strategies such as pulse width modulation are often employed to regulate the output voltage .Step up/down choppers find applications in various fields including power supplies , electric vehicles and renewable energy systems facilitating efficient voltage conversion .The main components of step up/down chopper include :

• DC source : The input to the chopper is a DC source which can be a battery or a rectified AC power source .
• Switching device : The switching device is a semiconductor switch that controls the flow of current through the circuit .It rapidly switches on and off to control the output voltage .
• Inductor : The inductor is connected in series with the load and stores energy in the form of a magnetic field . It smoothens the output current and plays a crucial role the energy transfer process .
• Capacitor : In some chopper circuits , a capacitor may be added to filter the output voltage , reducing ripple and improving the stability of the output .
• Load : The load is a device or system that is powered by the chopper . It could be a motor , an electronic circuit or any other electrical device .
• Freewheeling diode : It provides a path for inductor current when the chopper switch is turned off . The diode allows the energy stored in the inductor to circulate through the load , preventing voltage spikes and ensuring continuous current flow during the switch off period , thereby maintaining a stable output voltage and enhancing overall efficiency of the chopper circuit .

Working of Step Up/Down Chopper

A buck-boost chopper, also known as a buck boost converter or regulator , is a type of DC-DC converter used to efficiently regulate the output voltage of a power supply. It is capable of both stepping up and stepping down the input voltage to achieve the desired output voltage . This flexibility makes it a versatile choice for various applications , such as battery-powered devices and renewable energy systems . The operation of a buck boost converter involves the use of semiconductor switches to control the energy flow. The key components include an inductor, a capacitor and the switching devices .The converter operates in two main modes: the buck mode and the boost mode .

Buck mode : In buck mode , the chopper is configured to step down the input voltage. The switch is closed , allowing current to flow through the inductor , creating a magnetic field . The inductor stores energy during this time. The output voltage is the sum of the input voltage and the voltage across the inductor. The diode is reverse biased preventing current from flowing through it .Here the duty cycle is adjusted to provide a lower output voltage than the input voltage . This is beneficial in situations where a reduction in voltage is needed, such as in power supplies for electronic devices or in applications requiring variable speed drives for motors.

Boost mode: In the boost mode, the switch is open, causing the inductor to discharge . The diode becomes forward biased and the stored energy is released to the load. The output voltage is now the difference between the input voltage and the output voltage across the inductor. Here the duty cycle is adjusted to provide a higher output voltage than the input voltage. This capability is useful in applications where an increase in voltage is required , such as in battery charging systems or in certain ,motor drive applications .

Working of Step Up/Down Chopper with Low Pass Filter

A step up/down chopper is used to either increase or decrease the output voltage from a DC power source . The chopper operates by controlling the duty cycle of a switch , typically a semiconductor device like a transistor . The key component in achieving a stable output voltage is the integration of a low pass filter. In the step up operation , when the switch is closed , the input DC voltage is applied to the load , resulting in energy storage in the inductor .When the switch is open , the inductor discharges energy into the load , effectively raising the output voltage . Conversely , in step-down chopper when the switch is closed the inductor stores energy from the input DC voltage . When the switch opens , the inductor discharges energy into the load , lowering the output voltage .

To smooth the pulsating output voltage and obtain a more constant and stable signal , a low pass filter is integrated into the chopper circuit . This filter typically consists of an output capacitor and an inductor . The capacitor smoothens the voltage pulses by storing and releasing energy during the on and off cycles of the switch , while the inductor filters out high frequency components , allowing only the desired low frequency variations in the output . The low pass filter minimizes voltage ripple , reduce harmonics and ensures a more continuous and regulated output voltage . This is especially crucial in applications where a stable and constant DC voltage is required , such as in power supplies for electronic devices .

step up/down chopper circuit

When switch S is ON , Diode D is reverse biased and inductor starts discharging . V_L = V_s

When switch S is OFF , Diode D is forward biased and inductor starts charging . V_L = V₀

Since the average voltage across the inductor is zero.

Vs = Supply voltage

V_L = Load voltage

I _L =Load current

V _c = Capacitor voltage

V₀ =Average output voltage

D = Duty cycle

V_s(DT_s) + V₀(1 – D) T_s =0

V ₀= -[D/(1 – D) ] V_s

step up/down chopper waveform

Ripple in the Inductor Current

During ON period:

V _L= V_s

L (di _L /dt _on ) = V_s

L ( Δi _L/DT_s) = V_s

ΔI _L = DV_s/L f

Ripple in the Capacitor Voltage

During ON period , capacitor is feeding the load.

I _c = -I₀

C (d V _c/dt ) = -I₀

C ( ΔV/DT_s ) = – I₀

ΔV _c = DI₀ / f C

Advantages of Step Up/Down Chopper

• It can perform voltage increase or decrease with a minimal number of components .
• The chopper can be used in both step up and step-down modes providing flexibility in applications without the need for different types of converters .
• During the switch off periods energy stored in the inductor is transferred to the load . This energy recovery feature contributes to overall system efficiency .
• In comparison to the other voltage regulation methods , step up/down choppers can offer a cost effective solution for various power electronic devices .
• Choppers can respond quickly to changes in load conditions or input voltage making them suitable for applications with dynamic and variable power requirements .
• It offers lower operating duty cycle.
• Buck boost choppers can regulate the output voltage efficiently .
• Their flexibility in regulating both higher and lower voltages makes them adaptable to a wide range of power supply needs.
  

Disadvantages of Step Up/Down Chopper

• There is no isolation from input side to output side which is crucial for many applications.
• High gain cannot be achieved with this converter type as efficiency is poor for high gain.
• Rapid switching of the semiconductor devices in the buck boost converter generates high frequency noise.
• The rapid switching in chopper can generate electromagnetic interference , potentially causing issues with other electronic devices in the vicinity . EMI mitigation techniques such as shielding and filtering may be required to comply with electromagnetic compatibility standards .
• The trade-off between achieving a high step up ratio and maintaining efficiency can limit the overall performance of the chopper .

Applications of the Step Up/Down Chopper

• Step up/down choppers are commonly used in battery charging systems to provide a controlled and adjustable charging voltage to rechargeable batteries .
• UPS systems can use choppers to maintain a stable output voltage regardless of fluctuations in the input voltage .
• In electric vehicles step up/down choppers are employed to control the output voltage supplied to electric motors allowing for variable speed and efficient energy usage .
• Step up/down choppers are used in solar power systems to adjust the voltage generated by solar panels to match the requirements of the load or the energy storage system .
• In space applications , step up/down choppers are used to control and regulate power systems adapting to varying conditions encountered in space .
• In electric or hybrid trains , step up/down choppers are used for traction control , allowing for efficient and variable speed operation .

Conclusion

The step up/down chopper is a fundamental component in power electronics , plays a pivotal role in efficiently adjusting and stabilizing DC output voltages. Its versatility in both stepping up and stepping down voltages makes it applicable in a wide range of electronic systems and devices . The mechanism involves the controlled switching of a semiconductor device typically a transistor to modulate the duty cycle and consequently , the average output voltage. This technique is identified as pulse width modulation . To ensure smooth and continuous output voltage a low pass filter is integrated into the chopper circuit . This filter comprised of an output capacitor and an inductor minimizes voltage ripples , reduce harmonics and enhances the overall stability of the output voltage .The capacitor serves to store and release energy during the switching cycles . while the inductor filters out high frequency components , allowing only the desired low frequency variations to pass through . The step up/down chopper with its associated low pass filter , finds applications in various fields such as motor drives , power supplies , and voltage regulators . Its efficiency in power conversion and ability to deliver stable DC voltages make it an indispensable tool in modern electronics . As technology continues to advance , the step up/down chopper remains a key player in enabling energy efficient and reliable power management in diverse electronic systems .

Solved Example on Step Up/Down Chopper

The output of a buck boost converter is 100V with input voltage 120V . If the frequency of the switch is 10KHz then the turn off time of the switch will be ?

Given output voltage = 100V

input voltage = 120V

Frequency f = 12KHz

For step up/down chopper = *

100 = *120

D = 0.45

= 0.45

T = = = 100 µs

Ton = 0.45 * 100

Ton =45 µs

Toff = T – Ton = 100 – 45

Toff =55 µs

FAQs on Step Up/Down Chopper

1. Where is step up/down chopper commonly used ?

Step up/down choppers find applications in various electronic devices such as battery powered systems , solar power systems and electric vehicles where the input voltage may vary and a stable output voltage is required .

2. Can a step up/down chopper handle both step up and step down operations simultaneously ?

No , a step up/down chopper can operates either in step up or step down mode based on the relationship between the input and output voltages . It cannot perform both operations simultaneously .

3. What is the duty cycle in step up/down chopper ?

The duty cycle represents the proportion of time the switch is in the “on” state compared to the entire switching period. This parameter is critical for regulating the output voltage of a step-up/step-down chopper.