- Chopper Definition: A chopper in power electronics is defined as a device that converts fixed DC voltage to a variable one through high-speed switching.
- Types of Choppers: Various choppers like buck, boost, and buck-boost converters cater to different needs—reducing, increasing, or dynamically changing the voltage level.
- Operation Mechanics: Choppers function by controlling the duration a switch is ON or OFF, directly influencing the voltage output.
- Application Significance: Choppers are integral in applications where precise voltage control is required, such as in electric vehicles and renewable energy systems.
- Quadrant Operations: Understanding the quadrant operation of choppers helps in selecting the right chopper type for specific directional current and voltage needs.
A DC to DC converter changes power from one DC level or polarity to another. It lets a fixed voltage source supply equipment that needs a controlled output. Applications include traction systems, industrial drives and battery powered vehicles. A switched converter is traditionally called a chopper.
The power switch operates repeatedly at high frequency. Inductors and capacitors store and transfer energy, while the controller adjusts switch timing to regulate voltage or current. The topology determines whether the result is lower, higher, inverted or bidirectional.
A chopper can step voltage down or up only when its circuit is arranged for that function. It transfers energy between ports of a DC circuit without a line-frequency transformer, although isolated DC-DC topologies may include a high-frequency transformer.
Devices Used in a Chopper
Self-commutated switches: An IGBT, power BJT, GTO or Power MOSFET can be turned on and off by its drive circuit. MOSFETs and IGBTs are common in modern hard-switched converters.
Line- or load-commutated switches: A Thyristor or SCR latches on and needs the main current to fall below its holding current before it turns off.
The diagrams use an ideal switch symbol. A real device has conduction and switching loss plus voltage and current limits. It may also include a reverse diode. The arrow marks the defined current direction.
1) Step-down chopper:
A buck converter produces an average output below its input in normal operation. The adjacent circuit shows the basic switching arrangement.
With ideal switch CH on, source Vs is applied to the load, so Vo = VS.
With CH off in this simplified waveform, source Vs is disconnected and Vo = 0.
The waveform uses these quantities:
TON is the switch on-time
TOFF is the switch off-time
VS is the input voltage
Vo is the output voltage
T is the switching period, equal to TON + TOFF
Step-Down Chopper with a Resistive Load
When CH is on, Vo = VS
When CH is off, Vo = 0
Here D is the duty cycle, D = TON/T.
For the ideal switched-resistor circuit, varying TON from 0 to T gives 0 ≤ D ≤ 1 and changes average Vo from 0 to VS.
The average output is therefore no greater than the input. A practical filtered buck converter has ripple and losses, so feedback adjusts duty cycle to hold the required output under changing input and load.
Step-Down Chopper with an Inductive Load
With CH on, Vo = VS
With the ideal freewheel path conducting, Vo = 0
During the Chopper On-Time
The corresponding peak-to-peak change in load current is
During the Chopper Off-Time
A sufficiently large inductance can keep load current continuous over the full switching period. When CH turns off, the inductor changes its terminal voltage to maintain current, which freewheels through diode FD.
Equating the on-time and off-time current changes gives
Substitution in the on-time relationship gives
These relationships assume ideal components and continuous conduction. At lighter load, current may reach zero and the discontinuous-mode conversion relationship also depends on inductance, switching frequency and load.
2) Step-up chopper or boost converter:
A boost converter raises a positive DC input to a higher output under its normal operating conditions. The following diagrams show its switching states and waveforms.
Operation of a Step-Up Chopper
When CH is on, it connects the inductor to the input and the diode isolates the output. The output capacitor, not a short circuit, supports the load during TON. The inductor voltage is VS = VL
Here ΔI is the peak-to-peak inductor-current ripple.
When CH is off, the input and inductor both deliver energy through the diode. Their voltages combine, so VS and VL produce the output relationship
Equating the on-time and off-time inductor-current changes gives
For an ideal boost converter in continuous conduction, VO exceeds VS for 0 < D < 1. The mathematical result tends to infinity as D approaches one, but a real converter cannot operate there. Minimum off-time, resistance, switch limits, inductor saturation and control constraints set a finite maximum output.
Buck-Boost or Step-Up/Step-Down Converter
The circuit shown is an inverting buck-boost converter. It can produce an output magnitude below or above the input, but its output polarity is opposite to the input reference.
Operation of Buck-Boost Converter
When CH is on, the source applies voltage across inductor L and stores energy.
Thus VL = VS
When CH is off, the inductor changes polarity and delivers energy through the load and diode.
Equating the on-time and off-time changes gives
The magnitude is
These ideal continuous-conduction equations use 0 ≤ D < 1.
At D = 0, Vo = 0
At D = 0.5, the magnitude of Vo equals VS
As D approaches 1, the ideal magnitude of Vo tends to infinity, but D = 1 is not a valid energy-transfer state
For 0 ≤ D ≤ 0.5, the magnitude lies in 0 ≤ VO ≤ VS, which is step-down operation.
For 0.5 ≤ D < 1, the magnitude lies in VS ≤ VO, which is step-up operation. The actual output is negative with respect to the input reference.
Classification by output-voltage and current direction
Semiconductors used in choppers have defined conduction directions, but bridge and diode arrangements can produce different output-voltage and output-current directions. This leads to a quadrant classification.
The Vo – Io plane uses the signs of output voltage and current.
The arrows for Io and Vo in Figure 1 define the positive references.
In quadrant 1, Vo and Io are positive. Power flows from source to load, so positive Vo and Io correspond to forward motoring for a DC drive.
In quadrant 2, Vo is positive and Io is negative. The operating point therefore has positive Vo and lies in the Vo – Io second quadrant, and power returns from the load to the source during forward regenerative braking.
In quadrant 3, both Vo and Io are negative. Negative Vo with negative Io gives positive load power and reverse motoring.
In quadrant 4, voltage is negative while output current is positive. Negative Vo and positive Io return power during reverse regenerative braking.
A chopper may support one or two quadrants. A full bridge can support all four. Its switch and diode arrangement determines the allowed voltage, current and power-flow directions.
Type-A Chopper
A Type-A chopper operates in quadrant 1 of the Vo – Io plane.
When CH is on, Vo and Io are positive, so power flows from source to load.
When CH is off, inductive current freewheels through the diode. Ideal Vo is then zero while Io remains positive.
The average Vo and Io are positive, and average Vo is below the source. Type A is a step-down, one-quadrant converter used for forward motoring.
Type-B Chopper
A Type-B chopper operates in quadrant 2 of the Vo – Io plane.
The load must contain a DC voltage source, such as motor back-emf E.
When CH is on, Vo is zero and load current builds in the negative reference direction, storing energy in the inductance.
When CH turns off, the inductive voltage raises the load terminal above source VS, forward-biasing diode D and returning energy to the source.
Therefore Io is negative while Vo is positive or zero. Their product is negative, so power flows from load to source on the Vo – Io plane. Type B provides forward regenerative braking.
Type-C Chopper
A Type-C chopper operates in quadrants 1 and 2 of the Vo – Io plane. It combines Type-A and Type-B paths as shown.
For quadrant 1, CH1 switches the source to the load. Output Vo and current Io are positive. When CH1 turns off, inductive current freewheels through D1; Io keeps its positive direction while output voltage is zero. CH1 therefore provides Type-A forward-motoring operation.
For quadrant 2, CH2 provides the current-building interval. Output Vo is zero while Io flows in the negative reference direction. When CH2 turns off, the load terminal rises according to
When this exceeds source VS, negative current flows through diode D2. Thus Vo remains positive while Io is negative. CH2 provides Type-B regenerative operation.
The combined Type-C chopper supports forward motoring and forward regenerative braking. The controller must prevent both active switches from creating a direct source short circuit.
