Summary: Choppers

Table of Contents
1. Choppers
2. Types
3. Principle of Chopper Operation
4. Classification of Choppers
5. Class E - Four Quadrant Operation
6. Thyristor Chopper Circuits - Commutation
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Choppers

• Chopper provides adjustable DC from a fixed DC source; it is analogous to an AC transformer.

Types

• AC link chopper
• DC chopper

Principle of Chopper Operation

• Uses a high-speed ON/OFF semiconductor switch (e.g., SCR).
• Inductor L and FWD keep load current continuous.
• Timing terms:
  • T = T_ON + T_OFF
  • f = 1/T is the chopping frequency
  • δ = T_ON/T is the duty cycle
• Average output voltage V0 is controlled by changing T_ON or f.

Classification of Choppers

• Class A chopper - Step-down chopper operating in the first quadrant. Load current and load voltage are positive; current flows from source to load.
• Class B chopper - One-quadrant (second quadrant) operation: load voltage positive, load current negative; current flows from load back to source. Thyristor acts as a switch S (closed when conducting, open when off). When S is closed battery drives current through inductance; when S opens, inductance energy returns to supply through diode D and current decreases.
• Class C chopper - Two-quadrant device using S1, S2 (thyristors) and D1, D2 (diodes). Load voltage always positive; load current can be positive or negative. Combination of class A and B actions. Used for motoring and regenerative braking. Do not turn S1 and S2 on together (would short the source).
• Class D chopper - Load current always positive; load voltage can be positive or negative (first or fourth quadrant). When switches conduct voltage is positive; when switches off diodes conduct and voltage becomes negative. Power can flow both directions.
• Class E chopper - Four-quadrant chopper (both voltage and current can be positive or negative). Formed by combining two class C choppers. Operation is described by four quadrant modes below.

Class E - Four Quadrant Operation

• Quadrant I - S4 on, S2 and S3 off, S1 operated. When S1 and S4 conduct IL flows from source to load. When S1 turns off IL freewheels through S4 and D2. Both voltage and current positive.
• Quadrant II - S2 operated, others off. Negative current flows through L, S2, D4, E; inductance stores energy. When S2 turns off current is returned to source through diodes (energy feedback to source).
• Quadrant III - S1 and S4 off, S2 on and S3 operated. With S3 on the load connects to source via S2 and S3; both voltage and current are negative. When S3 turns off negative current freewheels through S2 and D4.
• Quadrant IV - S1, S2, S3 off, S4 operated; supply polarity reversed. When S4 on positive current charges L; when S4 turns off current is fed back to source through D3 and D2. Load current positive but load voltage negative (fourth quadrant).

Thyristor Chopper Circuits - Commutation

• Commutation = turning off a conducting thyristor. DC choppers require separate commutation circuitry to force the SCR to regain blocking capability.
• A conducting thyristor is turned off by reducing anode current below holding current and applying reverse voltage.
• Commutation methods are broadly classified:
  • 1. Forced Commutation - external L and C (not carrying load continuously) are used. Two schemes:
    • Voltage commutation - a large reverse voltage (usually from a charged capacitor) is applied to the conducting thyristor to reduce anode current rapidly and assist turn-off.
    • Current commutation - an external reverse current pulse (usually from a charged capacitor) greater than the load current is passed to make net thyristor current zero and turn it off. An anti-parallel diode is often used so the diode drop reverse biases the main SCR; commutation time is longer than voltage commutation. Both schemes use an auxiliary SCR to initiate commutation.
  • 2. Load commutation - the conducting thyristor turns off when the load current either becomes zero due to load circuit parameters or is transferred to another device.