What is a Class D (two-quadrant) chopper?
A Class D chopper uses two controlled switches and two diodes. When both switches are turned on together the source is applied directly across the load as +Vs; when both are turned off, the inductive load current forces the two diodes to conduct and the load is connected across the source with reversed polarity, −Vs. The output voltage therefore swings between +Vs and −Vs, and its average Vo = (2D−1)·Vs can be positive or negative while the load current stays in one direction. That makes it a two-quadrant converter covering Quadrant I (positive voltage) and Quadrant IV (negative voltage), both at positive current.
Output voltage, current & ripple
Here D is the fraction of each period both switches are on. At D = 0.5 the average output is zero; for D > 0.5 it is positive and for D < 0.5 negative. Because the output alternates between +Vs and −Vs every period, the current ripple is larger than in a Class A chopper for the same average. This simulator integrates the real R-L-E load through both states, runs to steady state, and compares the measured average to (2D−1)·Vs in the accuracy panel.
Continuous vs discontinuous conduction
Enough inductance keeps the current continuous (CCM) and Vo = (2D−1)·Vs holds. If the average voltage is close to the back-EMF and the inductance is small, the current can reach zero within a period (DCM); the simulator flags this and stops comparing against the formula.
Advanced options in this simulator
- Duty & frequency: sweep the duty through 0.5 to swing the average output from −Vs toward +Vs.
- Motor model & device model: R-L-E load with switch/diode drops; the accuracy check stays locked to the ideal formula.
- Four-quadrant map: Class D moves between Quadrant I and Quadrant IV as the duty crosses 0.5.
- Ripple spectrum & export: FFT of voltage/current and CSV/report/PNG export.
The five chopper classes
| Class | Quadrants | Average output | Use |
|---|---|---|---|
| Class A | I (V+, I+) | Vo = D·Vs | Forward motoring (step-down) |
| Class B | II (V+, I−) | Vo = (1−D)·Vs | Regenerative braking |
| Class C | I & II | Vo = D·Vs | Motoring + braking |
| Class D | I & IV | Vo = (2D−1)·Vs | ±V, one current direction |
| Class E | I–IV | Vo = (2D−1)·Vs | Full four-quadrant drive |
Explore the others: Class A, Class B, Class C and Class E chopper simulators. For the full theory see the Class D chopper tutorial.
Applications
Drives that need a reversible output voltage with one-directional current — certain servo and positioning drives, and as a building block of the four-quadrant Class E chopper.
Frequently asked questions
What is the average output voltage of a Class D chopper?
Vo = (2D−1)·Vs. It is zero at D = 0.5, positive above and negative below.
Why is it a two-quadrant chopper?
The output voltage can be positive or negative while the current stays one direction, so it covers Quadrants I and IV.
Why is the ripple higher than Class A?
Because the output swings the full ±Vs every period instead of Vs to 0, the volt-second swing across the inductor — and hence the current ripple — is larger.
When does discontinuous conduction occur?
When the average output is near the back-EMF with small inductance, the current can reach zero within a period; the simulator flags DCM.