Four-Quadrant Operation
Describing a supply by its quadrants is the precise way to state what it can do. The four quadrants come from the two signs of voltage and the two signs of current; how many a supply covers tells you exactly which test scenarios it can handle.
Reading the V-I plane
Plot output voltage on one axis and output current on the other. The plane divides into four quadrants by the sign of each:
| Quadrant | Voltage | Current | Power flow | Role |
|---|---|---|---|---|
| Q1 | + | + | Out of supply | Sourcing (positive) |
| Q2 | + | − | Into supply | Sinking (positive rail) |
| Q3 | − | − | Out of supply | Sourcing (negative) |
| Q4 | − | + | Into supply | Sinking (negative rail) |
The product of voltage and current gives power; its sign tells you whether energy leaves the supply (sourcing) or enters it (sinking). A unit's quadrant coverage is simply which of these four boxes its output can reach.
One, two and four quadrants
A plain supply lives in a single quadrant: positive voltage, positive current, sourcing only. A two-quadrant supply adds the ability to sink at one voltage polarity — typically Q1 and Q2 — so it can both charge and discharge a device while voltage stays positive. This covers most battery and unipolar DC-bus work.
A four-quadrant supply reaches all four boxes: it can present positive or negative voltage and source or sink at either. That means it can produce a true bipolar output and absorb energy regardless of polarity. The added capability is what enables bipolar device testing and arbitrary waveform reproduction across zero volts.
What each capability enables
- Two-quadrant (Q1+Q2): battery charge/discharge cycling, energy-storage testing, holding a DC bus while absorbing inverter regeneration — all at positive voltage.
- Four-quadrant: bipolar power-electronics testing, emulating a source whose voltage crosses zero, driving a load through a full positive-to-negative swing, and absorbing energy in either polarity.
Choosing between them is a question of whether your device under test ever needs the output to go negative. If it does not, a two-quadrant bidirectional supply is sufficient and simpler; if it does, four-quadrant is required. Ultra Power Systems specifies the quadrant coverage for each N35500 configuration.
Quadrants and seamless crossover
Quadrant capability is only useful if the supply can move between quadrants cleanly. Crossing from Q1 to Q2 means current reverses through zero; crossing from Q1 to Q3 means voltage reverses through zero. A capable four-quadrant supply makes both transitions without a dead band or glitch, so a test waveform that sweeps through the origin stays smooth. That continuity is what links quadrant theory to the practical source/sink crossover discussed in the operation article.
It also matters how fast the boundaries can be crossed. Reproducing an arbitrary current or voltage waveform that passes through zero requires the loops to reverse polarity within their response window — on the order of milliseconds — while holding accuracy. If the crossover is slow or noisy, the reconstructed waveform distorts exactly at the origin, which is often the most sensitive region of a power-electronics test. Quadrant coverage on paper therefore has to be backed by genuine bipolar transient performance, not just static reach into each box.
Frequently asked questions
What is the difference between two-quadrant and four-quadrant operation?
Do I need four quadrants to test a battery?
What does the sign of power tell me in the V-I plane?
Specifying a bidirectional DC supply?
The articles here describe the N35500 platform from Ultra Power Systems.