What is the output current of a single-phase CSI?

With full square-wave operation the load current is a square wave of amplitude ±Id: fundamental peak 4Id/π, fundamental RMS 2√2·Id/π ≈ 0.90·Id, total RMS = Id and THD ≈ 48.3%. Inserting a notch (zero-current interval) of half-angle β on each half-cycle gives a quasi-square wave whose fundamental RMS becomes (2√2/π)·cosβ·Id, so β directly controls the output.

How is the fundamental of a CSI controlled?

By pulse-width (notch) control. Instead of switching a full 180-degree square wave, the bridge holds a zero-current interval (a bypass or freewheel state) of half-width β at each zero crossing. The fundamental of the resulting quasi-square current is proportional to cos β, so increasing the notch angle reduces the output current fundamental without changing the DC-link current.

How is a single-phase CSI different from a VSI?

The CSI is the dual of the VSI. A VSI has a large DC capacitor (stiff voltage) and produces a square-wave voltage; a CSI has a large DC inductor (stiff current) and produces a square-wave current. CSI devices must block reverse voltage (so series diodes or symmetric devices are used) and an output capacitor is added for inductive loads to give the current a path and smooth the load voltage.

Single-Phase Current-Source Inverter (CSI) Simulator

Q: What is a single-phase current-source inverter?

A single-phase CSI is fed from a stiff DC current — a large inductor Ls in series with the DC source forces a nearly constant current Id — and an H-bridge of four devices steers that current back and forth through the load. Diagonal pair T1-T2 sends +Id through the load and pair T3-T4 sends −Id, producing a square-wave load current. It is the current-based dual of the single-phase voltage-source (H-bridge) inverter.

An advanced, physics-accurate simulator of the single-phase H-bridge current-source inverter (CSI) — a large inductor L_s makes the input a stiff current source I_d, and the four thyristors T₁–T₄ steer that current through the load. Diagonal pair T₁-T₂ drives +I_d and pair T₃-T₄ drives −I_d, giving a square-wave load current — or a quasi-square wave when you add a notch angle β to control the fundamental. Watch the true output current, load voltage and T₁–T₄ gating on a real-time oscilloscope, validated live against I₁ = (2√2/π)·cos β·I_d, with %THD and a real harmonic-spectrum FFT.

Single-phase H-bridge current-source inverter (CSI) circuit diagram: DC source Vin with series inductor Ls forming a stiff current source Is, feeding a four-thyristor H-bridge T1 T3 top and T4 T2 bottom that steers the current through the load to make a square-wave load current with fundamental peak 4Id/pi — Single-phase CSI — inductor L_s makes I_d stiff; T₁-T₂ steer +I_d, T₃-T₄ steer −I_d through the load. Square-wave fundamental I₁ = 4I_d/π.

Parameters

DC-link current I_d 20 A

Set by the large series inductor L_s — the input behaves as a stiff current source.

Frequency 50 Hz

Notch (pulse-width) angle β 0°

β = 0 → full square wave. Increasing β inserts a zero-current notch that reduces the fundamental as cos β.

Output filter & load

Output capacitor C 0 µF

0 = pure resistive load (as drawn). Adding C filters the stepped current into a smoother load voltage.

Load resistance R 10 Ω

Load inductance L 0 mH

Needs C > 0 to give the inductive current a path.

Sampling & display

Sampling rate 1200 pts/cycle

Periods shown

Run speed (DSO) 1.0×

Show sample points

Presets

Waveforms to display

T1–T4 gating Output current Load voltage Load current

Waveforms — one period Square

output current io fundamental load voltage vo load current

LIVE

Notch (pulse-width) fundamental control (topic-specific)

A CSI cannot vary its DC-link current quickly, so the output fundamental is set by pulse-width control: a zero-current notch of half-angle β at each zero crossing scales the fundamental by cos β. β = 0° gives the full square wave (maximum output); β = 60° cuts the fundamental to half.

Harmonic spectrum

Harmonics shown 40

Output-current THD (relative to fundamental)—

A single-phase square wave contains only odd harmonics (3rd, 5th, 7th…). The notch angle and any output filter reshape the spectrum.

Measurements

Live accuracy check — simulation vs closed-form theory

Output-current fundamental

Output-current total RMS

What is a single-phase current-source inverter (CSI)?

A current-source inverter (CSI) is the dual of the voltage-source inverter. Instead of a large capacitor holding a stiff DC voltage, a large series inductor L_s holds a stiff DC current I_d. The four-device H-bridge then steers this constant current through the load: diagonal pair T₁-T₂ sends +I_d and pair T₃-T₄ sends −I_d, so the output is a square-wave current, not a square-wave voltage.

Output current, fundamental and RMS

For full square-wave operation the load current alternates between +I_d and −I_d every half-cycle:

i1(peak) = 4Id/π ≈ 1.273·Id · I1(rms) = 2√2·Id/π ≈ 0.900·Id · I(rms) = Id · THD ≈ 48.3%

Only odd harmonics (3rd, 5th, 7th…) are present. This simulator forces the exact switching current and integrates the actual load network — the load resistance in series with any inductance, in parallel with an optional output capacitor — so every waveform, RMS and THD is exact.

Notch (pulse-width) control of the fundamental (topic-specific)

The DC-link current cannot change quickly, so the CSI controls its output by pulse-width (notch) control. A zero-current interval of half-angle β is inserted at each zero crossing — the bridge is put into a bypass/freewheel state so the load current is momentarily zero. The resulting quasi-square wave has a fundamental:

I1(rms) = (2√2/π)·cos β·Id

so β directly and smoothly reduces the output fundamental without touching I_d. At β = 0° the output is the full square wave; at β = 60° the fundamental is halved.

CSI vs VSI — the duality

Feature	Current-source inverter (CSI)	Voltage-source inverter (VSI)
DC link	Large inductor → stiff current I_d	Large capacitor → stiff voltage V_dc
Output shape	Square-wave current	Square-wave voltage
Fundamental	4I_d/π (peak)	4V_dc/π (peak)
Control	Notch / pulse-width (cos β)	Phase-shift / PWM
Devices	Reverse-blocking (series diode)	Anti-parallel diode

Compare with the three-phase CSI, and the single-phase voltage-source half-bridge and full-bridge inverters.

Applications

Induction heating, current-fed resonant links, wireless-power drivers and machine drives where a robust, inherently current-limited source is desirable.

Frequently asked questions

What is a single-phase current-source inverter?

An H-bridge fed from a stiff DC current (large series inductor) instead of a stiff DC voltage. It produces a square-wave load current of amplitude ±I_d.

What is the output current fundamental?

For a full square wave the fundamental peak is 4Id/π, fundamental RMS 2√2·Id/π ≈ 0.90·Id, total RMS = Id and THD ≈ 48.3%.

How is the output controlled?

By notch / pulse-width control: a zero-current interval of half-angle β scales the fundamental by cos β.

How is a CSI different from a VSI?

The CSI is the dual of the VSI: inductor vs capacitor DC link, square-wave current vs voltage, reverse-blocking devices vs anti-parallel diodes.

Power4All · Single-Phase CSI interactive simulator. All waveforms are produced by numerical integration of the actual switching circuit and validated against closed-form theory.