What is a PWM current source converter?

A pulse width modulation (PWM) current source converter (CSC) maintains a near-constant DC-link current using inductive energy storage while dynamically adjusting AC-side voltage through PWM switching. PWM CSCs provide safe, reliable, and precise power control for various applications, such as large synchronous motor drives, specialized uninterruptible power supplies (UPS), and industrial heating systems. This article explores the fundamentals of PWM CSC architecture and key functionality. It also explains the differences between PWM CSCs and PWM voltage source converters (VSCs) and reviews PWM CSC-specific applications.

Exploring PWM CSC functionality

PWM techniques control power delivery in electronic devices by varying the duty cycle, which is the ratio of “on” time to the total period of a periodic signal. As shown in Figure 1, precisely adjusting the duty cycle enables PWM to regulate the average voltage or current delivered to a load, enabling efficient energy management and minimizing switching losses.

Figure 1. A graphical representation of PWM, illustrating how a modulated switching signal determines the average output voltage over time. (Image: Thompson Linear)

PWM-based CSCs leverage this technique, regulating current rather than voltage. A large DC-link inductor maintains a near-constant DC input current, ensuring stability even during voltage or load fluctuations.

As shown in Figure 2, the converter employs PWM techniques to modulate its switching devices. This modulation shapes the AC-side output current in current source inverters (CSIs) or the AC input current in current source rectifiers (CSRs) into a sinusoidal waveform with minimal harmonics. CSIs actively control AC output current, while CSRs regulate AC input current to achieve sinusoidal waveforms and improve power factor.

Figure 2. A PWM CSC interfaces a superconducting magnetic energy storage (SMES) system with the power grid, regulating DC input current while converting DC to AC. (Image: Research)

Both CSIs and CSRs historically incorporated series diodes for reverse voltage blocking, though earlier PWM-based CSCs relied on gate turn-off thyristors (GTOs) or symmetrical gate-commutated thyristors (SGCTs) for high-power applications. Modern designs have transitioned to reverse-blocking insulated gate bipolar transistors (IGBTs) with integrated diodes, improving efficiency and reducing switching losses.

In CSIs, a three-phase capacitor bank at the AC output filters voltage spikes and stabilizes the waveform. Conversely, as shown in Figure 3, CSRs use three-phase inductors at the AC input to shape sinusoidal current and optimize power factor.

Figure 3. Six-switch and three-switch PWM CSR topologies illustrate AC input filtering with three-phase inductors and DC-link energy storage with an inductor-capacitor filter. (Image: Semantic Scholar)

Both topologies depend on a large DC-link inductor, rather than capacitors, to regulate current ripple, store energy, and maintain a near-constant DC current.

Key PWM CSC functionality

PWM CSCs deliver:

Reliable short circuit protection
Four-quadrant regenerative operation capability
Nearly sinusoidal current outputs
Lower voltage transition rates, resulting from the filtering effect of output capacitors, which reduce electromagnetic interference (EMI) and motor losses

Notably, the large DC-link inductors in PWM CSCs increase system size, weight, and cost. Their non-minimum phase behavior complicates the closed-loop control design, as the output initially responds inversely to sudden input changes before stabilizing. Additionally, the inertia of the DC-link inductor reduces dynamic performance, limiting the converter’s ability to adapt quickly to abrupt load or input voltage variations.

Comparing PWM CSCs and VSCs

PWM CSCs and PWM VSCs differ fundamentally in operation, topology, and application. PWM CSCs maintain a near-constant DC-link current, stabilized by a large inductor while adjusting the AC-side voltage to meet load demands. In contrast, PWM VSCs regulate a constant DC-link voltage using capacitors and modulate the AC-side current to match load requirements. CSCs rely on DC-link inductors for energy storage and current ripple suppression, whereas PWM VSCs leverage DC-link capacitors to stabilize voltage.

Modern PWM CSC designs incorporate IGBTs with integrated diodes, improving efficiency and reducing switching losses. Similarly, PWM VSCs employ IGBTs or silicon carbide (SiC) MOSFETs, the latter offering faster switching speeds and lower conduction losses. CSCs mitigate harmonics using AC output capacitors (in inverters) or AC input inductors (in rectifiers).

PWM CSCs require closely matched load impedance, particularly motor inductance, limiting their ability to support multi-motor configurations. In contrast, PWM VSCs offer greater flexibility, accommodating varied motor parameters while delivering faster dynamic response to load or speed changes. PWs operate efficiently across a broad power range, including low- and high-power applications, such as modular multilevel converters for high-voltage direct current (HVDC) systems.

PWM VSCs excel in applications requiring rapid adjustments, modularity, and high efficiency, such as EV motor control, renewable energy systems, and heating, ventilation, and air conditioning (HVAC) systems. While PWM VSCs are typically more compact and efficient, CSCs provide superior ruggedness and fault tolerance, ensuring reliable performance in harsh industrial environments.

Key applications and use cases

PWM CSCs are designed for high-power industrial systems, including steel mills and mining drives, as shown in Figure 4, where precise torque control, low voltage rise rates (dv/dt), and inherent short-circuit protection help reduce motor insulation wear and minimize electromagnetic interference (EMI).

PWM — Figure 4. A high-power mining drive is used in mineral processing, where precise torque control and regenerative braking optimize operational efficiency. (Image: International Mining)

Their four-quadrant operation also enables regenerative braking in high-power applications without additional hardware.

Key PWM CSC use cases include:

Motor drives: power large synchronous motor drives used in mining conveyors, steel rolling mills, compressors, pumps, and cranes. Support variable-speed drives for high-power, non-automotive systems requiring precise control.
Power systems: enable regenerative drives, where four-quadrant operation recovers energy in heavy machinery. Deliver power factor correction (PFC) through CSRs to mitigate harmonics in industrial plants.
Specialized UPS: provides high-power backup solutions where DC-link inductor-based short-circuit protection takes priority over compact design.
Heating systems: support industrial induction heating applications, such as metal forging, requiring precise current regulation and reliable operation under high-power demands.

Summary

PWM CSCs equipped with RB-IGBTs offer improved efficiency and reduced size compared to their predecessors. Widely used in select high-power systems, PWM CSCs ensure low dv/dt stress, inherent fault tolerance, and reliable operation in harsh environments.

Broader adoption beyond niche high-power applications remains limited due to the lack of bidirectional voltage-blocking switches, which restricts operational flexibility. Additionally, the inertia of the DC-link inductor reduces dynamic performance, slowing response speed in applications requiring rapid voltage or current transitions. PWM VSCs, with their compact design and faster dynamic response, are preferred for applications requiring rapid adjustments or modular configurations.