In power electronics, the ability to precisely control voltage and current efficiently is paramount. Since the days of linear regulation are largely behind us due to efficiency concerns, nearly all modern power converters and inverters rely on **Pulse Width Modulation (PWM)**. PWM is the foundational technique that allows a simple ON/OFF switch (like a MOSFET or IGBT) to deliver a regulated amount of average power to a load.
The Core Principle of PWM
At its heart, PWM is a digital technique used to generate an analog result. It involves varying the duration, or **width**, of a voltage pulse while keeping the pulse's frequency constant. The amount of power delivered is directly proportional to the **Duty Cycle ($D$)**, which is defined as the ratio of the pulse ON time ($T_{ON}$) to the total period ($T_{P}$):
$$D = \frac{T_{ON}}{T_{P}}$$
By changing the duty cycle, the average value of the output voltage is controlled. For instance, a 50% duty cycle results in an average output voltage of $V_{in}/2$. The magic happens because inductors and capacitors in the power converter act as low-pass filters, smoothing this train of pulses into a stable DC or AC waveform.
Common PWM Techniques in Power Electronics
While the core principle remains simple, its application varies widely depending on the type of converter and the desired output waveform quality.
1. Carrier-Based PWM (CB-PWM)
This is the most common technique for voltage-source inverters (VSI). It generates the switching signals by comparing a low-frequency **Modulating Wave** (e.g., a sinusoidal waveform for AC generation) with a high-frequency **Carrier Wave** (typically a triangle or sawtooth wave).
- **Single-Phase:** Simple comparison used for basic DC-AC conversion.
- **Three-Phase (Sinusoidal PWM):** Requires three modulating waves (120 degrees phase shift) to control the three legs of the inverter bridge.
2. Space Vector Modulation (SVM)
Space Vector Modulation is generally considered superior to traditional Sinusoidal PWM for three-phase inverters. Instead of focusing on each phase voltage individually, SVM treats the three-phase voltages as a single rotating **Voltage Space Vector**. It utilizes the eight possible switching states of a three-phase bridge to synthesize the required voltage vector over a sampling period.
"SVM provides a higher DC bus utilization (around 15% more voltage) and generates lower harmonic distortion compared to standard Sinusoidal PWM, making it preferred in high-performance motor drives and grid-tied systems."
PWM in Bidirectional Converters
For the **Bidirectional DC-DC Converter** discussed in the previous post, PWM controls the magnitude of the power flow. In a Dual Active Bridge (DAB) converter, a variation called **Phase-Shift PWM** is used. Here, the duty cycle is typically fixed at 50% to maximize voltage utilization, and the power flow direction and magnitude are instead controlled by the **phase shift** between the square wave voltages applied to the two sides of the high-frequency transformer. This technique is key to achieving the soft-switching (ZVS) necessary for high efficiency.