Inverter Circuits — Switching DC into AC

0. What this article explains

• Half-bridge and full-bridge topologies that create single- and three-phase AC from a DC link.
• How sinusoidal PWM and space-vector PWM create an average voltage.
• Control, losses, dead time, EMI, and protection in motors, PV PCS, and EVs.

1. Bottom line: what is an inverter?

An inverter is a power converter that rapidly switches a DC source and uses filtering or motor inductance to synthesize AC. An ideal switch only connects or disconnects a voltage; it does not directly generate a sine wave. Pulse width, switching frequency, and phase are controlled so the load sees the desired fundamental component.

2. Inputs, outputs, and topologies

Inputs include a battery, rectifier, or photovoltaic DC link V_{dc}. A single-phase full bridge uses four switches; a three-phase two-level bridge uses six. Simultaneously turning on upper and lower devices shorts the DC link, so dead time is inserted. NPC, T-type, multilevel, and modular multilevel arrangements share voltage across devices at high voltage and power.

3. PWM mathematics

Compare a sinusoidal reference v^*(t)=mV_{dc}\sin(\omega t) with a triangular carrier. In the linear modulation region the fundamental amplitude scales with V_{dc} and modulation m. The ideal switching average is

Three references are shifted by 120 degrees. Zero-sequence injection and SVPWM improve DC-bus utilization. Motor inductance or an LC filter suppresses current ripple, but parasitic capacitance still creates common-mode current.

4. Basic architecture

Figure 1 — The controller chooses switch states so the load receives its desired fundamental. Voltage and current sensors close the loop.

5. Control loops and motors

In a motor drive, an outer speed or position loop commands q-axis current and an inner current loop updates PWM duty. A simplified PMSM torque relation is

so torque follows i_q. At high speed, field weakening reduces back EMF. Induction motors require flux and slip-frequency estimation. Sensor resolution, current-sampling instant, and dead-time compensation determine low-speed torque ripple.

6. Loss, heat, and EMI

Switching and conduction loss are approximated by

Higher f_s reduces filter and motor ripple but raises loss, gate-drive power, and EMI. SiC and GaN enable speed, but parasitic-inductance overshoot, false turn-on, and creepage/clearance become strict layout requirements. Estimate junction temperature with T_j=T_c+P_{loss}R_{th,jc} under coolant temperature and short overload.

7. Topology comparison

8. Difficult conditions and protection

A short circuit can destroy a switch in microseconds, so DESAT gate-driver detection, current sensors, fuses, and hardware shutdown are layered. Excessive dead time creates voltage error and low-speed torque distortion. Long motor cables create reflected surges and bearing currents. Grid connection additionally requires anti-islanding, phase lock, current harmonics, and disconnection on loss of grid.

9. Practical choices

• EV and servo: evaluate continuous and peak current, regenerative voltage, cooling, and position sensing over the duty cycle.
• PV PCS: check MPPT range, grid voltage, power factor/reactive power, harmonics, and anti-islanding.
• UPS: prioritize transfer time, bypass, inrush current, and redundancy.
• High-frequency compact supply: layout, EMI, and insulation testing come before exploiting GaN speed.

10. Three-line recap

An inverter designs the average of switch states with PWM to synthesize AC from DC.
Motor drives close current, speed, and position loops; grid inverters close phase, current, and anti-islanding loops.
Efficiency must be verified together with dead time, heat, EMI, short-circuit interruption, and insulation.

References

• IEEE Power Electronics Society
• IEC 61800 adjustable-speed drive systems
• NREL power electronics research
• Wolfspeed SiC resources