This article describes the architecture and layout choices for a three-phase inverter board tested at 500 W continuous power. The main design concerns include switching loops, heat removal, gate drive, and protection.

The linked inverter board provides the project context. The sections below focus on its supporting circuits and physical layout.

System Architecture

The core topology is a three-phase, two-level voltage-source inverter (VSI). The supporting circuits handle sensing, gate drive, and filtering.

graph TD
    DC[DC Input Bus] --> Caps[DC-Link Capacitors]
    Caps --> Bridge[3-Phase VSI Bridge]
    
    MCU[Control MCU / DSP] -->|PWM Signals| Iso[Digital Isolation]
    Iso --> Gate[Isolated Gate Drivers]
    Gate -->|High/Low Side Drive| Bridge
    
    Bridge --> Filter[On-Board LC Filter]
    Filter --> Load[3-Phase Load / Motor]
    
    Bridge -.->|Current Sense| MCU
    DC -.->|Voltage Sense| MCU

The DC-Link and Decoupling

The DC-link capacitor bank supplies switching current near the bridge. Excess series resistance (ESR) raises capacitor heating, while loop inductance (including capacitor ESL) contributes to voltage overshoot during switching.

To mitigate this, the board supports multiple capacitor footprint options, allowing for a mix of bulk electrolytic capacitors (for low-frequency energy storage) and high-frequency ceramic/film capacitors placed immediately adjacent to the switching nodes.

Gate Drive and Isolation

Gate-drive timing affects switching loss, voltage overshoot, and electromagnetic interference (EMI). Faster edges can reduce transition loss but may increase ringing.

     +HV DC
       |
     | D |  (High-Side Switch)
     +---+
       |--------> Phase Output (U, V, W)
     +---+
     | D |  (Low-Side Switch)
       |
      GND

The gate-drive path uses:

1. Digital isolators to separate control signals from the power stage.
2. Gate drivers sized for the selected MOSFETs and switching rate.
3. Overcurrent protection provisions, whose sensing and shutdown method depend on the final switch and driver choices.

Output Filter Design

PWM output contains switching-frequency components. Where the load or interface requires lower ripple, an output filter can attenuate those components; its values depend on the application.

This board includes on-board filter options.

• The inductor ($L$) limits current ripple and must be checked for saturation at the expected peak current.
• The capacitor ($C$) provides a high-frequency current path; its connection and rating depend on the chosen filter topology.

The filter is integrated on the board to keep high-frequency current paths short and make EMI easier to manage.

Physical Layout & Thermal Management

PCB layout affects voltage spikes, thermal behavior, and reliability, even when the schematic is sound.

Key Layout Principles Applied:

1. Minimizing Commutation Loops: The loop from the DC-link capacitor, through the high-side switch, through the low-side switch, and back to the capacitor must be physically as small as possible to minimize parasitic inductance ($L_{loop}$). High $L_{loop}$ combined with high $di/dt$ causes destructive voltage spikes ($V = L \cdot di/dt$).
2. Thermal Planes: The copper pours carrying phase currents are sized not just for electrical resistance, but as thermal radiators.
3. Trace Clearance: Ensuring adequate creepage and clearance distances between the high-voltage DC bus and the low-voltage control circuitry to prevent arcing.

Conclusion

The board is a test platform for motor-control and power-conversion work. Switching speed, layout, thermal design, and clearances all need to be considered together.

Microcontrollers handle tasks from sensor sampling to motor control. PWM generation is one example where the timer configuration matters as much as the application code.

For projects needing more timer channels, interfaces, or processing headroom, a 32-bit STM32 can be a useful choice. This example uses the STM32F103x8 family, which has a 72 MHz Cortex-M3 core, timers, and 12-bit ADCs; peripheral availability varies by STM32 model. See the STM32F103x8 data sheet for device limits.

View the related STM32CubeMX/HAL PWM project on GitHub

The Development Ecosystem

For an STM32F103x8 PWM project, a typical toolchain includes:

• Software: STM32CubeMX can configure clocks and pins and generate initialization code. Keil uVision is one option for building and debugging C/C++ firmware.
• Hardware: An ST-Link v2 can flash the device and support Serial-Wire Debugging (SWD).
• Libraries: ST’s Hardware Abstraction Layer (HAL) can simplify setup. The timer reference manual is useful when checking update timing or debugging a configuration.

The Physical Layer of PWM

For edge-aligned PWM, the timer counts to an Auto-Reload Register (ARR) value, while a Capture/Compare Register (CCR) sets the compare point. The linked project uses STM32CubeMX and HAL; the following separate register-level illustration assumes TIM2 has a 72 MHz timer input clock and that the peripheral clock and output pin are already configured.

// Example Bare-Metal Timer Configuration
TIM2->PSC = 71;        // 72 MHz timer clock / 72 = 1 MHz tick
TIM2->ARR = 999;       // Auto-reload value (Period = 1ms -> 1kHz frequency)
TIM2->CCR1 = 500;      // Duty cycle = 50% in PWM mode 1
TIM2->CCMR1 |= TIM_CCMR1_OC1M_1 | TIM_CCMR1_OC1M_2; // PWM Mode 1
TIM2->CCER |= TIM_CCER_CC1E; // Enable Output
TIM2->CR1 |= TIM_CR1_CEN;    // Start Timer

Why It Matters in Power Electronics

Timer clock, prescaler, and register update behavior set the available PWM resolution and timing. In a power converter, also check when preload values take effect and configure dead time and protection where the switching topology requires them.

Power lost in a component appears mostly as heat. Estimating that loss and the path from the component to ambient helps identify where thermal validation is needed.

The Fundamentals

Power dissipation in a resistive element is defined by:

P = I² × R  =  V² / R  =  V × I

For a 10 V to 5 V linear regulator supplying 1 A, the IC dissipates about 5 W. A buck converter delivering the same 5 W output at an assumed 90% efficiency would dissipate about 0.56 W. Actual efficiency depends on the device and operating point.

Where Power Actually Goes

In a real system, losses appear across multiple domains:

Trace Resistance

Using bulk copper resistivity as a first estimate, a 10 mm trace that is 0.3 mm wide and 35 µm thick has:

R = ρ × L / A
R = (1.72×10⁻⁸) × 0.01 / (0.3×10⁻³ × 35×10⁻⁶)
R ≈ 16.4 mΩ

At 3 A, that is about 147 mW. Actual resistance also depends on copper thickness, temperature, and geometry.

Thermal Resistance Chain

With a heatsink, a simplified steady-state thermal path is:

T_junction = P_dissipated × (θ_jc + θ_cs + θ_sa) + T_ambient

Where:

• θ_jc — Junction-to-case (from datasheet)
• θ_cs — Case-to-heatsink (depends on TIM/pad)
• θ_sa — Heatsink-to-ambient (heatsink selection)

Without a heatsink, use the package’s junction-to-ambient estimate instead of θ_sa. For example, 8 W with θ_ja = 60°C/W would imply a 480°C temperature rise in this simple model, far beyond a practical operating limit. With a heatsink and illustrative values θ_jc = 2°C/W, θ_cs = 0.5°C/W, and θ_sa = 5°C/W, the same 8 W at 40°C ambient gives an estimated junction temperature of 100°C. These values must come from the chosen device, mounting method, heatsink, and airflow. See TI’s thermal model guide for the distinction between θ_ja and a heatsink path.

My Verification Workflow

1. Schematic phase: Annotate every power path with expected I, V, and P
2. Layout phase: Check resistance and temperature rise on relevant power traces
3. Thermal simulation: Build a thermal model in MATLAB or LTspice
4. Lab validation: Thermistor + thermal camera after 30-min steady state soak

Estimate the losses, measure temperatures under representative loads, and update the model where measurements differ.

A capacitor’s value depends on its geometry and dielectric. If pressure deflects a diaphragm or compresses a dielectric, the resulting capacitance change can be measured. This article describes the design principle; it does not report a calibrated sensor test.

The Governing Physics

Capacitance of a parallel-plate capacitor:

C = ε₀ × εᵣ × A / d

Where:

• A — plate overlap area (m²)
• d — plate separation (m)
• εᵣ — dielectric permittivity

With a fixed plate area and dielectric, reducing a 10 µm gap to 9 µm raises the ideal parallel-plate capacitance by about 11%. Fringing fields, diaphragm shape, and material changes make the response of a real sensor less simple.

Two Structural Approaches

1. Diaphragm-Type (MEMS Style)

A thin conductive membrane suspended over a fixed plate. Applied pressure deflects the membrane, decreasing the air gap:

ΔC / C₀ ≈ -Δd / d₀    (for small changes in gap)

MEMS devices use this principle at micrometre scale. A flexible electrode over a cavity is one possible prototype approach, but its pressure range and sensitivity depend on the mechanical design and must be calibrated.

2. Dielectric Compression

Soft dielectrics between plates may compress under load. Both gap and permittivity can change, so the capacitance-to-pressure relationship needs measurement and calibration.

Readout Circuit

One readout option converts capacitance to an oscillator frequency; the frequency equation depends on the circuit used. A capacitance-to-digital converter is another option. For example, the AD7746 data sheet specifies a ±4 pF changing-capacitance range and up to 17 pF of common-mode capacitance. Its 4 aF resolution figure describes noise under specified conditions, not overall measurement accuracy. The sensor and readout must be matched for baseline capacitance and expected change.

Noise and Stability Concerns

• Parasitic capacitance: Keep sense connections short and consider driven guards or shielding where the readout supports them.
• Humidity: Moisture can change dielectric properties and leakage; packaging choices depend on the intended environment.
• Temperature drift: Characterize the sensor and readout across the operating range, then calibrate or compensate as needed.

Practical Build Notes

A prototype could use two electrodes separated by a flexible film. The next steps would be to measure baseline capacitance, apply known pressures, repeat loading cycles, and check drift and hysteresis. No pressure-to-capacitance measurement is reported here.

The geometry sets the ideal sensitivity; calibration and environmental testing determine whether a particular build is useful as a pressure sensor.