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Technology / Technical guide
PWM Switching Frequency, Carrier & Fundamental
A PWM waveform can switch rapidly while representing a slower intended signal. The switching frequency, fundamental frequency and speed of each edge describe different properties. Confusing them can produce a misleading description of voltage quality or converter losses.
Count cycles and describe edges separately.
Fast switching is not the same quantity as a fast transition, and neither alone establishes a clean fundamental waveform.
01 / Two time scales
Distinguish the pulse pattern from the intended signal.
A carrier-based PWM example compares a reference with a repeating carrier to generate pulses.
A carrier-based PWM example compares a reference with a repeating carrier to generate pulses. The carrier repeats at the switching time scale, while a sinusoidal reference can vary at a slower fundamental frequency.
TI’s inverter discussion illustrates that the fundamental follows the reference frequency in its stated modulation model. A carrier label therefore does not identify a grid-frequency output, and a fundamental label does not identify semiconductor switching frequency.
02 / Duty variation
Understand an average representation within conditions.
The proportion of on-time can vary across switching cycles to represent a changing average.
The proportion of on-time can vary across switching cycles to represent a changing average. Over a short interval relative to the reference variation, the pulse average can approximate the intended slower value in the applicable model.
That representation has a limited modulation region and depends on the available switching states. Arbitrarily extending an amplitude relation beyond its model can change waveform shape rather than preserve a larger undistorted fundamental.
03 / Frequency content
Keep switching components outside the fundamental label.
The raw switched waveform includes more than the fundamental.
The raw switched waveform includes more than the fundamental. Carrier-related components and sidebands depend on modulation, topology and operating conditions. A single fundamental RMS value cannot describe their entire distribution.
The harmonic guide explains measurement definitions. Not every high-frequency component is simply an integer multiple of the fundamental, and not every instrument analysis includes the complete switching spectrum.
04 / Ripple trend
Connect cycle duration with energy-storage response.
For an ideal inductor segment under a stated voltage, current change is proportional to the duration of that segment.
For an ideal inductor segment under a stated voltage, current change is proportional to the duration of that segment. Shorter repeating intervals can therefore reduce a per-cycle current excursion if the other relevant conditions remain unchanged.
That is a conditional physical trend, not proof that a higher switching frequency always improves every quality metric. Filtering, current level, modulation and the connected system influence the actual result.
05 / Frequency and edge
Separate repetition from transition slope.
Switching frequency describes how often cycles occur.
Switching frequency describes how often cycles occur. Edge duration describes how quickly a voltage or current changes within one transition. Two waveforms can share the same repetition frequency and have different edge slopes.
Faster edges can change high-frequency coupling, while more transitions can change the loss balance. The noise-mode guide addresses paths; the loss guide addresses energy per event.
06 / Scope boundary
Keep model vocabulary apart from hardware claims.
A PWM explanation does not identify the actual device, control method, output filter or spectrum of a supplied converter.
A PWM explanation does not identify the actual device, control method, output filter or spectrum of a supplied converter. Fundamental, carrier and edge descriptions remain useful only when their waveform boundary is clear.
No MARSTON PWM implementation, carrier value, switching device or output-quality result is asserted here. This page explains time scales and cause-and-effect relationships without changing switching settings or prescribing a modulation strategy.
Continue the review with Semiconductor Conduction & Switching Loss Mechanisms.
Continue the review with LC Filter Resonance: Natural Frequency and Damping.
Continue the review with Common-Mode & Differential-Mode Electrical Noise.
Extend the converter switching discussion with space vectors and ideal inverter states and inverter dead time and waveform error.
General background: Texas Instruments Analog Applications Journal: reference, carrier and inverter waveform components. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Fundamental | Intended slow reference component | Not the carrier repetition rate. |
| Carrier frequency | Repeating modulation time scale | Depends on the stated PWM model. |
| Duty variation | Cycle proportion representing a changing average | Valid within the modulation conditions. |
| Edge duration | Time for an individual transition | Separate from events per second. |
| Spectrum | Fundamental, carrier-related and other components | Depends on topology and waveform boundary. |
Engineering questions.
Does a high carrier frequency mean high fundamental frequency?
No. They describe different time scales.
Does a pulse average describe every waveform component?
No. The raw waveform also contains faster variations.
Does increasing switching frequency always improve quality?
No. The conditional ripple trend is only part of the system behaviour.
Are frequency and edge speed the same?
No. Repetition rate and individual transition duration are different.
Can a fundamental RMS value describe the whole switching spectrum?
No. Its scope is the defined fundamental component.
Is a PWM setting specified for MARSTON?
No. No implemented carrier value or adjustment is given.
Technical enquiry
Discuss the technical question.
A PWM waveform can switch rapidly while representing a slower intended signal. Share the model and applicable operating or design information for an engineering discussion.