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Tech Tips

The crest factor is the ratio of wave height, which is one of the quality parameters in Alternating Current (AC) power sources and Bipolar power supplies.

In the field of electronics, it is used to describe the state of AC power source supplied from an outlet. The crest factor is expressed as the ratio of the peak value to the rms value of the alternating current (AC) waveform.

The equation can be expressed as follows.

When the peak value of each waveform is set to 1
Waveform Effective value
(RMS value)
Mean value Waveform factor Crest factor
DC DC waveform Effective value (RMS value) of DC is 1 Mean value of DC is 1 Waveform factor of DC is 1 Crest factor of DC is 1
Sine wave Sine wave Effective value (RMS value) of Sine wave is 0.707 Mean value of Sine wave is 0.637 Waveform factor of Sine wave is 1.11 Crest factor of Sine wave is 1.1414
Full wave rectified sine wave Full wave rectified sine wave Effective value (RMS value) of Full wave rectified sine wave is 0.707 Mean value of Full wave rectified sine wave mean value is 0.637 Waveform factor of Full wave rectified sine wave is 1.11 Crest factor of Full wave rectified sine wave is 1.1414
Half-wave rectified sine wave Half-wave rectified sine wave Effective value (RMS value) of Half-wave rectified sine wave is 0.5 Mean value of Half-wave rectified sine wave is 0.318 Waveform factor of Half-wave rectified sine wave is 1.571 Crest factor of Half-wave rectified sine wave is 2
Triangle wave Triangle wave Effective value (RMS value) of Triangle wave is 0.577 Mean value of Triangle wave is 0.5 Waveform factor of Triangle wave is 1.154 Crest factor of Triangle wave is 1.732
Sawtooth wave Sawtooth wave Effective value (RMS value) of Sawtooth wave is 0.577 Mean value of Sawtooth wave is 0.5 Waveform factor of Sawtooth wave is 1.154 Crest factor of Sawtooth wave is 1.732
Square wave Square wave Effective value (RMS value) of Square wave is 1 Mean value of Square wave is 1 Waveform factor of Square wave is 1 Crest factor of Square wave is 1
PWM signal PWM signal Effective value (RMS value) of PWM signal is square(t1/T) Mean value of PWM signal is t1/T Waveform factor of PWM signal is square(T/t1) Crest factor of PWM signal is square(T/t1)
Waveform Factor = RMS value / average value
Crest Factor = Peak value / RMS value

Usually, we don't care about the current value when plugging an electric appliance into an outlet. And we are even less conscious of the crest factor. So why should we consider the crest factor?

A high crest factor is called "poor power factor" and can cause the following problems.

  • Heat generation and burnout of power receiving equipment
  • Malfunction of electronic equipment
  • Input current increases due to higher apparent power

As you can see from the above equation, the higher the peak value of the numerator, the higher the value of the crest factor. A waveform with a high peak value occurs when the current flows in a sharp and pointed manner as shown in the figure below.
This is mainly seen when a circuit method known as a capacitor-input type is used.

crest factor image

A sharp current causes a large current to flow through the capacitor. Capacitors with repeated large currents can make a roaring noise, generate heat, and sometimes burn out. This is the first problem.

Extreme peak current waveforms also contain high-frequency components.
Some electronic devices can react to high frequency components and cause malfunctions.

Third, as can be seen from the equation, a higher crest factor results in a higher peak current, so the wiring must be correspondingly thicker. If the current = power / voltage is calculated without considering the crest factor, the current capacity of the wiring will be insufficient, and the cause of heat generation and the impedance of the wiring will lead to distortion of the voltage waveform.

To prevent these drawbacks, a power factor correction circuit (PFC) is used to reduce the peak current by bringing the input current closer to a sine wave.
However, PFC is also efficient, so the choice has advantages and disadvantages. If you consult with our sales staff, including your operating environment and usage, we will propose a power supply that matches your needs.