Use the simplest and cheapest means to achieve frequency jitter in SMPS and achieve lower EMC

If the SMPS switching frequency is designed to be non-fixed and allowed to vary around a certain frequency, its EMC electromagnetic waves will spread out and will not produce large peaks at the switching frequency and its multiples.

The range around the center frequency cannot be too large, otherwise the system regulation performance will be deteriorated. Generally speaking, the frequency variation range is suitable within 5% of the switching center frequency.

For example: a 100KHZ SMPS has a frequency jitter range of 97.5--102.5khz.

Nowadays, many new ICs have integrated frequency jitter function, but most of them, including classic ICs such as UC3843 and L4891, do not have this function.

So how can you design and transform your product in the simplest way? Here's how to do it with just a few cents.

Figure 1 above (for ICs that use RT/CT oscillation to achieve switching frequency, such as UC4843):

Simply add 3 components.

Principle: PWM outputs square waves, and the duty cycle is not constant, but is affected by many factors such as load, power supply voltage, temperature, etc. R2, C2 is a simple smoothing filter circuit, and its average voltage changes with the PWM duty cycle. It is connected to the R1, C1 oscillator through R3, so as to slightly adjust its oscillation frequency and realize the frequency jitter function.

Figure 2 (for frequency setting using a single mirror resistor, such as IR1150):

This type of IC simplifies the oscillator and controls the frequency of the internal oscillator through an external mirror resistor.

This resistor can be divided into two, as shown in the figure R1, R2.

Similarly, the voltage at point B also changes with the PWM duty cycle, and is connected to R1 and R2 through R4, thereby changing the current value of the mirror resistor and also changing the frequency.

PS: IR1150 is a PFC control IC, and the PWM duty cycle changes all the time, so this structure can achieve frequency jitter very efficiently. The frequency can also be controlled by other external signals, such as changes in AC power supply.

A very simple circuit, at very low cost, can give you huge results, easily reducing 10db of electromagnetic interference.