Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

An important generating source of conducting and launching EMI is electrical equipment of connecting AC power cord, such as computers and switching power supply, and electrical equipment with electric motors, such as refrigerators, air conditioners and motorcycles.

How to control EMI, generally consider the following factors:

1. ” PCB design - "sensitive components, power and ground layers of isolation"
2. Circuit current - "EMI radiation increases with the increase of current"
3. Frequency, including the slew rate - "EMI radiation can cause frequency increasing"
4. Bandwidth
5. Circuit loop area - "kept to a minimum."
6. Shielding / filtering - "with reasonable design, filtering, shielding and other techniques, to control EMI to a required level with the lowest cost method.
7.Spread Spectrum Clock - "the right spreading frequency number and modulation frequency"
8. Jittering center frequency in the clock application, to expand radiation energy to multiple bands, rather than all the energy of radiation to a frequency.

The methods of controlling and reducing EMI

Controlling and reducing EMI has two basic ways: suppression and absorption. The most common method of noise reduction is included a reasonable circuit design, shielding, grounding, filtering, isolation, separation and orientation, circuit impedance level control, cable design and noise cancellation. These methods require to use passive and active components such as filters, chokes, ferrite beads, foil and   Pieces, combined with the PCB design rules, and spread-spectrum clock generator (SSCG).

Solve the EMI problem at the source

A basic principle of EMC design is to reduce EMI at the source of the PCB. Spread spectrum method is to expand the radiation energy generated in the special bandwidth to the frequency are, resulting in a higher bandwidth signal. Exhibits clock generator (SSCG) can perform this function.

When choose the spread-spectrum clock to weaken EMI of consumer electronics products, the developer must ensure the following points:

1), The system must pass EMI testing.
2), Even if there are side effects of spread spectrum, but also to maintain system performance.
3), To minimize the impact on the total cost of system

Spread Spectrum Clock Generator (SSCG)

Spread Spectrum Clock Generator (SSCG) can be divided into two kinds of programmable and non-programmable, can also be classified according to have Hershey Kiss or not , or triangle spread-spectrum frequency .

Because the non-programmable spread spectrum clock chip is customized for specific applications, and the frequency range and expanded amount have only a few fixed options, to maximize the cost / performance while meeting the requirements of the optimal spread spectrum, it becomes very difficult.

Most of the fixed-function clock chip in the market have multiple fixed selectable input frequency range (eg 20-40MHz, 40-80MHz and 80-160MHz) and the expansion rate (such as 0.5%, 1%, 2% and 3% ). To achieve optimization, we need two sets of PLL parameters, one set for EMI suppression, another set for the PLL performance.

In contrast, a programmable spread spectrum clock generator is able to provide support for on-site programming universal clock, combined with on-chip nonvolatile memory, the dynamic of the spread spectrum parameters reset, eliminating the need for manufacturers to spend a lot of time and cost of the chip changes.

Figure 4 as above shows how to use a 4PLL type clock chip with two spread-spectrum PLL, by frequency modulation to easily optimize EMI reduction 3 ~ 4 dB's. These expanded PLL have two separate modes to be chosen.

Most developers prefer to use the Hershey Kiss Spread Spectrum Clock for achieving preferably EMI suppression performance, but many clock suppliers provide only a linear spread spectrum clock. Ideally, a SSCG must provide Hershey Kiss and linear spread spectrum clock at the same time. Figure 3 shows the Hershey Kiss spread spectrum clock shown is in the above 4PLL clock chip testing conditions to reduce EMI 1.67dB at a time.

In addition, the important clock parameters, such as PLL charge pump current, VCO gain and output drive strength, all must be able to be programmed and set. The flexibility can greatly improve system performance, reduce system development time, will change limit to a minimum and reduce risk.