The influence of IC chip on EMI design

When considering EMI control, design engineers and PCB board-level design engineers should first consider the choice of IC chip. Some characteristics of integrated circuit such as packaging type, bias voltage and chip technology (such as CMOS, ECI) have a great impact on electromagnetic interference. The following will focus on the influence of IC on EMI control.

Integrated circuit EMI source

EMI sources of integrated circuit in PCB mainly include: EMI signal voltage and signal current caused by square wave signal frequency generated at the output end, capacitance and inductance of the chip itself caused by electric field and magnetic field during the conversion of digital integrated circuit from logic high to logic low or from logic low to logic high.

The square wave generated at the output of IC chip contains sine and harmonic components with a wide frequency range, which constitute EMI frequency components concerned by engineers. The highest EMI frequency, also known as EMI emission bandwidth, is a function of the signal rise time (rather than the signal frequency).

The formula for calculating EMI emission bandwidth is: F=0.35/TR

Every voltage value in the circuit corresponds to a certain current, and every current has a corresponding voltage. When the output of IC is transformed from logic high to logic low or from logic low to logic high, these signal voltages and signal currents will generate electric and magnetic fields, and the highest frequency of these electric and magnetic fields is the transmission bandwidth. Electric field and magnetic field intensity and the percentage of the external radiation, is not only the function of signal rise time, at the same time also depends on the source to the load point signal channel between the stand or fall of capacitance and inductance control, therefore, the signal source is located inside of PCB, and load is located inside other IC, the IC may be on the PCB, could also not on the PCB. In order to effectively control EMI, it is necessary to pay attention not only to its capacitance and inductance, but also to the capacitance and inductance existing on PCB.

When the connection between signal voltage and signal circuit is not close, the capacitance of the circuit will decrease, and the inhibition of the electric field will be weakened, thus increasing EMI. The same is true for the current in the circuit. If the current is not well combined with the return path, it is bound to increase the inductance on the circuit, thus enhancing the magnetic field and ultimately leading to the increase of EMI. This fully demonstrates that poor control of the electric field usually results in poor magnetic field inhibition. The measures used to control the electromagnetic field in a circuit board are generally similar to those used to suppress the electromagnetic field in an IC package. Just like PCB design, IC packaging design will greatly affect EMI.

A considerable part of electromagnetic radiation in the circuit is caused by voltage transients in the power bus. When the output stage jumps and the connected PCB line is logically "high," the sink chip absorbs current from the power supply and provides the energy required for the output pole. For the uhf current generated by IC continuous conversion, the roll-off network on the power bus PCB stops at the output pole of the sink. If the signal rising time of the output pole is 1.0ns, IC should absorb enough current from the power supply to drive the transmission line on the PCB in such a short time as 1.0ns.

Since IC pin and internal circuit are part of the power bus, and the time to absorb current and output signal also depends on the technology to a certain extent, the selection of appropriate sink can control the three elements mentioned in the above formula to a large extent.

The role of encapsulation characteristics in emi control

The IC package usually consists of a silicon-based chip, a small internal PCB, and a solder pad. Silicon chip installed on small PCB 64, silicon chips are implemented by binding wire and connection of bonding pad, also can be directly connected in some of the encapsulation of small PCB to realize the signal on the silicon chip and power supply and corresponding to the connection between the pins on the packaging, thus realize the silicon chip signal and power source node outward.