Electronic Design - PCB circuit design and EMC device selection

1. PCB circuit design and EMC device selection

At the beginning of new design and development projects, the correct selection of active and passive components and perfect PCB circuit design technology will help to obtain EMC certification at the lowest cost, reducing the additional cost of products due to shielding and filtering. Volume and weight. These technologies can also improve the integrity of digital signals and the signal-to-noise ratio of analog signals, and can reduce the reuse of hardware and software at least once. This will also help new products meet their functional technical requirements and enter the PCB market as soon as possible. These EMC technologies should be regarded as part of the company's competitive advantage and help companies obtain maximum commercial benefits.

1.1 Digital device and EMC circuit design

1.1.1 Selection of device

Most digital IC manufacturers can produce at least a certain series of devices with lower radiation, and they can also produce several ESD-resistant I/O chips. Some manufacturers supply VLSI with good EMC performance (some EMC microprocessors are better than ordinary products. Radiation is lower than 40dB); most digital circuits use square wave signals for synchronization, which will produce high-order harmonic components, as shown in Figure 1. The higher the clock rate, the steeper the edge, and the higher the frequency and harmonic emission capability. Therefore, under the premise of meeting product technical indicators, try to choose a low-speed clock. Never use AC when HC can be used. Do not use HC when CMOS4000 can. Choose integrated circuits with high integration and EMC characteristics, such as:

* The power and ground pins are close

* Multiple power and ground pins

* Low output voltage fluctuation

* Controllable switching rate

* I/O circuit matching the transmission line

* Differential signal transmission

* Low ground reflection

* Immunity to ESD and other interference phenomena

* Small input capacitance

* The drive capacity of the output stage does not exceed the requirements of the actual application

* Low transient current of power supply (sometimes called penetration current)

The maximum and minimum values of these parameters should be specified by the manufacturer one by one. Devices with the same model and index produced by different PCB manufacturers may have significantly different EMC characteristics. This is very important to ensure that the products produced successively have stable electromagnetic compatibility compliance.

1.1.2 IC holder is not suitable

IC sockets are very unfavorable to EMC. It is recommended to solder surface mount chips directly on the PCB. IC chips with shorter leads and smaller volumes are better. BGA and similar chip package ICs are currently the best choice. The emission and sensitivity characteristics of the programmable read-only memory (PROM) mounted on the socket (and worse, the socket itself has a battery) often deteriorate an otherwise good design. Therefore, a surface-mount programmable memory directly soldered to the circuit board should be used.

A motherboard with a ZIF socket and a spring-mounted heat sink on the processor (which can be easily upgraded) requires additional filtering and shielding. Even so, it is beneficial to choose a surface mount ZIF socket with the shortest internal lead.

1.1.3 Circuit Technology

* Use level detection for inputs and keys (not edge detection)

* Use a digital signal with the slowest and smoothest leading edge rate possible (not exceeding the distortion limit)

* On the PCB template, it allows the signal edge speed or bandwidth to be controlled (for example, using soft ferrite beads or series resistors at the drive end)

* Reduce the load capacitance, so that the open collector driver close to the output end is easy to pull up, and the resistance value is as large as possible

* The processor heat sink is isolated from the chip by a thermally conductive material, and the processor is grounded at multiple points of radio frequency.

* High-quality RF bypass (decoupling) of the power supply is important at each power supply pin.

* High-quality power monitoring circuits need to be resistant to power interruptions, drops, surges and transient interference

* Need a high-quality watchdog

* Never use programmable devices on watchdog or power monitoring circuits

* Power monitoring circuits and watchdogs also need appropriate circuits and software technology to make them adapt to most unexpected situations, depending on the critical state of the product

* When the rise/fall time of the logic signal edge is shorter than the signal transmission time in the PCB trace, the transmission line technology should be used:

A. Experience: The time for a signal to transmit one round trip per millimeter of trajectory length is equal to 36 picoseconds

B. In order to obtain the best EMC characteristics, use transmission line technology for trajectories that are much shorter than the experience suggested in a

Some digital ICs produce high-level radiation, and their matching small metal boxes are often soldered to the PCB ground wire to achieve a shielding effect. The cost of shielding on the PCB is low, but it is not suitable for devices that require heat dissipation and good ventilation.

The clock circuit is usually the most important source of emission, and its PCB trace is the most critical point. The layout of the components must be made so that the clock traces are the shortest, while ensuring that the clock line is on one side of the PCB but does not pass through the vias. When a clock must pass a long path to reach many loads, a clock buffer can be installed next to the load, so that the current in the long track (wire) is much smaller. Here, the relative distortion is not important. The clock edge in the long trajectory should be as smooth as possible, or even a sine wave, and then shaped by the clock buffer next to the load.

1.1.4 Spread Spectrum Clock

The so-called "spread spectrum clock" is a new technology that can reduce the measured value of radiation, but it does not really reduce the instantaneous transmit power. Therefore, it may still cause the same interference to some fast-reacting devices. This technology modulates the clock frequency by 1% to 2%, thereby spreading the harmonic components, so that the peak value in the CISPR16 or FCC emission test is lower. The measured emission reduction depends on the bandwidth and the integration time constant of the test receiver, so this is a bit speculative, but this technology has been accepted by the FCC and is widely used in the United States and Europe.

1.2 Analog device and PCB circuit design

1.2.1 Select analog device

Choosing an analog device from the EMC perspective is not as straightforward as choosing a digital device. Although it is also hoped that the emission, conversion rate, voltage fluctuation, and output drive capability should be as small as possible, for most active analog devices, noise immunity is very important. Factors, it is quite difficult to determine a clear EMC ordering feature.

Operation amplifiers of the same model and index from different manufacturers can have significantly different EMC performance, so it is very important to ensure the consistency of subsequent product performance parameters. Manufacturers of sensitive analog devices provide signal-to-noise processing techniques or PCB layouts on EMC or PCB circuit design, which shows that they care about the needs of users, which helps users weigh the pros and cons when buying.

1.2.2 "Prevent demodulation problems

The immunity problem of most analog equipment is caused by radio frequency demodulation. Each pin of the op amp is very sensitive to radio frequency interference, which has nothing to do with the feedback circuit used. All semiconductors have a demodulation effect on radio frequency, but the problem in the analog circuit is more serious. Even low-speed op amps can demodulate signals at mobile phone frequencies and above.