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5 Suggestions For Surge Protection In PCB Design
gesptechnologypcbJuly 21, 2020 Blog 0 Comment350views

It estimated that transients and surges cause 75% of electronic product failures. Voltage transients and surges are everywhere. Power grids, lightning strikes, blasting, and even people walking on carpets can generate tens of thousands of volts of electrostatic induction voltage. These are the invisible, deadly killers of electronic products. Engineer always worried about that but still need more work on circuit board design.

Here will show you 5 suggestions for surge protection in PCB Design

Therefore, in order to improve the reliability of electronic products and the safety of the human body, protective measures must be taken against voltage transients and surges in PCB design.

There are many reasons for a surge. A surge is a spike with a high rate of rising and a short duration. Power grid overvoltage, switch ignition, reversed power source, static electricity, motor/power supply noise, etc. are all factors that generate surges.

The surge protector provides a simple, economical and reliable protection method for the power surge protection of electronic equipment.

As we all know, electronic products often encounter unexpected voltage transients and surges in use, resulting in damage to electronic products. The cause of the damage is semiconductor devices in electronic products (including diodes, transistors, thyristors and integrated circuits, etc.) is burn or broken down.

1. One of the methods is to ground the whole machine and the system. The ground (common end) of the whole machine and the system should separate from the Earth. Each subsystem in the whole machine and the system should have an independent common end. When data or signals need to transmitted in time, the ground should use as the reference level, and the ground wire (surface) must be able to flow a large current, such as several hundred amperes.

2. The second protection method is to use protective devices for voltage transients and surges in the whole machine and critical parts of the system (such as computer monitors, etc.) so that voltage transients and surges can bypass to the substation through the protective devices. System ground and ground, so that the transient voltage and surge amplitude entering the whole machine and the system significantly reduce.

3. The third protection method is to use a combination of much voltage transient and surge protection devices for essential and expensive whole machines and systems to form a multi-level protection circuit.

Surge protectors provide a simple, economical and reliable protection method for the power surge protection of electronic equipment. Through the anti-surge element (MOV), the surge energy can quickly transmit in the lightning induction and operating overvoltage Earth, protect equipment from damage.

Methods of protection against surges

(1) The parallel surge protector connected in parallel on the power supply line

Under normal conditions, the varistor in the lightning protection module is in a high resistance state. When the power grid struck by lightning or the switching operation has transient surge overvoltage, the lightning protector responds within nanoseconds, and the varistor is in a low resistance state, quickly limiting the overvoltage to very low amplitude.

When there is a long-term continuous pulse or continuous over-voltage in the line, the performance of the varistor deteriorates and heats up to a certain degree to trip the thermal tripping mechanism to avoid fire and protect the equipment.

(2) The Series Filter Type Surge Protector connected in series into the power supply line

In order to provide a safe and clean power supply for valuable electronic equipment, in addition to enormous energy, lightning waves also have extremely steep voltage and current rise rates. Parallel surge protectors can only suppress the amplitude of lightning waves, but cannot change their sharply rising front—the Series Filter Type Power Surge Protector connected in series on the power supply line.

In the case of overvoltage, MOV1 and MOV2 respond within nanoseconds to clamp the overvoltage; at the same time, the LC filter reduces the steep voltage and current increase rate of the lightning wave by nearly 1000 times, and the residual voltage reduces by 5 times, thus Protect sensitive user equipment.

(3) Install pressure-sensitive limiter type components between phases and lines of the power line to limit surge overvoltage

The first method has a better protection effect on electrical equipment such as lighting, elevators, air conditioners, motors and other electrical equipment with high impulse voltage resistance levels. However, for modern electronic equipment with high integration and compact structure, the actual protection effect is not satisfactory. The reasons are as follows:

Take the induction lightning protection of single-phase 220V AC power supply as an example. The standard method is to install suitable pressure-sensitive components between the zero and ground wires to absorb and limit the spike voltage generated by the induced lightning. The lightning protection effect of the power line depends entirely on the selection of the pressure-sensitive device parameters and the reliability of the pressure-sensitive device.

The choice of the voltage-sensitive limiting value was base on the peak 310V of the mains, plus 20% of the influence of grid fluctuations, 10% of the device dispersion error, and 15% of the reliability of heat, moisture, and component ageing caused by long-term work. The factor compensation is generally 470V~510V, and various spike interference voltages such as induced lightning are limited to 470V. For voltages below 470V, the varistor does not operate.

The power frequency withstand voltage value of ordinary low-voltage electrical equipment (machine tools, elevators, lighting, air-conditioning, etc.) is generally AC 1500V, and the instantaneous withstand voltage peak value can reach 2500V or more, so the voltage of 470V is very safe.

However, the working voltage of modern electronic equipment composed of large-scale integrated circuits is generally between ±5V and ±15V, and the highest withstand voltage generally does not exceed 50V. Therefore, the high-frequency peak voltage less than 470V superimposed on the mains will directly send Into the load, through the space coupling capacitor, the interlayer and interpolar capacitance of the transformer disproportionately transferred to the switching power supply or integrated circuit chip, which can cause malfunction.

Although high-frequency switching power supplies and electronic equipment have corresponding anti-spike interference measures, they limited by cost and size, coupled with the intensity of spike interference such as induced lightning strikes, and the frequency spectrum changes significantly, so the protective effect is not ideal.

This effect obtained when the varistor limiting element is ideal. In fact, due to the influence of the residual voltage of the varistor element and the lead inductance, under intense induced lightning strikes, the actual limiting voltage peak value may rise to 800V ~1000V above, which threatens the subsequent electronic equipment.

(4) Strengthen the protective effect of electronic equipment, connect an ultra-isolation transformer (also called isolation method) in series between the power supply and the load to isolate high-frequency spike interference, and at the same time make the secondary equipotential connection easy.

The isolation method mainly uses an isolation transformer with a shielding layer. Since common-mode interference is a relatively ground interference, it mainly transmitted through the coupling capacitance between the transformer windings.

If the shielding layer inserted between the primary and secondary and grounded well, the interference voltage can be shunted through the shielding layer, thereby reducing the interference voltage at the output.

Theoretically, a transformer with a shielding layer can make the attenuation of about 60dB. However, the isolation effect often depends on the process of the shielding layer.

It is best to use a 0.2 mm thick copper sheet, with a shielding layer on the primary side and the secondary side. Usually, the shielding layer of the first side is connected to the shielding layer of the secondary side through a capacitor, and then connected to the ground of the secondary side. The shielding layer of the first side can connect to the ground wire of the first side, and the shielding layer of the secondary side can connect to the ground wire of the side. Moreover, the cross-sectional area of ​​the ground lead should be more significant. It is an excellent method to use an isolation transformer with a shielding layer, but the volume is large.

Due to the single function of the transformer, the relatively large size and weight of this method, the installation is not very convenient, and it is not effective in protecting medium and low-frequency spikes and surges. Therefore, the market is limited, and there are not many manufacturers. Therefore, it is generally not used on non-special occasions.

(5) Absorption method

The absorption method mainly uses absorbing devices to absorb the surge spike interference voltage. Wave absorbing devices have the same characteristics, that is, they present high impedance below the threshold voltage, and once the threshold voltage exceeded, the impedance drops sharply, so it has a specific inhibitory effect on the peak voltage.

Such absorbing devices mainly include varistors, gas discharge tubes, TVS tubes, and solid discharge tubes.

Different wave absorbing devices also have their limitations in suppressing spike voltage. If the current absorption capacity of the varistor is not large enough, the response speed of the gas amplifying tube is slow.

In the design of electronic products, surge protection design is a relatively important knowledge point. Many siege lions have fallen here, so pits can be prepared in advance to complete the project more efficiently and with high quality.

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