Functions / Requirements of Direct-Off-Line SMPS -- AC POWERLINE SURGE PROTECTION

1. INTRODUCTION

With the advent of "direct-off-line" switchmode power supplies using sensitive electronic primary control circuits, the need for input AC powerline transient surge protection has become more universally recognized.

Measurements carried out by the IEEE over a number of years have demonstrated, on a statistical basis, the likely frequency of occurrence, typical amplitudes, and waveshapes to be expected in various locations as a result of artificial and naturally occurring electrical phenomena. These findings are published in IEEE Standard 587-1980 and are shown in Table 1. This work provides a basis for the design of AC powerline transient surge protection devices.

2. LOCATION CATEGORIES

In general terms, the surge stress to be expected depends on the location of the equipment to be protected. When equipment is inside a building, the stress depends on the distance from the electrical service entrance to the equipment location, the size and length of connection wires, and the complexity of the branch circuits. IEEE Standard 587-1980 proposes three location categories for low-voltage AC powerlines (less than 600 V). These are shown in Fig. .2.1, and described as follows:

1. Category A, Outlets and Long Branch Circuits. This is the lowest-stress category; it applies to

a. All outlets more than 10 m (30 ft) from Category B with #14 to #10 wires.

b. All outlets at more than 20 m (60 ft) from the service entrance with #14 to #10 wires. In these remote locations, far away from the service entrance, the stress volt age may be of the order of 6 kV, but the stress currents are relatively low, of the order of 200 A maximum.

2. Category B, Major Feeders and Short Branch Circuits. This category covers the highest stress conditions likely to be seen by a power supply. It applies to the following locations:

a. Distribution panel devices

b. Bus and feeder systems in industrial plants

c. Heavy appliance outlets with "short" connections to the service entrance

d. Lighting systems in commercial buildings Note: Category B locations are closer to the service entrance. The stress voltages may be similar to those for category A, but currents up to 3000 A may be expected.

3. Category C, Outside and Service Entrance. This location is outside the building. Very high stress conditions can occur, since the line and insulator spacing is large and the flashover voltage can be greater than 6 kV. Fortunately, most power supplies will be in category B or A locations within a partially protected environment inside the building, and only protection to category A and B stress conditions is normally required.

Most indoor distribution and outlet connectors have sparkover voltages of 6 kV or less, and this, together with the inherent distribution system resistance, limits the stress conditions inside the building to much lower levels.

Where power supplies are to be provided with surge protection, the category of the protection should be clearly understood and specified in accordance with the expected location. Since the protection devices for category B locations can be large and expensive, this protection category should not be specified unless definitely required.

Where a number of supplies are to be protected within a total distributed power system, it is often more expedient to provide a single transient surge protection unit at the line input to the total system.

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TABLE 1 Surge Voltages and Currents Deemed to Represent the Indoor Environment and Recommended for Use in Designing Protective Systems

For high-impedance test specimens or load circuits, the voltage shown represents the surge voltage. In making simulation tests, use that value for the open-circuit voltage of the test generator.

† For low-impedance test specimens or load circuits, the current shown represents the discharge current of the surge (not the short-circuit current of the power system). In making simulation tests, use that current for the short-circuit current of the test generator.

‡ Other suppressors which have different clamping voltages would receive different energy levels.

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FIG. .2.1 Circuit location categories, as defined by IEEE Standard 587-1980.

3. LIKELY RATE OF SURGE OCCURRENCES

Since some transient protection devices (metal oxide varistors, for example) have a limited life, dependent on the number and size of the stress surges, the likely exposure level should be considered when selecting protection devices. Figure 1.2.2 (from For example, in a medium-exposure location, a 5-kV spike can be expected at least once a year and, perhaps of greater concern, hundreds of transients in the range of 1 to 2 kV can occur in the same period. Since even these lower stress levels are quite sufficient to damage unprotected equipment, it is clear that some form of protection is essential in any electronic equipment to be connected to the supply lines.

IEEE Standard 587-1980 describes the exposure locations as follows:

1. Low Exposure. Systems in geographical areas known for low lightning activity, with little load switching activity.

2. Medium Exposure. Systems in geographical areas known for high lightning activity, with frequent and severe switching transients.

3. High Exposure. Rare but real systems supplied by long overhead lines and subject to reflections at line ends, where the characteristics of the installation produce high spark over levels of clearances.

FIG. .2.2 Rate of surge occurrences versus voltage level at unprotected locations.

IEEE Standard 587) shows, statistically, the number of surges that may be expected per year, as a function of the voltage amplitude, in low-, medium-, and high-exposure locations.

FIG. .2.3 Proposed 0.5-µs, 100-kHz ring wave (open-circuit voltage).

FIG. .2.4 Unidirectional waveshapes (ANSI/IEEE Standard 28-1974).

4. SURGE VOLTAGE WAVEFORMS

The IEEE investigation found that although surge voltage waveforms can take many shapes, field measurements and theoretical calculations indicate that most surge voltages in indoor low-voltage systems (AC lines less than 600 V) have a damped oscillatory shape, as shown in Fig. .2.3. (This is the well-known "ring wave" referred to in IEEE Standard 587.) The following quotation from this standard describes the phenomenon well: A surge impinging on the (distribution) system excites the natural resonant frequencies of the conductor system. As a result, not only are the surges typically oscillatory, but surges may have different amplitudes and wave shapes at different places in the system. These oscillatory frequencies of surges range from 5 kHz to more than 500 kHz. A 30 kHz-100 kHz frequency is a realistic measurement of a "typical" surge for most residential and light industrial ac line networks.

In category B locations (close to the ser vice entrance), much larger energy levels are encountered. IEEE Standard 587 recommends two unidirectional waveforms for high- and low-impedance test specimens. These two waveforms are shown in Fig. .2.4a and b.

For this category, the transient protection device must be able to withstand the energy specified in these two waveforms ( Table 1). In addition to the unidirectional pulses, ring-wave oscillatory conditions can also occur.

For these, the voltage can be of the order of 6 kV and the current 500 A. The various stress conditions are tabulated in Table 1.

The impedance of the protection circuit is often difficult to define, since a number of devices operating in different modes and different voltages are often used in the protection unit. To satisfy both high- and low-impedance conditions, the test circuitry is usually configured to generate the voltage waveform specified on an open circuit and the current waveform specified on a short circuit before being applied to the test specimen.

5. TRANSIENT SUPPRESSION DEVICES

The ideal transient suppression device would be an open circuit at normal voltages, would conduct without delay at some slight voltage above normal, would not allow the voltage to increase during the clamping period, would handle unlimited currents and power, would revert back to an open circuit when the stress has gone, and would never wear out.

At the time this is written, there is no single transient suppression device that approaches this ideal for all the stress conditions specified in IEEE Standard 587. Hence, at present efficient transient protection requires the use of a number of devices, carefully selected to complement each other and thus cover the full range of voltage and current stress conditions.

For the lower-stress category A locations, silicon varistors, in combination with transient suppressor diodes, filter inductors, and capacitors, are commonly used. In the higher-power category B locations, these devices are supplemented with much higher-current-rated gas discharge tubes or spark gaps. When gas-discharge devices are used, fast-acting fuses or circuit breakers will also be fitted.

For efficient matching of the various suppressor devices, their general performance characteristics should be fully understood.

6. METAL OXIDE VARISTORS (MOVS, VOLTAGE-DEPENDENT RESISTORS)

As the name implies, varistors (MOVs) display a voltage-dependent resistance characteristic. At voltages below the turnover voltage, these devices have high resistance and little circuit loading. When the terminal voltage exceeds the turnover voltage, the resistance decreases rapidly and increasing current flows in the shunt-connected varistor.

The major advantages of the varistor are its low cost and its relatively high transient energy absorption capability. The major disadvantages are progressive degradation of the device with repetitive stress and a relatively large slope resistance.

The limitations of the varistor for transient suppressor applications in medium-and high risk locations are fairly marked. Under high-exposure conditions, the device can quickly age, reducing its effective clamping action. This is a somewhat insidious process, as the degradation is not obvious and cannot be easily measured. Further, the varistor's relatively high slope resistance means that its clamping action is quite poor for high-current stress conditions (even low-voltage varistor devices have terminal voltages over 1000 V at transient currents of only a few tens of amperes). As a result, damagingly high voltages may be let through to the "protected" equipment if MOVs are used on their own. However, varistors can be of great value when used in combination with other suppressor devices.

Figure 2.5 shows the typical characteristics of a 275-V varistor. Note that the terminal voltage is 1250 V at a transient current of only 500 A.

7. TRANSIENT PROTECTION DIODES

Various transient suppressor diodes are available. These may be unidirectional or bidirectional as required. In general terms, silicon suppressor diodes consist of an avalanche voltage clamp device, configured for high transient capability. In a bipolar protector, two junctions are used in series "back to back." (An avalanche diode exhibits a normal diode characteristic in the forward direction.) The transient suppressor diode has two major advantages, the first being the very high speed clamping action-the avalanche condition is established in a few nanoseconds. The second advantage is the very low slope resistance in the conduction range.

In the active region, the slope resistance can be very low, with terminal voltage increasing by only a few volts at transient currents running into hundreds of amperes. Consequently, the transient suppressor diode provides very hard and effective voltage clamping at any transient stress up to the diode's maximum current capability. The characteristics of a typical 200-V bipolar transient suppressor diode are shown in Fig. .2.6. Note that the terminal voltage is only 220 V at 200 A.

FIG. .2.5 Metal oxide varistor (MOV) performance characteristics.

FIG. .2.6 Transient suppressor diode performance characteristics.

The major disadvantages of the transient suppressor diode are its relatively high cost and limited current capability. However, if the diode is overstressed, it is designed to fail to a short-circuit condition; this would normally clear the external fuse or circuit breaker, while maintaining protection of the equipment.

8. GAS-FILLED SURGE ARRESTERS

Much larger transient currents can be handled by the various gas-discharge suppressor devices. In such suppressors, two or more electrodes are accurately spaced within a sealed high-pressure inert gas environment. When the striking voltage of the gas tube is exceeded, an ionized glow discharge is first developed between the electrodes. As the cur rent increases, an arc discharge is produced, providing a low-impedance path among all internal electrodes. In this mode, the device has an almost constant voltage conduction path with a typical arc drop of 25 V. The characteristics of the gas arrester are shown in Fig. .2.7. Note the large striking voltage and low arc voltage.

FIG. .2.7 Gas-filled surge arrester (SVT) performance characteristics. (Siemens AG.)

When it strikes, the gas arrester effectively short-circuits the supply, with only a small volt age being maintained across the electrodes. Because of the low internal dissipation in this mode, a relatively small device can carry currents of many thousands of amperes. With this type of suppressor, protection is provided not so much by the energy dissipated within the device itself, but by the device's short-circuit action. This forces the transient energy to be dissipated in the series resistance of the supply lines and filter.

A disadvantage of the gas arrester is its relatively slow response to an overvoltage stress. The plasma development action is relatively slow, and the striking voltage is dv/dt-dependent. Figure 2.8 shows the striking voltage as a function of dv/dt for a typical 270-V device. The effect is quite marked at transient edge attack rates as low as 10 V/µs. Hence, for fast transients, the gas arrester must be backed by a filter or faster-action clamp device.

A major disadvantage of the arrester is its tendency to remain in a conducting state after the transient condition has ceased. On ac lines, the recovery (blocking action) should normally occur when the supply voltage falls below the arc volt age at the end of a half cycle. However, the line source resistance can be very low, and if the current rating of the device is exceeded, the high internal temperature may prevent normal extinction of the arc, so that the device remains conducting. The follow-on current, provided by the line sup ply after the transient has finished, will soon destroy the arrester. Hence, with this type of device, it is essential to provide some form of current limiting, fast-acting fuse or fast acting (magnetic) circuit breaker in the supply line.

Many manufacturers and designers advocate fitting a limiting resistor in series with the gas tube. This will reduce the follow-on current after the gas tube has struck. This technique satisfies the need to limit the follow-on current, and allows plasma extinction as the supply voltage passes through zero. However, the series resistance degrades the transient suppressor performance, since even a small (say 0.3-7) resistor would develop a voltage drop of 1000 V for the 3000A IEEE Standard 587 high-current stress condition. The author prefers not to fit series resistors, but to rely on filter and external circuit resistances to limit the suppressor current; this retains the excellent clamping capability of the gas device. For extended stress conditions, a fast circuit breaker or fuse will finally clear the line input if the gas device remains conducting.

The gas arrester is still undergoing development at the time this is written, and many ingenious techniques are being developed to improve its performance.

FIG. .2.8 Variation in sparkover voltage with applied dv/dt for gas-filled SVPs. (Siemens AG.)

9. LINE FILTER, TRANSIENT SUPPRESSOR COMBINATIONS

As mentioned above, the various transient suppressor devices have limited current capability.

Because the line impedance can be extremely low, it is often necessary to include some limiting resistance in series with the supply lines to reduce the stress on the shunt-connected suppressors. This also permits efficient voltage clamping action.

Although the series limiting may be provided by discrete resistors, in the interest of efficiency, inductors should be used. If inductors are used, it is expedient to provide additional filtering in the transient suppressor circuit at the same time. This will help to reject line-borne noise and filter out power supply-generated noise. Also, the winding resistance and inductance can provide the necessary series impedance to limit the transient current for efficient transient suppression. Consequently, transient suppression is often combined with the EMI noise filtering circuits typically required with switchmode supplies.

10. CATEGORY A TRANSIENT SUPPRESSION FILTERS

Figure 2.9 shows a typical combination of line filter and transient suppressor devices that may be found in a category A protection unit.

FIG. .2.9 Line-to-line and line-to-ground transient over-voltage protection circuit with noise filter, using MOV and SVP protection devices (low- to medium-power applications).

The inductors L1(a) and L1(b) and capacitors C1 through C4 form the normal noise filter network. At the input to this filter network, varistors MOV1 through MOV3 provide the first level of protection from line-borne transient stress. For very short lived high-voltage transients, the clamping action of the varistors, together with the voltage dropped across the series inductance, holds off the majority of the transient voltage from the output.

For more extended stress conditions, the current in L1(a) and L1(b) will increase to the point where the output capacitors C2 and C3 are charged to a voltage at which suppressor diodes ZD1, ZD2, and ZD3 are brought into conduction. These diodes prevent the output voltage from exceeding their rated clamp values for all stress currents up to the failure point of the suppression diodes. If this level is reached, the diodes fail to a short circuit, clearing the protection fuse FS1, and the unit fails to a safe condition. However, this very high level of stress should not occur in a category A location.

It should be noted that the suppressor unit also prevents voltage transients generated within the driven equipment from feeding back into the supply line. This can be an important advantage when several pieces of equipment in a system are connected to the same supply.

In this example, protection has been provided for differential- (line to neutral) and common-mode (line and neutral, to ground) stress. It will be shown later that although differential protection is often the only protection provided, common-mode stress conditions often occur in practice. Hence, protection for this condition is essential for full system integrity.

The wisdom of common-mode transient suppression has been questioned as possibly being dangerous, because of the voltage "bump" on the earth return line under transient conditions. It will be shown later that this effect is almost inevitable; it should be dealt with in other ways if full protection is to be provided.

11. CATEGORY B TRANSIENT SUPPRESSION FILTERS

Although the circuit shown in Fig. .2.9 could be used for category B locations if suitably large devices were selected, it is more expedient to use the small low-cost gas-discharge suppressors to provide the additional protection.

Figure 2.10 shows a suitable circuit arrangement. This circuit combines the advantages of all three types of protection device, and also has a full common-and series-mode filter network.

The common-mode filter inductor L3 (a and b) has been supplemented with additional series-mode inductors L1 and L2. These inductors, together with capacitors C1 through C5, provide a powerful filter for common- and series-mode line-conducted transient and RFI noise. This unit may be used to supplement or replace the normal line filter of the switchmode supply.

In addition to the voltage-dependent resistors (varistors) and output transient suppressor diodes, the three-terminal gas-discharge arrester tube (GT1) is shown fitted at the interface between the series-mode and common-mode inductors.

This arrangement combines the advantages of all three suppressor devices in a most effective manner. For very fast transients, once again, the input varistors V1, V2, and V3, together with L1, L2, and L3 (a) and L3 (b) and capacitors C1 through C5, provide efficient attenuation of the transient. For medium-stress conditions of longer duration, the current in the inductors will increase and the output voltage will also increase to the point where the output clamping diodes D1, D2, and D3 are brought into conduction, protecting the load.

The major advantage of this category B suppressor is that for very large and extended stress conditions, the gas arrester GT1 will be brought to the striking voltage, effectively short circuiting all lines (and the transient) to ground.

An advantage of the three-terminal gas arrester is that, irrespective of which line the original stress appears on, all lines are shorted to ground. This tends to reduce the inevitable ground return "bump" voltage.

Extensive stress testing of this circuit has shown that in most cases, the supply line impedance, combined with the current limiting action of L1 and L2, will prevent excessive buildup of current in the gas arrester after ignition. As a result, the arrester recovers to its nonconducting state after the transient has passed, during the following zero crossover of the supply line. Hence, under the rare conditions when a gas device is called on to conduct, in most cases the power to the load is interrupted for less than a half cycle.

FIG. .2.10 Line-to-line and line-to-ground transient protection circuit with noise filter, using MOV, SVP, and transient protection diodes (for medium- to high-power applications).

Because of the energy storage and holdup ability of the typical switchmode supply, a half cycle line dropout will not result in an interruption of the DC output to the loads. In the rare event of a gas tube continuing to conduct, the fast-acting magnetic circuit breaker will operate in less than a cycle, clearing the line input from the filter.

12. A CASE FOR FULL TRANSIENT PROTECTION

The major cause of high-voltage transients is direct or indirect lightning effects on the external power system. Irrespective of the initial cause of the transient, be it a direct strike to one or another of the supply lines or the induced effects of a near miss, the initial stress attenuation is provided by flashover between lines and from line to ground at various points throughout the distribution system.

As a result of these flashovers, the transient that arrives at a remote location will tend to be common-mode, appearing between both supply lines and ground. Even if the neutral is connected to ground near the service entrance of the building, the stress can still tend to be common-mode at the protected equipment because of flashover in the building cables, distribution boxes, and receptacles. (It is this flashover that reduces the stress between category C and category A locations.) Consequently, transient suppressors which provide protection between line and neutral only are not protecting the equipment or common-mode capacitors against the line ground, neutral ground stress conditions.

13. THE CAUSE OF "GROUND RETURN VOLTAGE BUMP" STRESS

A voltage stress which appears between both supply lines and the ground return is called a common-mode transient. When a common-mode transient arrives at the suppressor unit, the current is diverted to ground through one or more of the transient suppressor devices. As a result, considerable currents can flow through the ground return during a transient. Because of the resistance and inductance between the transient suppressor and the service entrance, this ground current can elevate the potential of the local system ground with respect to real ground. Hence, a possible shock hazard now exists between the case of the protected equipment and real earth. (This voltage is referred to as an earth return "bump.") It is possible to argue, therefore, that transient suppressors which return the stress current to the ground line are a shock hazard and should not be used. This is a viable argument only if the load can be guaranteed not to break down to ground during the stress in the absence of a transient suppressor. In practice, the equipment is likely to fail in this mode, and the hazard of ground return bump will still exist, even without the suppressor. In addition, the load will not have been protected and may well be damaged.

The possibility of an earth return voltage bump under high-stress conditions should be considered an inevitable hazard with or without transient protection. Measures should be taken to reduce the voltage by ensuring a very low resistance ground return path. If an operator has access to the equipment, all equipment within the operator's reach must be grounded to the same return. In computer rooms, the need for a good ground return may include the furniture and very fabric of the building itself.

14. QUIZ

1. Why is it important to provide AC powerline surge protection in direct-offline switch mode power supplies?

2. Give some typical causes of AC line transients.

3. Give the number of an IEEE standard which describes the typical amplitudes and waveshapes to be expected on various line distribution systems in office and domestic locations.

4. Describe stress locations A, B, and C, as described in IEEE Standard 587-1980.

5. Explain the meaning of exposure locations, as described in IEEE Standard 587-1980.

6. How does IEEE Standard 587-1980 indicate the likely rate of surge occurrence and voltage amplitude at various locations?

7. What would be the typical waveform and transient voltage to be expected in a class A location?

8. What surge waveforms may be expected in a class B location?

9. Describe three transient protection devices commonly used in input line protection filters.

10. Describe the advantages and limitations of metal oxide varistors, transient protection diodes, and gas-filled surge suppressors.

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