Protecting the Protector: Adding Circuit Protection to UPSs

Downtime has taken on enormous importance as companies struggle to compete in a global market and a global downturn. Managers are in hot pursuit of technologies that reduce business interruptions, no matter how elusive. And when your product is an uninterruptible power supply (UPS) then, let's face it, it's all about reliability.

UPS design engineers have to make sure that the batteries don't start a fire and that the signal lines that communicate alarms and data continue to function after sustaining potentially disruptive transient events such as lightning. Circuit protection for a UPS can be tricky, and designers need to be a bit foxy about how they apply it. The right choices make the difference between a UPS that prevents an interruption and a UPS that is itself a source of interruption.

To meet UL safety standards, a UPS needs basic overcurrent protection to prevent fires, which is usually handled by a fuse on the ac input. But for reliability, the design engineer is concerned about protecting the UPS against transient voltage events that can disrupt internal circuitry. A UPS in outdoor applications is exposed to voltage surges caused by nearby lightning strikes and carried on power lines. Similarly, a UPS located in a building near the service entrance (for example, a computer server room) may have large and short power-input lines. This is a typical environment for a commercial-grade UPS, which means there usually is a very-low-impedance path between a nearby lightning strike and the UPS ac input. Also, equipment within a building, such as electric welders or large motors (machinery or HVAC air handler blowers) may cause voltage transients large enough to disrupt sensitive electronic components within the UPS. This calls for protection.

UPS CIRCUIT PROTECTION CONSIDERATIONS

If a UPS is called into service, whatever overvoltage event knocked out the power (such as nearby lightning) must not also prevent the UPS from coming on line and communicating the occurrence of such an event to its supervisor system. So protecting the UPS from overvoltage events at the ac mains power input is critical. The usual solution is the use of one or more metal oxide varistors (MOVs).

MOVs respond quickly and have high surge ratings. With energy ratings from 0.1 to 10,000 joules and typical peak current ratings from 40 A to 70,000 A, MOVs divert transient currents to ground and away from sensitive circuits. They are clamping devices, so they don't short the ac input line when they activate (doing so would trip the overcurrent protection), and they recover automatically when the overvoltage is gone.

However, MOVs are ceramic devices that degrade slightly with each successive surge activation. This aging characteristic of MOVs must be kept in mind. Many commercial-grade UPSs sit inactive for weeks or years, and with each passing overvoltage event (from lightning or motor switching) the MOVs will slowly degrade. So choosing MOVs with long life ratings is a key consideration. Some newer types of circuit protection, such as silicon-based devices, do not suffer from such aging characteristics. Silicon components may cost more than MOVs (which are the usual solution) but the performance of silicon-based transient-voltage suppression (TVS) diodes don't degrade with use.

Silicon avalanche diodes and bidirectional TVSs also provide protection against high-voltage transients. Typically, peak power ratings range from 400 W to 50 kW, and reverse standoff voltages from 5 V to 376 V. Their precision clamping voltages offer better consistency than MOVs. Some devices offer a clamping voltage lower than the avalanche voltage (but above the rated working voltage). Therefore, any voltage rise due to increased current conduction is minimized, providing superior protection.

Few designers know that exotic silicon devices are available with peak power ratings exceeding 600 kW (10×1000) or 6,000 kW (8×20). Larger in size than a typical TVS device, yet much smaller than a similarly rated MOV, these unusual-looking devices are made by stacking up to 14 layers of silicon between heavy copper plates (Fig. 1).

Whatever type device is chosen, it should be hefty. For example, a ceramic overvoltage-protection device that is acceptable in a consumer product, such as an appliance, probably won't be robust enough for a commercial UPS application. Many consumer products can be protected by a 14-mm MOV because they are usually connected to a branch circuit using a small power cord. Commercial UPSs, because they are connected via heavy-duty low-impedance wiring, should use 34-mm MOV devices at a minimum.

If a surge event does knock out the power and cause the UPS to come on line, the alarm signal line back to the supervisor system still has to work, or the supervisor will never know that the UPS has been activated. So make sure to put circuit protection on signal lines that leave the enclosure as well as on the internal logic I/O lines. This will protect the lines from ESD and voltage transients. The usual solution is to use ceramic multi-layer varistor (MLV) devices.

MLVs provide protection in sensitive equipment operating at 0 to 120 Vdc, with peak current ratings from 20 to 500 A and peak energy ratings from 0.05 to 2.5 joules. As SMT devices, they don't have a significant amount of parasitic inductance and can offer response times as low as 1 to 5 ns. However, their comparatively large capacitance (up to 6,000 pF) renders them unsuitable for high-frequency lines. Silicon protection-array devices are more appropriate for high-speed data lines due to their low capacitance.

CIRCUIT DESIGNS THAT AVOID AGING

When selecting devices for power-input protection, be mindful of the degradation of MOVs that are exposed to high-voltage surges. With ratings around 40 kA, an MOV can handle repeated large surges, but each surge ages the MOV and, over time, its break-over voltage will begin to rise, reducing its effectiveness.

One strategy is to put up to 20 MOVs on the power input so that a high surge will be shared across multiple MOVs, preserving their low clamping-voltage ratings (Fig. 2). Derate the parallel combinations by 20% because the MOVs may not perfectly share the current due to slight differences in individual characteristics.

Many designers of UPS systems are not familiar with the new silicon devices emerging for UPS applications, and therefore they often overlook this option. Silicon-based surge protectors will block surges repeatedly with no degradation in performance. However, with typical ratings up to only 10 kA, a large voltage surge could destroy a standard silicon-based device.

Therefore the ideal arrangement is to combine the high surge rating of the MOV in series with the non-aging reliability of the silicon device (Fig. 3). An inductor is placed between the two devices. The inductor coil resists changes in the current, which protects the silicon device from large surges. The inductors must be air-core devices so they do not saturate during surges.

TVSS CIRCUIT PROTECTION

Transient voltage surge suppressors (TVSSs) are similar to UPSs in form and function. When it comes to circuit protection, many of the same design considerations apply. The use of TVSSs is broadening, as they are used increasingly in new home and building construction at the service entrance panel.

Here, they are more exposed to lightning-generated transient voltages than if they were located far from the panel, where the natural resistance of wiring in the building offers a small measure of protection. When designing a TVSS, be sure it is fortified with proper protection devices.

POINTS OF CIRCUIT PROTECTION IN A UPS DESIGN

LET'S REVIEW THE BASIC DESIGN of a UPS (Fig. 4), and note where circuit protection is needed. Protection starts with a fuse on the ac main: the power input. The chosen fuse must be heavy enough to withstand lightning surges yet remain the “weakest link” so that it will open safely if any downstream circuit component fails.

1. :The ac input drives a switch-mode power supply (SMPS), which creates the battery recharge circuitry and provides power for the logic of the device (in the controller circuitry). There isn't much need for overvoltage protection inside the SMPS because it is already protected on both sides. It does have its own fuse on the power input, but no MOVs.
2. The control circuitry has I/O lines and it usually has a signal line, such as a LAN connection. All of these ports require protection, although it is sometimes overlooked.
3. The battery has a fuse, similar to an automotive fuse. Match the rating to the protection needs of the wiring.
4. Next is an inverter/oscillator that recreates the 60 Hz.
5. Finally is the switch that switches the UPS to battery power when it's needed. One side is the output for the regular ac power, and the other side is the output for the inverter-created ac.
6. No circuit protection is needed at the inverter or the switch, because they are protected by devices upstream.