Power System Integration Part 3: DC UPS

In Part 1 of this four-part series on power system integration, we covered how redundancy and multiple input sources form a fault tolerant power system in the article, “Single Source Fault Tolerant Systems,” published in June of this year. Last month, we continued with Part 2, “Battery Backup,” describing how battery-based standby online UPSs protect against power outages. This month, we'll continue our discussion on power system integration, emphasizing the advantages the dc UPS (DUPS) has over its ac UPS counterpart.

A DUPS is actually a power supply (mostly SMPS) with battery backup. The backup involves either a battery placed across the output of this power supply or a high frequency dc-dc converter embedded within the SMPS. In any case, this system does not have double conversion when powered by an electric utility, so it avoids expensive sinewave generation. Its overall efficiency while on utility power is continuously 75% to 80%, as opposed to 56% to 60% for a UPS-SMPS combination. More importantly, the overall price is only 10% to 20% higher than the price of a standard SMPS. As such, it provides backup at a fraction of the cost of an ac UPS, and the electricity consumption cost for the years of service is much smaller.

Due to its higher efficiency, a DUPS also requires less battery capacity than a standard ac UPS. In the DUPS, you can accomplish battery backup in a variety of ways — some direct and simple, and some more complex. Let's focus on these methods, starting with those not requiring conversion.

DUPS with LVD

Often, an SMPS with a single output of 24V (commercial, communications system, or industrial applications) or 48V (telecom applications) requires a battery backup for a few minutes to a few hours. The host system in this case may be able to tolerate the battery voltage range of 21V to 30V for the 24V case, or 42V to 56V for the 48V case.

It makes sense to connect the battery across the SMPS output. Thus, if the utility power (or SMPS) fails, the battery takes over. Fig. 1, on page 26, shows a DUPS consisting of a single output with a low voltage disconnect (LVD) circuit and external battery in such a backup configuration. In this arrangement, a lengthy blackout can totally drain the battery — unless you can cut off the load before deep discharge starts. The LVD circuit uses a MOSFET switch, which is preferable to a bipolar transistor because the MOSFET's body diode has no extra cost. This circuit allows the discharge of battery current into the load until the battery reaches its low threshold. At the low threshold point, logic cuts off the MOSFET's gate drive, preventing further discharge. If the utility power fails, the SMPS sends an “AC Fail” signal (a standard feature in most SMPS) to the host system, notifying it that the battery now powers the output.

By knowing the discharge time, you can program the host system to shut down when feasible, and without any crashes. When utility power returns, the LVD resets, and the battery starts to charge. This method is the most simple and least costly. It's also the most reliable and efficient, wasting no battery energy for conversion. The battery is simply charged by the host system's power supply. The Photo, above, shows a redundant hot swap power system with a battery backup LVD configuration.

Despite its advantages, the main disadvantage of the LVD method is it's only suitable for specific host systems:

• Those with a single output power supply of 12V, 24V, 48V, or some other battery standard voltage, i.e. multiples of 12V.
• The host system must be able to work well from the low to high battery voltage range, which is approximately ±15% of nominal. This holds regardless of the battery type. If the host system's active circuitry needs tight regulation, the method isn't applicable.
• The battery should be relatively insensitive to the charging method and tolerant of a high charge after a discharge. For this reason, the recommended battery is a maintenance-free, sealed, lead-acid type. These batteries are rugged, abuse-tolerant, and insensitive to charge. They're also relatively inexpensive — compared with others — and maintenance free.

Instead of a MOSFET, the LVD can use a relay or a contactor to cut off the load before deep discharge. The LVD can be either a latching type (cutting off when reaching the low threshold, with an automatic reset when the ac utility comes back); or it can be a simple hysterisis comparator (sensing the battery voltage and when it cuts off, will not reset until the battery voltage rises 4V to 5V above the cutoff point). This feature avoids cycling because the battery voltage jumps up when the load is removed. For a 24V battery, the cut off should be 21V, with automatic restoration at 25V to 26V; for a 48V battery it's 42V and 46V.

Often, you can backup a multi-output SMPS by using the LVD method. You can accomplish this if the main output in the SMPS is 12V, 24V, or 48V, and the other outputs are derived from it. Once the main output is backed, the others will also exist.

Multi-Output DUPS

You will encounter a more complex situation when the SMPS provides several independent outputs. In this case, it's not possible to place a battery (with LVD) across the output, since there's backup for only one output and the rest will not have backup. Fig. 2 shows the method of backup for a DUPS with a multi-output SMPS.

A multi-output DUPS receives ac input from the utility, as well as dc input from a battery. If the ac power fails, a signal activates a dc-dc converter module contained within the SMPS. This converter produces 300Vdc to 350Vdc, replenishing the primary side dc rail to keep the SMPS and all of its outputs alive. Activation of the dc-dc converter is quick (2 ms to 5 ms) and well within the SMPS hold-up time. The large capacitors on the dc rail store a significant amount of energy (with high energy-to-volume ratio) due to the high voltage. Hence, it needs no extra storage to facilitate a smooth, seamless transition between utility and battery power. All the outputs continue uninterrupted, and the dc rail remains good for the useful range of the battery. The dc-dc converter module can operate open loop, requiring no regulation. It can be a simple self-oscillating circuit without PWM. Fig. 3, on page 28, shows a 1 kW dc-dc converter module (switching at 80 kHz) embedded in a SMPS to provide DUPS operation.

To understand the DUPS advantages, consider that a host system SMPS employs a 1kW dc-dc converter switching at 60 kHz to 100 kHz, measures 5 in.×5.25 in.×2 in., and weighs less than 2 lb. This is less than 10% the size of an electronics part of an ac UPS. The dc-dc converter remains inactive when utility voltage exists, but activates when the utility fails. Therefore — under normal conditions — it doesn't dissipate power, and the SMPS works strictly as a power supply.

When the dc-dc converter operates, the unit as a whole works at 65% to 70% efficiency as opposed to 50% to 55% in ac UPS combination. While the ac UPS dissipates power all the time, and is therefore costly to maintain for years of operation, the dc-dc converter or LVD circuits don't dissipate any power in the standby mode. This results in a savings of about $1000 in energy costs over a period of five years of continuous operation at a 1kW load.

The LVD circuit or the dc-dc converter in the above DUPSs can be added options in the SMPS, if the SMPS can accept them with no noticeable difference.

Battery Charging

In either the LVD or dc-dc converter, the SMPS must charge the battery. You can use a simple battery charging circuit for this function as an added SMPS output, or, you can use a small high-frequency switching regulator. A rectified secondary voltage of the power transformer within the SMPS can produce a good charging voltage, if regulated by inexpensive three-terminal linear regulators. The cost of these components is barely $1.50 for a 1A charger.

Fig. 4 shows a multi-output SMPS that contains the module shown in Fig 3. Simplifying the DUPS design even further is possible if the dc-dc converter circuit shares the same power transformer as the SMPS. Consider a simple microprocessor circuit that controls all the SMPS functions. This controller senses the failure of utility voltage and automatically activates the power transistors of the dc-dc converter stage. In this scheme, there's only one power transformer, resulting in better weight and volume efficiencies as well as lower cost.

For a quick implementation of a DUPS with LVD or embedded dc-dc converter, the following four considerations must receive priority during the design stage.

First, if only one output is necessary, it should be as close as possible to the nominal voltage of a standard battery i.e. 12V, 24V, 48V, etc. This will make it possible to connect a backup battery at the output if an LVD is included in the SMPS.

Second, if the major output is suitable for a DUPS with an LVD and one more small output is necessary, it costs less to produce the second output from the main DUPS output.

Third, if multiple outputs are necessary, consider the cost comparison between a number of dc-dc converters driven from a main output (i.e. “distributed power” style) and an embedded central dc-dc converter module that facilitates DUPS configuration.

Last, it's advisable to build the SMPS in such a way that an LVD or a small backup dc-dc converter may be included in it as an option. This will not add too much to the volume, but will make it readily feasible to offer the SMPS to DUPS at later stage or for those users who request it.

To complete the discussion on input source redundancy, we'll conclude our series on power system integration in the coming months with Part 4. This future article will discuss multiple input ac sources for backup against utility failure, covering the design criteria for the transfer switch.

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