In many applications, a reliable power supply is essential to ensure that there’s no disruption due to grid outages. This might be as simple as keeping your computer running when the lights go out, or providing a complex, highly structured supply to a hospital or datacentre. Similar applications exist throughout the industry, in telecoms systems, and many more.
This demand is met by uninterruptible power supply (UPS) systems, which typically use batteries to ensure power is always available. This might be just to cover short-term outages or to provide power for long enough for a diesel generator to start up or for essential equipment to shut down properly.
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The different types of UPS implementations
There are two fundamental types of UPS: offline and online.
Offline systems are typically low-power and can deliver up to about 0.5 kW. Their disadvantage is that they take about 10ms to provide power in response to a failure in the AC supply, which is too long for many critical applications. This is because the load is connected directly to the AC power source, and is powered by the AC in normal operation, only switching – using a relay – to the UPS’ battery when the outage occurs.
An option to consider is a Line Interactive UPS, which is another type of offline UPS. Typically providing between 0.5 kW and 5 kW, the Line Interactive UPS also actively regulates the supply voltage in normal use and achieves faster back-up times of around 4 to 10 ms.
The second main type of UPS is the online system, which provides instantaneous backup power with no break in supply due to starting up. Online UPS are typically used in high power applications, often 10 kW and above, according to EN 62040-3 Class 1.
The market for offline, low-power systems is typically cost-driven and highly commoditised, meaning the opportunities for design and differentiation are in the online, high-power segment.
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High-power UPS systems
In an online UPS, the DC-AC inverter is always connected to the load (Figure 1), and the bidirectional battery charger is always able to provide power instantly, which means there are no delays in switching to battery power.
In an online UPS, the AC input first passes through a rectifier which converts it to DC, as well as providing power factor correction (PFC) and filtering. The bidirectional charger then uses this DC power to charge the battery, while an inverter also converts the DC to a stable AC voltage and isolates the load from the power grid.
With this topology, the AC supply is converted to DC to charge the battery, and then back to AC if needed to power the load. This is known as a ‘double conversion’ UPS. It is worth noting that these types of UPS have the inevitable disadvantage of reducing efficiency due to two power conversion stages.
Market drivers and design considerations for UPS systems
As we have discussed, the best choice of UPS system depends on your application’s requirements – with offline and online systems providing a range of options for different power capacities, switchover times, and costs. Whatever the chosen UPS system, it needs to provide high efficiency, excellent reliability (for example, a minimum 10-year lifetime), and low dynamic losses for critical equipment operating 24/7.
Figure 1: Online UPS (Source: onsemi)
While the online UPS is the most expensive option, it provides the best protection for high-power critical loads. Options include monolithic UPS, typically supplying over 50 kW, and using multiple modular UPS (typically 10 kW to 50 kW). The choice depends on the particular use case – monolithic UPS can have a lower initial purchase price, while modular UPS provides redundancy and higher availability.
Any cost discussion needs to look at total cost of ownership (TCO), a key driver of purchase decisions. Components of TCO to consider include the fixed and manufacturing costs of purchasing a suitable UPS system, which are influenced by power density and efficiency, as well as variable/operational costs such as energy consumption, space usage costs related to footprint, and the cost of any downtime from the UPS.
Power topologies in a UPS
Within UPS systems, several different topologies can be chosen for both the rectification (AC-DC conversion) and inversion (DC-AC conversion).
A popular choice for AC-DC conversion is the ‘Vienna’ rectifier, a three-level topology. Compared to a basic two-level rectifier, it reduces the voltage stress on components and operates in a continuous conduction mode (CCM), which minimizes voltage spikes and fluctuations, thus improving reliability.
For DC-AC conversion, two-level and three-level inverters are popular, but two-level designs can suffer from higher harmonic distortion and voltage stress. Three-level inverters, such as NPC1 and NPC2, offer lower voltage stress, and the NPC2 topology also offers lower conduction losses and higher efficiency than NPC1.
SiC vs silicon
While silicon has been the traditional material of choice in UPS, today’s silicon carbide (SiC)-based devices present some key advantages. Firstly, they have significantly better efficiency than silicon, with lower conduction and switching losses. This reduces heat and increases efficiency, meaning that a UPS can power a load for longer with the same battery size.
The high switching frequency of SiC can also enable a reduction in the size and weight of passive components, thus saving space and weight in the overall UPS system.
Finally, SiC devices can operate at greater temperatures and higher voltages and currents than silicon devices, reducing the need for cooling systems and enabling reliable high-power operation.
Picking the right material or topology for your UPS application can be complicated. Avnet Silica’s experts can provide the advice you need to make the best decision. Get in touch with us.
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