DC UPS and Transfer Times

Following on from Klara V’s article on UPS Transfer Times I thought I’d expand on this with regard to DC UPS Systems. A DC UPS system works very much in the way that an online UPS works up to a point. That is, it takes incoming AC and converts this into DC power. Whereas the online UPS converts this into a high DC level to power the bus from which the inverter works, the iPower DC UPS converts this into a lower 12V. It does this both when powered from AC or its internal DC battery.

This 12V is a regulated output to within 5%. However there still needs to be a transition, as the unit will need to move from a primary AC power source to the energy stored in the battery – a DC power source. Hence the internal circuit needs to switch from AC to DC conversion to DC to DC conversion. Since this transition is unplanned, eg an unexpected power cut, there will be some transition time from when the unit detects the mains failure and starts up the DC to DC converter.

During this time the output voltage falls dependent upon the amount of power being taken by the load. The output of the iPower only has so much capacitance it can contain in a small enclosure. It is this capacitance, and the length of time taken for the DC to DC converter to start up which results in a period of unregulated voltage. Note the output voltage does not disappear, so there is not a break in power as such. There may just be a slight dip in the output voltage.

Example

Take the following image. This is measuring the output voltage when powering a 12V 1A router. The vertical grid (voltage) is 2V per square and the horizontal grid (time) is 5milliseconds per square. In the trace, the input voltage disconnects at two squares (10msec) in.

Transition of DC UPS

The output voltage which is perfectly on 12V starts to fall. After 10milliseconds, the DC converter starts to power up arresting the fall in voltage. A further 10milliseconds later, the voltage has fully recovered. There is a slight overshoot due to the control circuit which is still within the regulation of the iPower. Note the fall in the output voltage is limited to around 1.6V and lasts for only 20 thousands of a second.

Unlike offline or line interactive AC UPS where there is a definite loss of power for a period, the iPower DC UPS does not lose power totally. Instead there is a period of unregulated voltage, where the output voltage will fall. The amount it falls depends on the load connected. The higher the load, the more the drop. For a typical router this is around 1.5V.

Does this matter?

For the vast majority of 12Vdc powered equipment this variation in voltage is not a problem. Most equipment will have some in built regulation or capacitance which can ride through this very short term power drop. In fact, out of the many thousands of iPowers sold we have only had an issue with one such router – the Huawei HG633 as used by TalkTalk. This router, being the free ISP provided model seems to be made a cheap as possible – see this review on the HG633 router. Especially since other Huawei routers work absolutely fine with the iPower DC UPS. That being said, our engineering team have already made some improvements to the iPower to make the transition time even faster. Our prototype of the next generation iPower DC UPS works with the HG633 as well. Available in 2018.

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