UPS Design – Part Two
In order not to confuse we will stick to three types of UPS and the electric motor-alternator or frequency converter, which is; the standard inverter or standby UPS, three phase motor generator unit (better known as rotary frequency/converters), the line-interactive UPS and the double-conversion / On Line UPS types. The inverter in its simplest form was known as an inverter / charger as it would charge the battery when power was good and switchover to inverter status when power was down. It consisted of a multivibrator circuit running at upwards of 50Hz (square-wave), a buffer circuit which in junction with the multivibrator would drive power bipolars or FETs coupled to a transformer which was in most cases a laminated type. A relay acted as the switchover. It was a simple abut in it’s simplicity came it’s biggest drawback – the time taken to switchover when mains power dropped would cause the load to lose power momentarily, not nearly good enough for electronic equipment, especially computers or data retrieval systems. They were then used more often than not as a simple means of backup power often in caravans and for camping.
The rotary coverter was oft seen on naval vessels in the run up to static inversion. (See fig 1) and more reliable semiconductors used in high power switching. Rotary converters were used for powering the gyro compass and radar equipment – they were medium power units which had an output frequency of a few hundred hertz. They were reliable and ran continuously. Usually maintenance was carried out according to a time schedule, possibly once per annum and this would have only been for the fitment of new bearings and brushes. The output frequency had to be stable however because the gyro relied on the high frequency output to get the motor running at optimal speed. A deviation off the norm would cause the gyro either to topple or be off true north. When this happened however the rotary converter was well passed it’s service date or was caused by premature bearing failure. Motors could be DC or AC, for emergency power the motor could be powered directly off a battery. Rotary converters usually are powered by mains as they are known as frequency converters, the derived frequency change was the required outcome – I have added this in because it can be used to provide a very reliable form of backup power although no where near as efficient as modern day inverter circuitry where there are no mechanical/friction losses. I will cover this more in Part Three.
Inverters used for regulating output voltage.
Over time it became apparent that semiconductors, moreso SCRS or silicon controlled rectifiers could be used reliably in inverter circuits and some very stable low frequency 50 or 60H inverters were built although not really true sine until bipolar transistors started proving themselves both in wattage and reliability. I found Germanium transistors used in inverter circuits particularly nasty devices. These were costly however and manufacturers quickly cottoned on to modified sine wave inverters which used cheaper parts (silicon, FET) and ran cooler. The first of these units to become commercially acceptable would be the line-interactive UPS which as the name implies boosted or dropped (bucked) mains input voltage to regulate the output. This switching back and forth between mains and inverter to maintain a steady output has one drawback with this type of UPS – the time delay during the switchover period. The rotary converter didn’t have such problems but then again they weren’t really designed as back up power supplies.
Line interactive UPS can be picked up for about 50U$ for a 600VA device – being cheap it stands to reason that they are also very popular. Yet, this alone should not be a reason to rush out and buy. Many of the preferred companies manufacturing UPS give excellent background and white papers to their products and one would such would be advised to look up the topology requirement – online versus off-line/line-interactive UPS are vastly different yet at grass roots level seem to offer the same solution. Parts-Ring is using the Gigatech/Kstar PRO OFFICE 500 UPS as a design example – it is a stock standard design, relable and robust. The drawing can be obtained here.
The heart of the UPS is the microcontroller – we will look at the MC68HACDW, which is the EPROM version of the MC6C05P6. It is a four channel 8bit A/D converter, 16bit timer with serial port comm capability. Download the full spec and datasheet here: http://www.datasheetarchive.com/MC68HC705P6-datasheet.html
If the image is clear on your PC you will see input / outputs of the uController are pretty straightforward, clocked at 6MHz. Moving to the PWM which drives the gates of the switching devices through AB (pins 11 and 14), pin 10 [INH INVERTER] is inhibited when there is a error and the outputs AB are set to zero. Datasheet for the IRF1010 can be seen here.
Battery charger is through the IC U1 3843N, Q2 and TX2, a simple inverter, rectification circuit.
It is out of the scope of this article to cover the full workings of this particular UPS, more to explore the schematic and components used. In reality the circuit is straightforward and follows standard design, emphasis being on output protection and regulation. From a design point of view there is very little chance of failure, the semiconductors are reliable and depending on the source of passive components will outlive that of the battery. The most common failure of a UPS is through battery degeneration – sealed lead acid accumulators do not have an infinite life span although I have come across APC units where the batteries are still good after 7-8 years. (almost all were Smart UPS). It is advisable to change these batteries however. Some cheap and nasty UPS units are cheaper to replace than the battery. Again, this is often because the batteries installed are cheap to keep manufacturing prices down – cheap UPS batteries normally last the warranty of the unit +/- one year only. Always use the best batteries you can afford. Not all manufacturers are known for quality products – Japanese products are usually excellent. The standout point of this particular UPS, a line-interactive unit is RL1 which is the switchover relay. When comparing line-interactive UPS to the Online UPS, two things will immediately become apparent once the bonnet is open (1) the power rectifier and filter circuits are mammoth and (2) huge heatsinking. It is not often cost effective to make smaller UPS using online topology.
The online UPS can be explained simply as an inverter that runs all the time and is not dependent on mains voltages at all. In Part One it may contradict my statement about swollen reservoir capacitors and huge input voltages but in fact, online UPS are just as susceptible to damage as line-interactive units, hence the reason why the high power and more costly units have to be installed and signed off by a qualified engineer. In fact the most common problems I have picked up with failed online UPS has been destructive input voltages and in lesser cases tinkerers trying to install extended runtime batteries through switchover circuits. (incorrectly wired). The Online UPS needs to charge the battery at such a rate that it will not discharge, this makes circuitry more sophisticated and the unit itself quite inefficient due to the conversion processes.
Figure 2 shows a typical block diagram of a double conversion UPS – from mains, to DC charger – from battery to AC. Double conversion UPS have been around for many years, more often than not just the weight alone of the unit points to the unit being a little more than an average UPS. Of course there are true sine wave UPS which carry massive inductors making for even 600VA units being very bulky. Because the Online UPS is an inefficient beast engineers found a way to overcome this by developing what is known as a Delta Conversion UPS. Excellent write-up by APC here.
I think getting back to Part One of this UPS series we need to explore the reasons why UPS is no longer seen as being that important when in fact electronic equipment is getting more commonplace in the household, no less.The APC website is a brilliant source for data and not only that, it is a wonderful advertisment for all UPS. As a portal for UPS, undoubtedly so. Although there are other UPS manufacturers whom claim market dominance it’s highly unlikely, at least not from a brand perspective. Are APC UPS the best there is? From an innovation point of view I’d say yes. From a warranty backup and support point of view? I’d say yes. But definitely not the only manufacturer out there with good products – APC knows how to market and they target the entire spectrum of users, from small to corporate. Sales staff should learn the topologies and more to the point what goes where. A notebook can be damaged by a faulty peripheral through the comms port if there is a supply rail fault. Supply rails can and do go faulty when there is inadequate protection overall on the equipment. Ask your insurer.
To sum up, one can never be too careful, especially when you are using a U$5000.00 Vaio or iMac powered by a dodgy power source. I wouldn’t.