USB Power Supply Output
Under open circuit condition, the supply outputs 5.12V with 60mVPP of ringing and 100mVPP of switching transient noise. At 1A output current, I got exactly 5V at the USB plug and the pictured waveforms across my load. Switching transients died down as soon as there was any load on the output, which is good. We can clearly see the 50-70mVPP turn-on transients at the oscilloscope's trigger point, followed by more small transients and a slight ringing with less than 10mV initial magnitude at turn-off.
Having that 10µF capacitor I mentioned on the USB connector board would have eliminated most transients from the USB output. A common-mode choke does you no good when there is no load attached to close the loop.
For Comparison's Sake
Here is the output from my 2012 Nexus 7's original AC adapter. The thing is over three years old, has been plugged in nearly 24/7 all along, yet still produces output with negligible ripple, negligible ringing and 30mVPP transients under 1A load.
Genuine OEM adapters usually carry high premium prices as replacement parts or accessories. But some of them are legitimately over-engineered. The LX1500's USB output is almost that good, at least for now.
The Main Board
There are four distinct areas on the circuit board: the low-power, low-voltage control area with the daughterboard and output current-sensing transformer in the top-right corner; the line voltage section with AVR/bypass relays and four 14mm MOVs in the bottom-right corner; the UPS' internal power supply, which also provides charging in the bottom-left corner; and the inverter bridge in the top-left to drive the transformer. All electrolytic capacitors come from Su'scon and include the low-voltage HG series, UX for the battery charger output and SE for the mains input.
The only EMI filtering provided by this UPS is what it needs to suppress noise from its own power supply.
Main PCB Bottom
Soldering quality looks good across the board. Aside from a bucket of resistors, capacitors, some diodes and transistors, its residents also include three 817-style photocouplers. One of those couplers is used for the charger's feedback loop and the two others drive the inverter's MOSFETs.
Driving 1500VA out of the transformer requires pushing nearly 70A. If you have no idea what this translates to in terms of circuit board layout, here is a hint: it is not subtle.
Some Delegation Required
The analog and digital magic required to bring the LX1500 to life requires higher circuit density than what can be accommodated on a single-sided board. Instead of making the main board double-sided and finding space for these components, CyberPower put them on a riser card.
What is on there? A microcontroller, a pair of 324 quad op-amps, a buzzer and a linear regulator on the front. The back side is covered in the usual complement of resistors, capacitors, diodes and transistors.
How much power does an EnergyStar UPS draw while doing nothing? The LX1500 had a few hours to top off its batteries and the initial 40W died down to 8.8W. To meet EnergyStar UPS requirements, a UPS must have 96.7 percent weighed average efficiency across 25, 50, 75 and 100 percent load test points weighed 0.2, 0.2, 0.3 and 0.3, respectively. With 8.8W of internal power draw and 5W of weighed wiring losses, the LX1500 should achieve an overall efficiency of 98.5 percent.
If you were wondering what a "modified sine wave" with 46 percent of THD and 34 percent peak harmonic looked like, this is what the LX1500 produces. Its output goes to 190V with 40 percent duty cycle, which yields 125VRMS with a slight amount of ringing at each transition. During battery operation, a slight buzzing can be heard from the transformer.
Why do some power supplies with APFC have trouble with this waveform? It is usually because their APFC controller shuts down or misbehaves when it detects a non-sinusoidal input. When the main converter is designed to expect APFC-boosted input voltage, losing the input boost may be sufficient to make it shut down. At 190V, though, most converters should have little trouble operating without it.
The BX1000's electronic inverter produces perfectly flat plateaus at 160V with a 57 percent duty cycle resulting in 121V RMS. Here, what may look like ringing is actually a choke getting switched in and out to soften the slopes. In operation, the BX1000 generates a distinctly audible 120Hz buzz.
Two completely different implementations of the same general waveform, nearly identical results. But it's still looking good for a 10-year-old device. I just need to investigate why it's excessively sensitive to noise.
Non-PFC Current Waveform
Many people are wary of connecting electronic equipment to a modified sine wave UPS out of fear that the fast edges might damage input rectifiers or capacitors. I decided to have a look.
Here, I am using the SL300 supply I repaired a few months ago to power a 15-foot 12V 80W LED strip. While intuition may spontaneously dictate that the "square wave" plotted in dark green should have worse peak current than smooth AC plotted in dark red, practice shows that flat tops keep capacitors charged longer than AC peaks do. With capacitors having less time to discharge between pulses, peaks remain of similar magnitude but much shorter duration.
Try to avoid connecting large iron core transformers (“linear adapters”) and AC motors to UPS though: the harmonics will greatly increase core losses.
APFC Current Waveform
Some people also believe that an APFC power supply requires a "pure sine wave" UPS. For this round, I used my PC as the load since its EA-650 is the only APFC supply I currently own. As a bonus, it is one of the pre-Green Delta-made EarthWatts models that allegedly had issues with modified sine wave UPS.
As expected under AC, the current follows line voltage, albeit a little noisily. Under battery power, though, the EA's APFC circuit does not skip a beat. The momentary peak at over 5A is caused by APFC trying to follow the rising edge. Once it figures out that the slope is too steep for a sine wave, it switches to boost regulator mode for the remainder of the pulse.