USB Power Board – Top
This “Mt. Whistler II” board has a couple of chunky ceramic capacitors before and after the PS54425 buck regulator chip rated for up to 4A, tons of ground plane area to heat-sink that chip, a generous amount of via stitching to transfer heat to the bottom, and a pair of SOT23-5 USB power switches with unreadable part numbers, providing over-current protection and possibly over-voltage disconnect, each followed by an aluminum capacitor. A typical $1 Chinese DC-DC USB adapter won’t have half as many components, and it'll rely entirely on the regulator’s internal current limit instead of using dedicated protection ICs.
Every through-hole component and the bulky surface-mounted inductor also get the squiggle treatment.
USB Power Board – Bottom
Quality does not stop at the component count and protection on the top side. To keep output noise low, the input and output grounds meet at a single point next to the chip, and the ground plane covering most of the bottom makes the whole board act as a heat sink.
The only flaw I can see in this design is the power trace for the 1A USB port. See that fat horizontal trace near the middle of the board? Vias on the right have been doubled and tinned to provide extra current-carrying capacity, but are not matched on the left where only a single via covered in solder mask can be found. There should have been two vias there, preferably tinned to match the other end.
USB Power Measurements
How good are the USB outputs? It's time to dust off my shunt regulator from last year and find out. I got 1.1A on the 1A port and 1.4A on the 1.5A port before voltage at the port dipped below 5V from 5.15V open-circuit voltage. With the shunt tweaked to 1.5A from the 1.5A output, I connected my oscilloscope and found about 30mVPP worth of noise on there. That's not quite as good as my original N7 AC adapter or the LX1500, but still much better than most third-party adapters you can get under $10.
Considering that these USB power outlets are a bonus feature on a $30 UPS, I’d call our measurements mildly impressive.
Main Board – Top
As I wrote earlier, this is one busy little board, at least from the top side. Power comes in from the bottom-right corner where basic filtering and the bypass relay live. It feeds the power supply in the bottom-middle. And the analog magic happens mainly in the bottom-left corner, while digital goodies occupy the upper-left corner. The remaining components in the top-right quadrant make up the power stage of this electronic inverter.
In the input section, AC power comes in through the two spade connectors in the bottom-left corner, passes by the 14D471K MOV, through a small common-mode choke, hits a 1µF X-class capacitor, then goes off to power the loads through the bypass relay and internal electronics.
What's with the large number of small resistors? They make up four voltage dividers used to monitor input and output voltage. Why so many? Because sensing is done without isolation, which means that if the divider network fails before anything else causes you to scrap the UPS first, you can end up with full AC voltage on the USB outputs. Take the required number of resistors based on their voltage withstanding specification and double that for safety.
APC’s labeling tradition continues: every power terminal, connector, or wire is marked with component designator, wire color, and wire function.
Power Supply – Primary Side
In the BGE90’s power supply primary side, we find an NTC inrush-limiting resistor hidden in a piece of heat-shrink tubing, a rectifier bridge in DIP6-4 packaging, a pair of Jianghai (“JH”) CD264 22µF 400V capacitors for the input energy reservoir, a Power Integration TNY280 monolithic flyback regulator, an RCD snubber circuit for the transformer’s primary winding, a pair of 817C photo-couplers, and a Y-class capacitor.
Information on Jianghai capacitor reliability is scarce despite the company having been around since 1958, which hopefully means there aren’t many failures that get traced back to them.
What Is That HS2 Component?
HS2 is exactly what its designator implies: just a heat sink. While the TNY280 controller is capable of providing up to 28W, it requires some thermal assistance to do so across its full 85-240VAC range in an enclosed area. Help comes in the form of a small sink close to the TNY’s four source pins and copper pours on both sides of the circuit board to help move heat along.
Power Supply – Secondary Side
On the secondary side, we have an MB310 diode, a 330µF 35V Lelon RXK capacitor, and a handful of low-value resistors to measure battery charging current, which would explain why the feedback circuit contains two 817C opto-couplers. One provides current limiting during charging, while the other provides float voltage regulation.
I do not like that Lelon capacitor. On top of being a brand near the bottom of the reputation ladder, the capacitor APC uses is only rated for 1ARMS. This does not bode well in a flyback application, especially if people choose to use the USB ports to continuously power 10W worth of devices.
Probing For Answers
After triple-checking that the only connection between the low-voltage side and mains was through those mega-ohm resistor strings, I concluded that it would be relatively safe (as safe as poking around live line-powered equipment gets) to directly probe the capacitor and peek at how much ripple voltage it actually sees.
How Much Ripple?
For testing purposes, I am drawing 1.5A from the USB power outlet once again. According to Lelon’s datasheet, the capacitor has an impedance better than 0.062Ω and is almost entirely to blame for the ~250mVPP worth of ripple seen here. Decomposing the waveform into triangles to approximate its RMS value yields about 36mVRMS and divided by the capacitor’s impedance, we get 580mARMS of ripple current. While this is within the capacitor’s rating, it is far from what it could have been with a better-quality capacitor. As I have written in the past, flyback and boost converters are cruel taskmasters to their capacitors.
MORE: APC BE550G Tear-Down