10 Inexpensive Automotive USB Adapters Tested -- Are They Any Good?

Second Batch: Bigger Bullet Duo

Stepping up to a larger caliber, these two units tout an output rating of 2100 mA, sport a pair of stacked USB power outlets and look effectively identical from the outside aside from different text screen printed on their body. Both units are held together by press-fitted plastic stakes and glue or solvent welding. The unbranded freebie showed evidence of a significant glue or solvent spill on the inside, and it gave me a much harder time popping it open than the Fosmon unit. Since glue is the main structural component keeping these and the spring-loaded contact nub together, the tip of the first one I popped open caught me unaware and launched across the room. I will need to re-glue or tape them in some way if they turn out to be worth using.

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On the top side, slightly different component placement tells me these two aren’t exact clones. The generic unit has a bent-over input capacitor and a shoehorned-in spool core inductor slapped on what appears to be a much larger inductor footprint. The Fosmon unit’s toroidal inductor has fewer turns of what appears to be slightly thicker wiring, which should lend it a significant lead on efficiency. On the bottom side, it also has beefier copper fills connected to the regulator’s pads. Based on the previous results, I expect that to translate into better thermal performance. The Fosmon adapter has three ceramic capacitors where the nameless unit has none, leading me to expect lower noise from the Fosmon if those capacitors are across input and output supplies to ground. Both units also use a tiny (but bright) blue surface-mount LED powered from the 5 V output via a current-limiting resistor for power indication.

Someone had a hard time soldering the spring to the Fosmon unit’s PCB. First, its slot was too close to the board’s edge and the thin strip of PCB broke off. Then, without a copper pad on the other side of the hole to help hold the solder bead in place until it solidifies and locks the spring into place, the employee used tons of flux and increased soldering temperature to get the solder to hold onto the spring with what was left of the pad. This looks so bad that I could not find any angle that clearly did it justice.

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What chips do these units use? We’ll never know without reverse-engineering Fosman's circuit and cross-referencing the pin-out to find potential matches since its markings have been thoroughly removed. Given the lack of tooling marks, perhaps the chip was never marked in the first place, and the wet-looking spots are simply flux. For the other adapter, we have an XL1410E1 from XLSemi, one of those Chinese chip manufacturers that hosts its website on what seems like a dial-up connection. When I tried visiting the site simply to confirm the country of origin, it took five minutes to load the main page.

Based on component selection alone, neither unit is suitable for use in a 24 V system. The Fosmon adapter's input aluminum capacitor is only rated up to 25 V, while the nameless unit’s switching IC is only rated up to 20 V absolute maximum and 18 V operating input voltage. That's uncomfortably close to a 12 V vehicle’s normal system voltage, if you ask me. On the capacitor side, Fosmon uses known lower-tier capacitor brands, while the other one uses brands I do not remember ever seeing anywhere before, with “szwx” making me bat my eyebrows for a while. In case you were wondering, the company is called Shenzhen Wangxing and there isn’t much information about the company online aside from Chinese and other vendors claiming that it is the “best quality.”  Shenzhen being what it is, take that with a grain of salt. Jackcon, on the other hand, is a Taiwanese brand well known from the motherboard capacitor plague years. Here, I believe the smart money would be on Fosmon and its mystery chip.

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Where noise is concerned, this appears to be a clear win for the nameless unit. At a glance, it has roughly one-third as much peak-to-peak noise amplitude as the Fosmon adapter. As far as transients go, both adapters show negligible deviation from the baseline waveform, and regulation against line voltage looks spot-on in both cases. From the density of peaks on these waveforms, it looks like the Fosmon unit has somewhat of a flutter in its switching modulation, while the unbranded unit has more uniform noise output. Will Fosmon’s extra components pay off elsewhere or did I get mislead by thinking that its ceramic capacitors would give it an advantage? Time to check the numbers.

Once again, neither adapter meet its claimed 2100 mA output current specification, with the no-name unit only managing about half of the output current before its thermal shutdown periodically kicks in. When cold, both units momentarily max out my dummy load’s 3.7 A maximum current. But after warming up, they only go up to 2.5 A even into a dead short. What does a “thermal shutdown” mean? It means that the chip has an internal sensor that turns off the internals and output when the chip exceeds a given temperature and turns everything back on after it cools down a little. The “small bullet” adapters also have thermal protection in the form of a temperature-dependent current limit, similar to how modern processors throttle clock rate to rein in power dissipation under excessive temperature conditions to prevent damage. Between its better thermals and more efficient toroidal inductor, Fosmon’s design does pull ahead on efficiency by as much as 7% at each units’ respective maximum sustainable output power.

The figure I used as the maximum was determined by dialing down the current in 100 mA increments with each shutdown, starting from the maximum rated current until thermal shutdowns stopped occurring within three minutes from the latest step down. As with the previous trio, I suspect that Fosmon’s higher sustainable current delivery is partly thanks to its copper fill providing better heat dissipation and lower losses (mainly wiring) in its toroidal inductor, allowing its regulator to maintain a lower duty cycle. In both designs, there was a missed opportunity of using the 12 V socket’s ground contacts as heat sinks to significantly improve thermal performance.

On the noise side, the difference is not as large as it seemed on the waveforms. Why? While the Fosmon has about three times the peak-to-peak noise based on the waveforms, those peaks are very narrow. This reduces their contribution to the RMS value. While these adapters produce two to four times as much noise as the first trio, they also provide two to three times as much power at 20-25% higher efficiency. Despite their superior efficiency, these units do get warm while supplying this much extra power and the smell of toasty electronics asserts itself again.

Would you entrust your expensive mobile gadgetry to adapters that are barely built to survive normal voltages on a 12 V automotive electrical system? I guess not, so that’s two more for the fail bin. At least the Fosmon fails with some style by managing to keep efficiency above 80% across all three test points, and 1.8 A should be enough to power the majority of mobile gadgetry in a pinch.