Cellphone charger has no transformer?

Question

I opened up a really tiny mobile phone charger I have, to see how it is designed. The entire "charger" is integrated into a small 2-pin mains plug, 1 x 1.25 x 0.5 inch, that has an USB socket for the phone's USB charging cable.

I could not find anything that seems to be a transformer anywhere in the circuit, and yet I have tested that it is an isolated 5 volt well-regulated output. The tiny flexible PCB has merely a dozen or so SMD parts, ranging from 0402 to 4516 (metric), plus the connectors at the two ends, for mains and for USB. The SMD parts all have part numbers sanded off.

How do they manage the isolation in these chargers?

Responses to comments: This is a no-name "Hi-Standard USB phone charger, Extra Powerful!" I have just bought in Korea, that is supposed to work with any USB-charged cellphone. They have pictures of a half dozen different cellphones on the box, and a hydra USB cable inside that has mini-USB, micro-USB and some other types of connectors.

I bought it just to see whether it is safe or not. That is why I tested for isolation first.

Answer 1

Although the question has provided limited details, this answer presents a somewhat different hypothesis from the standard assumption that there's an inductive coil hidden in there somewhere.

The charger in question possibly uses a Piezoelectric Transformer instead of the magnetic (inductive) transformers usually seen for isolation.

Does the charger looks somewhat like this?

If yes, the designers have used a Piezo transformer instead of a conventional one. Interestingly, the source of this image is a paper in a Korean academic publication. This makes the hypothesis even more apt.

A piezoelectric transformer designed for Mhz operation, 500 mA secondary current with 5 Volt signals, using Polyvinylidene Fluoride (PVDF) as the piezoelectric medium, could be fabricated as a thick 1210 SMD part. Since the question mentions SMD parts up to 4516 metric i.e. 1806 imperial, one of the largest of those components is probably the piezoelectric transformer providing the isolation as per the question.

Some interesting information gleaned while investigating this mystery charger:

• Piezoelectric transformers deliver 80% to (recent experimental versions) 90% efficiency, impressive in transformer terms
• These transformers can provide galvanic isolation at multi-kV levels - of course, not in a SMD 1210 size, where the contacts would be too close together.
• PVDF exhibits piezoelectricity several times greater than quartz. Hence it is ideal for making Piezo transformers.
• Many LCD display CCFL backlights are made using Piezoelectric transformers instead of the inductive coil ballast used in earlier versions. So it isn't really new technology.
• Equipment used in magnetism-sensitive areas (e.g. MRI labs) are expected to transition to non-magnetic electronics, hence Piezo transformers where a transformer is needed. (n.b. Any current flow, however, will still generate some magnetism courtesy H fields)

Some articles of interest:

• Piezoelectric Transformer With Very High Power Density
• Panasonic's Piezo transformers for LCD backlights - Of course, much bigger than the dimensions usable in USB chargers.
• A comparison of magnetic and Piezo transformers by Texas Instruments

---

Full disclosure: I have never worked with, or even seen Piezoelectric transformers before today - the above information was a new learning for me, in the process of investigating the mystery charger.

Answer 2

The isolation is managed by a transformer. However, it's just very small; it's probably one of the bigger surface-mount components you see in there. See How do these new-fangled phone chargers achieve step-down? for a more detailed description of the technique involved. Note the transformer in the middle of the circuit.

Answer 3

How do they manage the isolation in these chargers?

A high-frequency transformer can be very small, especially when triple-insulated wire for one of the windings. The device is most likely only designed to work at 120VAC so the 2mm clearance needed for a 150VAC circuit rated at pollution degree 1 (since the charger is essentially a sealed box) allows for very small construction.

I bought it just to see whether it is safe or not. That is why I tested for isolation first.

If you want to judge safety, the first step is to check for any regulatory agency marks on the package (UL, TUV, CSA, etc.) and cross-reference the file number with the respective agency to make sure it's not fraudulently applied. If there is a valid safety approval, the device would have been subjected to isolation and other safety tests before release to market.

If the device doesn't have any safety marks, or has a fraudulent mark on it, the onus is on you to establish if it's safe to use in your jurisdiction (by getting a special inspection certificate for the device, for example) which means the prudent thing to do is throw it away and buy a safe device given the price of the device vs. the price of a special inspection.

Answer 4

I just bought a "night light" which is an LED that is in a 240V plug. It comes on in the dark and goes out in daylight. It contains a 47muF capacitor in series on the active wire followed by two diodes and then a NPN transistor controlled by the light sensitive resistor. The LED is across the transistor and that's about it. There is a electrolytic capacitor across the diodes for smoothing and a 70K resistor also in series but in parallel with the 47muF cap.

It works. There is a detectable 50hz blinking but if you don't look away, the LED appears steady enough. So there's a DC couple of volts from 240V AC definitely with no transformer.