USB Isolator by Analog Devices

This is something that I was long awaited for. Analog Devices introduced the first digital isolator for USB ports, ADuM4160, meaning that now complete decoupling is possible between USB host and device, including the ground common being separated. Why is it useful?

• The possibility for removing ground loops thus achieving lower noise levels which is handy for low noise instrumentation or high end audio electronics (in fact, it is especially useful for external sound cards plugged on USB);
• Decoupling the device from the computer host might protect the computer if anything goes wrong in the device, i. e. high voltage level appears in the digital parts due to malfunction.

The principle of operation is discussed here, the main point is that a CMOS-integrated transformer couples the output to the input. This is not that simple for USB however, as data flow has to be ensured in both directions, so additional logic circuit is necessary. 

There is one additional goal: to ensure power supply for bus-powered USB devices. For this purpose, I used ADuM5000, which shares the same transformer coupling technique for transferring power to the decoupled part. Sadly enough, the efficiency is quite low, only around 33%. The maximum output is 100mA at 5V but this can be improved by hooking up these circuits in parallel as suggested by the datasheet. I used two devices in the scheme to make use the entire 500mA available on each port which leads to around 200mA maximum output which is still enough for a plenty of kinds of devices.

The schematics is quite straightforward, the most important thing is to decouple power supply pins with 100nF capacitors near the integrated circuit. The PCB is easy as well, a small two sided board is enough. The only guideline is to make sure that there are no nearby copper lines from the parts which could endanger the isolation for high voltages.

I tested the circuit and it worked pretty well with my 1GB flash drive, and it made use of the 12Mb/s Full Speed bandwidth. There were other devices, including USB mice that ended up with a "USB device not recognized" error message upon plug-in. The reason I suspect is the power supply: USB devices can allocate up to 500mA on each port, which should prevent overloading the port. I assume if much more is used just after plugging in the device, the USB controller might inhibit the device in question. Here this could easily happen, as the power supply efficiency is only around 30% (factor of three!).

To check this, I used a self-powered USB hub between the isolator the device (i.e. mouse), and in this fashion, everything worked out for all devices I could test.

There is, however another side effect of the low efficiency of the ADuM5000: if operated on the edge, they can heat up quite easily. I think this can be addressed by gluing some sort of heat sink on the top of the IC.Yet, the heat sink has to be isolated from everything as well!

Update: I tested the board with a small piece of heat sink glued on the two ADuM5000s. I put some thermally conductive adhesive on the top of the ICs, installed the heat sink and glued it together with some two component epoxy which is also useful against accidental shorts and other problems with an unprotected PCB. I tested the device afterward and not it is stable no matter what the load is.

In conclusion, the circuit worked pretty well for me, the only improvement could be a better power supply, or a USB2.0 high speed device... But still, this kind of portfolio is quite unique on the market.

Inside a LED lamp

Using LED lamps instead of incandescent bulbs and fluorescent lamps is argued to be power efficient thus economically favorable. Longer life span is also usually cited as an advantage. I wanted to look into the details for a long time and recently I ordered some LED lamps on ebay, so I took one of them apart to see what is inside...

The lamp is a 48 LED device, and is written to use 4.5W on a 230V power supply. My intention is to present the schematics of the lamp so that we can see the details of its operation.

LED basics:

Just to understand the main building block of the lamp: LEDs (light emitting diodes) are semiconductor heterostructure devices meaning that they have rectifying properties so that current only passes through one direction, while in the other way around it doesn't. While this is true for all diodes, the plus for LEDs is that light emission will happen only when they are conducting. Even if this is the case there is a certain threshold for the light emission to start which is around 3.5V for blue and white LEDs. Above this threshold the current through the device rises exponentially so it needs to be regulated which is usually achieved by either an external resistor in series or a current generating power supply circuit.

LED lamp:

Let's see what we have here: Regulation on the AC side is done by C1 as it acts as a finite impedance at 50Hz. The point is that the this is achieved without any unnecessary dissipation as there is no heat generated on C1. D1 is an integrated Graetz-bridge which rectifies the input voltage. The role of C2 is smoothing the output and R4 is an additional protection for the LED block. R1, R2 and R3 discharge the capacitors after switching the lamp off (most likely due to safety reasons). I also put some measurement data on the schematics: the 48 LEDs in series operate from a sum of 179V meaning that each LED has a voltage drop of 3.73V on average, in good agreement with our expectations. The 110mV voltage across the 10 Ohm resistor tells that 11mA flows through the LEDs meaning that the dissipation of the block is 11mA×179V=1.97W which is way lower than the expected 4.5W! Indeed it is not surprising that the light output of this lamp is not enough especially if compared to compact fluorescent lamps. The power factor can also be calculated: the voltage drop over C1 is 124V leading to a  power factor of 0.84. This value of course becomes important if LED lighting is to take a major role in households.

In conclusion it turns out that the electrical circuit is as simple as possible leading to a high average lifetime of the device (I expect it to be comparable to the couple of thousand hours of compact fluorescent lamps). No 50Hz wobbling of the light output is also a benefit of the device. However the efficiency of the principle described above is given by the fact that many low power LEDs was put in series leading to a voltage drop of 179V on the LED block with only 11mA current consumption. High power LEDs cannot be used this way since each single (!) chip has a light output better then this entire block while consuming several hundred mA-s at around 3.5V (a good example is Cree's XLamp series). This means that a more complicated power supply is necessary for such a LED lamp which together with the more expensive LED means more expensive device but way better light output as well (especially true for devices equipped with three or four high power LEDs), so I recommend buying such a lamp instead of these cheap ones.