Hardening a meter against magnetic tamper attacks: Cap-drop based high-side power supplies
In order to use the AMC1304 for isolated shunt sensing, it is necessary to provide power to the high-side power supply of the AMC1304. One option for this high-side power supply is to use a cap-drop supply to power the AMC1304’s high side from Mains. This module covers the pros and cons of using a cap drop power supply to power the high-side of a AMC1304. Details will be provided on the TIDA-01094 reference design, which uses a cap-drop power supply to power the high-side of the AMC1304.
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The other alternative to power the AMC1304 high side is to use a cap drop base supply instead of a transformer base supply. Looking at the data-sheet of the AMC1304, we find that the high side of each AMC1304 takes only a maximum of 6.5 milliamps. This 6.5 milliamps is assuming maximum modulation clock frequency of 20 megahertz. If we lowered the modulation clock frequency down to 5 megahertz, we could further reduce the current consumption of the AMC1304 high side, as shown in the figure on the right.
This current assumption is small enough so that a cap drop power supply could be used to easily power the AMC1304's high side. In addition, since the AMC1304 already has an internal LDO, the cap drop supply does not need an external LDO, as long as the voltage fed into the LDOIN pin of the AMC1304 is between 4 to 16 volts. There are various advantages to using a cap drop power supply to power the high side of the AMC1304. You
First, using the cap drop power supply allows us to have a lower system cost than the SN6501 based solution. Second, with the cap drop supply our solution is now completely transformer-less, which is good for magnetic immunity. In addition, cap drop supplies inherently have better performance when it comes to emissions in general. Finally, since each AMC1304's high side power is derived from [? mains ?] directly, instead of controller aside power supply like what was done for the TIDA-00601 design, there would be less current drawn from this controller side power supply.
There is one disadvantage compared to the SN6501 based solution is that this cap drop supply does take more room. Also, if there is an issue with phase or neutral, then the high side power supply for one of the AMC1304s could be disabled.
We most recently released a new design that takes the original TIDA-00601 TI design and replaces the high side power supply with a half-bridge cap drop supply. With this power supply, this design is now completely transformer-less. The high side for the AMC1304 is powered directly from mains. In addition, the data isolation in the AMC1304 is based on TI's capacitive based technology.
With the lack of transformers, this design is completely immune to magnetic tamper attacks, instead of only being resistant to a certain extent. In addition, in-design support was added for both the AMC1304 [? ML5, ?] which has an integrated LDO as well as the AMC1305, which doesn't have an internal LDO but uses external TLV70450 LDO.
This slide shows the active energy error for the TIDA-01094 design. Measured from a current range of a 100 milliamps to 90 amps, these tests were conducted with a voltage to current phase shift of 0 degrees, minus 6 degrees, and plus 6 degrees. From these results, everything was within 0.5%. A voltage variation test was also performed. In this test, the active energy error is measured across a range from 75 volts to 270 volts.
This error was less than 0.2% across this range. This test is important for two reasons. First, it shows that the 10-bit [INAUDIBLE] used to measure mains voltage has sufficient accuracy. Second, it shows the voltage range at which the AMC1304 would operate, which is more important for the design than TIDA-00601 design because the high side of each AMC1304 is now powered from mains instead of controller side power supply.
So to summarize, shunt current sensors are the best sensors for magnetic immunity. For polyphase systems, isolation must be added to shunts. Isolated shunt sensing can be implemented using a isolated [INAUDIBLE] modulator such as the AMC1304. A power supply for the high side [? is advised, ?] such as a transformer base or cap drop base supply, as well as a metrology [INAUDIBLE] controller that has a [INAUDIBLE] filter that could take in external bitstream as well as additional ADCs for measuring phase voltages.
This metrology [INAUDIBLE] controller could be the M230F67641. There are two options for a high side power supply-- a transformer based supply, as well as a cap drop option. The transformer based solution is implemented in the TIDA-00601 and TIDA-01088 TI designs. These power supplies are relatively compact. However, they do require special magnetically tolerant transformers so that the system could be magnetically tolerant to magnetic tamper attacks.
The other option for a power supply is to use a cap drop power supply. If you compare this to a transformer based solution such as a switching transformer based power supply, the cap drop power supply is lower cost. It is completely magnetically immune, inherently would have less conducting and radiating emission issues, as well as does not draw any current from the controller power supply so that the specs on this controller power supply could be relaxed.