Chipset address offers inexpensive isolation for multi-phase power meters

Smart, accurate, tamper-proof power meters are an essential ingredient of the next-generation power grid. Isolation issues can prove a challenge for polyphase power meters. For a single-phase system, a non-isolating shunt resistor is an inexpensive, reliable choice for sensing current, but for a multi-phase system, with significant voltage differentials between the three phases, the voltage and current signals must be isolated from the data collection/metering IC.

Maxim believes it’s solved the problem with a new chip set that allows meters to use inexpensive shunt current sensors on polyphase meters, while still providing three-phase isolation.

Take, for example, a delta-connected load: To measure directly across each of the branches (A-B, B-C, C-A) requires some kind of an inductive device to provide isolation. Current transformers and Rogowski coils are the most popular current sensor types, but they have problems: They’re bulky, expensive, and susceptible to tampering.

Maxim’s MAXQ3108/DS8102 chipset takes a different approach. Instead of isolating at the measurement device, isolation occurs at the interface between the ADC and the MAXQ3108 supervisory microcontroller. Each phase has its own DS8102 two-channel sigma-delta modulator that floats with its respective phase. The output of each DS8201 is a digital bit stream that contains the I-channel measurement, the V-channel measurement and a synchronization signal. The composite bit stream is then Manchester-encoded so that it can pass over a non-DC continuous medium that serves as the high-voltage isolator..

Ben Smith, a design engineer on Maxim’s Power Metering Products team, describes one example of how that medium can be implemented: “In our reference meter, we use a PCB-etch capacitor. We’ve engineered the capacitor, which is made from standard FR-4 PCB material, to withstand up to 5kV. Consequently, the three phases are isolated from one another and from the supervisory microcontroller.

“This scheme frees the designer to use any current and voltage transducer method that fits the application. Some will choose a current transformer due to power dissipation concerns, but others will elect to use a shunt for the lower cost and reduced size. [The] chipset doesn’t care since each phase is in its own potential domain.”

I’ve heard recently from several companies in stealth mode about how they’re planning on doing poly-phase power isolation. Maxim‘s approach of focusing on what’s cheap and reliable is a strong contender.

The MAXQ3108 is packaged in a 28-pin TSSOP and the DS8102 in a 16-pin TSSOP. Prices are $2.43 for the MAXQ3108 and $1.17 for the DS8102.