Energy Ideas #132: A low-cost and high-accuracy e-meter answer

Introduction

Energy provides in information facilities that measure the enter energy in actual time and report the measurement to the host are conducting what’s often called electrical metering (e-metering). An e-meter has turn into a typical requirement in power-supply models over the past decade as a result of it brings these benefits to information facilities [1]:

Identifies abnormally low or excessive power utilization and potential causes, supporting such practices as peak shaving.
Facilitates capability planning round area and energy utilization.
Helps monitor and handle power prices; verifies power payments; and prioritizes, validates, and reduces power prices by means of improved power effectivity and power administration.
Permits quantitative assessments of knowledge middle efficiency and benchmarking of that efficiency throughout a stage enjoying subject.
Helps develop and validate energy-efficiency methods, and identifies alternatives to enhance power effectivity by reducing power and operational prices.
Fee and detect faults in bodily programs and diagnose their causes.

For all of those causes, an e-meter should be exceptionally correct. Determine 1 exhibits the Modular {Hardware} System-Frequent Redundant Energy Provide (M-CRPS) e-meter accuracy requirement [2], which requires an enter energy measurement error inside ±1% when the load is bigger than 125 W, inside ±1.25 W when the load is between 50 W and 125 W, and inside ±5 W when the load is under 50 W.

Determine 1 The M-CRPS e-meter accuracy specification which requires an enter energy measurement error: inside ±1% when the load is bigger than 125 W; inside ±1.25 W when the load is between 50 W and 125 W, and inside ±5 W when the load is under 50 W. Supply: Texas Devices

To realize such excessive measurement accuracy, historically the e-meter operate is carried out by means of a devoted metering system [3], as proven in Determine 2. A present shunt positioned on the ability issue correction (PFC) enter facet senses the enter present, with a voltage divider (not proven in Determine 2) throughout the AC line and AC impartial senses the enter voltage. A devoted metering system receives this present and voltage info and calculates the enter energy and enter root-mean-square (RMS) present info, sending the outcomes to the host.

Determine 2 Conventional e-meter and PFC management configuration the place: a present shunt is positioned on the PFC enter facet to sense the enter present, a voltage divider (not proven) senses the AC line, and AC impartial senses the enter voltage. Supply: Texas Devices

To manage the PFC enter present, one other present sensor, such because the Corridor-effect sensor proven in Determine 2, senses the enter present, then sends the enter present info to an MCU for PFC current-loop management. Nevertheless, each the Corridor-effect sensor and devoted metering system are costly.

On this energy tip, I’ll talk about a low-cost however extremely correct e-meter answer that makes use of a single present sensor for each e-metering and PFC current-loop management. Integrating e-meter performance into PFC management code eliminates the necessity for a devoted metering system, not solely decreasing system price, but in addition simplifying printed circuit board (PCB) format and expediting the design course of.

E-meter answer

Determine 3 exhibits the proposed e-meter configuration. A present shunt senses the enter present; an remoted delta-sigma modulator AMC1306 measures the voltage drop throughout the present shunt. The delta-sigma modulator output is distributed to the PFC controller MCU. This present info can be used for each e-metering and PFC current-loop management. A voltage divider senses the enter voltage, which is then measured by the MCU’s analog-to-digital converter (ADC) straight, simply as in conventional PFC management.

Determine 3 New e-meter and PFC management configuration the place: a present shunt senses the enter currnet, an remoted delta-sigma modulator measures the voltage dropp acorss the shunt, and the output of the modulator is used to e-metering and PFC current-loop management. Supply: Texas Devices

Delta-sigma modulator

In comparison with the successive approximation register (SAR) type ADC, which nearly all digital PFC controller MCUs use, a delta-sigma modulator can present high-resolution information. The modulator samples the enter sign at a really excessive price to provide a stream of 1-bit codes, as proven in Determine 4.

Determine 4 Delta-sigma modulator enter and output; the next optimistic enter sign produces ones on the output the next proportion of the time whereas a decrease unfavourable enter sign produces ones a decrease proportion of the time. Supply: Texas Devices

The ratio of ones to zeros represents the enter analog voltage. For instance, if the enter sign is 0 V, the output has ones 50% of the time. A better optimistic enter sign produces ones the next proportion of the time, whereas a decrease unfavourable enter sign produces ones a decrease proportion of the time. Not like most quantizers, the delta-sigma modulator pushes the quantization noise to larger frequencies [4] making it appropriate for high-precision measurements.

Delta-sigma digital filter

The C2000 MCU has a built-in delta-sigma digital filter which decodes the 1-bit stream. The efficient variety of bits (ENOB) of the filter output relies on the filter sort, oversampling price (OSR), and delta-sigma modulator frequency [5]. Sometimes, the next OSR will lead to the next ENOB for a given filter sort; nevertheless, the trade-off is elevated filter delay.

It is very important select the fitting filter configuration by finding out the optimum pace versus decision trade-offs. For PFC current-loop management, a brief delay is extra necessary, as a result of it could actually assist improve the control-loop section margin and scale back the entire present harmonic distortion. Then again, high-resolution present information is critical to realize excessive accuracy for e-metering. Because of this, the answer proposed right here makes use of two delta-sigma digital filters: one configured with excessive pace however a comparatively low decision for PFC current-loop management, and the opposite configured with excessive decision however a comparatively low pace for e-metering; see Determine 5.

Determine 5 The proposed delta-sigma filter configuration makes use of two filters: one for high-speed however with a low decision for PFC current-loop management and one other with low-speed for e-metering however with a excessive decision. Supply: Texas Devices

Firmware construction

Determine 6 is the firmware construction, which consists of three loops:

A foremost loop used for gradual and non-time-critical duties.
A quick interrupt service routine (IRS1) operating at 100 kHz for the ADC, delta-sigma information studying, and current-loop management.
A slower ISR2 operating at 10 kHz for voltage-loop management and e-meter calculation.

For the reason that e-meter calculation is in ISR2, it has no impact on the PFC present loop. Integrating e-meter performance into PFC management code with this construction doesn’t have an effect on PFC efficiency.

Determine 6 Firmware construction that consists of three loops: a foremost loop for low non-time-critical duties; a 100 kHz IRS1 loop for ADC, delta-sigma information studying, and present loop management; and 10 kHz ISR2 lopo for voltage-loop management and e-meter calcuation. Supply: Texas Devices

E-meter calculation

Now that there’s each enter present information (by means of the delta-sigma modulator) and enter voltage information (by means of the MCU’s ADC), it’s time to carry out e-meter calculations. Equation 1 calculates the enter voltage RMS worth:

the place V_in(n) is the V_in ADC pattern information and N is the entire variety of ADC samples in a single AC cycle.

The enter present RMS worth calculation consists of two steps. Step one is to calculate the measured present (inductor present) RMS worth, as proven in Equation 2:

the place I_in(n) is the delta-sigma digital filter output.

Referring again to Determine 3, as a result of the shunt resistor is positioned after the EMI filter, the reactive present attributable to the X-capacitor of the EMI filter shouldn’t be measured. Due to this fact, Equation 2 doesn’t symbolize the entire enter present. This case worsens at excessive line and light-weight load, the place the reactive present shouldn’t be negligible; correct enter present reporting requires its inclusion.

As a way to calculate the reactive present of the EMI capacitor, you first have to know the enter voltage frequency. The ADC measures the AC line and impartial voltage; evaluating the road and impartial voltage values will discover the zero crossing. For the reason that enter voltage is sampled at a set price, it’s doable to calculate the AC frequency by counting the variety of samples between two consecutive zero-crossing factors. As soon as the enter voltage frequency, Equation 3 calculates the reactive present of the EMI capacitor:

the place C is the entire capacitance of the EMI filter and f is the enter AC voltage frequency.

I_EMI is a reactive present that leads the measured present (I_L) by 90 levels; due to this fact, Equation 4 calculates the entire enter present as:

Enter energy calculation additionally consists of two steps. First, calculate the measured energy, as proven in Equation 5:

For the reason that enter voltage is measured after the EMI filter, the ability loss attributable to the EMI filter shouldn’t be measured. Whereas this energy loss is often very small, it’s possible you’ll want to incorporate it for purposes requiring extraordinarily correct measurements.

The overall DC resistance of the EMI filter is R. Equation 6 calculates the ability loss on the EMI filter as:

Lastly, including the EMI filter energy loss to the measured energy obtains the entire enter energy (Equation 7):

Check outcomes

I carried out the proposed e-meter operate in a 3.6 kW (1.8 kW at low line) totem-pole bridgeless PFC. Determine 7, Determine 8 and Determine 9 present the take a look at outcomes at low line, excessive line and DC enter, respectively. This implementation achieved <0.5% measurement error, which is 2 instances higher than the M-CRPS e-meter specification. Furthermore, the implementation makes use of solely 1-point calibration, which considerably reduces calibration time and value.

Determine 7 E-meter take a look at outcomes at 1.8 kW low line with Vin set to 115 VAC exhibiting an e-meter accuracy significantly better than the M-CRPS accuracy specification. Supply: Texas Devices

Determine 8 E-meter take a look at outcomes at 3.6 kW excessive line with Vin set to 230 VAC exhibiting an e-meter accuracy significantly better than the M-CRPS accuracy specification. Supply: Texas Devices

Determine 9 E-meter take a look at outcomes at DC enter exhibiting an e-meter accuracy significantly better than the M-CRPS accuracy specification. Supply: Texas Devices

Low-cost, high-accuracy e-meter

This text described a low-cost and high-accuracy e-meter answer: an remoted delta-sigma modulator measures the enter present which is then despatched to an MCU for each e-metering and PFC current-loop management. The proposed answer achieves wonderful measurement accuracy with solely 1-point calibration. In comparison with a conventional e-meter answer, it not solely saves price, but in addition simplifies PCB format and expedites the design course of.

Bosheng Solar is a Techniques Engineer in Texas Devices, give attention to creating digital managed excessive efficiency AC/DC options for server and business utility. Bosheng acquired the M.S. diploma from Cleveland State College, Ohio, USA in 2003, the B.S diploma from Tsinghua College, Beijing, China in 1995, each in Electrical Engineering. He holds 5 US patents.

Associated Content material

References

S. Division of Power, (2017, Feb. 7). Knowledge Heart Metering and Useful resource Information. [Online]. Out there: https://datacenters.lbl.gov/websites/default/recordsdata/DataCenterMeteringandResourceGuide_02072017.pdf.
Modular {Hardware} System – Frequent Redundant Energy Provide (M-CRPS) Base Specification. Open Compute Undertaking, Model 1.00, Launch Candidate 4, Nov. 1, 2022.
Analog Units. 78M6610+PSU {Hardware} Design Tips. (2012). [Online]. Out there: https://www.analog.com/media/en/technical-documentation/user-guides/78m6610psu-hardware-design-guidelines.pdf.
Bonnie Baker, “How Delta-Sigma ADCs Work.” Texas Devices Analog Utility Journal, August 2011.
Texas Devices. TMCS320F28003x Actual-Time Microcontrollers Technical Reference Guide. (2022). [Online]. Out there: https://www.ti.com/product/TMS320F280039C.

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