Introduction

Power supplies in data centers that measure the input power in real time and report the measurement to the host are conducting what’s known as electrical metering (e-metering). An e-meter has become a common requirement in power-supply units over the last decade because it brings these advantages to data centers [1]:

• Identifies abnormally low or high energy usage and potential causes, supporting such practices as peak shaving.
• Facilitates capacity planning around space and power utilization.
• Helps track and manage energy costs; verifies energy bills; and prioritizes, validates, and reduces energy costs through improved energy efficiency and energy management.
• Enables quantitative assessments of data center performance and benchmarking of that performance across a level playing field.
• Helps develop and validate energy-efficiency strategies, and identifies opportunities to improve energy efficiency by lowering energy and operational costs.
• Commission and detect faults in physical systems and diagnose their causes.

For all of these reasons, an e-meter must be exceptionally accurate. Figure 1 shows the Modular Hardware System-Common Redundant Power Supply (M-CRPS) e-meter accuracy requirement [2], which requires an input power measurement error within ±1% when the load is greater than 125 W, within ±1.25 W when the load is between 50 W and 125 W, and within ±5 W when the load is below 50 W.

Figure 1 The M-CRPS e-meter accuracy specification which requires an input power measurement error: within ±1% when the load is greater than 125 W; within ±1.25 W when the load is between 50 W and 125 W, and within ±5 W when the load is below 50 W. Source: Texas Instruments

To achieve such high measurement accuracy, traditionally the e-meter function is implemented through a dedicated metering device [3], as shown in Figure 2. A current shunt placed on the power factor correction (PFC) input side senses the input current, with a voltage divider (not shown in Figure 2) across the AC line and AC neutral senses the input voltage. A dedicated metering device receives this current and voltage information and calculates the input power and input root-mean-square (RMS) current information, sending the results to the host.

Figure 2 Traditional e-meter and PFC control configuration where: a current shunt is placed on the PFC input side to sense the input current, a voltage divider (not shown) senses the AC line, and AC neutral senses the input voltage. Source: Texas Instruments

To control the PFC input current, another current sensor, such as the Hall-effect sensor shown in Figure 2, senses the input current, then sends the input current information to an MCU for PFC current-loop control. However, both the Hall-effect sensor and dedicated metering device are expensive.

In this power tip, I’ll discuss a low-cost but highly accurate e-meter solution that uses a single current sensor for both e-metering and PFC current-loop control. Integrating e-meter functionality into PFC control code eliminates the need for a dedicated metering device, not only reducing system cost, but also simplifying printed circuit board (PCB) layout and expediting the design process.

E-meter solution

Figure 3 shows the proposed e-meter configuration. A current shunt senses the input current; an isolated delta-sigma modulator AMC1306 measures the voltage drop across the current shunt. The delta-sigma modulator output is sent to the PFC controller MCU. This current information will be used for both e-metering and PFC current-loop control. A voltage divider senses the input voltage, which is then measured by the MCU’s analog-to-digital converter (ADC) directly, just as in traditional PFC control.

Figure 3 New e-meter and PFC control configuration where: a current shunt senses the input currnet, an isolated 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 control. Source: Texas Instruments

Delta-sigma modulator

Compared to the successive approximation register (SAR) style ADC, which almost all digital PFC controller MCUs use, a delta-sigma modulator can provide high-resolution data. The modulator samples the input signal at a very high rate to produce a stream of 1-bit codes, as shown in Figure 4.

Figure 4 Delta-sigma modulator input and output; a higher positive input signal produces ones at the output a higher percentage of the time while a lower negative input signal produces ones a lower percentage of the time. Source: Texas Instruments

The ratio of ones to zeros represents the input analog voltage. For example, if the input signal is 0 V, the output has ones 50% of the time. A higher positive input signal produces ones a higher percentage of the time, while a lower negative input signal produces ones a lower percentage of the time. Unlike most quantizers, the delta-sigma modulator pushes the quantization noise to higher frequencies [4] making it suitable 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 effective number of bits (ENOB) of the filter output depends on the filter type, oversampling rate (OSR), and delta-sigma modulator frequency [5]. Typically, a higher OSR will result in a higher ENOB for a given filter type; however, the trade-off is increased filter delay.

It is important to choose the right filter configuration by studying the optimal speed versus resolution trade-offs. For PFC current-loop control, a short delay is more important, because it can help increase the control-loop phase margin and reduce the total current harmonic distortion. On the other hand, high-resolution current data is necessary to achieve high accuracy for e-metering. For this reason, the solution proposed here uses two delta-sigma digital filters: one configured with high speed but a relatively low resolution for PFC current-loop control, and the other configured with high resolution but a relatively low speed for e-metering; see Figure 5.

Figure 5 The proposed delta-sigma filter configuration uses two filters: one for high-speed but with a low resolution for PFC current-loop control and another with low-speed for e-metering but with a high resolution. Source: Texas Instruments

Firmware structure

Figure 6 is the firmware structure, which consists of three loops:

• A main loop used for slow and non-time-critical tasks.
• A fast interrupt service routine (IRS1) running at 100 kHz for the ADC, delta-sigma data reading, and current-loop control.
• A slower ISR2 running at 10 kHz for voltage-loop control and e-meter calculation.

Since the e-meter calculation is in ISR2, it has no effect on the PFC current loop. Integrating e-meter functionality into PFC control code with this structure does not affect PFC performance.

Figure 6 Firmware structure that consists of three loops: a main loop for low non-time-critical tasks; a 100 kHz IRS1 loop for ADC, delta-sigma data reading, and current loop control; and 10 kHz ISR2 lopo for voltage-loop control and e-meter calcuation. Source: Texas Instruments 

E-meter calculation

Now that there’s both input current data (through the delta-sigma modulator) and input voltage data (through the MCU’s ADC), it’s time to perform e-meter calculations. Equation 1 calculates the input voltage RMS value:

where V_in(n) is the V_in ADC sample data and N is the total number of ADC samples in one AC cycle.

The input current RMS value calculation consists of two steps. The first step is to calculate the measured current (inductor current) RMS value, as shown in Equation 2:

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

Referring back to Figure 3, because the shunt resistor is placed after the EMI filter, the reactive current caused by the X-capacitor of the EMI filter is not measured. Therefore, Equation 2 does not represent the total input current. This situation worsens at high line and light load, where the reactive current is not negligible; accurate input current reporting requires its inclusion.

In order to calculate the reactive current of the EMI capacitor, you first need to know the input voltage frequency. The ADC measures the AC line and neutral voltage; comparing the line and neutral voltage values will find the zero crossing. Since the input voltage is sampled at a fixed rate, it is possible to calculate the AC frequency by counting the number of samples between two consecutive zero-crossing points. Once you know the input voltage frequency, Equation 3 calculates the reactive current of the EMI capacitor:

where C is the total capacitance of the EMI filter and f is the input AC voltage frequency.

I_EMI is a reactive current that leads the measured current (I_L) by 90 degrees; therefore, Equation 4 calculates the total input current as:

Input power calculation also consists of two steps. First, calculate the measured power, as shown in Equation 5:

Since the input voltage is measured after the EMI filter, the power loss caused by the EMI filter is not measured. While this power loss is usually very small, you may need to include it for applications requiring extremely accurate measurements.

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

Finally, adding the EMI filter power loss to the measured power obtains the total input power (Equation 7):

Test results

I implemented the proposed e-meter function in a 3.6 kW (1.8 kW at low line) totem-pole bridgeless PFC. Figure 7, Figure 8 and Figure 9 show the test results at low line, high line and DC input, respectively. This implementation achieved <0.5% measurement error, which is two times better than the M-CRPS e-meter specification. Moreover, the implementation uses only 1-point calibration, which significantly reduces calibration time and cost.

Figure 7 E-meter test results at 1.8 kW low line with Vin set to 115 VAC showing an e-meter accuracy much better than the M-CRPS accuracy specification. Source: Texas Instruments

Figure 8 E-meter test results at 3.6 kW high line with Vin set to 230 VAC showing an e-meter accuracy much better than the M-CRPS accuracy specification. Source: Texas Instruments

Figure 9 E-meter test results at DC input showing an e-meter accuracy much better than the M-CRPS accuracy specification. Source: Texas Instruments

Low-cost, high-accuracy e-meter

This article described a low-cost and high-accuracy e-meter solution: an isolated delta-sigma modulator measures the input current which is then sent to an MCU for both e-metering and PFC current-loop control. The proposed solution achieves excellent measurement accuracy with only 1-point calibration. Compared to a traditional e-meter solution, it not only saves cost, but also simplifies PCB layout and expedites the design process.

Bosheng Sun is a Systems Engineer in Texas Instruments, focus on developing digital controlled high performance AC/DC solutions for server and industry application. Bosheng received the M.S. degree from Cleveland State University, Ohio, USA in 2003, the B.S degree from Tsinghua University, Beijing, China in 1995, both in Electrical Engineering. He holds 5 US patents.

Related Content

• Delta-sigma ADCs in a nutshell
• ADC noise article and all about delta-sigma converters
• Power Tips #124: How to improve the power factor of a PFC
• Power Tips #116: How to reduce THD of a PFC
• Power Tips #131: Planar transformer size and efficiency optimization algorithm for a 1 kW high-density LLC power module
• Power Tips #130: Migrating from a barrel jack to USB Type-C PD

 References

1. S. Department of Energy, (2017, Feb. 7). Data Center Metering and Resource Guide. [Online]. Available: https://datacenters.lbl.gov/sites/default/files/DataCenterMeteringandResourceGuide_02072017.pdf.
2. Modular Hardware System – Common Redundant Power Supply (M-CRPS) Base Specification. Open Compute Project, Version 1.00, Release Candidate 4, Nov. 1, 2022.
3. Analog Devices. 78M6610+PSU Hardware Design Guidelines. (2012). [Online]. Available: https://www.analog.com/media/en/technical-documentation/user-guides/78m6610psu-hardware-design-guidelines.pdf.
4. Bonnie Baker, “How Delta-Sigma ADCs Work.” Texas Instruments Analog Application Journal, August 2011.
5. Texas Instruments. TMCS320F28003x Real-Time Microcontrollers Technical Reference Manual. (2022). [Online]. Available: https://www.ti.com/product/TMS320F280039C.

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