### Abstract

The 78M6610+PSU is an energy measurement processor that is embedded in high-power switch-mode power-supply units (PSUs) to measure and report the real-time power used by the system. As with any measurement device, the sense circuit used with the 78M6610+PSU must be calibrated before its measurements can be accepted as accurate. This application note demonstrates how to calibrate a measurement subsystem using the 78M6610+PSU.

### Introduction

### Scaling Values

In order to calibrate the voltage and current measurements and to allow the 78M6610+PSU to perform proper power calculations, the user must first set the correct scaling values to match the selected sensors and LSB size.

- Setting the expected full-scale values for the measurements.
- Setting the resolution, or least significant bit (LSB) value, for the reported values.

### Determining Scaling Values

#### ADC Input Range

_{3P3A}). The first step in determining the scaling values for voltage and current is to determine the peak input voltage and current (at the sensor input) that produce a ±250mV peak to the 78M6610+PSU’s ADC input. Consider the following example.

**Figure 1**.

*Figure 1. Voltage-divider for inlet voltage sensing.*

_{ADC}, can be calculated as:

V_{ADC} = V_{LINE} × (750/(750 + 2 × 10^{6})) |
(Eq. 1) |

V_{MAX} = ±0.25V/0.00375 ±667V_{PEAK} |
(Eq. 2) |

_{PEAK}, or 472V

_{RMS}, assuming a sinusoidal voltage waveform.

I_{MAX} = (±0.250V)/0.004Ω ±62.5A_{PEAK} |
(Eq. 3) |

_{RMS}. It is necessary to consider the peak current rather than the RMS current when determining the maximum current to be measured. In power-supply applications, the current waveform at high loads is usually almost sinusoidal due to the power factor correction. Because of that, assuming a crest factor (i.e., the ratio of peak current to RMS current) between 1.414 and 1.5 is reasonable.

#### Measurement Resolution

VScale = V_{MAX}/(Voltage LSB Weighting) |
(Eq. 4) |

IScale = I_{MAX}/(Current LSB Weighting) |
(Eq. 5) |

**Tables 1**and

**2**.

Table 1. VScale Examples | ||

Maximum Peak Voltage (V) | Voltage Resolution (V) | VScale Register |

667 | 1 | 667 |

667 | 0.001 | 667000 |

170 | 1/2048 | 348160 |

Table 2. IScale Examples | ||

Maximum Peak Current (A) | Current Resolution (A) | IScale Register |

32 | 1/2048 | 65536 |

62.5 | 0.001 | 62500 |

62.5 | 0.0000078125 | 8000000 |

#### PScale Value

PScale = (V_{MAX} × I_{MAX})/(Power LSB Weighting) |
(Eq. 6) |

_{MAX}= 200V and the voltage resolution is 10mV/LSB; I

_{MAX}= 20A and the current resolution is 1mA/LSB; and power has an LSB weight of 1mW, PScale is calculated as:

PScale = (200 × 20)/0.001 = 4000000 | (Eq. 7) |

^{23}- 1). If a calculated scaling value is greater than 8,388,607, then the maximum values, the LSB weighting, or both must be adjusted to generate a positive value that can be expressed as a 24-bit signed integer. The default scaling for the 78M6610+PSUEVK is shown in

**Table 3**.

Table 3. The 78M6610+PSUEVK Default Scaling | |||

Maximum | Resolution | Scaling Value | |

Voltage | 667V | 0.001V | 667000 |

Current | 62.5A | 0.0000078125A (1/128mA) | 8000000 |

Power | 41687.5W | 0.005W | 8337500 |

#### TScale, PFScale, and FScale

### Measurement Considerations

**Figure 2**. In this case, the current flowing in the filter capacitors—commonly referred to as X and Y capacitors—and the voltage drop across PCB traces and common mode chokes are not measured by the 78M6610+PSU, but should be accounted for in the measurements.

*Figure 2. Typical measurement locations in power supplies.*

### General Calibration

**Figure 3**. After calibration has completed, the new coefficients can be saved into flash memory as defaults by issuing the Access (ACC) command.

*Figure 3. The 78M6610+PSU Command register bit map.*

### Voltage and Current Gain Calibration

**Figure 4**, and a stable AC supply and load must be applied to the sensors to be calibrated. The value corresponding to the applied AC supply (usually measured with a power meter) must be entered into the relevant target register (e.g., VTarget, ITarget).

*Figure 4. Typical connections for system calibration.*

### Offset Calibration

### Calibration of the X+Y Capacitor and R Compensation Coefficients

**Figure 5**. The current in the filter capacitors (I

_{CAP}) preceding the 78M6610+PSU cannot be measured. Since the current is 90° phase shifted with respect to the voltage, there is no effect on the power measurements. However, in order to obtain high accuracy in the current measurement, the current in the capacitors should be compounded in the total current (I

_{RMS}) calculation.

*Figure 5. Use of line input filter to minimize EMI in a typical calibration setup.*

### On-Chip Temperature Calibration

### External Temperature Calibration

### Calibration Procedures

**Figure 6**has no exit arrow shown, it means that once that calibration function has completed, the firmware does not initiate any further calibration operations until another calibrate command (0xCA0000 + request bits) is written to the Command register. For example, if chip temperature (bit 10), current (bit 11), and voltage (bit 12) were all requested simultaneously, only the temperature calibration (bit 10) would be performed; current (bit 11) and voltage (bit 12) would remain set.

*Figure 6. Flowchart of the 78M6610+PSU calibration command process.*

### Recommended Calibration Sequence

- Calibrate the current measurement at an AC inlet voltage that is near the low end of the design voltage range for the PSU. The PSU should be loaded to provide a power factor of 0.9 or greater, as shown on the reference meter. When the current calibration is done at low line voltage, the current in the X and Y capacitors, which is measured by the reference meter but not by the metering device, is minimized.
- Calibrate the voltage measurement at high line voltage and low current (I
_{RMS}< 500mA). The voltage and current measurements are independent. Calibrating voltage at low input current minimizes the voltage drop in the PCB and the components that are between the AC power inlet and the 78M6610+PSU’s measurement point. - Calibrate X+Y capacitor compensation at high line voltage and low current. At high voltage, the current in the X and Y capacitors is increased, and at low current, the ratio of the capacitor current to the current measured by the 78M6610+PSU is larger, which improves the results of the X+Y capacitor compensation calibration.
- Calibrate R compensation at low input voltage and high current. The high current increases the magnitude of the voltage drop in the PCB and the EMI filters; the low input voltage increases the ratio of the voltage drop between the inlet and the measurement point to the voltage measured by the 78M6610+PSU.
- Save the updated calibration values to flash memory.

Table 4. Calibration and Scaling Checklist | |

Step | Description |

1 | Calculate the new voltage, current, and power scaling values based on the sensors used in the system and the desired resolution. Set the VScale, IScale, and PScale registers accordingly and store these parameters in flash memory. |

2 | Calculate the frequency and power factor scaling parameters to obtain the desired resolution (if required). Set the relevant FScale and PFScale registers and store these parameters in flash memory. |

3 | Connect source and load for voltage and current calibration. Set the calibration point values. |

4 | Set the VTarget, ITarget, and TTarget registers. |

5 | Perform VCAL , ICAL, and TCAL. |

6 | If necessary, perform XYComp and RComp calibrations. |

7 | Store the newly calculated coefficients in flash memory. |

### Example of the Calibration Procedure Using the 78M6610+PSUEVK

**Figure 7**shows the power-supply input stage (EMI filters) and the EV board.

*Figure 7. Test setup for calibration example.*

- AC source: Chroma
^{®}model 6430 - Power meter: Chroma model 66202
- DC load: Chroma model 6314, 63106 (DC load mainframe and DC electronic load module)
- Computer with the standard GUI that is provided with the 78M6610+PSUEVK

**Step 1. Current Gain Calibration**

**Figure 8**), the current gain is calibrated. The value of the X+Y capacitor compensation coefficient must be set to zero. The power-supply output must be loaded in order to obtain a power factor approximating unity; the power factor is measured through the power meter. The power-supply input should be set to the lower range of the working voltage (e.g., 100VAC). By doing so, the effect of the current on the filter capacitors is minimized and thus, a greater accuracy can be obtained. The value of the current read through the power meter must be entered as a new target current, and the calibration command must be entered.

*Figure 8. Calibrating the current gain in the 78M6610+PSUEVK GUI.*

**Step 2. Voltage Gain Calibration**

**Figure 9**) consists of calibrating the voltage gain. In this step, the output load can be reduced. The input voltage should be set to the upper range.

*Figure 9. Calibrating the voltage gain in the 78M6610+PSUEVK GUI.*

**Step 3. X+Y Capacitor Compensation Coefficient Calibration**

**Figure 10**), the compensation coefficient for the X+Y capacitor is set.

*Figure 10. Calibrating the X+Y capacitor compensation coefficient in the 78M6610+PSUEVK GUI.*

**Step 4. Storing the Newly Calculated Coefficients in Flash as Defaults**

### Accuracy Results

**Figures 11**and

**12**.

*Figure 11. Load line at 120V*

_{RMS}.*Figure 12. Load line at 230V*

_{RMS}.### Conclusion