Intelligent energy-saving socket designed with CSE7780

　　Smart socket system design

　　This design consists of three parts: metering module, display module and control module (see Figure 1). This article will focus on the design of the key metering module.

　　Figure 1: Block diagram of CSE7780-based smart socket system

　　1. Metering function design

　　This system uses CSE7780, which can provide active power, active energy, current RMS, voltage RMS, line frequency, zero-crossing interruption and other functions, and provides full digital gain, phase, bias current calibration, and active energy pulse output from the PF pin. In addition, CSE7780 provides an SPI serial interface that can communicate with an external MCU, and has an internal power monitoring circuit to ensure the normal operation of the chip.

　　As shown in Figure 2, the measurement of this system includes two parts: current and voltage sampling.

　　(1) Current signal sampling

　　In the current sampling circuit, when the current flows through the manganese copper shunt, a voltage drop will be generated on the current sampling channel of the metering chip. Different current signals will form different voltage drops on the shunt. The metering chip collects the voltage signal formed on the shunt to realize the collection of the current signal.

　　(2) Voltage signal sampling

　　Voltage sampling usually collects the signal on the neutral line. Since the voltage signal is large, this system design directly samples the voltage signal by reducing the voltage through a resistor network.

　　Figure 2: Metering circuit based on CSE7780 smart socket

　　2. Display module design

　　The display part of this system design adopts the HT1621 LCD driver control chip, which can drive 4*32 LCD segments and fully meet the requirements of display driver. It can display information such as power, voltage RMS, current RMS, and active power.

　　3. Power module

　　Considering the space factor of the product, the power supply designed for this system adopts a non-isolated power supply, and the power supply circuit can provide a current of about 60mA.

　　Smart socket software design

　　1. Calculation of electrical parameters

　　To design a rated voltage of 220V (Un), 10 (60) For example, the current specification and constant 1600imp/KWh socket, since the maximum input signal allowed by the current input channel is ±700mV peak-to-peak (effective value is 495mVrms), the 10(60)A table can select 200~250μΩ manganese copper considering the heating of channel A. If 250μΩ manganese copper is used for sampling, when Imax=60A, the sampling signal of channel A is 60A*250μΩ=15mV. Since the maximum input signal allowed by current channel A is 495mV, the gain selection of the current channel can be configured to 16, and channel B uses a 2500:1 mutual inductor; the load resistance is 10Ω, and the gain of current channel B is set to 1. The maximum input signal allowed by the voltage channel is ±700mV peak-to-peak. Considering that the voltage will have an overvoltage of 130%Un, the voltage sampling signal can be reduced to about 220mV through a network resistor, and the gain of the voltage channel is selected to be 1.

　　Based on the above discussion, we need to set the gain of current channel A to 16 and the gain of voltage channel A to 1, so the SYSCON register should be set to 0080H.

　　The configuration flow of CSE7780 registers is shown in Figure 3.

　　2. Setting of HFConST register

　　Constant EC is 1600imp/KWh; Vu=0.22V; Vi=10A*0.00025Ω*16=40mV; EC=1600; Un=220V;

　　Ib=10A. According to the formula HFConst= INT[39.3143*Vu*Vi*10^11/(EC*Un*Ib)], HFConst=2664H, so the value written into the HFConst register should be 2664H.

　　3. Other metering control register configuration

　　Configuration of starting current. In the case of Un and Ib, the value of the active power register PowerA is 1A375D7H. According to the requirement, it can start normally under 0.4%Ib. Then the Pstar register can be configured to the value corresponding to 0.2%Ib active power Pstar=00D6H (Pstart corresponds to the upper 24 bits of PowerA, and the calculated Pstart is 16'h00D6).

　　Configuration of energy accumulation mode. Since we need to measure the positive and negative active energy, we must configure the energy accumulation mode so that both positive and negative power are accumulated. The accumulation method is algebraic sum. Negative power is indicated by the REVQ sign. Enable PF pulse output and active energy register accumulation. Then, configure EMUCON to 0001H.

　　4. Configuration of calibration registers

　　(1) Active power correction

　　a. Power gain correction

　　When the input signals are Un and Ib, the error of channel A obtained from the calibration station is err, then formula 1. If Pgain>=0, then GPQA=INT[Pgain*2^15], otherwise if Pgain<0, then GPQA=INT[2^16 +Pgain*2^15].

　　Power calibration of Channel B can be accomplished by configuring GPQB in the same manner as Channel A.

　　b. Phase correction

　　When PF=0.5L and the input signal is 100%Un, 100%Ib, the error obtained from the calibration table is err, and the phase error compensation is formula 2. For a 50Hz power grid, PHSA has a relationship of 0.02^0/LSB, then: if θ>=0, PHSA =INT(θ/0.02^0); if θ<0, PHSA =INT(2^8+θ/0.02)-96.

　　Phase correction of channel B can be achieved by configuring PHSB in the same way as that of channel A.

　　c. Active bias current correction

　　In the case of small signal 1.0, if the small signal deviation is large, the deviation of the small signal can be corrected by adjusting the active power bias current correction register. In the case of PF=1.0, Vu=Un, Vi=0, wait for the update of DPUDIF, get the value of PowerA through MCU, read several times to average, and write the last 4 bits of the complement of the average value into the APOSA correction register.

　　Active bias current correction for Channel B can be achieved by configuring APOSB in the same way as for Channel A.

　　(2) Voltage and current calculation

　　The correction process of the effective value is to correct the bias current of the current first. After correcting the bias current, calculate the current conversion coefficient KIA and the voltage conversion coefficient KU. Read the value of the IARMS register when PF=1.0, 100%Un, Ib. According to the formula KIA=IARMS/Ib, the conversion coefficient of the current channel A can be obtained. The conversion coefficient KU of the voltage channel can be obtained in the same way.

　　Figure 4: Main program flow chart of the intelligent energy-saving socket system based on CSE7780

　　Conclusion

　　The smart energy-saving socket based on CSE7780 has been successfully applied in batches by many companies. After testing, the energy-saving socket system has shown good control effect, can sensitively detect whether the load is overloaded and the standby state, effectively protect the safety of electrical appliances, and is favored by manufacturers of such products and has begun to be sold overseas in batches.