Three-phase smart meter switching power supply solution

　　background:

　　Smart meters are smart terminals of smart grids. In addition to the basic electricity metering function of traditional energy meters, in order to adapt to the use of smart grids and new energy, they also have intelligent functions such as electricity information storage, two-way multi-rate metering, user-side control, two-way data communication in multiple data transmission modes, and anti-electricity theft. Smart meters represent the development direction of smart terminals for energy-saving smart grid end users in the future. Reducing the power consumption of smart meters and improving their operating efficiency have become important links in current smart meters. Switching power supplies are different from other devices in smart meters. Large-scale and standardized production may improve quality, reduce production costs, and optimize production processes. Although switching power supplies for smart meters have received attention, there are still many problems in the development of switching power supplies in China, such as lack of basic theory, inability of industrial level to keep up with demand, and immature production processes. In addition, the explosion phenomenon caused by switching power supplies has always been one of the important reasons that trouble and hinder its widespread application. Other reasons include long-term reliability. At present, the power supply for smart meters in China is still mainly based on traditional industrial frequency transformers, while some foreign products have gradually used switching power supplies. The main reason is that after the meter function is enhanced, the power supply power requirement increases, and the power frequency transformer is difficult to meet the requirements. At the same time, considering the installation and transportation costs, the switching power supply will have great advantages.

　　Internal power supply structure of three-phase smart meter:

　　Requirements for switching power supply in smart meters:

　　This article only proposes solutions to a few important requirements:

　　Very wide input voltage range

　　Multiple output regulation

　　Various abnormalities

　　Cascade ordinary flyback solution:

　　For low-power switching power supply applications with conventional input voltage (85Vac-265Vac), the flyback topology is the most common in terms of comprehensive efficiency and cost. Structurally, a controller can be used with an external switching device, or considering the integration level, there are also integrated controllers and switching devices in one package. The withstand voltage level of the switching device is usually 650V, 700V and 800V. If for three-phase applications, considering the reflected voltage and leakage inductance of the transformer and the design margin, this type of device cannot meet the requirements. However, if a high-voltage switching device is used alone, such as a power switching device above 1000V or 1200V, there is not much room for selection and the cost is also high. Therefore, the first design issue to be considered in a three-phase meter is how to solve the withstand voltage problem under high input voltage.

　　Let's take a specific specification as an example:

　　Specification:

　　Due to the characteristics of multiple outputs and low power output, flyback is more suitable for the power supply topology. The control chip in this article is Infineon ICE3AR2280JZ. In addition to the current mode controller with an operating frequency of 100KHz, it also integrates 800V CoolMOS, with an on-resistance of 2.2ohm and a DIP7 package. The chip also integrates an 800V high-voltage startup unit. At an ambient temperature of 50 degrees and a conventional wide voltage input (85Vac-265Vac), the maximum input power can reach 28W. At the same time, the chip also has protection functions such as overcurrent, overvoltage, input undervoltage, overtemperature, and a burst mode to improve light load efficiency. In view of factors such as low-power applications, transformer size and loop compensation, it is usually recommended that the system operate in current discontinuous mode (DCM) in the full load section.

　　Schematic diagram:

　　Principle description:

　　The input voltage passes through the common-mode filter L1, C20, C21 and two rectifier bridges BR1 and BR2 of the front stage; the varistors RV1, RV2, RV3 and CX11, CX12, CX13 form an overvoltage protection circuit; the power resistors R1, R2, R3 are used to suppress surge current. In order to simplify the design, the filter inductor is placed after the rectifier bridge to save costs. Considering the input phase loss situation, that is, as long as any two wires exist, whether the live wire is the neutral wire or the live wire is the live wire, the system can still work normally, and two rectifier bridge outputs are used in parallel. After rectification, since the maximum peak voltage can reach 780V, two 450V electrolytic capacitors are used in series, and considering the voltage balance, R13, R14, R15, R16 are connected in parallel on both sides of the capacitor.

　　The primary switching circuit consists of a transformer, a clamping circuit, a switching tube, a CoolSET, a TVS, a Zener diode, etc.

　　At startup, the current flows through R19, R20, R21, and R22 through the Zener diode D10 and enters the high-voltage startup unit connected to the drain of CoolSET. The high-voltage startup unit inside CoolSET is 800V. Due to the existence of the external TVS diode, the ultra-high voltage will be clamped at a specific voltage to protect CoolSET. However, when CoolSET is turned on, the source of the external MOSFET is pulled to ground, so that the Zener diode D10 is reverse biased, thereby turning on the external MOSFET; when CoolSET is turned off, the inductor current first charges the drain-source capacitance of the MOSFET inside CoolSET until the Vds voltage reaches the clamping voltage of the external TVS diode, and the current begins to discharge the gate-source capacitance of the external MOSFET until the forward voltage of the Zener diode between GS exceeds 0.7V, the external MOSFET is turned off, and the current will flow through the Zener diode D10 to the external TVS diode or R19, R20, R21, and R22. Depends on the impedance of the two loops, since the Vgs of the external MOSFET is close to zero, the MOSFET will be completely turned off; for inputs exceeding the rated voltage of the external TVS tube, the CoolSET voltage stress is the clamping voltage value of the external TVS. For example, if a 550V TVS diode and an 800V external MOSFET are used, the flyback withstand voltage capability is: 550V+800V=1350V. As a design, considering the worst case, it can be roughly estimated that the time from the internal MOSFET to the external MOSFET turning off is the time flowing through the external TVS diode, and the average current flowing through the TVS is easily obtained using the peak current at maximum load. Therefore, the loss of the TVS diode is the product of the average current and the clamping voltage;

　　The output circuit is composed of Schottky diode, absorption circuit and filter. In order to meet the ripple requirements, a two-stage filter is used. Output 1 is mainly 5V, which is grounded with 12V, and another 5V reference ground is isolated from outputs 1 and 2. Considering the cross-adjustment problem of multi-channel output loads, the 12V reference is superimposed on the 5V output. In this way, the adjustment accuracy of the 12V output is improved. Because the influence of the forward voltage drop of the diode on the 5V output with the change of current is avoided. Based on the consideration of dynamic stability, the 12V output capacitor C8 is placed on the 5V output, which can avoid the instability of the 12V output caused by the large dynamic load jump of the 5V output.

　　The feedback circuit is composed of a voltage divider network, a compensation network, TL431 and an optocoupler . The compensation part is composed of C10, C11 and R10, where R10 and C10, C11 respectively form two poles and a zero point to compensate for the current-type flyback.

　　Considering the size of the transformer, EE20-PC40 core is selected. Comprehensive duty cycle, select

　　Design considerations:

　　The maximum duty cycle of ICE3AR2280JZ is 0.7. In order to reasonably utilize the duty cycle to cover an ultra-wide voltage range, the reflected voltage is taken as 150V. According to the minimum input voltage, the maximum duty cycle under full load conditions is 0.62. Therefore, the inductance is: 1.024mH

　　Select the primary side turns Np=72, the secondary side main 5V turns Ns1=3, the chip Vcc turns Nvcc=8; Considering that the output adopts the DC stacking method, the 12V winding turns are 4 (the 12V winding is superimposed on the main 5V output, not the 5V winding end). The transformer structure is as follows:

　　Load regulation and input regulation:

　　Load cross regulation rate: (The horizontal axis is mainly 5V output from 5% to 100% load, and the various color curves are 12V output from 5% to 100% load)

　　efficiency:

　　Switching waveforms and output ripple of upper and lower MOSFETs:

　　Conclusion:

　　Through testing, it can be seen that when the Vds voltage of the internal MOSFET of CoolSET reaches about 550V, the voltage is clamped by the TVS; the external MOSFET is completely turned off by the freewheeling of the primary current, so that the entire shutdown voltage stress is shared by the two MOSFETs in series. After using the secondary LC filter, the output ripple is: 24mV (5V), 79mV (12V), 20mV (isolated 5V); the cross-regulation rate can achieve a cross-regulation of less than 10% (>10% load) without adding an external linear regulator to the output. For higher voltage designs, multiple TVS series can be used. Taking 800V CoolSET and 800V CoolMOS as examples, the maximum withstand voltage can reach 1600V. It can fully meet the requirements of high voltage input applications.