Design and Analysis of Single Chip Microcomputer Power Supply Module

introduction

　　There are two design schemes for special monolithic switching power supplies: the first is to use a general monolithic switching power supply integrated circuit (such as TOPSwitch-Ⅱ, TOPSwitch-FX, TOPSwitch-GX series), and then design it with peripheral circuits such as voltage control loop and current control loop. Its characteristics are large output power but complex peripheral circuits; the second is to use the recently launched LinkSwitch series of high-efficiency constant voltage/constant current three-terminal monolithic switching power supply chips, or use LinkSwitch-TN series, DPA-Switch series monolithic switching power supply dedicated ICs, which can greatly simplify the circuit and reduce costs, and is suitable for forming medium and small power special switching power supplies.

1 2.5W constant voltage/constant current charger module

　　The following is a 2.5W constant voltage/constant current charger module composed of LNK500. It is suitable for mobile phone battery chargers, personal digital assistants (PDA, Personal Digital Assistant), portable audio equipment, electric shavers, built-in power supplies of household appliances (such as backup power and bias power supplies of color TVs) and other fields.

1.1 Performance characteristics and technical indicators

　　1) Using high-efficiency constant voltage/constant current monolithic switching power supply LNK500, the AC input voltage range is 85~265V. When the AC input voltage is 265V, the leakage current is <5μA, the rated output voltage is 5.5V, the maximum output current is 0.45A, and the output power is 2.5W.

　　2) Low power consumption, high efficiency, no-load power consumption <0.3W, the typical value of power efficiency η≈68%.

　　3) At the peak power point, the output voltage is allowed to have an error of ±10%. When the error of the primary inductance Lp is ±10%, the output current has an error of ±25%.

　　4) The circuit is simple and inexpensive. The power supply only requires 23 components and does not require a secondary feedback circuit. Constant current/constant voltage output can be achieved using the primary circuit, allowing the use of a low-cost, small-size EE13 type magnetic core.

　　5) With overheat protection, output short circuit protection and open loop protection functions.

　　6) Comply with the international electromagnetic compatibility standard CISPR22B/EN55022B.

1.2 Circuit Design of 2.5W Constant Voltage/Constant Current Charger Module

　　The internal circuit of the 2.5W constant voltage/constant current charger module composed of LNK500 is shown in Figure 1. FR is a self-recoverable fuse resistor, which has a current limiting protection function and can limit the impact current when power is on. VD1~VD4 constitute a bridge rectifier, and the inductor L1, L2 and the capacitor C1, C2 form a low-power π-type filter, which can filter out electromagnetic interference. L2 can use a 3.3μH magnetic bead. When the power MOSFET inside LNK500 is turned on, the output rectifier tube VD6 is turned off, and the electric energy is stored in the high-frequency transformer. When the power MOSFET is turned off, VD6 is turned on, and the energy stored in the high-frequency transformer is output through the secondary circuit. VD6 uses a 1A/100V Schottky diode SB1100. R4 and C7 are connected in parallel at both ends of VD6 to prevent VD6 from self-excited oscillation in the high-frequency switching state. C6 is the output filter capacitor. R5 is a 22kΩ load resistor.

Figure 1 Internal circuit of a 2.5W constant voltage/constant current charger module

　　由R1、C3和VD5构成的RCD型箝位电路具有以下功能：

　　1）当功率MOSFET关断时，对初级感应电压进行箝位；

　　2) It can simplify the design of the feedback circuit.

　　The feedback current of the control terminal is set by resistor R2. When the power is just started, the control terminal capacitor C4 supplies power to LNK500, and C4 also determines the automatic restart frequency.

　　In order to reduce electromagnetic interference, the primary of the high-frequency transformer is designed with two windings, NP1 and NP2. NP2 is called the "cancellation winding", which is connected to the primary return end through R3 and C5 to reduce electromagnetic interference in the primary circuit. In addition, a shielding layer needs to be added between the primary and secondary.

　　LNK500 is only suitable for working in discontinuous mode, and its output power is determined by formula (1).

　　PO≈0.5ηLPIP2f（1）

　　Where: PO is the output power;

　　η is the power efficiency;

　　LP is the primary inductance of the high-frequency transformer;

　　IP is the peak current of LNK500;

　　f is the switching frequency.

　　It is not difficult to see that PO is proportional to LP, and the size of IP2f is controlled by LNK500. [page]

　　The high-frequency transformer uses EE13 core with 8-pin skeleton. The primary winding NP1 is wound with 90 turns of φ0.13mm enameled wire, NP2 is wound with 22 turns of φ0.16mm enameled wire, and the secondary winding is wound with 5 turns of two strands of φ0.25mm triple insulated wire. Three strands of φ0.25mm enameled wire are wound with 5 turns between the primary and secondary windings as a shielding layer. The primary inductance LP=2.3mH (±10% error is allowed). The resonant frequency of the high-frequency transformer is not less than 300kHz.

　　The output characteristics of a 2.5W constant voltage/constant current charger are shown in Figure 2.

Figure 2 Constant voltage/constant current output characteristics of a 2.5W charger

2 15W DC/DC power converter module with Ethernet interface
2.1 Performance characteristics of Ethernet power supply
　　Ethernet (Ethernet Network) is currently the most commonly used local area network. Ethernet power supply is referred to as POE (Power Over Ethernet). It can provide data and power supply to users at the same time through only one Ethernet cable, without the need for additional wiring. The power supply device in the Ethernet power supply is referred to as PD, which has the following characteristics:
　　- It can provide PD detection and classification signals;
　　- It can provide a soft start interface to the DC/DC power converter;
　　- It has overcurrent protection, overvoltage protection, overheating protection and other functions.
　　According to the POE specification, PD should have the following three basic functions.
　　1) Ability to identify signal impedance When an input voltage is applied to the PD, it must present the correct identification signal impedance within the specified voltage range. When an Ethernet device requests power, it first sends a 2.5-10V voltage signal to the Ethernet. After the valid PD detects this voltage signal, it places a 23.75-26.25kΩ resistor on the power supply loop. The current will change with the input voltage. By detecting this current, it is confirmed that there is a valid Ethernet device that needs power at the Ethernet cable terminal. If the placed resistor value is in the range of 12-23.75kΩ or 26.25-45kΩ, the Ethernet device is considered valid but does not need power. Resistance values ​​in other ranges mean that the detected Ethernet device is invalid.
　　2) Type There are different types of PDs, and each type corresponds to a certain current. For example, the current of a "0" type PD is 0.5-4mA. After the PD detects a valid signal, it is classified. The specific method is to increase the voltage sent to the network link to 15.5-20.5V so that the PD obtains a fixed current, and then complete the PD classification according to the current range.
　　3) Switch connection There are two main types of switches for connecting Ethernet power supply. One is a bipolar transistor switch, which has high power efficiency and low cost; the other is a MOSFET switch, which has extremely high power efficiency (close to 100%).
　　The following introduces a synchronous rectification 15WDC/DC power converter module with Ethernet interface circuit, which can be widely used in network and communication equipment.
2.2 Circuit design of 15W Ethernet power module
　　The internal circuit of the 15WPOE module composed of bipolar switch tube and DPA424P is shown in Figure 3. The power supply consists of two parts, namely the Ethernet interface circuit (represented by a dotted box in the circuit) and the DC/DC power converter. The module contains POE identification signal impedance (24.9kΩ, DC 2.5~10V) and "Class 0" type circuit (0.5~4mA, DC 15~20V). When using a bipolar transistor switch or a MOSFET switch, the efficiency of the POE interface is η≥87% or η≥97% respectively.

2.2.1 Working Principle of Ethernet Power Interface Circuit
　　The working process of the Ethernet power interface circuit can be divided into three stages: In the first stage, when the input voltage is applied to the PD, it must present the correct identification signal impedance within the voltage range of DC 2.5 to 10V, and the resistor R13 (24.9kΩ) can provide this impedance; in the second stage, when the DC input voltage is 15 to 20V, the PD uses a specified current to identify the device type, for example, the "Class 0" current range is 0.5mA to 4mA, which is also completed by R13; in the third stage, the input voltage is connected to the DC/DC power converter through the bipolar switch tube (VT), and the power converter allows the input of a DC voltage exceeding 30V (28V + UR14). At this time, the voltage regulator tube VDZ1 is reversely broken down, and the base current is provided to VT through R14. The role of R15 is to prevent the power supply from being turned on under other conditions. Once the power is turned on, the high-frequency voltage signal output by the auxiliary winding passes through the coupling capacitor C3, the rectifier tube VD2 and the current-limiting resistor R16 to increase the DC bias of VT and increase the base current. VD1 is turned on in the negative half cycle to ensure that the bias voltage applied to the base is always positive.
　　As shown in Figure 4, a switching circuit using MOSFET (V3) is shown. VDZ4 and VDZ5 use 28V and 15V voltage regulator tubes respectively. When the input voltage exceeds 28V, VDZ4 is reversely broken down, causing V3 to turn on and turn on the power supply. When the input voltage exceeds 43V, VDZ5 is also reversely broken down, which can limit the gate-source voltage of V3 and play a protective role. R15 can prevent V3 from being mis-turned on. The relationship curve between the identification signal impedance and the input voltage of the Ethernet power module is shown in Figure 5, and the identification voltage range is 2.5~10V. [page]

2.2.2 Working principle of 15W DC/DC power converter
　　The main performance indicators of the DC/DC power converter are as follows:
　　1) It uses DPA424P type monolithic switching regulator to form a forward, isolated, 3-way output DC/DC power converter module. The DC input voltage range is 36~75V, the 3-way output is 5V/2.4A, 7.5V/0.4A and 20V/10mA respectively, the total output power is 15.2W, and the switching frequency is 400kHz;
　　2) Multiple outputs, good voltage regulation performance, in the worst case, the load regulation rate indicators of each output are shown in Table 1,

Table 1 Load regulation index of each output

　　3 路输出电压UO/V  5  7.5  20
　　负载变化范围/％  20～100  0～100  100
　　负载调整率SI/％  ≤±1  －4～＋8  －3～＋6
　　3）采用电容耦合式同步整流技术，DC／DC电源变换器的效率高达88％；
　　4）能精确设定输入线路的欠电压、过电压值；
　　5）具有输出过载保护、开环保护和过热保护功能。
　　图3中，输入端EMI滤波器由C1，L1和C2构成。R1为欠电压值／过电压值设定电阻，所设定的UUV=33.3V，UOV=86.0V。R1还能自动减小最大占空比，防止磁饱和。R2为极限电流设定电阻，取R2=13.3kΩ时，所设定的漏极极限电流ILIMIT′=0.57ILIMIT=0.57×2.50A=1.425A。稳压管VDZ2可将漏极电压箝位在安全范围以内。V1的等效栅极电容能给高频变压器提供最佳复位。
　　该电源以5V输出作为主输出，其他两路输出都是在此基础上获得的。由C11，R11，R12和MOS场效应管V2及V1构成5V主输出的电容耦合式同步整流器。稳压管VDZ3起箝位作用。在没有开关信号时，通过下拉电阻R13使V2关断。储能电感L2回扫绕组的电压经过VD4和C9整流滤波后，获得20V输出。高频变压器次级绕组（8－5）的电压经过VD3和C10整流滤波后获得7.5V输出。将6.8V稳压管VDZ4和二极管VD7反极性串联后作为7.5V输出的负载电阻，以改善空载稳压特性。空载时输出电压一旦超过7.5V，VDZ4就被反向击穿，利用VDZ4和VD2上的压降可将输出电压箝制在大约7.5V上。正常工作时，辅助绕组的输出电压经过VD6、C5整流滤波后给光耦合器PC357提供12～15V的偏压。R5、VD8和C16组成软启动电路，能防止在启动过程中输出过冲。

　　The relationship curve between the power efficiency and input voltage of the power module is shown in FIG6 .

2.2.3 Key points of circuit design
　　1) Use bipolar power switch tube (VT)
　　(1) Select bipolar switch tube VT, which can withstand high voltage and provide sufficient current, and its current amplification factor should be high enough.
　　(2) Select R14 to provide a large enough base current to ensure that the DC/DC power converter can be turned on.
　　(3) Select R16 (typical value is 10~20Ω) to limit the peak current generated during the switching process.
　　(4) It is recommended to use the TIP29C bipolar medium power switch tube produced by Fairchild. Its main parameters are as follows: collector-emitter breakdown voltage UU (BR) CEO = 100V, maximum base allowable current IBM = 0.4A, maximum collector current ICM = 1A, maximum collector power consumption PCM = 30W, hFE = 75 times, fT = 3.0MHz.
　　2) Use power MOSFET (V3)
　　(1) Select R14 to limit the power consumption of voltage regulator tubes VDZ4 and VDZ5.
　　(2) Select R15 to ensure that V3 can be turned off when the input voltage is lower than 28V.
　　(3) Select the voltage regulation value of VDZ4 to prevent V3 from turning on when the input voltage is lower than 28V.
　　(4) Note that the resistance values ​​of R14 and R15 also affect the loss of VDZ4.
　　(5) Select the voltage regulation value of VDZ5 to limit the maximum gate-source voltage of V3 (the typical value should be 15V).
　　(6) It is recommended to use the IRF530N type N-channel power MOSFET produced by Philips. Its main parameters are as follows: drain-source breakdown voltage U(BR)DS=100V, maximum drain power dissipation PDM=79W, drain-source on-state resistance RDS(ON)=80mΩ, transconductance gFS=11S, turn-on time tON=36ns, turn-off time tOFF=12ns.
3 Conclusion
　　There are many types of special integrated switching power supplies, and new products such as LED driver power supplies, terrestrial digital TV broadcasting (DVB-T) power supplies and power adapters can also be designed; and the advent of the LinkSwitch series of high-efficiency constant voltage/constant current three-terminal monolithic switching power supplies, LinkSwitch-TN series and DPA-Switch series monolithic switching power supplies has created favorable conditions for the optimized design of medium and small power special switching power supplies.