Abstract: Non-isolated power supplies are permitted in the control power supplies of some household appliances, as well as in smart energy meters and residential heating controllers. This paper introduces a LinkSwitch-TN series monolithic switching power supply, which can replace traditional RC step-down linear power supplies, creating favorable conditions for the optimized design of high-efficiency, energy-saving, low-power switching power supplies. Keywords: Energy saving; Monolithic switching power supply; Buck circuit; Buck-Boost circuit; Design Introduction Some electronic devices and household appliances do not require completely isolated input and output switching power supplies. For example, the drive power supply of DC motors, and the regulated power supply in air conditioners, frost-free refrigerators, and microwave ovens are themselves isolated systems, and therefore can be powered by non-isolated switching power supplies, but these power supplies require simple circuitry and high power efficiency. In January 2004, PI Corporation launched the LinkSwitch-TN series of four-terminal non-isolated, energy-saving monolithic switching power supply ICs. It is specifically designed to replace low-power linear power supplies used in household appliances and industrial fields, eliminating the need for bulky power transformers and overcoming the poor load characteristics of RC step-down linear power supplies. The LinkSwitch-TN series includes six models: LNK304P/G, LNK305P/G, and LNK306P/G, with a maximum output current of 360mA. These are suitable for control power supplies in household appliances and LED drivers. 1. Performance Characteristics of the LinkSwitch-TN Series Monolithic Switching Power Supplies: 1) The LinkSwitch-TN series can be configured as a non-isolated, energy-saving switching power supply with a minimal number of external components. Compared to traditional passive (capacitor-based voltage reduction) solutions, LinkSwitch-TN utilizes EcoSmart energy-saving technology, achieving higher efficiency and improved power factor compared to capacitor-based linear regulators. 2) Flexible application: It can be designed as a positive output buck circuit, a negative output buck or boost circuit, or a buck LED constant current drive circuit to meet the needs of different users. 3) Wide input voltage range: It has good voltage and load regulation within the AC 85–265V range. Two operating modes are available: Continuous Mode (CCM) and Discontinuous Mode (DCM). Discontinuous Mode is preferred in most cases. 4) Strong anti-interference capability and low power consumption. The LinkSwitch-TN has a switching frequency of 66kHz and a frequency jitter range of 4kHz. Frequency jitter technology reduces electromagnetic interference by 10dB and also reduces the power consumption of the EMI filter. The power MOSFET can turn on quickly without overshoot. When the power supply is unloaded and the input voltage is 230V, the power consumption using the self-powered buck circuit is only 80mW; the power consumption using the external bias circuit is as low as 12mW. 5) Comprehensive protection functions. The chip has an internal protection circuit for automatic restart after a short circuit, an open-loop fault detection and protection circuit, a current limiting protection circuit, and an overheat protection circuit with hysteresis characteristics. The maximum output current of the LinkSwitch-TN at a fixed AC input of 230 (1±15%)V or a wide-range AC input (also known as a universal input) is shown in Table 1. Table 1 Maximum Output Current Values ​​of LinkSwitch-TN Series Products 2. Wiring Methods of LinkSwitch-TN Series Monolithic Switching Power Supplies 2.1 Seven Circuit Wiring Methods for LinkSwitch-TN The seven wiring methods for LinkSwitch-TN are shown in Figures 1(a) to (g). Users can select one circuit according to their needs. 2.1.1 Positive Terminal Buck Direct Feedback Circuit The positive terminal buck direct feedback circuit is shown in Figure 1(a). Its main characteristics are as follows: ——Output depends on input (the same applies below, no further explanation needed); ——Positive voltage output; ——UO ——Simple circuit, low cost; ——UO accuracy is approximately ±10%. 2.1.2 Positive Terminal Buck Optocoupler Feedback Circuit The positive terminal buck optocoupler feedback circuit is shown in Figure 1(b). Its main characteristics are as follows: ——Positive voltage output; ——UO ——Uses optocoupler feedback circuit, the accuracy of UO is determined by external reference voltage, the output terminal does not need to be connected to a load resistor, and the power consumption is the lowest under no-load conditions. 2.1.3 Negative-terminal buck optocoupler feedback circuit The negative-terminal buck optocoupler feedback circuit is shown in Figure 1(c). Its main characteristics are as follows: —Positive voltage output; —UO —Using an optocoupler feedback circuit, the accuracy of UO is determined by an external reference voltage, and no load resistor is required at the output terminal. 2.1.4 Negative-terminal buck LED constant current drive circuit The negative-terminal buck LED constant current drive circuit is shown in Figure 1(d). It is suitable for driving LEDs, and other characteristics are the same as in 2.1.3. 2.1.5 Positive-terminal boost/buck direct feedback circuit The positive-terminal boost/buck direct feedback circuit is shown in Figure 1(e). Its main characteristics are as follows: —Negative voltage output; —Boost or buck output. When using boost output, |UO|>UI; when using buck output, |UO| —Simple circuit, low cost, and the accuracy of UO is approximately ±10%; —Even if the power MOSFET fails, the input voltage will not be applied to the output terminal and damage the load. 2.1.6 Positive Terminal Boost/Buck LED Constant Current Driver Circuit The positive terminal boost/buck LED constant current driver circuit is shown in Figure 1(f). This circuit is suitable for driving LEDs. It has higher accuracy in constant current driving than the circuit shown in Figure 1(d) and is less affected by ambient temperature. Other characteristics are the same as in 2.1.5. 2.1.7 Negative Terminal Boost/Buck Optocoupler Feedback Circuit The negative terminal boost/buck optocoupler feedback circuit is shown in Figure 1(g). Its main characteristics are as follows: — Negative voltage output; — Boost or buck output. When using boost output, |UO|>UI; when using buck output, |UO| — Using an optocoupler feedback circuit, the accuracy of UO is determined by an external reference voltage, and no load resistor is needed at the output terminal; — Even if the power MOSFET fails, the input voltage will not be applied to the output terminal; — Lowest power consumption under no-load conditions. 2.2 Basic Circuit Structure When constructing a non-isolated power supply using the LinkSwitch-TN series, there are two basic circuit structures: the Buck converter and the Buck-Boost converter, as shown in Figure 2(a) and Figure 2(b), respectively. RF is a fusible resistor, VDIN1 and VDIN2 are input stage rectifier diodes. VDFW is an ultra-fast recovery diode. CIN1 and CIN2 are input stage filter capacitors, LIN is an input stage inductor. CBP is a bypass capacitor, and RBAIS is a bias resistor. RFB, CFBB, and VDFB are the feedback resistor, feedback capacitor, and feedback diode, respectively. L is the output stage inductor, and RPL is the load resistor. The main difference between the circuits shown in Figure 2(a) and Figure 2(b) is the wiring position of VDFW and L. In the Buck converter, VDFW is connected in parallel between the source and the negative terminal of the input voltage, and L is connected in series between the source and the positive terminal of the output voltage. The Buck-Boost converter does the opposite. For a given LinkSwitch-TN chip and inductor value, choosing a Buck topology not only maximizes output power but also reduces the voltage across the LinkSwitch-TN chip and decreases the average current through the filter inductor. [align=center] [/align] 3 Typical Applications of LinkSwitch-TN Series Monolithic Switching Power Supplies Figure 3 shows a circuit for a +12V/120mA non-isolated switching power supply constructed from LNK304, with an output power of 1.44W. This circuit is suitable for controlling household appliances such as air conditioners, dishwashers, and rice cookers. It can also be used for nightlights, LED drivers, smart meters, and residential heating controllers, where non-isolated power supplies are permitted. The input circuit consists of a fusible resistor RF, diodes VD1 and VD2, capacitors C4 and C5, and inductor L2. The fusible resistor has the following functions: —It provides current-limiting protection for VD1 and VD2; —It reduces common-mode noise interference; —When other components experience a short circuit, RF is quickly melted, cutting off the input voltage. The advantage of using a fusible resistor instead of a fuse is that it does not produce sparks or smoke when it blows, making it both safe and interference-free. Connecting diodes VD1 and VD2 in series increases the withstand voltage to 2kV and ensures that noise current only flows when the diodes are conducting. The power supply regulation circuit consists of an LNK304, a UF4005 ultra-fast recovery diode VD3, an output energy storage inductor L1, and a filter capacitor C2. The peak current of inductor L1 is limited by the limiting current of the LNK304P, and its control scheme is similar to the on/off controller in TinySwitch. Since VD4 (glass-passivated 1N4005GP) and VD3 have the same forward voltage drop, the voltage across C3 can follow changes in the output voltage. The voltage across capacitor C3 is divided by resistors R1 and R3 and then sent to pin FB of the LNK304. To achieve the desired output voltage, UFB should be equal to 0.65V. The LNK304 regulates the output voltage by skipping cycles. When the output voltage increases, the current IFB flowing into pin FB also increases. If the current IFB > 49μA, subsequent cycles will be skipped until IFB < 49μA. Therefore, many cycles will be skipped when the load is light, and fewer cycles will be skipped when the load is heavy. If an output overload or short-circuit fault occurs, the LinkSwitch-TN switch enters the automatic restart phase, reducing the output power to POM×6%, thus limiting the average output power. R2 is the load resistor, which controls the error between the output voltage under light load or no load and the rated output voltage within ±10%. With R2 = 2.4kΩ, the preset load current is 5mA. The measured load adjustment curve of this switching power supply is shown in Figure 4. 4. Circuit Design Considerations The following section uses Figure 3 as an example to introduce the key points of LinkSwitch-TN circuit design. 4.1 Freewheeling Diode VD3 When using discontinuous mode, VD3 should be an ultra-fast recovery diode with a trr ≤ 75ns as the freewheeling diode; when using continuous mode, trr ≤ 35ns is required. UF4005 is an ultra-fast recovery diode with a trr = 30ns, which meets the requirements of both operating modes. Do not use fast recovery diodes, because their reverse recovery time is several hundred ns. During startup, this will cause LinkSwitch-TN to always be in continuous operating mode, resulting in a high-rising-edge current spike that forces the switching cycle to end prematurely, preventing the output from reaching a stable state. 4.2 Feedback Diode VD4 The feedback diode VD4 can be a low-cost rectifier diode, such as the 1N4005 type, but a glass-sealed diode is preferred due to its shorter reverse recovery time. Furthermore, the forward voltage drops of VD4 and VD3 should be equal. 4.3 Inductor L1 It is recommended that L1 be an inductor with a ferrite core to reduce cost and audio noise. The inductance of L1 should be greater than or equal to the design value, and a certain margin should be allowed for the effective current it can withstand. 4.4 Output Stage Filter Capacitor C2 The main function of C2 is smoothing and filtering. Since the output ripple voltage is a function of the equivalent series resistance (ESR) of C2, a capacitor with low ESR should be selected. 4.5 Feedback Resistor (R1) and Bias Resistor (R3) The resistor divider formed by R1 and R3 should maintain the voltage at pin FB at 1.65V. R3 can be a resistor with a nominal resistance of 2kΩ/±1%. 4.6 Feedback Capacitor C3 C3 can be a common electrolytic capacitor with a "sample and hold" function. During the LinkSwitch-TN off time, the voltage on C3 is charged to the output voltage value. The capacitance range of C3 is 10–22 μF. If the capacitance is too small, the voltage regulation performance under low load conditions will be reduced. 4.7 Load Resistor R2 When the minimum load current is less than 3mA, the direct feedback circuit requires a load resistor to maintain output voltage stability. Choosing R2=4kΩ allows IOmin=3mA. Furthermore, in the optocoupler feedback circuit, a current-limiting resistor (RZ) needs to be connected to the external Zener diode to limit its operating current to 1–2mA, thereby reducing the ripple voltage at no-load, as shown in Figure 1(c). 5 Conclusion The LinkSwitch-TN series monolithic switching power supplies have advantages such as advanced performance, flexible use, simple circuitry, and low cost, and have good application prospects. LinkSwitch-TN can also be used to design multi-output switching power supplies, characterized by the total output voltage and total output current being equal to the sum of the individual outputs (absolute values ​​should be taken for negative voltages).