Linear adapter circuit design principles: stable and efficient power supply solutions

　　As a key device for converting AC power into stable DC power, linear adapters are widely used in small electronic devices, instrumentation and other fields. Its design must take into account output stability, energy efficiency performance and safety and reliability, and follow a series of core principles to ensure that the circuit continues to operate efficiently under various working conditions.

　　I. Principle of priority of core performance indicators

　　(I) Output voltage accuracy control

　　The core function of the linear adapter is to provide stable DC output, and the output voltage accuracy must be controlled within ±2% (precision equipment scenarios require ±1%). In the design, a high-precision reference voltage source (such as TL431, with an accuracy of up to 0.5%) and a high-gain operational amplifier are selected to form a feedback loop to monitor the output voltage changes in real time and adjust the conduction state of the adjustment tube. For example, when the input voltage fluctuates or the load changes and the output voltage increases, the feedback circuit will quickly reduce the conduction degree of the adjustment tube and reduce the output voltage to the set value; otherwise, the conduction degree will be increased to ensure voltage stability. In circuit layout, the feedback sampling point needs to be close to the load end to reduce the impact of line voltage drop on accuracy. Especially in high current output scenarios, thickened wires or copper-clad designs are required to reduce the error caused by wire resistance.

　　(II) Low ripple and noise suppression

　　The output ripple voltage of the linear adapter needs to be controlled below 10mV peak-to-peak (5mV or less is required in sensitive scenarios such as audio equipment). The ripple mainly comes from the rectification residue of the input AC power and the switching noise of the adjustment tube. In the design, it can be suppressed by a multi-stage filtering circuit: a large-capacity electrolytic capacitor (usually 100-470μF) is connected in parallel to the high-voltage DC end after rectification to absorb low-frequency ripple, and an RC filter network (resistance 10-100Ω, capacitance 1-10μF ceramic capacitor) is connected in series at the output end to filter out high-frequency noise. In addition, the adjustment tube uses a device with a low noise coefficient (such as a PNP transistor with lower noise than an NPN transistor), and a ground shield layer is set around the reference voltage source to further reduce noise coupling.

　　(III) Optimization of load regulation and linear regulation

　　The load regulation reflects the output stability when the load changes, and should be controlled within ±1% (full load 10% - 100% variation range); the linear regulation reflects the stability when the input voltage changes, and needs to be controlled within ±0.5% (input voltage ±10% fluctuation range). The design is optimized by improving the bandwidth and gain of the feedback loop, such as using a common emitter-common base combination adjustment tube structure to improve the open-loop gain of the circuit; low temperature drift resistors (such as metal film resistors, temperature drift ±25ppm/℃) are selected as sampling resistors to reduce the impact of temperature changes on feedback accuracy. In load mutation scenarios (such as sudden startup of the device), a small-capacity tantalum capacitor (10 - 100μF) can be connected in parallel at the output end to suppress voltage spikes using its fast charging and discharging characteristics.

　　II. Circuit topology and device selection principles

　　(I) Adjustment tube working state design

　　The core of the linear adapter is that the adjustment tube works in the linear region (amplification region) and achieves voltage regulation by continuously adjusting the conduction degree. In the design, the rated power consumption of the adjustment tube should be selected according to the maximum output current (P = (Vin_max - Vout) × Iout_max), and 2-3 times of margin should be reserved to cope with instantaneous overload. For example, in a circuit with an output current of 1A, an input of 24V, and an output of 5V, the power consumption of the adjustment tube can reach 19W. It is necessary to select a high-power transistor in TO-220 package (such as 2N3055, with a rated power consumption of 115W) and a heat sink (thermal resistance ≤5℃/W) to ensure that the junction temperature does not exceed 150℃. For low voltage drop scenarios (such as input 5V, output 3.3V), a low voltage drop linear regulator (LDO, such as LM1117, voltage drop 0.5V@1A) can be selected to reduce power consumption and improve efficiency.

　　(II) Matching of rectifier and filter circuits

　　The front-end rectifier circuit needs to select a suitable rectifier bridge according to the input voltage and output power. The rated current should be greater than 1.5 times the maximum output current, and the reverse withstand voltage should be greater than 1.2 times the input voltage peak (a rectifier bridge with a withstand voltage of ≥400V should be selected for 220V AC input). The capacity calculation of the filter capacitor must meet C ≥ Iout × Δt / ΔV, where Δt is the half-cycle time (50Hz AC corresponds to 10ms), and ΔV is the allowable ripple voltage. For example, when the output current is 1A and the allowable ripple is 100mV, the filter capacitor needs to be ≥1000μF (1A×10ms / 0.1V = 1000μF). In actual selection, the capacity can be appropriately increased (such as 2200μF) to improve stability.

　　(III) Co-design of protection circuits

　　In order to prevent the circuit from being damaged by overcurrent, overtemperature, short circuit and other faults, multiple protection mechanisms need to be integrated. Overcurrent protection is achieved by connecting a sampling resistor in series at the output end. When the current exceeds the threshold, the comparator is triggered to control the adjustment tube to cut off. Overtemperature protection uses the negative temperature coefficient characteristics of the transistor (such as placing a thermistor near the adjustment tube) to reduce the output current or shut down the circuit when the temperature is too high. Short-circuit protection needs to be designed as a "hiccup mode", that is, the output is quickly shut down when a short circuit occurs, and an attempt to restart is made at intervals of hundreds of milliseconds to avoid long-term overcurrent damage to the adjustment tube. The response time of the protection circuit must be controlled within 100μs to ensure that the dangerous energy is cut off at the moment of the fault.

　　III. Reliability and environmental adaptation principles

　　(I) Wide input voltage compatibility

　　Linear adapters need to adapt to grid fluctuations in different regions (usually designed for 85-265V AC input). The front end needs to absorb grid spikes through a varistor (such as 14D471K, with a withstand voltage of 470V) to avoid high voltage damage to the back-end circuit. For fixed input scenarios (such as 12V DC input), a diode must be connected in series at the input end to prevent reverse connection, and a TVS tube (such as SMBJ15A) must be connected in parallel to resist transient overvoltage.

　　(II) Temperature and electromagnetic compatibility optimization

　　The circuit operating temperature range must cover -20℃ - 70℃ (industrial grade requirements -40℃ - 85℃), and wide temperature devices (such as military-grade operational amplifier LMV7219, operating temperature -55℃ - 125℃) must be selected. The layout must be optimized through thermal simulation software (such as ANSYS Icepak) to avoid placing high-power devices in close proximity to sensitive circuits (such as reference sources). In electromagnetic compatibility (EMC) design, the input end needs to be connected in series with a common-mode inductor (inductance 10 - 100mH) and an X capacitor (0.1 - 0.47μF) to suppress differential and common-mode interference, and the output end needs to be connected in parallel with a Y capacitor (1000pF - 10nF) to reduce ground noise to ensure that it passes CE, FCC and other certification standards.

　　(III) Derating design and redundancy considerations

　　Key components need to be used at a reduced rating: the operating voltage of the capacitor does not exceed 70% of the rated voltage, the resistor power does not exceed 50% of the rated power, and the junction temperature of the semiconductor device does not exceed 80% of the rated junction temperature. For example, a capacitor with a rated voltage of 16V is used in a 10V circuit, and a 1/4W resistor is used in a 1/8W power consumption scenario to improve long-term working reliability. At the same time, a certain amount of output power redundancy is reserved (usually 10% - 20%) to avoid performance degradation during full-load operation and extend service life.

　　IV. Application scenario adaptation principles

　　(I) Consumer electronics scenario

　　Linear adapters designed for mobile phone chargers and small household appliances should give priority to cost and volume, and use integrated regulators (such as LM317) in SOP packages to simplify the circuit structure; the output current is usually 0.5-2A, and the ripple is controlled below 20mV to meet the audio noise-free requirements. For example, the adapter of a portable Bluetooth speaker uses 5V/1A output, reduces costs by streamlining the protection circuit (only retaining short-circuit protection), and the shell uses flame-retardant ABS material to meet safety requirements.

　　(II) Industrial instrumentation scenario

　　Industrial linear adapters need to emphasize stability and anti-interference capabilities, with an output voltage accuracy of ±0.5%, support 1-5A high current output, and be equipped with complete over-current and over-temperature protection. For example, the power adapter of the PLC controller uses a 12V/3A output, a surge suppression circuit is added to the input end (withstanding 1.2/50μs 4kV surge), and a low ESR capacitor (such as a solid capacitor) is used at the output end to reduce ripple to ensure accurate instrument data collection.

　　(III) Medical equipment scenario

　　Medical-grade adapters must comply with the IEC 60601 standard, the leakage current is controlled below 10μA, and an isolated linear regulator (such as a circuit with optocoupler feedback) is selected to achieve high and low voltage isolation to avoid the risk of electric shock to patients. The output ripple must be less than 5mV to ensure that devices such as ECG monitors are not affected by power supply noise. For example, the adapter of a portable infusion pump uses a 9V/0.5A output, and the housing is drip-proof (IP22 protection level) to meet the cleaning requirements of the medical environment.

　　Linear adapter circuit design is a system engineering that balances performance, cost and reliability. Following the above principles can ensure that the circuit has the ability to cope with complex environments while meeting basic functions. From device selection to layout and routing, the optimization of each link directly affects the quality of the final product, enabling it to continue to play a core role in stable power supply in consumer, industrial, medical and other fields.

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