Selection and Technical Guide for Isolated Power Supply ICs

^{September 4, 2025 News — With the acceleration of Industry 4.0 and automotive intelligence, the demand for high-performance isolated power solutions continues to grow. Texas Instruments' SN6505BDBVR low-noise transformer driver is becoming an industry focus due to its exceptional isolated power performance. The chip delivers up to 1A of output drive capability, supports a wide input voltage range of 2.25V to 5.5V, and enables multiple isolated output voltages through external transformers, making it perfectly suited for various demanding industrial application environments.}

^{The SN6505BDBVR is a low-noise, low-EMI push-pull transformer driver designed for compact isolated power supplies. It drives thin, center-tapped transformers using a 2.25V to 5V DC power source. Its ultra-low noise and EMI characteristics are achieved through controlled slew rate of the output switching voltage and spread spectrum clocking (SSC) technology. Housed in a small 6-pin SOT23 (DBV) package, it is suitable for space-constrained applications. With an operating temperature range of -55°C to 125°C, it adapts to harsh environments. The device also features soft-start functionality to effectively reduce inrush current and prevent high surge currents during power-up with large load capacitors.}

^{1.The SN6505BDBVR demonstrates excellent load regulation under 5V input conditions, maintaining stable output voltage across a wide load range from 25mA to 925mA, ensuring reliable operation of the isolated power supply.}

^{2.The device achieves peak efficiency exceeding 80% within the 300-600mA load range. This high-efficiency conversion significantly reduces system power consumption and thermal management requirements, providing advantages for compact end-product designs.}

^{1.Power Supply and Enable: Supports a wide input voltage range of 2.25V to 5.5V. Start/stop control via the EN pin, with shutdown current below 1µA.}

^{2.Oscillation and Modulation: Built-in 420kHz oscillator with integrated spread spectrum clocking (SSC) technology, effectively reducing electromagnetic interference (EMI).}

^{3.Power Output: Utilizes two 1A N-MOSFETs in a push-pull configuration to directly drive the primary winding of the transformer.}

^{4.Comprehensive Protection: Provides 1.7A overcurrent protection, undervoltage lockout, and 150°C thermal shutdown to ensure system safety.}

^{5.Soft-Start Control: Built-in soft-start and slew rate control circuits to suppress inrush current and optimize EMI performance.}

^{Core Workflow}

• ^{Input voltage is supplied via VCC, and the chip activates after the EN pin is set high.}
• ^{The oscillator (OSC) generates a high-frequency clock, which is transmitted to the drive logic after spread spectrum modulation (SSC).}
• ^{The drive circuit controls the alternating conduction of two MOSFETs (push-pull operation), generating an AC signal on the transformer primary.}
• ^{The transformer secondary outputs an isolated voltage, which is rectified and filtered to power the load.}
• ^{The protection circuit continuously monitors current and temperature, immediately shutting down the output in case of abnormalities.}

^{​Application Scenarios}

^{Core Circuit Architecture}

^{The typical application circuit of the SN6505BDBVR is shown in the figure. It adopts a push-pull topology to achieve DC-AC conversion, delivering isolated power output through a transformer. The design primarily consists of the following components:}

^{1.Input Power: Supports 3.3V/5V DC input (range 2.25V-5.5V), filtered with a 10μF electrolytic capacitor in parallel with a 0.1μF ceramic capacitor.}

^{2.Drive Core: Drives the transformer primary through D1 and D2 pins, providing 1A output capability with a switching frequency of 420kHz.}

^{3.Rectification and Filtering: Utilizes an MBR0520L Schottky diode for rectification, combined with an LC network for efficient filtering.}

^{4.Regulated Output: Optionally integrates a TPS76350 LDO for precise voltage regulation, achieving ±3% output accuracy.}

^{Key Circuit Module Analysis}

^{1.Input Power Filtering:}

^{The VCC pin requires a 10μF electrolytic capacitor (low-frequency filtering) and a 100nF ceramic capacitor (high-frequency filtering), placed as close as possible to the chip pins.}

^{2.Transformer Drive:}

^{OUT1 and OUT2 conduct alternately with a 180-degree phase difference to drive the primary winding of the transformer.}

^{Switching frequency: 420kHz for SN6505B, 350kHz for SN6505A.}

^{3.Rectification Circuit:}

^{Utilizes a full-wave rectification topology with two Schottky diodes (MBR0520L).}

^{Diode selection requirements: Fast recovery characteristics and low forward voltage drop.}

^{4.Output Filtering:}

^{LC filtering network, with capacitors recommended to be low-ESR type.}

^{Output ripple: Typically <50mV.}

^{Design Guidelines and Component Selection}
 

^{Transformer Specifications:}

^{Type: Center-tapped transformer}

^{Turns Ratio: Calculated based on input/output requirements (e.g., 1:1.2 for 5V to 6V conversion)}

^{Saturation Current: >1.5A}

^{Recommended Models: Würth 750315240 or Coilcraft CT05 series}

^{Application Design Considerations}

^{1.Layout Recommendations:}

^{Place input capacitors as close as possible to VCC and GND pins.}

^{Keep traces from the transformer to OUT1/OUT2 short and wide.}

^{Maintain ground plane integrity.}

^{2.Thermal Management:}

^{Ensure ambient temperature remains below 85°C during continuous full-load operation.}

^{Add copper foil for heat dissipation if necessary.}

^{3.EMI Optimization:}

^{Utilize the chip's built-in spread spectrum clock (SSC) feature.}

^{Appropriately add RC snubber circuits.}

^{Left: Module Block Diagram}

^{The diagram illustrates the core functional modules and signal flow within the SN6505 chip. The functions of each section are as follows:}

^{1.OSC (Oscillator): Generates the original oscillation signal (frequency foscfosc​), serving as the "clock source" for the entire circuit.}

^{2.Frequency Divider: Divides the oscillator output signal to generate two complementary signals (labeled S‾S and SS), providing the fundamental timing for subsequent control logic.}

^{3.Output Transistors (Q1Q1​, Q2Q2​): Controlled by G1G1​ and G2G2​ to achieve "alternating conduction/cutoff," ultimately outputting signals from D1D1​ and D2D2​.
4.Power and Ground (VCCVCC​, GND): Provide operating power and reference ground for the chip.}

^{Right: Output Timing Diagram}

^{The right-side chart uses time as the horizontal axis to show the conduction/cutoff states of Q1Q1​ and Q2Q2​ over time. The key point is to understand the manifestation of "Break-Before-Make":}

^{1.In the timing diagram, the blue and red waveforms correspond to the control signals (or conduction states) of Q1Q1​ and Q2Q2​, respectively.}

^{2.Observation along the time axis reveals that Q2Q2​ only turns on ("Q2Q2​ on") after Q1Q1​ is completely off ("Q1Q1​ off"); similarly, Q1Q1​ only turns on after Q2Q2​ is completely off.}

^{3.This timing sequence of "break one before making the other" is a direct manifestation of the "Break-Before-Make" principle, effectively preventing faults caused by simultaneous conduction of both transistors.}

^{SN6505BDBVR sets a new benchmark for industrial isolated power supply design with its high switching frequency of 420kHz, over 80% conversion efficiency, and excellent EMI performance. Its compact SOT-23 package and highly integrated features significantly simplify peripheral circuit design while substantially improving system reliability and power density. The demand for efficient and miniaturized isolated power supplies will continue to grow.}

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