In industrial/communication applications, such as telecom 48V systems, PoE switches, industrial stepper servo systems, BMS, GPS trackers, and other terminal applications, power supply systems are usually required to provide approximately 5W of isolated power output. This isolated power mainly supplies power for isolated communication interfaces, isolated drivers, and other loads.

As shown in Figure 1, the traditional method typically uses low-power isolated power modules to take power from the system's 5V/12V and provide isolated output.

Figure 1 Common Isolated Power Architecture

ISO-BUCK, due to its simple design, lower cost, and isolated output features, has been increasingly used in these application scenarios. ISO-BUCK evolved from synchronous buck converters, replacing the inductors of the primary buck converter with coupled inductors or transformers, forming a topology similar to a flyback, achieving primary/secondary isolation, thereby helping users save on isolated power costs.

Figure 2 ISO-BUCK Isolated Power System Architecture

ISO-BUCK Application Introduction

Figure 3 SCT2A23 Single-Channel Isolated Output Application Schematic

As shown in Figure 3, SCT uses a synchronous buck converter to implement the ISO-BUCK application.

The circuit uses SCT2A23's own feedback sampling control, with magnetic coupling to achieve voltage regulation for the isolated output, eliminating the need for optocouplers or additional transformer windings for feedback control, making the design simple.

The primary feedback power supply can meet the power requirements of the primary side system, providing high-precision voltage output. The secondary side can provide isolated power to the system, supplying power to communication chips and isolated driver chips.

Different transformer winding selections can be used in different applications to achieve single or multiple isolated outputs, positive or negative voltage outputs, as shown in Figure 4.

Figure 4 SCT2A23 Multiple Isolated Output Application Schematic

ISO-BUCK Considerations

Since the ISO-BUCK topology does not have an isolated optocoupler feedback loop, attention must be paid to whether the secondary side output voltage accuracy meets the actual application requirements.

As shown in Figures 5/6, for single-channel isolated output, when the primary/secondary side has a load greater than 100mA, the secondary side output can be relatively stable.

Figure 5 5V->5V Output, Secondary Side Voltage Accuracy with Different Load Currents on Primary/Secondary Side

However, in the following two cases:

1. When the primary side current is large (>0.5A), and the secondary side current is small (<0.1A);
2. When the primary side power is small (0A~0.1A) or the input voltage fluctuates significantly.

The secondary side output voltage accuracy is relatively poor. In such applications, if higher accuracy of the secondary side voltage output is required, it is recommended to adjust the transformer turns ratio of the primary/secondary side to raise the secondary side output voltage and add a secondary voltage regulation circuit (such as an LDO or Zener diode) to achieve high-precision voltage regulation.

Figure 6 8V->12V Output, Primary Side No-Load, Secondary Side Voltage Accuracy with Different Load Currents on Secondary Side

Application Scenarios:

• Isolated RS-232 and RS-485 communication channels, line drivers, isolated amplifiers, sensors, and CAN transceivers. In such communication and data acquisition applications, isolated power provides good noise immunity;
• In servo applications, it can provide isolated gate drive power for IGBTs;
• In medical applications, providing isolation to ensure safety;
• In battery-powered applications like BMS, it can extend battery standby time;
• Small power isolated supply under 10W offers a good competitive advantage.

Product Description

• Input voltage range: 4.5V-100V
• Continuous output current: 1.2A
• Supports peak current of 2A
• Feedback voltage: 1.2V ±1%
• Integrated 600mΩ upper MOSFET and 300mΩ lower MOSFET
• Static current with VCC diode: 15µA
• Static current without VCC diode: 150µA
• Selectable PFM/USM/FPWM light-load operating modes
• Built-in soft-start time: 3.6ms
• Fixed switching frequency: 300KHz
• COT control mode
• FPWM mode supports Iso-buck topology
• Precise enable voltage, programmable UVLO threshold, and hysteresis
• Cycle-by-cycle current limiting
• Overvoltage protection
• Over-temperature protection
• ESOP-8 package

The SCT2A23 is now in mass production. If needed, sample testing can be provided. To request samples or place orders, please contact sales or email: sales@silicontent.com.