A simple, low component count and high efficiency solution for isolated power supplies
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Non-synchronous flyback topologies are commonly used in isolated power supplies, ranging from under 1W up to tens of watts. Linear Technology's opto-coupler-free isolated flyback converter family simplifies the design of isolated power supplies by using a proprietary master-side sensing technique that eliminates the need for optocouplers or third windings on the transformer for output regulation. The new LT8300 is the first low-power device in this family, offering improved efficiency at light loads and reducing no-load input standby current to around 200μA.
The LT8300 operates across an input voltage range of 6V to 100V, delivering up to 2W of isolated output power. Its 150V integrated DMOS power switch removes the need for external snubbers in most applications. By directly sampling the isolated output voltage from the main flyback waveform, the LT8300 avoids the use of optocouplers or third windings for regulation. Output voltage is set using a single external resistor.
Internal loop compensation and soft start features further reduce the number of required external components. Operating in boundary mode under heavy load allows for the use of smaller magnetic components while maintaining excellent load regulation. Low ripple Burst Mode® operation ensures high efficiency under light loads and minimizes output voltage ripple. All these features are integrated into a compact 5-pin TSOT-23 package, with a high-voltage pin pitch that meets IPC-2221 standards.
As shown in Figure 1, the LT8300 is available in a 5-lead TSOT-23 package, which provides sufficient high-voltage pin spacing between pins 4 and 5.
Performance and Simplicity
Figure 2 illustrates a complete isolated flyback solution with a footprint smaller than 1 x ½ inches. Figure 3 shows a typical application using the LT8300, converting a 36V to 72V input into a 5V isolated output. The design requires only five external components (input capacitor, output capacitor, transformer, feedback resistor, and output diode), along with two optional undervoltage lockout resistors.
Figure 2: The LT8300 Isolated Flyback Converter Solution Sizes Less Than 1" x 1/2" in the Standard Demo Board DC1825A
Figure 3: 5V/300mA Micropower Isolated Flyback Converter with a 36V to 72V Input
Despite its simplified design, the LT8300 delivers exceptional performance. Figure 4 shows the power efficiency (up to 85%) of the 5V application circuit in Figure 3. Figure 5 demonstrates the load and voltage regulation performance (±0.5%) of the same circuit. Figures 6 and 7 show the load step transient response and the startup waveform under a 1mA resistive load, respectively.
Figure 4: Power efficiency of the 5V application circuit shown in Figure 3.
Figure 5: Output load and voltage regulation performance of the 5V application circuit shown in Figure 3.
Figure 6: 50mA to 250mA load step transient waveform for the 5V application circuit shown in Figure 3.
Figure 7: 1mA resistive load start-up waveform for the 5V application circuit shown in Figure 3.
Post Regulator Eliminates Output Temperature Deviation
In a typical LT8300 application, the output voltage can be expressed as:
The first term of the VOUT formula is temperature-independent, but the forward voltage of the output diode (VF) has a significant negative temperature coefficient (-1mV/°C to -2mV/°C). This can lead to an output voltage change of approximately 200mV to 300mV over the full operating temperature range.
For higher voltage outputs like 12V or 24V, the effect of the diode’s temperature coefficient is negligible. However, for lower voltages such as 3.3V or 5V, the impact on output regulation increases by 2% to 5%. To achieve tight output voltage regulation over temperature, a micropower low dropout linear regulator can be added to post-regulate the LT8300 output. The LT8300 should be set slightly above the “regulation voltage + LDO drop voltage.â€
Figure 8 shows the combination of the LT8300 with an LT3009-3.3 post regulator to produce a 3.3V/20mA isolated output from an 18V to 32V input. As shown in Figure 9, the no-load input standby current is less than 250μA, meeting the requirements of the DEF-STAN61-5 standard.