Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Traditional high-voltage isolated flyback converters use optocouplers to transmit voltage regulation information from the secondary-side reference power supply circuit to the primary side, thereby achieving accurate voltage regulation. The problem is that optocouplers will greatly increase the complexity of isolation design: there are propagation delays, aging, and gain changes, all of which will complicate power loop compensation and reduce reliability. In addition, during the startup process, a bleeder resistor or a high-voltage startup circuit is needed to initially start the IC. Unless an additional high-voltage MOSFET is added to the startup component, the bleeder resistor will consume a lot of power.

Author: ADI Yuchen Yang, Senior Application Engineer, William Xiong Application Engineer

Introduction

Traditional high-voltage isolated flyback converters use optocouplers to transmit voltage regulation information from the secondary-side reference power supply circuit to the primary side, thereby achieving accurate voltage regulation. The problem is that optocouplers will greatly increase the complexity of isolation design: there are propagation delays, aging, and gain changes, all of which will complicate power loop compensation and reduce reliability. In addition, during the startup process, a bleeder resistor or a high-voltage startup circuit is needed to initially start the IC. Unless an additional high-voltage MOSFET is added to the startup component, the bleeder resistor will consume a lot of power.

The LT8316 is a micro-power, high-voltage flyback controller that does not require optocouplers, complicated secondary-side reference power circuits or additional startup components.

Extended power supply voltage

The LT8316 is packaged in a thermally enhanced 20-pin TSSOP, with 4 pins removed to show the high voltage interval. By sampling the isolated output voltage of the third winding, there is no need to use an optocoupler for voltage stabilization. The output voltage is programmed through two external resistors and a third optional temperature compensation resistor. Quasi-resonant boundary mode operation helps achieve excellent load regulation, small transformer size, and low switching losses, especially at high input voltages. Since the output voltage is detected when the secondary side current is almost zero, there is no need to use external load compensation resistors and capacitors. Therefore, the LT8316 solution uses a small number of components, which greatly simplifies the design of the isolated flyback converter (see Figure 1).

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 1. A complete 12 V isolated flyback converter, suitable for a wide range of output from 20 V to 800 V, with a minimum starting voltage of 260 V.

The rated working voltage of the LT8316 is 600 V maximum, but it can be expanded by replacing the Zener diode in series with the VIN pin. The voltage of the Zener diode will reduce the voltage supplied to the chip, making the power supply voltage more than 600 V.

Figure 1 shows the entire schematic diagram of a flyback converter with an input voltage of 18 V to 800 V. For a detailed component selection guide, please refer to the LT8316 data sheet. When a 220 V Zener diode is connected in series with the VIN pin, the minimum starting voltage is 260 V. In view of the voltage tolerance of the Zener diode, this value may have a slight difference. Note that after startup, the LT8316 generally works with a supply voltage lower than 260 V.

Figure 2 shows the efficiency under different input voltages, and the peak efficiency of the flyback converter reaches 91%. Even without an optocoupler, the load adjustment under different input voltages remains accurate, as shown in Figure 3.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 2. The efficiency of the flyback converter in Figure 1.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 3. Load and voltage regulation of the flyback converter in Figure 1.

Low starting voltage design

Although the previous solution extended the input voltage to 800 V, the Zener diode increased the minimum starting voltage to 260 V. The challenge is that some applications require both high input voltage and low starting voltage.

Figure 4 shows an alternative 800 V maximum input voltage solution. This circuit uses a Zener diode and a diode to form a voltage regulator. The input voltage can be steadily increased to 800 V, while the voltage of the VIN pin is kept stable at around 560 V. The advantage of this circuit is that it allows the LT8316 to start with a lower supply voltage.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 4. Schematic diagram of isolated flyback converter: 20 V to 800 V input is converted to 12 V, with low starting voltage.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 5. Schematic diagram of a non-isolated buck converter with a supply voltage of up to 800 V.

Non-isolated buck converter

The high-voltage input function of the LT8316 can be easily implemented in a simple non-isolated step-down converter without the need for an isolation transformer. Use relatively inexpensive off-the-shelf inductors as electromagnetic components.

For non-isolated buck applications, the ground pin of the LT8316 is connected to the switch node of the buck topology, and its voltage is variable. The LT8316 uses a unique detection method to only detect the output voltage when the switch node is grounded, so the step-down schematic is quite simple.

Like the flyback converter, the power supply voltage of the buck converter can also be expanded. Figure 5 shows the schematic diagram of a buck converter with an input voltage of up to 800 V. There is a 220 V Zener diode between the LT8316’s power supply voltage and the VIN pin. In view of the voltage tolerance of the Zener diode, the minimum starting voltage is 260 V. After starting, the LT8316 continues to operate normally with a lower supply voltage. Figure 6 shows the efficiency under different input voltages, and the peak efficiency of the buck converter reaches 91%. Figure 7 shows the load and voltage regulation.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 6. The efficiency of the buck converter in Figure 5.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 7. The load and voltage regulation of the buck converter in Figure 5.

Extend the power supply voltage of 600 V input, non-optocoupler isolated flyback controller to 800 V or higher

Figure 8. Schematic diagram of an 800 VIN non-isolated buck converter with low start-up voltage.

Similar to the flyback converter in Figure 4, a voltage regulator can be added between the power supply voltage and the VIN pin to enable the buck converter to achieve a low startup voltage. It should be noted that there is a body diode between the GND pin and the VIN pin, which will increase the emitter voltage of the transistor and cause basic emitter breakdown. To prevent this, we add two diodes to protect the transistor. Figure 8 shows the low starting voltage solution.

in conclusion

The LT8316 works in quasi-resonant boundary mode, which can achieve excellent voltage regulation without the use of optocouplers. In addition, it has a wealth of features, including low-ripple Burst Mode® operation, soft start, programmable current limit, undervoltage lockout, temperature compensation and low quiescent current. High integration simplifies the design of high-performance solutions with a small number of components, covering a wide range of applications, from battery-powered systems to automotive, industrial, medical, telecommunications power supplies, and isolated auxiliary/home power supplies.

About the Author

Yuchen Yang is a senior application engineer at Analog Devices. He is responsible for various non-isolated and isolated converter power product applications. He holds a bachelor’s degree in Electronic engineering from Tsinghua University, and a master’s and doctorate degree in electronic engineering from Virginia Tech. He joined ADI in 2018. Contact information:[email protected]

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