The power supply voltage is extended from 600V to 800V. How to do it in 5 minutes

The LT8316 is available in a thermally enhanced 20-lead TSSOP package with four pins removed to expose high voltage spacing. By sampling the isolated output voltage from a third winding, no optocoupler is required for regulation. The output voltage is programmed with two external resistors and a third optional temperature compensation resistor. Quasi-resonant boundary mode operation contributes to excellent load regulation, small transformer size, and low switching losses, especially at high input voltages.

Since the output voltage is sensed when the secondary-side current is almost zero, no external load compensation resistors and capacitors are required. Therefore, the LT8316 solution uses a low component count and greatly simplifies the design of an isolated flyback converter (see Figure 1).

Figure 1. A complete 12 V isolated flyback converter for a wide range of 20 V to 800 V outputs with a minimum startup voltage of 260 V.

The LT8316 is rated for operation up to 600 V, but this can be extended by replacing the Zener diode in series with the V _IN pin. The Zener diode’s voltage drops the voltage supplied to the chip, allowing the supply voltage to exceed 600 V.

Figure 1 shows the entire schematic of the flyback converter for an input voltage range of 18 V to 800 V. For detailed component selection guidelines, refer to the LT8316 data sheet. When a 220 V Zener diode is placed in series with the V _IN pin, the minimum startup voltage of the circuit is approximately 260 V due to the voltage tolerance of the Zener diode. Note that after startup, the LT8316 can operate normally with voltages below 260 V.

Figure 2. Efficiency of the flyback converter in Figure 1.

Figure 2 shows the efficiency at different input voltages, with the flyback converter achieving a peak efficiency of 91%. Even without an optocoupler, load regulation remains accurate at different input voltages, as shown in Figure 3.

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

While previous solutions extended the input voltage to 800 V, the Zener diode increases the minimum startup voltage to 260 V. The challenge is that some applications require both a high input voltage and a low startup 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 increased steadily to 800 V, while the voltage at the V _{IN pin} remains stable at around 560 V. The advantage of this circuit is that it allows the LT8316 to start up with a lower supply voltage.

Figure 4. Schematic of an isolated flyback converter: 20 V to 800 V input to 12 V with low start-up voltage.

The high voltage input capability of the LT8316 can be easily implemented in a simple nonisolated buck converter without the need for an isolation transformer. Relatively inexpensive, off-the-shelf inductors are used as the 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 can be varied. The LT8316 uses a unique detection method that only detects the output voltage when the switch node is grounded, so the buck schematic is quite simple.

Like the flyback converter, the supply voltage of the buck converter can also be extended. Figure 5 shows the schematic of the buck converter with an input voltage up to 800 V. A 220 V Zener diode is placed between the supply voltage and the V _IN pin of the LT8316. Due to the voltage tolerance of the Zener diode, the minimum startup voltage is 260 V. After startup, the LT8316 continues to operate normally at a lower supply voltage. Figure 6 shows the efficiency at different input voltages, with the buck converter achieving a peak efficiency of 91%. Figure 7 shows the load and line regulation.

Figure 5. Schematic of a nonisolated buck converter with supply voltages up to 800 V.

Figure 6. Efficiency of the buck converter in Figure 5.

Figure 7. Load and line regulation of the buck converter in Figure 5.

Similar to the flyback converter in Figure 4, a voltage regulator can be added between the supply voltage and the V _{IN pin to achieve a low startup voltage for the buck converter. It should be noted that there is a body diode between the GND pin and the V IN} pin, which will increase the emitter voltage of the transistor and cause base-emitter breakdown. To prevent this, two diodes are added to protect the transistor. Figure 8 shows the low startup voltage solution.

Figure 8. Schematic of an 800 V _IN nonisolated buck converter with low start-up voltage.

The LT8316 operates in quasi-resonant boundary mode, achieving excellent regulation without the need for an optocoupler. In addition, it has a rich feature set, including low ripple Burst Mode ^® operation, soft-start, programmable current limit, undervoltage lockout, temperature compensation, and low quiescent current. The high level of integration simplifies the design of high-performance solutions with low component counts for a wide range of applications, from battery-powered systems to automotive, industrial, medical, telecom power supplies, and isolated auxiliary/home power supplies.

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