Traditional high-voltage isolated flyback converters use optocouplers to transfer regulation information from the secondary-side reference power circuit to the primary side, thereby achieving accurate regulation. The problem is that optocouplers greatly increase the complexity of the isolation design: there is propagation delay, aging, and gain variation, all of which complicate power loop compensation and reduce reliability. Additionally, during startup, a bleeder resistor or a high voltage startup circuit is required to initially start the IC. Unless an additional high-voltage MOSFET is added to the start-up assembly, the bleeder resistor will consume a lot of power.

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

Extended supply voltage

The LT8316 is available in a thermally enhanced 20-pin TSSOP package with 4 pins removed to reveal the high voltage spacing. By sampling the isolated output voltage of the tertiary winding, an optocoupler is not required for regulation. The output voltage is programmed through two external resistors and a third optional temperature compensation resistor. Quasi-resonant boundary-mode operation facilitates 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, external load compensation resistors and capacitors are not required. As a result, the LT8316 solution uses fewer components, greatly simplifying the design of an isolated flyback converter (see Figure 1).

Figure 1. Complete 12 V isolated flyback converter for a wide range of outputs from 20 V to 800 V with a minimum start-up voltage of 260 V.

The LT8316 is rated to operate up to 600 V, but can be extended by replacing the Zener diode in series with the VIN pin. The voltage of the Zener diode reduces the voltage supplied to the chip so that the supply voltage exceeds 600 V.

LT8316

Wide Input Voltage Range: 16V to 600V

Adjustment without opto-isolator

Supports Quasi-Resonant Boundary Modes

Constant current and constant voltage regulation

Low Ripple Light Load Burst Mode? Operation

Low Quiescent Current: 75μA

Programmable Current Limit and Soft-Start

Available in TSSOP Package with High Voltage Lead Pitch

Figure 1 shows the overall schematic 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. With a 220 V Zener in series with the VIN pin, the minimum start-up voltage of the circuit is about 260V due to the voltage tolerance of the Zener. Note that after startup, the LT8316 can operate normally at voltages below 260V.

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

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

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

Low startup voltage design

While the previous solution extended the input voltage to 800 V, the Zener diode increased the minimum startup voltage to 260 V. The challenge is that some applications require both high input voltage and 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 steadily increase up to 800 V, while the voltage at the VIN 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 converted to 12 V with low startup voltage.

Non-Isolated Buck Converters

The high voltage input capability of the LT8316 can be easily implemented in a simple non-isolated buck converter without the need for an isolation transformer. A relatively inexpensive off-the-shelf inductor is used as the electromagnetic component.

For non-isolated buck applications, the ground pin of the LT8316 is connected to the switch node of the buck topology, which has a variable voltage. The LT8316 uses a unique detection method that detects the output voltage only when the switch node is grounded, so the buck schematic is fairly simple.

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

Figure 5. Schematic of a non-isolated buck converter for supply voltages up to 800 V.

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

Figure 7. Load and voltage regulation for 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 VIN pin to allow the buck converter to achieve a low startup voltage. Note that there is a body diode between the GND pin and the VIN pin, which increases the transistor's emitter voltage and causes base-emitter breakdown. To prevent this, we add two diodes to protect the transistor. Figure 8 shows a low startup voltage solution.

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

in conclusion

The LT8316 operates in quasi-resonant boundary mode for excellent regulation without the need for an optocoupler. In addition, it has a wealth of features, including low ripple burst mode (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.