Flyback Converter Without Optocoupler: Available Options

Flyback Converter Without Optocoupler: Available Options

Flyback converters are typically used in applications that require electrical isolation of the supply Voltage and transmit relatively low power. A flyback converter is usually used for output power below 60 W.

Frederik Dostal
Analog Devices

Flyback converters are typically used in applications that require electrical isolation of the supply voltage and transmit relatively low power. A flyback converter is usually used for output power below 60 W.

For a galvanically isolated power supply, you must determine on which side of the primary or secondary the galvanically isolated controller IC will conduct. If it is on the secondary side, control of the primary side power switch must be provided through galvanic isolation.

Whether it is the controller on the primary side or the controller on the secondary side, a path for signal transmission across electrical isolation is required in both architectures. A common path is an optocoupler (or opto-isolator). However, they all come with some downsides. They are typically only rated at 85°C and their current transfer ratio (CTR) changes over time, meaning their transfer behavior changes over the life of the circuit. Additionally, other components are required to control the optocoupler. The feedback loop speed of isolated power supplies is usually slow if optocouplers are used. In recent years, some neat solutions to this problem have been developed. The first solution is a flyback controller, which does not measure the output voltage directly. By monitoring the voltage across the primary-side transformer winding, a sufficiently accurate criterion for the actual output voltage can be obtained. The accuracy of this regulation depends on the usual conditions of the application, including input and output voltages, load changes, and voltage changes.

However, for many applications, a regulation accuracy of ±10% to ±15% is sufficient. Figure 1 shows the LT8301. Due to the integrated power switch and the SOT23 package, the IC requires few external components. The isolation breakdown voltage of the circuit depends only on the transformer used. This provides great flexibility, especially when very high isolation voltages are required.

Flyback Converter Without Optocoupler: Available Options
Figure 1. The LT8301 flyback regulator does not require an isolated feedback path.

However, for applications that require higher output voltage control accuracy, another interesting solution has only recently become available. Analog Devices has brought to market a flyback controller, the ADP1071, that includes a fully integrated feedback path using iCoupler® technology.

Figure 2 shows a circuit that requires only a very small number of passive components. The ADP1071 includes a primary-side controller, a secondary-side active rectifier controller for improved conversion efficiency, and a fully integrated feedback path for a very fast feedback loop. With this solution, the output voltage regulation is very accurate and, more importantly, very fast, even with large load transients. The allowable operating temperature is up to 125°C silicon temperature.

Flyback Converter Without Optocoupler: Available Options
Figure 2. The ADP1071 flyback controller has an integrated feedback path for very precise regulation.

Its maximum isolation voltage depends on the selected transformer and the isolation technology used in the switching regulator IC. The maximum isolation voltage of the chip is 5 kV. Reinforced insulation classification class according to VDE V 0884-10 has been applied for.

The above interesting solutions can be used to develop electrically isolated power supplies. Depending on the application, either a solution without a feedback path or a solution with a fully integrated feedback path may be suitable. Since it is no longer limited by the optocoupler’s 85°C operating temperature, a compact power supply design with very high power density can be achieved.

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