This article will explore how to design isolated power supply circuits for gate drive, isolated sensing, and communication circuits with minimal parts, lowest complexity, and most cost effective methods. This circuit can be used when the input voltage is low and a small (5%) voltage deviation is allowed while the circuit is energized.
The example in Figure 1 demonstrates an IC developed for a simple isolated bias supply, and any synchronous buck circuit that allows a sink operaton can be used. This type of circuit is called asymmetrical half-bridge flybuck and operates in much the same way as a synchronous buck regulator. The FET totem pole output that connects the input voltage supplies an inductor-capacitor filter. The filter output is then adjusted through the voltage divider and the error amplifier negative input. The error amplifier controls the duty cycle of the FET totem pole output to maintain the DC voltage at the sense point.
The voltage of C6 is equivalent to the duty factor multiplied by the input voltage. Like the buck power stage, the voltage-second of the inductor must be equal to zero. However, this circuit adds a coupled winding to the inductor and uses a diode to correct the inductor voltage reflected by the low FET when it is started. Since the inductor voltage during this period is equal to the output voltage, the output of the circuit will be adjusted. However, the difference in voltage drop between the primary side and the secondary side will reduce the effect of the adjustment. In this circuit, the voltage regulation of the load will be affected by the forward voltage drop of diode D1. If the diode is changed to FET, the effect of load regulation can be improved. 
Figure 1: Synchronous Buck Circuit Provides Isolated Power Supply
Like the coupled inductor SEPIC, this topological parasitic component can also affect circuit performance. During the on-time, the circuit condition is quite good, and most of the current flows into the magnetizing inductance of the coupled inductor T1, charging C6. Output capacitor C3 supplies the load current. However, during shutdown, the two capacitors will be placed in parallel through the coupled windings of the inductor. These two capacitors have different voltages, and only parasitic components in the loop limit the current between the two. These parasitic components include the ESR of the two capacitors, the winding resistance of the coupled inductor, the impedance of the low-side MOSFET and the diode, and the leakage inductance of the coupled inductor.
Figure 2 shows the analog current for different leakage inductance values. The upper half is the current on the primary side of T1 and the lower half is the current on the output diode D1. The tightly coupled inductor 10 nH is different from the loosely coupled inductor 1 uH. For tightly coupled inductors, the peak current is high and is also limited by the loop impedance.
For loosely coupled inductors, the peak current is low. Higher leakage reduces RMS current and helps improve power supply efficiency. Figure 2 shows a comparison of the two. The loosely coupled inductor reduces current by up to 50% and reduces the loss of a few components by up to 75%. The disadvantage of loose coupling is the poor regulation of the output voltage.
Figure 2: Low leakage increases circulating current
Figure 3 shows the load regulation results presented by the converter of Figure 1. If the load current is limited, in most cases, this converter will provide sufficient regulation. At light loads, the effects of diode junction voltage variations and ringing can be seen. A minimum load or Zener clamp may be required to reduce these light load effects. Parasitic components of the circuit can reduce the effects of regulation during heavy loads. Therefore, reducing the number of components helps to improve the effect. For example, changing the diode to synchronous switching will greatly increase load regulation.
All in all, the Flyback converter is an attractive topology that provides a low-cost, simple, isolated power supply that withstands 5% to 10% of the output voltage change. The output efficiency of the diode rectifier at 5V is maintained at 80%, and the state of the synchronous rectifier is also improved.
Figure 3: The flyback load adjustment is good in most cases
All in all, the Flyback converter is an attractive topology that provides a low-cost, simple, isolated power supply that withstands 5% to 10% of the output voltage change. The output efficiency of the diode rectifier at 5V is maintained at 80%, and the state of the synchronous rectifier is also improved.
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