Designing Isolated Flyback Converter Circuits: Selecting Critical Components – IC Settings Etc.

Here we explain the selection of components to set parameters for the BM1P061FJ controller IC for switching power supplies used in this design. In an AC-DC converter circuit, in addition to the diode bridge and transformer for example as the basic components of the power supply circuit, components to set the protection function operating level, current limit and the like of the power supply IC are also necessary. A number have already been explained, here the remaining major components are discussed.

VH pin Resistor R1

The VH pin is the power supply pin of a startup circuit (starter) integrated in the IC, and has a rated voltage of 650 V. When an AC power supply is input, a voltage from the input filter, rectified by the diodes D1 and D2, is applied to the VH pin via R1, and operates the startup circuit within the IC. As a result, the IC begins switching operation, obtains a feedback from the output, and enters steady-state operation. The startup circuit operates only for the purpose of startup, and so becomes unnecessary once steady-state operation begins, and enters an idle state. Because of this circuit, extremely fast and reliable startup is made possible, and after startup the circuit enters the idle state, so that power consumption is reduced. This function is not present in all such ICs.

The current flowing into the VH pin has the function, together with capacitor C2 constituting the VCC circuit for the IC, of setting the startup time. (Refer to [Selecting Critical Components ? Vcc of IC]）. The VH pin inflowing current is specified by the “startup current 1”, “startup current 2”, and “OFF current” in the data sheet, and is at maximum 5 mA.

The value of the VH pin resistance R1 is determined taking into account these requirements as well as protection (current limiting) when the VH pin is short-circuited to GND. Specifically, from the voltage applied to VH (85 to 264 VAC ×√2), the 5 mA required for the VH pin, and the need for current limiting upon short-circuits, the value is set between 5 kΩ and 60 kΩ. For this circuit, a value of 10 kΩ is selected.

One matter requiring attention is that R1 must be able to withstand high voltages (85 to 264 VAC ×√2) and the power equal to the resistance value times the current.

Including empirical rules, calculating the values assuming 72 VAC for the startup voltage and 50 VAC for the voltage to stop operation when the AC voltage is falling, we obtain R2 = 3.9 MΩ, R3 = 39 kΩ.

Further, this function need not necessarily be used; in this case the resistor values are determined such that the voltage at the ACMONI pin is always between 1 V and 5 V.