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


Points of this article

・In a design that uses a power supply IC, in addition to basic components (diode bridge, transformer etc.), components are also necessary to set functions for the power supply IC and the like.

・The constants of the components used with the power supply IC should be determined according to the data sheet, application notes, design manuals, and the like.

・These are often used to set threshold levels of protection circuits, limiting values and the like; components are primarily resistors and capacitors.

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.

AC Start/Stop Voltage Settings: R2, R3

This power supply IC is provided with a “brownout function” for stopping operation when the input AC voltage has fallen. The start and stop voltages are set using the ACMONI pin.

The ACMONI pin has threshold values for voltage determination; the typical values are 1.0 V when rising and 0.7 V when falling. An input voltage is, similarly to the VH pin, a voltage from an AC input filter rectified by the diodes D1 and D2, and is the DC voltage equal to the AC input voltage ×√2. By dividing the voltage using R2 and R3 and inputting the result to the ACMONI pin, the AC input voltage is detected. The equation for calculation is as follows.



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.

Other Components

C5: Capacitor to reduce noise at the ACMONI pin

Noise is bypassed to regulate the start/stop voltages.

C4: Capacitor for FB pin regulation

Regulates the FB pin voltage. A value of approximately 1000 pF to 0.01 μF is recommended.

R10, C6: To address noise at the CS pin

When the blanking function does not completely eliminate noise, this RC filter is added. Even when the filter is unnecessary, it is recommended that R10 (approx. 1 kΩ) be inserted for surge protection.

R14, R15, R16: Output voltage setting resistors

The output voltage is set according to the following equation.
*Shunt regulator U2: Vref = 2.495 V (typ.)

C9, R13: Phase compensation circuit

Circuit to adjust the phase of the feedback loop.
With C9=0.1 μF and R13=10 kΩ to 30 kΩ or so, check the response in an actual device.

R11: Resistor to limit the optocoupler current

Set between 1 kΩ and 2 kΩ.

R12: Resistor to set the cathode current of the shunt regulator U2

When U2 is the TL431, a current of 1 mA is secured.
R12 should be set to VF (optocoupler)/1mA=1V/1mA=1 kΩ.

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Basic of AC-DC Conversion

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