Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? VCC of IC

In this section, we explain the diode D5, capacitor C2, and surge limiting resistor R9 which form a circuit to generate a power supply voltage VCC for the BM1P061FJ switching power supply controller IC that is used in this design.

In this AC-DC converter design, because the specifications stipulate input of 85 to 264 VAC, the rectified voltage on the primary side is 400 VDC or higher; this voltage cannot be used directly as the power supply voltage of a power supply IC. Hence a voltage suitable for use as the power supply voltage of a power supply IC must be generated from the input voltage. In this design, the power supply for the power supply IC is generated using the auxiliary windings (Nd) of the transformer.

From here on, nomenclature specific to this article is used. The power supply of the power supply IC is called VCC, and the auxiliary windings that generate VCC are called the VCC windings Nd. The VCC windings Nd have the number of windings set so as to generate 15 VDC.　(For Nd specifications, refer to [Transformer Design (Calculating numerical values)]. On the other hand, the VCC of the power supply IC is stipulated to have a maximum rating of -0.3 to 30.0 VDC and an operating range of 8.9 to 26.0 VDC. The VCC generated by the VCC windings Nd must be controlled such that the operating range is not exceeded, with 15 V as a target.

VCC is generated by the VCC windings Nd, the rectifying diode D5, the capacitor C2 for smoothing and regulation, and the resistor R9 to limit surge voltages. We see that the circuit is the same diode-rectifying circuit as on the secondary side that is the output.

Rectifying diode D5 for VCC generation and smoothing capacitor C2

As explained above, the diode D5 and capacitor C2 convert into DC the switched (chopped) voltage from the VCC windings Nd. In essence, a DC voltage for use as VCC can be generated using these two components.

A fast diode is suitable as the diode D5. The diode withstand voltage is calculated from the voltage Vdr applied to D5.

If the margin is 128.2 V/0.7 = 183 V, then a fast-recovery diode with the rated voltage of 200 V is selected.

The diode RF05VA2S shown in the circuit is a fast-recovery diode with the rated voltage of 200 V and an average rectified current of 0.5 A.

Apart from this operation, the capacitor also determines, together with the resistor R1 connected to the VH pin of the power supply IC, the startup time (soft start) for the IC upon power-on (the VH pin will be explained in future). This is specific to this particular IC; the same capacitor can be used for VCC and to set the startup time, so that the number of components can be reduced. Hence the capacitance of C2 must be a value that satisfies two requirements, for smoothing/regulation and regarding the startup time. To aid selection, the data sheet for power supply ICs provides a graph relating to the capacitance and startup time.

Capacitor C2：　2.2 μF or higher, rated voltage 50 V

In this design, an empirical rule indicates that at least 2.2 μF is necessary, and so 10 μF is selected. When setting a longer startup time, the graph is used for the calculation. As a rule, 30 V or higher is not applied as VCC, but adding a margin, a 50 V product is used.

Surge voltage limiting resistor R9 for the VCC windings

Due to the leakage inductance (Lleak) of the transformer, at the moment when the MOSFET switches from on to off, a large surge voltage (spike noise) occurs. This surge voltage is induced in the VCC windings, and it is expected that as the VCC voltage rises, the VCC overvoltage protection of the IC is activated. In order to alleviate the surge voltage induced in the VCC windings, the limiting resistor R9 is inserted in series. From 5 to 22 Ω is an appropriate range for R9, but the value should be adjusted after checking the rise of the VCC voltage in actual equipment.

With this, we can configure a circuit that generates a VCC voltage to be used as the power supply of the power supply IC. The VCC rating of the IC is 30 V, and so it is important that the rated voltage, including any surge voltage, is not exceeded. In contrast with a DC-DC converter, the input voltage is high, and therefore sufficient verification is necessary.