The selection of the main components in the circuit design of the insulated flyback converter-the VCC of the IC

This project will explain the switching power supply control IC BM1P061FJ used in this design, the circuit that generates the power supply voltage VCC, that is, the diode D5 and the capacitor C2, and the surge limiting resistor R9.

This project will explain the switching power supply control IC BM1P061FJ used in this design, the circuit that generates the power supply voltage VCC, that is, the diode D5 and the capacitor C2, and the surge limiting resistor R9.

The selection of the main components in the circuit design of the insulated flyback converter-the VCC of the IC

First, explain the purpose of building the circuit and how it works. Basically, no matter what kind of IC, it must use power to make itself work. Many power supply ICs use the input voltage as their own power supply. However, most of the power applied to the IC power pins is almost DC voltage. Generally, the voltage below 40VDC is selected, and the special high voltage specification products can reach 60~80VDC.

In the design of this AC/DC converter, the allowable input specification is 85~264VAC, so the rectified voltage on the primary side is 400VDC or more, but it cannot be used directly as the power supply voltage of the power supply IC. Therefore, the input voltage must be used to generate a voltage suitable for use as the power supply voltage of the power supply IC. In this design, the reference coil (Nd) of the transformer is used to generate the power supply of the power IC.

The titles in this manuscript will be used in the following. The power source of the power IC is called VCC, and the auxiliary winding that generates VCC is called the VCC winding Nd. The VCC winding Nd is set to the number of turns that can generate 15VDC. (Please refer to the item of “Transformer Design (Numerical Calculation)” for the specifications of Nd.) On the other hand, the maximum rating of the power supply IC’s VCC is -0.3 to 30.0 VDC, and the operating range is 8.9 to 26.0 VDC. The VCC generated by the VCC winding Nd must take 15V as the target value, and it must be controlled within the working range.

VCC is generated by the VCC winding Nd, the rectifier diode D5, the smoothing and stabilizing capacitor C2, and the surge voltage limiting resistor R9. The circuit is the same diode rectifier circuit as the output secondary side.

Rectifier diode D5 for VCC generation and smoothing capacitor C2

As mentioned above, the diode D5 and the capacitor C2 convert the voltage generated by the switching (chopping) of the VCC winding Nd into DC. Basically, these two components can be used to generate the DC voltage for VCC.

Diode D5 is suitable for high-speed diodes. The withstand voltage of the diode is calculated based on the voltage Vdr applied to D5.

The selection of the main components in the circuit design of the insulated flyback converter-the VCC of the IC

The margin is 128.2V/0.7=183V, and a fast recovery diode with a withstand voltage of 200V is selected.

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

The DC voltage rectified by D5 will of course include larger ripples, so connect the capacitor C2 for smoothing. In addition, C2 also has the function of stabilizing the current necessary for supplying the VCC of the power supply IC.

The selection of the main components in the circuit design of the insulated flyback converter-the VCC of the IC

Unlike this operation, R1 connected to the VH pin of the power supply IC (VH pin will be described later) determines the IC’s startup time (soft start) when the power is turned on. This is inherent to the IC itself. Sharing the capacitor for setting the startup time with the capacitor for VCC can reduce the number of parts. Therefore, the capacitance of C2 must be able to meet the two requirements of smoothing/stabilization and start-up time. The technical specifications of the power IC provide graphs related to capacitors and start-up time to help make choices.

Capacitor C2: 2.2μF or more, 50V withstand voltage

According to the rule of thumb, this design must reach more than 2.2μF, and 10μF is chosen. Also use the chart to calculate the start-up time. In principle, the voltage value applied to VCC does not exceed 30V, but considering the margin, it can be a specification product that can reach 50V.

Surge voltage limiting resistor R9 for VCC winding

Due to the leakage inductance (Lleak) of the transformer, when the MOSFET turns from ON to OFF, a large surge voltage (peak noise) will be generated. This surge voltage is caused by the VCC winding. After the VCC voltage rises, it is assumed that the IC’s VCC overvoltage protection will be activated. In order to reduce the surge voltage caused by the VCC winding, a limiting resistor R9 is inserted in series. R9 is suitable to be 5-22Ω. Please check and adjust the rise of VCC voltage with a real machine.
Therefore, the constituent circuit generates VCC as the power supply of the power supply IC. The IC’s VCC rating is 30V, including the surge voltage, and must not exceed the rating. It differs from DC/DC converters in that the input voltage is high, which must be verified.

The Links:   G190EG02VG LM170E03-TLG4

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