Critical checkpoint: Vcc voltage

Regarding “Critical check points” other than specifications that should be confirmed when evaluating the performance of isolated flyback converters, here we explain the “V_cc voltage”.

We will review the circuit first. In essence, any given IC requires a power supply for operation. Of course, the same is true of power supply ICs that become power supplies for other devices. In the case of a DC-DC converter, the input voltage is a DC voltage, and even if the input voltage is high, it is generally 100 V DC or lower. However, in the case of an AC-DC converter, the input is an AC voltage. According to domestic Japanese specifications, the input will be 100 V AC, and when tolerances are included for universal input, the input ranges from 85 to 264 V AC. In the design considered here, the input is the latter figure, but in any case a power supply IC for an ordinary AC-DC converter cannot directly use this AC voltage as V_cc.

In this circuit, the transformer is provided with third windings (auxiliary windings) in addition to the primary and secondary windings in order to generate a DC voltage suitable for V_cc, and a method is adopted in which the input AC voltage is stepped down and rectified, and converted to a low DC voltage.

The voltage generated in the third windings is rectified by the diode D4, but it includes large ripples, which are smoothed by the capacitor C5. R5 is a resistor that limits the rise in the V_cc voltage due to surges. To explore this in further detail, please refer to this link for the Design Edition.

From here we address the main subject. The V_cc voltage of this power supply IC has a recommended operating range of 8.9 V to 26 V. Of course this is a DC voltage. Here the checkpoint is whether or not this V_cc voltage is appropriate, and care must be taken to ensure that the resistor R5 that limits voltage increases is appropriate and functions adequately.

In normal operation, in the instant in which a MOSFET changes from on to off, a surge voltage occurs due to leakage inductance of the transformer. This surge voltage is induced by the third windings, and ultimately cases an increase in the V_cc voltage. The leakage inductance differs depending on the transformer specifications, but in the end it is essential that actual measurements be performed to definitely confirm that R5 holds the actual voltage increase to within the tolerated range. When the voltage increase causes the range to be exceeded, the value of R5 is increased somewhat, but if it is made too large, losses are increased, and so normally a value of approx. 5 to 22 Ω is appropriate.

The V_cc voltage waveform should be confirmed using an oscilloscope. When doing so, look at the AC waveform and the size of surges in the third windings, and if there are very large surges, it is best to identify their causes.

As operating conditions, whereas the recommended operating range is from 8.9 V to 26 V, the minimum voltage is set at 9.7 V. This is because if V_cc falls below 9.7 V, a V_cc charging function is started and V_cc is charged from the VH pin via the startup circuit, and thus unnecessary operations to raise V_cc should be avoided. This function is provided to ensure that startup of the power supply IC is reliable, and operation of the function itself is not a problem. However, when a stable state has been achieved after startup, the startup circuit is turned off to suppress wasteful power consumption, and there is no need to cause this circuit to be activated apart from its original purpose of coping with hindrances. For this reason, R5 is set such that the voltage does not drop below 9.7 V, at which the V_cc charging function is activated. Regarding the V_cc charging function of this power supply IC, please refer to page 9 of the Data Sheet. It should also be noted that the minimum voltage of the recommended operating range, 8.9 V, is the maximum value of the UVLO operation voltage when V_cc is falling.