Technical Information Site of Power Supply Design

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• Examination of Losses
• Controller IC Power Consumption Losses in a Synchronous Rectifying Step-Down Converter

2018.11.08 DC/DC

Controller IC Power Consumption Losses in a Synchronous Rectifying Step-Down Converter

Examination of Losses

This time, in succession to the discussion of “dead time losses” in the previous section, we consider losses due to power consumption in the controller IC itself.

Self Power Consumption Losses of the Controller IC

In this example, the power supply IC used is a synchronous rectifying controller IC, that is, a controller type IC that does not incorporate a power switch. This IC, which controls the power supply circuit, also requires a power supply for operation, and of course consumes power. This power consumption becomes a part of the device losses, corresponding to PIC in the graphic above.

Here the power consumed by the power supply IC in pure control operation is considered. The controller IC includes a gate driver that switches an external MOSFET, and so normally, when the power switch is continuously performing switching, power consumption by the gate driver is dominant. Hence in applications in which the power supply constantly provides a substantial load current, the power consumption of the control IC itself does not pose too much of a problem. However, in intermittent operation at a light load, or in PFM operation with an extremely long period, the power consumption of the IC itself dominates, greatly affecting the efficiency. Hence when efficiency at a light load must be considered, it is essential that losses due to power consumption of the IC itself be understood.

The calculation equation is extremely simple. This is the simplest calculation of the power consumption of the IC, but in some cases a number of related issues must be studied.

In order to conform with other sections of this series, we have shown the switching waveforms, but depending on the measurement conditions indicated on the data sheet for the IC, it may be necessary to perform measurements with switching stopped.

Moreover, due to the IC pinout, there are cases in which power supply pins for the control circuit are separate from power supply pins for the gate driver, and cases in which they are used in common. The same is true of GND. Thus it is not necessarily a simple matter to separate power consumption of the IC itself and power consumption of the driver. In any case, measurements must be made referring to the conditions for items relevant to the current consumption of the IC itself given on the data sheet.

The next article will address losses incurred in gate driving.

Key Points:

・Losses due to power consumption by the controller IC itself greatly affect the efficiency at a light load.

・Loss calculations are extremely simple, being the product of the power supply current and the power supply voltage.

・For measurement conditions, the data sheet for the IC should be referenced.