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Notes on a Boost Power Supply During ShutdownOperation During Shutdown of a Boost DC-DC Converter

2025.01.16

Points of this article

・In diode rectification boost DC-DC converters, the output does not go to 0 V at shutdown, and instead a voltage equal to VIN-VF is output.

・In synchronous rectification boost DC-DC converters as well, a voltage equal toVIN-VF is output during shutdown due to the parasitic diode in the FET used as a high-side switch.

・Because a low voltage is output during shutdown, erroneous operation of the load circuit occurs and the standby power consumption is increased due to the current flowing in the load circuit and the voltage setting resistors.

・Synchronous rectification boost DC-DC converters with a back gate control function can reduce the output voltage to 0 V during shutdown by canceling the parasitic diode, thus preventing erroneous operation of the load circuit and increases in the standby power consumption.

table of contents

Introduction

When a buck DC-DC converter shuts down and halts the switching operation, the output voltage goes to zero and the supply of power to the load circuit stops; power consumption from the input source is then only the self-current consumption during shutdown of the power supply circuit. However, in a boost DC-DC converter, even when boost operation stops due to a halt in switching, the supply voltage VIN is output without being boosted. As a result, due to the current that flows to the circuit connected to the output, a larger current may flow than the self-current consumption during shutdown of the power supply circuit. This may cause problems with calculation of the standby current consumption.

The effects of the output voltage at shutdown of a boost power supply, and countermeasures to address these effects, are explained in “Diode Rectification Boost DC-DC Converters”, “Synchronous Rectification Boost DC-DC Converters”, “Standby Current Consumption Due to Current Flowing in Voltage Setting Resistors”, and “Synchronous Rectification with Back Gate Control Function”.

Diode Rectification Boost DC-DC Converters

In a diode rectification boost DC-DC converter, even when switching is halted due to shutdown, a voltage (VIN–VF) is output to VOUT because there is a path from VIN through an inductor and rectifier diodes. Since a voltage continues to be applied to the load circuit, the load circuit may consume the current depending on the voltage VIN, increasing the standby current consumption. In addition, application of an inappropriate low voltage to the load circuit may cause erroneous operation, and thus an output disconnect circuit may be necessary.

Output of a diode rectification boost DC-DC converter when halted

Synchronous Rectification Boost DC-DC Converters

In a synchronous rectification boost DC-DC converter, an FET is used as a high-side switch, and upon shutdown there is no longer gate driving, so that the high-side switch is in the off state. However, a current path is formed by the parasitic diode in the FET, and thus a voltage of (VIN–VF) is still output to VOUT.

Output of a synchronous rectification boost DC-DC converter when halted

Standby Current Consumption Due to Current Flowing in Voltage Setting Resistors

The VOUT of a power supply is connected to a voltage divider that uses resistors to set the output voltage. During operation, a current equal to VOUT/(R1+R2) flows through the resistors. When the power supply shuts down, the VOUT voltage is falling, and so the current flowing in the voltage setting resistors also decreases, but it is added to the standby current consumption. This current can be reduced by increasing the resistor values; but a current of at least 100 times the input bias current of the error amplifier must be passed through these resistors. If this level of current is not passed, the output voltage VOUT will vary greatly due to input bias current fluctuations caused by temperature and other factors, affecting the error in the output voltage; thus the resistor values cannot be increased indiscriminately. Many of the power supply ICs before the year 2000 were bipolar devices, and the input bias current was on the level of several tens of microamps, so that it was necessary to pass currents on the order of several milliamps through voltage setting resistors. However, since 2000, power supply ICs have generally been CMOS devices, and input bias currents have been reduced to nanoamp order, so that the current to be passed through voltage setting resistors also dropped to the level of tens of microamps. Even so, when the power supply source is a battery in mobile equipment, even microamp-level current consumption can pose problems because it impacts operation time.

Current consumption by voltage setting resistors when operation is halted

To eliminate current consumption by voltage setting resistors and reduce the standby current consumption, some power supply devices incorporate a switch that disconnects the voltage setting resistors from GND upon shutdown.

Synchronous Rectification with Back Gate Control Function

There are power supply IC devices using synchronous rectification that incorporate a high-side switching FET within the IC, and that include a circuit configured to apply a bias to the FET back gate, which can be controlled to cancel the parasitic diode. The current path formed by the parasitic diode of the FET can be shut off, so that the output voltage is 0 V during shutdown. This prevents erroneous operation of the load circuit and ensures power supply reset. Current consumption by the voltage setting resistors and the load circuit can also be eliminated.

Output disconnected by the back gate control function

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Operation During Shutdown of a Boost DC-DC Converter
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