1. A protection circuit for a direct current (DC)-DC converter including a buck switch connected between a voltage input terminal and a first node, the buck switch being controlled by a switching cycle and a duty cycle of a switching signal; a buck inductor connected between the first node and a voltage output terminal; a buck diode connected between a second node and a ground; and a buck capacitor connected between the voltage output terminal and the ground, wherein the protection circuit is connected to the first node, the second node and the ground, and when the buck switch is switched between an On state and an OFF state according to the switching signal, the protection circuit is configured to supply a protection voltage between the first node and the ground so that voltage stress of the buck switch is smaller than an input voltage supplied between the voltage input terminal and the ground.

2. The protection circuit according to claim 1, wherein the protection circuit includes: a first protection capacitor connected between the first node and the second node; a second protection capacitor connected between a third node and the ground; a protection inductor connected between the second node and the third node; and a protection diode connected between the first node and the third node.

3. The protection circuit according to claim 2, wherein the protection diode is kept in the OFF state in a first period of time during which the buck switch is in the ON state, and is kept in the ON state in a second period of time during which the buck switch is in the OFF state. 19

4. The protection circuit according to claim 2, wherein the protection voltage is equal to a voltage of a series circuit of the first protection capacitor and the buck diode.

5. The protection circuit according to claim 2, wherein a voltage of a series circuit of the first protection capacitor, the protection inductor and the second protection capacitor is equal to a sum of a voltage of the buck inductor and an output voltage in a first period of time during which the buck switch is in the ON state, and the output voltage is a voltage between the voltage output terminal and the ground.

6. The protection circuit according to claim 2, wherein a voltage of the first protection capacitor is equal to a voltage of a series circuit of the protection inductor and the protection diode in the second period of time during which the buck switch is in the OFF state.

TECHNICAL FIELD
The present disclosure relates to technology for protecting a direct current (DC)-DC
converter from switching loss.
The present application claims priority to Korean Patent Application No. 10-2020-
0111845 filed on September 2, 2020 in the Republic of Korea, the disclosure of which is
incorporated herein by reference.
BACKGROUND ART
Recently, there has been a rapid increase in the demand for portable electronic
products such as laptop computers, video cameras and mobile phones, and with the extensive
development of electric vehicles, accumulators for energy storage, robots and satellites,
many studies are being made on high performance batteries that can be recharged repeatedly.
Currently, commercially available batteries include nickel-cadmium batteries,
nickel-hydrogen batteries, nickel-zinc batteries, lithium batteries and the like, and among
them, lithium batteries have little or no memory effect, and thus they are gaining more
attention than nickel-based batteries for their advantages that recharging can be done
whenever it is convenient, the self-discharge rate is very low and the energy density is high.
An electric vehicle includes a battery and a battery charger. The battery charger
generates the charge power for the battery using the input power from an external power
source when connected to the external power source through a charging cable. In general,
the battery charger includes a direct current (DC)-DC converter to generate the output
voltage that is lower than the input voltage.
To keep up with the recent trend toward lightweight electric vehicles, there is a
growing demand for lighter and smaller DC-DC converters. To reduce the weight and size
of the DC-DC converter, it is necessary to increase the switching frequency of a switching
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signal rather than reducing the size of each physical device included in the DC-DC converter.
FIG. 1 is a schematic diagram of a common DC-DC stepdown converter.
Referring to FIG. 1, the DC-DC converter includes a buck switch SWB connected between
a voltage input terminal Ni and a first node N1; a buck inductor LB connected between the
first node N1 and a voltage output terminal No; a buck capacitor CB connected between the
voltage output terminal No and the ground; and a buck diode DB connected between the first
node N1 and the ground. When the buck switch SWB is switched from an ON state to an
OFF state, the buck diode DB is turned on, and voltage which is, in substance, equal to 0V,
is supplied between the first node N1 and the ground. On the contrary, when the buck
switch SWB is switched from the OFF state to the ON state, the buck diode DB is turned off,
and voltage which is, in substance, equal to the input voltage Vin is supplied between the
first node Ni and the ground. As a result, each time the buck switch SWB is switched
between the ON state and the OFF state, voltage stress which is, in substance, equal to the
input voltage Vin, occurs across the buck switch SWB.

We Claim:
1. A protection circuit for a direct current (DC)-DC converter including a buck
switch connected between a voltage input terminal and a first node, the buck switch being
controlled by a switching cycle and a duty cycle of a switching signal; a buck inductor
connected between the first node and a voltage output terminal; a buck diode connected
between a second node and a ground; and a buck capacitor connected between the voltage
output terminal and the ground,
wherein the protection circuit is connected to the first node, the second node and the
ground, and when the buck switch is switched between an On state and an OFF state
according to the switching signal, the protection circuit is configured to supply a protection
voltage between the first node and the ground so that voltage stress of the buck switch is
smaller than an input voltage supplied between the voltage input terminal and the ground.
2. The protection circuit according to claim 1, wherein the protection circuit
includes:
a first protection capacitor connected between the first node and the second node;
a second protection capacitor connected between a third node and the ground;
a protection inductor connected between the second node and the third node; and
a protection diode connected between the first node and the third node.
3. The protection circuit according to claim 2, wherein the protection diode is
kept in the OFF state in a first period of time during which the buck switch is in the ON state,
and is kept in the ON state in a second period of time during which the buck switch is in the
OFF state.
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4. The protection circuit according to claim 2, wherein the protection voltage is
equal to a voltage of a series circuit of the first protection capacitor and the buck diode.
5. The protection circuit according to claim 2, wherein a voltage of a series
circuit of the first protection capacitor, the protection inductor and the second protection
capacitor is equal to a sum of a voltage of the buck inductor and an output voltage in a first
period of time during which the buck switch is in the ON state, and
the output voltage is a voltage between the voltage output terminal and the ground.
6. The protection circuit according to claim 2, wherein a voltage of the first
protection capacitor is equal to a voltage of a series circuit of the protection inductor and the
protection diode in the second period of time during which the buck switch is in the OFF
state.