Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Utility Patent
1999-12-14
2001-01-02
Nguyen, Matthew (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
Utility Patent
active
06169392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC—DC converter circuit for performing DC—DC conversion by switching an input voltage on and off and, more particularly, to a DC—DC converter circuit that achieves high conversion efficiency while, at the same time, making it possible to supply low voltages.
2. Description of the Related Art
Battery-operated apparatuses such as notebook-size personal computers are provided with a DC—DC converter circuit for converting a voltage from an AC adapter, a dry battery, etc. into a voltage needed by the load. To increase the utility of such battery-operated apparatuses, the conversion efficiency of the DC—DC converter circuit must be increased.
In the DC—DC converter circuits used in battery-operated apparatuses such as notebook-sized personal computers, switching regulators for performing PWM (pulse width modulation) are used to achieve as high conversion efficiency as possible.
For switching devices in such DC—DC converters, N-channel MOSFETs are often used. The reason for this is that N-channel MOSFETs have lower ON resistance, are less expensive, and offer a wider selection of products than P-channel MOSFETs.
However, to cause an N-channel MOSFET to turn on, the gate voltage must be made higher than the source voltage, and when switching the power line on and off, a voltage higher than the power line must be applied to the gate of the MOSFET.
To achieve this, the prior art provides a configuration, such as shown in
FIG. 14
, that uses an N-channel MOSFET as the main switching device Q
1
, and that performs DC—DC conversion by switching the main switching device Q
1
on and off in accordance with a PWM control signal generated by a PWM control circuit
100
. This configuration includes: a regulator circuit
200
which generates a predetermined voltage from an input voltage; a capacitor Cc which is provided between the regulator circuit
200
and the source of the main switching device Q
1
and is charged up by the voltage supplied from the regulator circuit
200
via a diode Dc; and a driver circuit
300
which, in accordance with the circuit configuration shown in
FIG. 15
, selects either the voltage of the capacitor Cc or the source voltage of the main switching device Q
1
, depending on the PWM control signal generated by the PWM control circuit
100
, and supplies the selected voltage to the gate of the main switching device Q
1
.
A flywheel diode Dd is included to provide a path for a current that flows from ground to inductance L when the main switching device Q
1
is off.
In this configuration, the capacitor Cc is charged up while the main switching device Q
1
is held in the off state by the driver circuit
300
selecting the source voltage for application to the gate in accordance with the PWM control signal. Then, when the driver circuit
300
selects the voltage of the capacitor Cc for supply to the gate in accordance with the PWM control signal, a voltage higher than the source voltage by the voltage of the capacitor Cc is input to the gate, causing the main switching device Q
1
to turn on.
In this way, in the above prior art, the gate voltage necessary to turn on the N-channel MOSFET is generated by using the capacitor Cc that is provided between the regulator circuit
200
and the source of the main switching device Q
1
and is charged up by the voltage supplied from the regulator circuit
200
.
The prior art shown in
FIG. 16
is also used. This prior art employs the configuration in which, after the output voltage of the DC—DC converter circuit has reached a predetermined voltage Vref, the capacitor Cc is charged up by the output voltage of the DC—DC converter circuit.
More specifically, the configuration includes: a comparator circuit
400
which outputs a high level when the output voltage of the DC—DC converter circuit is lower than the predetermined voltage Vref, and a low level when the output voltage reaches or exceeds the predetermined voltage Vref; a switching device Q
3
constructed from a P-channel MOSFET which couples the output voltage of the DC—DC converter circuit to the capacitor Cc when the comparator circuit
400
outputs a low level; and a switching device Q
4
constructed from a P-channel MOSFET which disconnects the capacitor Cc from the regulator circuit
200
when the comparator circuit
400
outputs a high level from its inverting output terminal. With this configuration, the capacitor Cc is charged up by the output voltage of the DC—DC converter circuit after the output voltage of the DC—DC converter circuit has reached the predetermined voltage Vref.
In the configuration of the prior art shown in
FIG. 16
, since the voltage drop across an N-channel MOSFET is smaller than the voltage drop across the flywheel diode Dd, the flywheel diode Dd is replaced by a synchronous commutation-type switching device Q
2
constructed from an N-channel MOSFET, with a view to improving the conversion efficiency.
The configuration also includes a driver circuit
500
which, in accordance with the PWM control signal generated by the PWM control circuit
100
, selects either ground potential or the drain voltage of the switching device Q
4
for application to the gate of the synchronous commutation-type switching device Q
2
. More specifically, when the main switching device Q
1
is turned off in accordance with the PWM control signal, the drain voltage of the switching device Q
4
is selected and applied to the gate of the synchronous commutation-type switching device Q
2
, causing the synchronous commutation-type switching device Q
2
to turn on; on the other hand, when the main switching device Q
1
is turned on in accordance with the PWM control signal, ground potential is selected and applied to the gate of the synchronous commutation-type switching device Q
2
, causing the synchronous commutation-type switching device Q
2
to turn off.
Turning back to
FIG. 14
, the prior art shown in the figure has the problem that the conversion efficiency drops because of a large loss in the regulator circuit
200
.
The regulator circuit
200
is used to generate a predetermined voltage, irrespective of the magnitude of the input voltage, and is usually constructed from a linear regulator. As is well known, loss in the linear regulator is expressed as follows:
Loss in linear regulator=Vin×Iq+(Vin−Vout)×Iout
where Vin: Input voltage
Iq: Current consumption of linear regulator
Vout: Output voltage of linear regulator
Iout: Output current of linear regulator
The loss here cannot be ignored since it is large enough to reduce the conversion efficiency of the DC—DC converter circuit. This problem is magnified when the output current of the DC—DC converter circuit is reduced, because the loss in the linear regulator becomes relatively large.
The loss in the regulator circuit
200
can be reduced by lowering the input voltage, but there is a limit to how much the input voltage can be lowered.
That is, the relation
Input voltage≦N-channel MOSFET drive voltage+Voltage drop across regulator circuit must be satisfied. Usually, about 0.5 V must be allowed for the voltage drop across the regulator circuit
200
(linear regulator), and this imposes a limit on how much the input voltage can be lowered. Accordingly, the loss in the regulator circuit
200
cannot be reduced below a certain level.
On the other hand, in the prior art shown in
FIG. 16
, the problem of reduced conversion efficiency of the DC—DC converter circuit due to the loss in the regulator circuit
200
does not occur because the regulator circuit
200
is disconnected once the output voltage of the DC—DC converter circuit has risen. However, since the N-channel MOSFET that can be used in practice as the main switching device Q
1
is one that operates with 5 V, the prior art has the problem that it cannot be applied to loads operating with voltages lower than 5 V.
Nowadays, battery-operated apparatuses operating with 2 to 3 volts are becoming widespread. If the DC—DC converter circuit shown
Armstrong, Westerman Hattori, McLeland & Naughton
Fujitsu Limited
Nguyen Matthew
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