Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1994-10-31
2001-03-27
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06208535
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to electronic devices, and, more particularly, to junction field effect transistor drivers.
Electronic components, such as integrated circuits and displays, typically have power requirements which differ from the primary power supply characteristics. For example, portable computers may contain integrated circuits operating with a 3.3 volt DC supply and a backlit display screen operating at 1500 volts but the primary power consists of rechargeable batteries whose output voltage at full charge may be 5 volts and which drops exponentially as the batteries are discharged. Hence, electronic systems typically will include power supplies with AC-DC converters, DC-DC regulators, or DC-AC inverters to provide output power with the required characteristics.
DC-DC regulators most commonly utlilize switching regulation and may use a push-pull arrangement as illustrated in
FIG. 1
a
with waveforms shown in
FIG. 1
b.
Basically, the pulse width modulated (PWM) driver alternately switches on and off power devices Q
1
and Q
2
to excite the transformer primary, and the transformer secondary feeds a self-commutating synchronous rectifier followed by an LC filter for output. The synchronous rectifier uses n-channel MOSFETs rather than diodes; this avoids the turn on voltage drop of diodes which can be significant for low output voltage power supplies. Resistor divider R senses the output voltage and feeds this back to the PWM driver. If the output voltage is too low, then the PWM driver increases the duty cycles of Q
1
and Q
2
, and conversely, if the output voltage is too high, the duty cycles of Q
1
and Q
2
are reduced. The PWM driver may simply be an error amplifier (amplifying the difference between desired and actual output voltages) feeding one input of a comparator with a sawtooth voltage having a fixed frequency feeding the other comparator input; the comparator output would be the input signal for a driver for power device Q
1
and a similar phase-shifted comparator would be the input signal for a driver for power device Q
2
.
FIG. 1
c
illustrates such a possible half of a PWM driver.
Switching power supplies for portable computers and other portable electronic equipment generally benefit from higher switching frequencies because the size and weight of the magnetic portions (transformers and inductors) can be reduced. And the trend towards lower operating voltages for integrated circuits to reduce power consumption requires the output rectifiers of related power supplies have low on resistance and minimal voltage drop.
Junction field effect transistors (JFETs) typically are depletion mode devices with n-type channels and require a negative gate voltage to pinch off the channel and turn off. Thus the driver for a JFET also requires a negative power supply. Further, injecting carriers from the gate of a JFET into the channel while the JFET is turned on can lower the channel and drift region resistance (R
ON
) and thereby minimizes ohmic losses. This “bipolar mode” of operation requires a small positive voltage to forward bias the gate, and so the JFET driver would further require a small positive power supply. See Baliga, Modern Power Devices pp.175-182 (Krieger Publ., Malabar, Fla. 1992). Thus the known drivers for JFET devices have problems including inefficient circuitry.
JFETs made be made of gallium arsenide to lower R
ON
due to the higher electron mobility of gallium arsenide as compared to silicon. JFETs frqeuently have a vertical channel structure and thus may be called VFETs.
FIGS. 2
a-b
heuristically illustrate in perspective and cross sectional elevation views VFET
100
as including a source
102
, multifinger gate
104
, channel region
106
between the gate fingers, drain
108
, source contact
112
, gate contact
114
, and drain contact
118
. U.S. Pat. No. 5,231,037 describes a method of fabrication for such VFETs.
SUMMARY OF THE INVENTION
The present invention provides a resonant switching for a field effect transistor and associated circuits such as power supplies with synchronous output rectifiers. The resonant switching for a junction field effect transistor invokes bipolar mode operation with a diode clamping of the gate.
This has the advantage of yielding bipolar mode operation without a separate bias power supply.
REFERENCES:
patent: 5140201 (1992-08-01), Uenishi
patent: 5264736 (1993-11-01), Jacobson
patent: 5265003 (1993-11-01), Kayser et al.
patent: 5296765 (1994-03-01), Williams et al.
patent: 5328845 (1994-07-01), Takahashi
Brady W. James
Hoel Carlton H.
Nguyen Matthew
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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