Transformer isolated driver

Details Transformer isolated driver Transformer isolated driver

2003-07-28

2004-10-19

Berhane, Adolf (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S131000

Reexamination Certificate

active

06807071

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a transformer isolated driver, and more particularly to a transformer isolated gate driver with an active switch (For example: a small power MOSFET) so as to achieve reliable transient performance.
BACKGROUND OF THE INVENTION
The transformer isolated drivers are used for the driving of the high side power switch (For example: a MOSFET). Please refer to
FIG. 1
, which is the circuit of a transformer isolated driver of the prior art. T
1
is an isolation transformer, QM is a switch (a MOSFET), the capacitor C
1
is an input capacitor of the transformer T
1
, and the reference directions of the voltages on the capacitors are shown in
FIG. 1
, and C
3
is an equivalent input capacitor of the switch QM.
FIG. 2
is the operating waveforms of the circuit shown in FIG.
1
. S
1
is the waveform of the output signal of the pulse width modulation (PWM) driver, S
2
is the waveform of the primary winding of the transformer, and S
3
is the output waveform of the transformer. Assuming that in the steady state, the period of the output signal of the PWM driver S
1
is T, the duty-cycle is D, the amplitude value is V
1
, and the turns ratio of the transformer T
1
is 1, then the steady state voltage on the input DC isolation capacitor C
1
is DV
1
. When S
1
is high, S
3
is also high, and the amplitude of S
3
is (V
1
−V
C1
), that is (1−D)V
1
. When S
1
is low, S
2
is negative, and the amplitude of S
2
is (−V
C1
), that is −DV
1
. Thus, the switch QM has a reversed gate driving voltage so as to have a strong characteristic of anti-interference. However, a shortcoming of the circuit is that when the duty cycle D is larger, the amplitude of S
3
, which is (1−D)V
1
, becomes smaller, which might cause the insufficient driving of the switch QM. Therefore, the circuit of this kind of drivers is not suitable for the situations having larger variations of duty cycle.
Please refer to
FIG. 3
, which is another transformer isolated driver of the prior art. The polarities of the transformer T
1
are shown in
FIG. 3
, capacitor C
1
is an input capacitor at the primary side of the transformer, capacitor C
2
is an output capacitor at the secondary side of the transformer, and C
3
is the equivalent input capacitor of a power switch QM (a MOSFET). Please refer to
FIG. 4
for operating waveforms of the circuit shown in
FIG. 3
, S
1
is the waveform of the output signals of the PWM driver, S
2
is the waveform on the primary of the transformer, and S
3
is the output waveform of the transformer. Assuming that in the steady state, the period of the output signal of the PWM driver S
1
is T, the duty-cycle is D, the amplitude value of S
1
is V
1
, and the turn ratio of the transformer is 1, then the voltage of steady state across the input capacitor C
1
at steady state is V
C1
=DV
1
, the voltage across the output capacitor C
2
is V
C2
=DV
1
, and the reference directions of the voltages across these two capacitors are shown in FIG.
3
. When S
1
is high, S
2
and S
3
are both high. The amplitude of S
2
is V
2
=(V
1
−V
C1
), that is. (1−D)V
1
. The amplitude of S
3
is V
3
=(V
2
+V
C2
), that is V
1
. Thus the amplitude of S
3
has no relationship with the duty-cycle D, and keeps unchanged as V
1
at any duty-cycle operating condition. When S
1
becomes zero, the input/output voltages of the transformer becomes −DV
1
, thus the diode DR is conducted, and the voltage across the equivalent input capacitor C
3
of the switch QM is kept zero.
However, the still existed drawback of the above-mentioned transformer isolated drivers is: that when the driving signal S
1
is disappeared due to certain reasons, S
3
will become zero immediately as mentioned above. But latter on, the input of the transformer T
1
is crossed by the bias of voltage (−DV
1
) from the capacitor C
1
, and will be saturated gradually, thus the amplitude of the input/output voltages of the transformer T
1
will become smaller starting from DV
1
. Since the voltage across the capacitor C
2
, is still DV
1
, therefore, the voltage across the equivalent input capacitor C
3
of the switch QM will be increased starting from zero. Until the transformer T
1
is saturated, then the primary and secondary side voltages of the transformer T
1
will become zero, and the voltage across C
3
will finally become DV
1
, which will cause the switch QM to suffer a mis-triggering in a longer time resulting in circuit damage. Please refer to
FIG. 5
for the recorded operating waveform of S
3
under circuit protection operating. Therefore, this kind of driving circuits does not have a better transient operating. When a power converter implemented with this driver is started up or is shut down, the power switch will suffer a mis-triggering, thus causing the break down of the whole power converter.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a transformer isolated gate driver with an active switch (a small power MOSFET) for driving of a power switch with reliable transient performance, and avoiding the power switch from suffering mis-triggering so as to prevent the break down of the whole power converter.
According to the aspect of the present invention, the transformer isolated gate driver electrically connected to a power switch for realizing the driving of the power switch includes: a PWM driver for producing a PWM signal; a transformer electrically connected to the PWM driver; an input capacitor electrically connected to a primary side of the transformer in series; an output capacitor electrically connected to a secondary side of the transformer in series; an output diode electrically connected to two output terminals of the transformer isolated gate driver in parallel; an active switch electrically connected to the output capacitor in series and having an input electrode electrically connected to a first terminal of the secondary side of the transformer, and a first electrode electrically connected to .a second terminal of the secondary side of the transformer; and the secondary side of the transformer, the output capacitor, the output diode, and the active switch are electrically connected in series, and when the PWM signal is disappeared, by discharging a plurality of charges across an equivalent input capacitor of the power switch through the secondary side of the transformer, the output capacitor, and a body diode of the active switch, a driving signal on the power switch is disappeared.
Preferably, the two polarities of the transformer are the same.
Preferably, the two polarities of the primary and secondary windings of the transformer are reversed.
Preferably, the power switch is a MOSFET.
Preferably, the active switch is a small power MOSFET, and the input electrode, the first electrode, and the second electrode of the active switch are a gate, a source, and a drain of a MOSFET respectively.
Preferably, the output capacitor is electrically connected between a cathode of the output diode and the input electrode of the active switch.
Preferably, the output capacitor is electrically connected between an anode of the output diode and the second electrode of the active switch.
Preferably, one selected from a group consisting of the input capacitor, the output capacitor, and the two output terminals of the transformer isolated gate driver is electrically connected to a resistor in parallel selectively.
Preferably, an input diode is electrically connected to the input capacitor in parallel, wherein an anode of the input diode is connected to a first end of the input capacitor connecting with the primary winding of the transformer, and a cathode of the input diode is connected to a second end of the input capacitor.
According to another aspect of the present invention, a transformer isolated circuit electrically connected to a PWM driver and a power switch for realizing the driving of the power switch includes: a transformer; an input capacitor electrically

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