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參數(shù)資料
| 型號： | AN-937 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | Gate Drive Characteristics and Requirements for HEXFET |
| 中文描述： | 門驅(qū)動器的特點及要求的HEXFET |
| 文件頁數(shù)： | 4/21頁 |
| 文件大?。?/td> | 418K |
| 代理商： | AN-937 |
AN-937 (v.Int)
decreasing drain-source voltage is reflected across the drain-gate capacitance, pulling a discharge current through it, and
increasing the effective capacitive load on the drive circuit.
This in turn increases the voltage drop across the source impedance of the drive circuit, and decreases the rate of rise of voltage
appearing between the gate and source terminals. Obviously, the lower the impedance of the gate drive circuit, the less this effect
will be. This also is a negative feedback effect; increasing current in the drain results in a fall of drain-to-source voltage, which in
turn slows down the rise of gate-source voltage, and tends to resist the increase of drain current. These effects are illustrated
diagramatically in Figure 4. This state of affairs continues throughout the period t
1
to t
2
, as the current in the HEXFET
rises to
the level of the current, I
M
, already flowing in the freewheeling rectifier, and it continues into the next period, t
2
to t
3
, when the
freewheeling rectifier goes into reverse recovery.
Finally, at time t
3
the freewheeling rectifier starts to support voltage and drain current and voltage start to fall. The rate of fall of
drain voltage is now governed almost exclusively by the Miller effect, and an equilibrium condition is reached, under which the
drain voltage falls at just the rate necessary for the voltage between gate and source terminals to satisfy the level of drain current
estab-lished by the load. This is why the gate-to-source voltage falls as the recovery current of the freewheeling rectifier falls,
then stays constant at a level corresponding to the drain current, while the drain voltage falls. Obviously, the lower the impe-
dance of the gate-drive circuit, the higher the discharge current through the drain-gate self-capacitance, the faster will be the fall
time of the drain voltage and the switching
losses.
Finally, at time t
4
, the HEXFET
is switched fully
on, and the gate-to-source voltage rises rapidly
towards the applied “open circuit” value.
Similar considerations apply to the turn-off
interval. Figure 5 shows theoretical waveforms
for the HEXFET
in the circuit of Figure 4 during
the turn-off interval. At t
o
the gate drive starts to
fall until, at t
l
, the gate voltage reaches a level
that just sustains the drain current and the device
enters the linear mode of operation. The drain-to-
source voltage now starts to rise. The Miller
effect governs the rate-of-rise of drain voltage
and holds the gate-to-source voltage at a level
corresponding to the constant drain
current.
Once again, the lower the impedance of the drive
circuit, the greater the charging current into the
drain-gate capacitance, and the faster will be the
rise time of the drain voltage. At t
3
the rise of
drain voltage is complete, and the gate voltage
and drain current start to fall at a rate determined
by the gate-source circuit impedance.
We have seen how and why a low gate drive
impedance is important to achieve high
switching performance. However, even when
switching performance is of no great concern, it
is important to minimize the impedance in the
gate drive circuit to clamp unwanted voltage
transients on the gate. With reference to Figure
6, when one HEXFET
is turned on or off, a step
of voltage is applied between drain and source of
the other device on the same leg. This step of
voltage is coupled to the gate through the gate-to-
drain capacitance, and it can be large enough to
turn the device on for a short instant (“dv/dt
induced turn-on”). A low gate drive impedance would keep the voltage coupled to the gate below the threshold.
A STEP OF VOLTAGE CAUSES
V
DS
Q
1
V
DS
Q
2
V
GS
Q
1
V
GS
Q
2
A TRANSIENT
ON THE GATE
Figure 6.
Transients of Voltage Induced on the Gate by Rapid
Changes on the Drain-to-Source Voltage
To Order
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