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| 型號(hào)： | AN-30 |
| 英文描述： | TOPSwitch-GX Forward Design Methodology |
| 中文描述： | 的TOPSwitch - GX系列正向設(shè)計(jì)方法 |
| 文件頁數(shù)： | 9/40頁 |
| 文件大小： | 311K |
| 代理商： | AN-30 |
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AN-30
9
B
12/02
from D
in Equation (8). In Equation (10), L
is the
leakage inductance of the secondary winding of the main
output, I
is the winding current required to turn off the
catch diode of the main output, and f
is the switching frequency.
Note that in the DC stacked connection for the auxiliary output,
the winding for the main output carries the current of the main
output plus the current of the stacked auxiliary outputs.
Next, compute the duty ratio D
NOM
that corresponds to the
nominal input voltage.
(11)
This allows a better estimate of the turns ratio that will produce
the desired independent output voltage.
(12)
Finally, compute the turns ratio for the bias winding so that the
bias voltage is greater than eight volts. This value is the
CONTROL pin voltage, 5.8 V, plus the 2.2 V saturation voltage
of the optocoupler’s phototransistor at V
DROPOUT
.
The turns ratio for the bias winding is then
(13)
where V
is the minimum DC bus voltage for regulation
and V
is the voltage drop on the rectifier for the bias voltage.
Check that the breakdown voltage on the phototransistor of the
optocoupler is higher than the bias voltage at the highest
transient input voltage.
Step 4. Calculate the primary current.
Find the peak and RMS values for the primary current. This is
a preliminary estimate from the system parameters. It allows
the designer to assess the suitability of his application for
TOPSwitch-GX
as early as possible.
Figure 7 shows typical primary current waveforms for forward
converters with and without a magnetic amplifier post regulator.
Figure 7(a) is without a magnetic amplifier, whereas
Figure 7(b) shows the effect of one magnetic amplifier post
regulator.
TOPSwitch-GX
determines the duty ratio D to
regulate the main output, whereas the post regulator sets D
independently by its own local feedback to regulate the output
voltage from the magnetic amplifier.
n
V
V
+
V
V
D
D
V
V
P
DROPOUT
DS
MAIN
DMAINC
MAX
MAX
MAIN
DMAINF
=
+
(
)
+
1
n
V
V
V
D
V
P
DROPOUT
DS
MAX
MAIN
DMAIN
=
+
(
)
D
V
V
V
n
V
V
NOM
MAIN
DMAINC
NOM
P
DMAINF
DMAINC
=
+
+
n
V
V
V
V
V
AUX
AUX
DAUXC
+
AUXREF
MAIN
DMAINC
=
+
δ
D
MAINLK MAINSEC
L
V
MAIN
MAX S
D
f
=
n
n
V
V
B
P
DB
DROPOUT
≥
+
8 volts
n
V
V
V
D
D
V
V
D
V
D
IND
IND
DINDF
NOM
DINDC
NOM
MAIN
DMAINF
NOM
DMAINC
NOM
=
+
+
+
+
1
(
)
(
)
1
where V
is the DC bus voltage at the end of the holdup
time and V
is the maximum drain-to-source voltage on the
TOPSwitch-GX
during operation. The minimum recommended
value for V
is 130 V, while V
is usually less than the
breakdown voltage of 700 V by a comfortable safety margin. A
safety margin of 15% is typical, giving 600 V for V
DSOP
.
The maximum duty ratio for the converter occurs at V
.
This must be reduced as a function of line voltage from the
DC
of
TOPSwitch-GX
by external circuitry in Step 15. The
recommended maximum duty ratio D
for the forward
converter application depends on the operating input voltage
range. For a 3:1 operating range (V
:V
) 70% is typical.
As the operating range reduces so does the value of D
.
For a 2:1 operating range a value of 50% would be selected.
First, compute the turns ratios for the primary and the auxiliary
winding. The turns ratio on the primary of the transformer is
(8a)
Where V
is the average drain-to-source voltage during the
on-time of
TOPSwitch-GX
:
When V
and V
DMAINC
are the same value V
DMAIN
, this
equation simplifies to:
(8b)
The turns ratio for the auxiliary winding is
(9)
Equation (8) is valid for systems where the leakage inductance
of the transformer is negligible. This is a reasonable assumption
because the leakage inductance must be minimized for low
power dissipation and proper operation of the clamp
circuit. Leakage inductance reduces the effective duty ratio on
the secondary circuits by delaying the turn-off of the catch
diodes. The effect can be significant in designs with very high
output currents. To compute the turns ratio for the primary
winding when leakage inductance is a consideration, subtract
the constant
(10)
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