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| 型號： | AN-30 |
| 英文描述： | TOPSwitch-GX Forward Design Methodology |
| 中文描述： | 的TOPSwitch - GX系列正向設(shè)計方法 |
| 文件頁數(shù)： | 19/40頁 |
| 文件大?。?/td> | 311K |
| 代理商： | AN-30 |
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AN-30
19
B
12/02
The remaining components are connected across the forward
diode on the main output. Energy from leakage inductance on
the secondary and magnetization inductance of the transformer
charges C
when the
TOPSwitch-GX
turns off. The energy
from C
is delivered to the output during the next switching
cycle. Resistor R
provides damping for oscillations that
would otherwise occur from the resonance of C
CS
with stray
inductance.
Typical values are in the neighborhood of 0.1
μ
F for C
and
1
for R
. The resistor must dissipate power that corresponds
to the charge and discharge of C
each cycle. It typically will
dissipate less than 1 watt. Proper values must be determined
empirically from evaluation of prototype hardware.
Step 18. Choose components for the feedback circuit.
The pulse width modulator in
TOPSwitch-GX
sets the duty
ratio according to the current into the CONTROL pin.
TOPSwitch-GX
senses the drain current for protection only, and
does not use it for control purposes. Thus, forward converters
with
TOPSwitch-GX
operate with a voltage-mode control that
modulates the converter’s duty ratio directly according to an
error signal from the regulated output voltage. Voltage mode
control provides sufficient loop bandwidth and is fully able to
meet all the specifications for PC Main and other high power
applications.
The general configuration of the feedback circuit for a forward
converter with
TOPSwitch-GX
is illustrated in Figure 11. It
shows a typical connection of a TL431 voltage regulator with
an optocoupler and components for frequency compensation.
There is an optional connection to V
to improve the regulation
of the auxiliary output by sharing regulation with the main
output. This general technique is common in all types of
multiple output regulators.
While the design of the feedback loop is beyond the scope of
this application note, it is useful to consider the general circuit
of Figure 11. The components are chosen to provide regulation
of output voltages and to shape the frequency characteristics of
the control loop. Proper design of the feedback components is
important not only for the stability of the system, but also for
transient response of the output.
Inductor L
with capacitor C
reduces high frequency noise on
the main output. As such, it introduces phase shift in the small
signal response that would make loop compensation difficult if
the only feedback for the main output were taken from the
voltage on C
. To avoid difficulties with the feedback loop,
information about the main output is taken from two places.
Low frequency information that is most important to the DC
regulation comes mainly through the path formed by resistor
divider of R5, R6 and R7. The voltage on R7 is the reference
voltage of the TL431 when V
MAIN
and V
AUX
are at their desired
values.
High frequency information that is most important in the
transient response comes through the path formed by the
optocoupler’s diode and R2. This same technique is commonly
used with
TOPSwitch-GX
in flyback converter applications.
The values of R1, R3, R4, C1 and C2 are chosen to shape the
frequency response. The choices are influenced by the
components on the CONTROL pin and equivalent series
resistance of the output capacitor, which can be important
features of the loop gain. Designers must make proper
measurements of loop gain and transient response on prototype
hardware to confirm that the converter performs as desired
under all specified conditions.
Evaluation of Prototype Hardware
The design that results from the steps of the previous section
contains the uncertainties of the initial assumptions. Performance
must be validated with measurements on prototype hardware
before the design is complete.
At this stage in the procedure, the designer will have enough
information to build a circuit that will operate at nominal
conditions for evaluation on the bench. The designer must test
the circuit at all the limits of specified performance.
Measurements will indicate which changes to the original
assumptions are necessary. A successful design is obtained
after repetition of the procedure with parameters adjusted from
measurements on the hardware.
The evaluation should include observation of the drain-to-
source voltage on
TOPSwitch-GX
under steady state operation
and transient conditions. Apply power to the converter slowly
with minimum loads. Then exercise the loads on the outputs in
different combinations, first at the nominal input voltage and
then at the extremes of input voltage.
Observe the behavior at various static loads before going to
transient loading. Check for excessive power dissipation in the
clamp circuit. A useful technique is to monitor the average
current in the string of Zener diodes in the clamp circuit with a
low value resistor in series. A capacitor in parallel with the
resistor will develop a voltage proportional to the average
current through the diodes. The product of this voltage and the
clamp voltage gives an indication of the power dissipation in the
Zener diodes.
Monitor the drain current when the output has steady-state
overload and during transient loading. The waveform will
provide important information about the operation of the
converter and the limits of the design. Check that the current
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