參數(shù)資料
型號(hào)： AN-22
英文描述： OBSOLETE when inventory is depleted. 10% tolerance no l
中文描述： 設(shè)計(jì)多路輸出電源的的TOPSwitch
文件頁(yè)數(shù)： 16/24頁(yè)
文件大?。?/td> 569K
代理商： AN-22
AN-22
C
5/98
16
Figure 11 (a), (b). Two Configurations to Get Negative Outputs.
additional measures are necessary to optimize the EMI
performance. This is particularly true when the outputs are
galvanically isolated from each other. In motor control circuits,
for example, several isolated outputs may be required to
supply high side drivers in an inverter output stage.
In these cases it is important that displacement currents driven
by the
TOPSwitch
DRAIN node through the transformer
’
s
interwinding capacitance have a low impedance return path
from a specific output to the primary side of the power supply.
This consideration demands that each isolated output provide
a low impedance path for common mode displacement
currents to return from its own return to the primary return
(
TOPSwitch
SOURCE potential). This low impedance path
can usually be provided from the output
’
s return through a
capacitor (suitably rated for the isolation voltage required on
a particular output) to the main secondary return, from where
a safety Y capacitor is connected to the primary return rail.
This configuration is shown in Figure 9, where the isolated
30 V output has a 500 V capacitor, C13, connected between
its return rail and that of the main power supply output.
If these low impedance capacitive paths are not provided on
each isolated output, then the common mode displacement
currents transferred through the transformer interwinding
capacitance will return to their source on the primary of the
transformer through any alternative route that is available.
The common mode currents may split many times on their
route to the DRAIN node. If a capacitive return path is not
present, there is the risk that enough of the displacement
current will flow through the AC input conductors to fail
regulatory emission specifications.
The need for additional capacitors in this type of circuit
depends on the transformer
’
s interwinding capacitance.
Additional capacitors from an isolated output may not be
necessary if its capacitance to the primary is low enough.
However, tests are essential to verify the necessity of additional
components.
One other EMI consideration related to output diode snubbers
is worthy of note. Output diodes are always a source of
additional noise that depends on their forward and reverse
recovery characteristics, particularly the di/dt and dv/dt during
recovery. Many diodes are now available with so called
‘
soft
recovery
’
characteristics which are designed to limit switching
noise. It is often desirable, however, to further snub the diode
characteristics with external components.
These external snubbers are usually a single capacitor, or series
resistor and capacitor in parallel with the output diodes.
In many cases the snubbing circuitry can be limited to a single
output diode to achieve the desired reduction in switching
noise. In such cases, the highest voltage winding with
significant loading should be chosen for the snubber circuitry.
In this example, the 12 V output diode would be chosen since
the capacitors on that output have lower ESR than the
capacitors on the 30 V output. It also has the best overall
coupling with the primary winding because it is physically
closest. During the primary switching events, these snubber
components are an AC current path in series with the output
electrolytic capacitors. They therefore provide a low
impedance AC path across the transformer output winding and
the output diode to confine the noise currents created by
primary switching events.
+V Output
(a)
(b)
PI-2130-120297
Output
RTN
–
V Output
DC Rail
DRAIN
Bias
Primary
RTN
+V Output
Output
RTN
–
V Output
DC Rail
DRAIN
Bias
Primary
RTN
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