TOP242-249
AhighVOR isrequiredtotakefulladvantageofthewiderDCMAX
of TOPSwitch-GX. An RCD clamp provides tighter clamp
voltage tolerance than a Zener clamp and allows a VOR as high
as 150 V. RCD clamp dissipation can be minimized by
reducing the external current limit as a function of input line
voltage (see Figure 21 and 35). The RCD clamp is more cost
effective than the Zener clamp but requires more careful design
(see quick design checklist).
For 10 W or below, it is possible to use a simple inductor in
place of a more costly AC input common mode choke to meet
worldwide conducted EMI limits.
Transformer Design
Itisrecommendedthatthetransformerbedesignedformaximum
operating flux density of 3000 Gauss and a peak flux density of
4200 Gauss at maximum current limit. The turns ratio should be
chosen for a reflected voltage (VOR) no greater than 135 V when
using a Zener clamp, or 150 V (max) when using RCD clamp
with current limit reduction with line voltage (overload
protection).
Output Diode
The output diode is selected for peak inverse voltage, output
current, and thermal conditions in the application (including
heatsinking, air circulation, etc.). The higher DCMAX of
TOPSwitch-GX along with an appropriate transformer turns
ratio can allow the use of a 60 V Schoktty diode for higher
efficiency on output voltages as high as 15 V (see Figure 41. A
12 V, 30 W design using a 60 V Schottky for the output diode).
For designs where operating current is significantly lower than
the default current limit, it is recommended to use an externally
set current limit close to the operating peak current to reduce
peak flux density and peak power (see Figure 20 and 34). In
most applications, the tighter current limit tolerance, higher
Bias Winding Capacitor
Due to the low frequency operation at no-load a 1µF bias
winding capacitor is recommended.
80
70
60
50
40
30
20
-10
0
TOPSwitch-II (no jitter)
Soft-Start
Generally a power supply experiences maximum stress at start-
up before the feedback loop achieves regulation. For a period
of 10 ms the on-chip soft-start linearly increases the duty cycle
from zero to the default DCMAX at turn on. In addition, the
primary current limit increases from 85% to 100% over the
same period. This causes the output voltage to rise in an orderly
manner allowing time for the feedback loop to take control of
the duty cycle. This reduces the stress on the TOPSwitch-GX
MOSFET, clamp circuit and output diode(s), and helps prevent
transformersaturationduringstart-up. Alsosoft-startlimitsthe
amount of output voltage overshoot, and in many applications
eliminates the need for a soft-finish capacitor.
EN55022B (QP)
EN55022B (AV)
-10
-20
0.15
1
10
30
Frequency (MHz)
Figure 46a. TOPSwitch-II Full Range EMI Scan
(100 kHz, no jitter)
EMI
80
The frequency jitter feature modulates the switching frequency
over a narrow band as a means to reduce conducted EMI peaks
associated with the harmonics of the fundamental switching
frequency. This is particularly beneficial for average detection
mode. AscanbeseeninFigure46, thebenefitsofjitterincrease
with the order of the switching harmonic due to an increase in
frequency deviation.
70
60
TOPSwitch-GX (with jitter)
50
40
30
20
-10
0
The FREQUENCY pin of TOPSwitch-GX offers a switching
frequency option of 132 kHz or 66 kHz. In applications that
require heavy snubbers on the drain node for reducing high
frequency radiated noise (for example, video noise sensitive
applications such as VCR, DVD, monitor, TV, etc.), operating
at66kHzwillreducesnubberlossresultinginbetterefficiency.
Also, in applications where transformer size is not a concern,
use of the 66 kHz option will provide lower EMI and higher
efficiency. Note that the second harmonic of 66 kHz is still
below 150 kHz, above which the conducted EMI specifications
get much tighter.
EN55022B (QP)
EN55022B (AV)
-10
-20
0.15
1
10 30
Frequency (MHz)
Figure 46b. TOPSwitch-GX Full Range EMI Scan (132 kHz,
with jitter) with Identical Circuitry and
Conditions.
E
7/01
August 8, 2000
30
POWERINT [ Power Integrations ]
