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| 型號： | AN-20 |
| 英文描述： | Transient Suppression Techniques for TOPSwitch Power Supplies |
| 中文描述： | 暫態(tài)的TOPSwitch電源抑制技術(shù) |
| 文件頁數(shù)： | 1/8頁 |
| 文件大小： | 112K |
| 代理商： | AN-20 |
June 1996
Introduction
AC power mains occasionally have transient surge voltages.
Lightning strikes and AC mains load switching are just two
examples of many possible conditions causing transient voltages.
Consequently, all off-line power supplies must provide some
level of protection to suppress the effects of such transient
voltages.
This application note presents design techniques which improve
TOPSwitch
power supply operation through most AC mains
transient surge voltages. Properly designed transformers, PC
boards, and EMI filters not only suppress the effects from
transient voltages but also reduce both conducted and radiated
EMI emissions as well. These techniques can also be used in
applications with DC input voltages such as Telecom and
Television Cable Communication (or Cablecom).
The ST202A Reference Design Board using the TOP202YAI
TOPSwitch
will be used as an example throughout this
application note. Refer to the ST202A data sheet as well as
AN-14 and AN-15 for additional information.
Typical Transient Test Voltages
Figure 1 shows a typical surge voltage waveform specified by
IEC 1000-4-5 (formerly IEC 801-4-5). Peak test voltages (U)
of 3kV are common but in some applications higher peak
voltages are specified. The surge voltage waveform has a 1.2
μ
S
front time T
1
and 50
μ
S time to half value T
2
as shown.
Figure 2 shows a typical ring wave voltage waveform specified
by IEEE-587. Peak test voltages (V
) of 3kV are common but
in some applications higher voltages are specified. The open
circuit ring wave voltage waveform has a 0.5
μ
S rise time to
90% of peak value and exponentially decays while oscillating
at 100 kHz with each peak being 60% of the preceding peak.
The transient test voltage may be applied both in common mode
and differential mode configurations. The common mode
configuration shown in Figure 3 applies the transient test
voltage first to one AC mains conductor and then the other with
respect to earth ground. The
TOPSwitch
power supply output
should be connected either directly to earth ground or AC
coupled through a capacitor to earth ground. This transient test
voltage causes high peak transient ringing currents to flow
between the
TOPSwitch
power supply primary and secondary.
Without proper attention to EMI filter design, transformer
design, and PC layout design, transient currents couple into
signal traces and generate voltage spikes capable of setting the
TOPSwitch
shutdown latch.
U
T
t
T1
T2
1.0
0.9
0.5
0.3
0.1
0.0
PI-1709-120595
Front Time
T1
= 1.67 x T = 1.2
μ
s ± 30%
Time To Half Value
T2
= 50
μ
s ± 20%
Figure 1. Waveshape of Open Circuit Voltage (1.2/50
μ
S)
From IEC-1000-4-5.
0.9 V peak
60% OF V peak
0.1 V peak
0.5
μ
s
T = 10
μ
s (f = 100 kHz)
V peak
PI-1710-120595
Figure 2. 0.5
μ
s-100 kHz Ring Wave (Open-Circuit Voltage)
From IEEE-587.
Transient Suppression Techniques
for
TOPSwitch
Power Supplies
Application Note AN-20
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