参数资料
型号: FSD176MRTUDTU
厂商: Fairchild Semiconductor
文件页数: 10/17页
文件大小: 0K
描述: IC OFF-LINE SWITCH PWM TO220-6
标准包装: 50
系列: FPS™
输出隔离: 隔离
频率范围: 61kHz ~ 73kHz
输入电压: 13 V ~ 26 V
输出电压: 650V
功率(瓦特): 90W
工作温度: -40°C ~ 125°C
封装/外壳: *
供应商设备封装: *
包装: *
4. Protection Circuits : The FSD176MRT has several
self-protective functions, such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP),
Output-Short Protection (OSP), Over-Voltage Protection
(OVP), and Thermal Shutdown (TSD). All the
protections are implemented as auto-restart. Once a
fault condition is detected, switching is terminated and
charge C FB slowly up . In this condition, V FB continues
increasing until it reaches 7.0 V, when the switching
operation is terminated, as shown in Figure 20. The
delay for shutdown is the time required to charge C FB
from 2.5 V to 7.0 V with 2.0 μ A. A 25 ~ 50 ms delay is
typical for most applications. This protection is
implemented in auto-restart mode.
When V CC falls to the Under-Voltage Lockout (UVLO)
the SenseFET remains off. This causes V CC to fall.
B B
stop voltage of 7.5 V, the protection is reset and the
startup circuit charges the V CC capacitor. When V CC
reaches the start voltage of 12.0 V, the FSD176MRT
resumes normal operation. If the fault condition is not
removed, the SenseFET remains off and V CC drops to
stop voltage again. In this manner, the auto-restart can
alternately enable and disable the switching of the
power SenseFET until the fault condition is eliminated.
Because these protection circuits are fully integrated
into the IC without external components, the reliability is
improved without increasing cost.
Figure 20. Overload Protection
4.2 Abnormal Over-Current Protection (AOCP) :
When the secondary rectifier
diodes or the
transformer pins are shorted, a steep current with
extremely high di/dt can flow through the SenseFET
during the minimum turn-on time. Overload protection
is not enough to protect the FSD176MRT in that
abnormal case; since severe current stress is
imposed on the SenseFET until OLP is triggered. The
FSD176MRT internal AOCP circuit is shown in Figure
21. When the gate turn-on signal is applied to the
power SenseFET, the AOCP block is enabled and
monitors the current through the sensing resistor. The
voltage across the resistor is compared with a preset
AOCP level. If the sensing resistor voltage is greater
than the AOCP level, the set signal is applied to the
Figure 19. Auto-Restart Protection Waveforms
4.1 Overload Protection (OLP) : Overload is defined
as the load current exceeding its normal level due to
an unexpected abnormal event. In this situation, the
protection circuit should trigger to protect the SMPS.
However, even when the SMPS is in normal
operation, the overload protection circuit can be
triggered during load transition. To avoid this
undesired operation, the overload protection circuit is
designed to trigger only after a specified time to
determine whether it is a transient situation or a true
overload situation. Because of the pulse-by-pulse
current limit capability, the maximum peak current
through the SenseFET is limited and, therefore, the
maximum input power is restricted with a given input
voltage. If the output consumes more than this
maximum power, the output voltage (V OUT ) decreases
below the set voltage. This reduces the current
through the opto-coupler LED, which also reduces the
opto-coupler transistor current, thus increasing the
feedback voltage (V FB ). If V FB exceeds 2.5 V, D1 is
blocked and the 2.0 μA current source starts to
? 2011 Fairchild Semiconductor Corporation
FSD176MRT ? Rev. 1.0.1
10
S - R latch, resulting in the shutdown of the SMPS.
Figure 21. Abnormal Over-Current Protection
www.fairchildsemi.com
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