FS7M0680TU Fairchild Semiconductor, FS7M0680TU Datasheet - Page 6

• Manufacturer Part Number: FS7M0680TU
• Description: IC SWIT PWM CM OVP UVLO HV TO3P
• Manufacturer: Fairchild Semiconductor

Specifications of FS7M0680TU

• Output Isolation: Isolated
• Frequency Range: 60 ~ 72kHz
• Voltage - Input: 9 ~ 30 V
• Voltage - Output: 800V
• Power (watts): 150W
• Operating Temperature: 25°C ~ 150°C
• Package / Case: TO-3P-5
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

FS7M0680
Pulse by pulse current limit
Figure 3 shows the pulse-width-modulation (PWM) block of
the FPS. Since the FPS employs the peak current mode con-
trol, the current through the power MOSFET is limited by
the inverting input voltage of PWM comparator (Vfb*).
Assuming that the 0.9mA current source flows only through
the internal resistor (2.5R + R
voltage drop is 0.7V, the anode voltage of diode D2 is about
3.2V. Since D1 is blocked when the feedback voltage (Vfb)
exceeds 3.2V, the maximum voltage of the anode of D2 is
3.2V. Therefore, the maximum value of Vfb* is about 0.7V,
which determines the maximum current through the power
MOSFET.
Over Load Protection
Overload means that the load current exceeds a pre-set level
due to the abnormal situation. In this situation, protection
circuit should be activated in order to protect the SMPS.
However, even when the SMPS is in the normal operation,
the over load protection circuit can be activated during the
load transition. In order to avoid this undesired operation, the
over load situation should be distinguished from the normal
load transition situation. As a measure against this problem,
over load protection circuit in the FPS is designed to be acti-
vated after a specified period to determine whether it is a
transient situation or an overload situation. The protection
circuit is allowed to shut down the SMPS only when the over
load condition continues longer than preset period. The
detailed operation principle is explained in figure 3. Because
of the pulse by pulse current limit circuit, the maximum cur-
rent through the FPS is limited, and therefore the maximum
6
·
= 2.8k) and the diode forward
Vo
Figure 3. Pulse-width-modulation (PWM) block
K A 4 3 1
3 . 2 V
7 .5 V
0
V f b
.
C f b
# 4
7 . 5 V
D1
5 uA
T h e r m a l
S h u t d o w n
t
2
=
C
D2
input power is restricted with a given input voltage. If the
output consumes beyond this maximum power, the output
voltage (Vo) decreases below the set voltage. This reduces
the current through the opto-coupler diode, which also
reduces opto-coupler transistor current increasing Vfb. If
Vfb exceeds 3.2V, D1 is blocked and the 5µA current source
starts to charge Cfb slowly compared to when the 0.9mA
current source charges Cfb. Vfb continues increasing until it
reaches 7.5V, and the FPS shuts down at that time. The delay
time for shutdown is the time required to charge Cfb from
3.2V to 7.5V with 5µA. When Cfb is 10nF (103), t2 is
approximately 8.6mS and when Cfb is 0.1µF (104), t2 is
approximately 86ms. These values are enough to prevent
SMPS from being shut down for most transient situations.
Just increasing Cfb to obtain a longer delay time may cause
problems, because Cfb is an important parameter for deter-
mining the response speed of the SMPS. To solve this prob-
lem, auxiliary capacitor in series with zener diode can be
used in parallel with Cfb. The breakdown voltage of the
zener diode should be about 3.9 ~ 4.7V. When Vfb is below
the zener voltage, the system dynamics is determined by
Cfb. When Vfb exceeds the zener voltage, the delay time is
determined by the auxiliary capacitor. By using large auxil-
iary capacitor, the delay time can be extended without sacri-
fice of the dynamic response.
Over voltage Protection (OVP) Circuit
The FPS has a self-protection feature against malfunctions,
such as feedback circuit open or short-circuit. When the
feedback terminal is open due to a malfunction in the sec-
ondary side feedback circuit or a defect of solder, the current
through the opto-coupler transistor becomes almost zero.
V f b *
0 . 9 m A
fb
5
×
µ
4
A
2 . 5 R
t2
3 .
R
V
P W M c o m p
R s e n s e
O S C .
R e s e t
V c k
S h u t d o w n
R
S
S
R
Q
I o f f s e t
Q
S h u t d o w n
S e n s e
t
F P S