FS7M0680TU Fairchild Semiconductor, FS7M0680TU Datasheet - Page 7

• 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

Then, Vfb continues increasing and the preset maximum
current flows through the primary side until the over load
protection circuit is activated. Since maximum current is
transferred to the secondary side, the secondary side voltage
becomes much higher than the rated voltage. If there is no
protection circuit against over voltage, the devices in the
secondary side will be damaged. In order to prevent this sit-
uation, the FPS has an over voltage protection circuit (pro-
tection against feedback circuit abnormalities). In general,
Vcc is proportional to the output voltage and FPS uses Vcc
instead of directly monitoring the output voltage to detect
over voltage situation. If V
vates the OVP circuit. Therefore, V
designed to be below 24V during normal operation to avoid
the undesired activation of OVP.
OCP (Over Current Protection)
Even though the FPS has OLP (Over Load Protection) and
pulse by pulse current limiting feature, these are not enough
to protect FPS when a secondary side diode short or load
short occurs. Therefore, FPS has internal OCP (Over Cur-
rent Protection) circuit as shown in figure 4. When the gate
turn-on signal is applied to the power MOSFET, the OCP
block is enabled and monitors the current through the sens-
ing resistor for 1us. The voltage across the resistor is com-
pared with the preset OCP level. If the sensing resistor
voltage is greater than the OCP level for longer than 200ns
within the allowed comparison time of 1us, the reset signal
is applied to the latch, resulting in the shutdown of SMPS.
Here, the additional delay of 100ns after the 200ns delay is
the time required for the operation of the protection circuit.
Soft start operation
At startup, the voltage of the PWM comparator inverting
input is saturated to its maximum value. In that case, the
power MOSFET current is at its maximum value and maxi-
mum allowable power is delivered to the secondary side
until the output voltage is established. It should be noted
that when the SMPS delivers maximum power to the sec-
ondary side during the startup, the entire circuit is seriously
stressed. By using a soft start function, such stresses can be
alleviated. Figure 5 shows how the soft-start circuit is
implemented. When it starts up, the soft start capacitor Cs
on pin 5 begins to be charged through the internal resistor
(Rss), which forces the comparator inverting input voltage
OCP Operating
Minimum Turn-on Time
Fiqure 4. OCP Function & Block
200ns
100ns delay
CC
exceeds 24 V, the FPS acti-
S
R
CC
C
Q
OCP time
R
should be properly
Latch signal
OCP Level
Rsense
to increase slowly, also increasing the duty ratio slowly.
When the voltage of C
is turned off and Cs continues being charged up to 5V
through Rss. Then, the voltage of the comparator inverting
input follows the feedback voltage of pin 4 instead of follow-
ing the voltage of C
protection circuits, C
resistor allowing C
starts up again.
1 0 V
5 u A
#4
V f b
Cfb
0 .9 m A
Figure 5. Soft Start Circuit
D1
S
S
to be charged from 0V when the SMPS
C S
. When the SMPS is shut down by the
S
S
5 V
is discharged through the internal
#5
reaches about 3.2V, PNP transistor
18.5K
D2
R
ss
Fa ir c hild P owe r
S witc h(FP S )
PWM
Comparator
FS7M0680
7