FSD210, FSD200
9
Functional Description
1. Startup
: At startup, the internal high voltage current
source supplies the internal bias and charges the external
Vcc capacitor as shown in figure 7. In the case of the
FSD210, when Vcc reaches 8.7V the device starts switching
and the internal high voltage current source is disabled (see
figure 1). The device continues to switch provided that Vcc
does not drop below 6.7V. For FSD210, after startup, the
bias is supplied from the auxiliary transformer winding. In
the case of FSD200, Vcc is continuously supplied from the
external high voltage source and Vcc is regulated to 7V by
an internal high voltage regulator (HVReg), thus eliminating
the need for an auxiliary winding (see figure 2).
Figure 6. Internal startup circuit
Calculating the Vcc capacitor is an important step to design-
ing in the FSD200/210. At initial start-up in both the
FSD200/210, the stand-by maximum current is 100uA, sup-
plying current to UVLO and Vref Block. The charging cur-
rent (i) of the Vcc capacitor is equal to Istr - 100uA. After
Vcc reaches the UVLO start voltage only the bias winding
supplies Vcc current to device. When the bias winding volt-
age is not sufficient, the Vcc level decreases to the UVLO
stop voltage. At this time Vcc oscillates. In order to prevent
this ripple it is recommended that the Vcc capacitor be sized
between 10uF and 47uF.
2. Feedback Control
: The FSD200/210 are both voltage
mode devices as shown in Figure 8. Usually, a H11A817
optocoupler and KA431 voltage reference (or a FOD2741
integrated optocoupler and voltage reference) are used to
implement the isolated secondary feedback network. The
feedback voltage is compared with an internally generated
sawtooth waveform, directly controlling the duty cycle.
When the KA431 reference pin voltage exceeds the internal
reference voltage of 2.5V, the optocoupler LED current
increases pulling down the feedback voltage and reducing
the duty cycle. This event will occur when either the input
voltage increases or the output load decreases.
Figure 7. Charging the Vcc capacitor through Vstr
3. Leading edge blanking (LEB)
: At the instant the inter-
nal Sense FET is turned on, there usually exists a high cur-
rent spike through the Sense FET, caused by the primary side
capacitance and secondary side rectifier diode reverse recov-
ery. Exceeding the pulse-by-pulse current limit could cause
premature termination of the switching pulse (see Protection
Section). To counter this effect, the FPS employs a leading
edge blanking (LEB) circuit. This circuit inhibits the over
current comparator for a short time (TLEB) after the Sense
FET is turned on.
Figure 8. PWM and feedback circuit
4. Protection Circuit
: The FSD200/210 has 2 self protec-
tion functions: over load protection (OLP) and thermal shut-
down (TSD). Because these protection circuits are fully
integrated into the IC with no external components, system
Vin,dc
Vstr
Vcc
HV
Reg.
Vin,dc
Vstr
Vcc
7V
Istr
Istr
FSD210
FSD200
8.7V/
6.7V
L
H
Vin
dc
,
dc
dc
Vstr
Istr
J-F
E
T
U
VLO
Vre
f
ma
x
1
1
00uA
i
=
Istr-ma
x
1
0 0 u A
i
=
i
=
x
1
Istr-ma
x
1
00uA
FSD2
xx
Vcc
UVLO
start
UVLO
stop
t
Vcc
Vcc must not drop
to UVLO stop
Auxiliary winding
voltage
4
OSC
Vcc
Vref
5uA
0.25mA
V
SD
R
FB
Gate
driver
OLP
Vfb
KA431
Cfb
Vo