ML4819CP_NL Fairchild Semiconductor, ML4819CP_NL Datasheet - Page 12

• Manufacturer Part Number: ML4819CP_NL
• Manufacturer: Fairchild Semiconductor

Specifications of ML4819CP_NL

• Start-up Supply Current: 600uA
• Operating Supply Voltage (max): 35V
• Operating Temp Range: 0C to 70C
• Operating Temperature Classification: Commercial
• Package Type: PDIP
• Pin Count: 20
• Mounting: Through Hole
• Lead Free Status / RoHS Status: Compliant

ML4819
OVERVOLTAGE PROTECTION (OVP) COMPONENTS
The OVP loop should be set so that there is no interaction
with the voltage control loop. Typically it should be set to
a level where the power components are safe to operate.
Ten to fifteen volts above V
This sets the maximum transient output voltage to about
395V.
By choosing the high voltage side resistor of the OVP
circuit the same way as above i.e. R
be calculated as:
Choose 4.53k , 1%.
Note that R
resistors such as 1% or better.
OFF-LINE START-UP AND BIAS SUPPLY GENERATION
The Start-Up circuit in Figure 12 can be either a “bleed
resistor” (39k , 2W) or the circuit shown in Figure 13. The
bleed resistor method offers advantage of simplicity and
lowest cost, but may yield excessive turn-on delay at low
line.
When the voltage on pin 15 (V
starts up. The energy stored on the C21 supplies the IC
with running power until the supplemental winding on T3
can provide the power to sustain operation.
ENHANCEMENT CIRCUIT
The power factor enhancement circuit (inside the dotted
lines) in Figure 11 is described in Application Note 11. It
improves the power factor and lowers the input current
harmonics. Note that the circuit meets IEC1000-3-2
specifications (with the enhancement circuit installed) on
the harmonics by a large margin while correcting the
input power factor to better than 0.99 under most steady
state operating conditions.
12
V
REF
R
8
R33
2k
V
START-UP
2k
CIRCUIT
R32
OVP
V
5
REF
, R
6
Figure 13. Start-Up Circuit
, R
V
R
REF
2N2222
7
7
Q5
and R
5
395
V
8
510k
C21
0.1
OUT
R31
should be tight tolerance
V
356
CC
seems to be adequate.
5
D16
22V
k
V
) exceeds 16V, the IC
IN
7
IRF821
1N4001
R30
4.3k
= 356K then R
4 564
Q6
D15
.
k
OUT
TO V
8
(20)
can
CC
PWM SECTION
The PWM section in Figure 12 is a two switch forward
converter, shown in Figure 14 below for clarity. This fully
clamped circuit eliminates the need for very high
voltage MOSFETs. Flyback topology is also possible with
the ML4819.
This regulator (Figure 12) uses current mode control.
Current is sensed through R24 and filtered for high
frequency noise and leading edge transient through T23
and C14. The main regulation loop is through PWM B. The
TL431 (U3) in the secondary serves as both the voltage
reference and error amplifier. Galvanic isolation is
provided by an optocoupler (U2) which provides a current
command signal on pin 8. Loop compensation is provided
by R29 and C20. The output voltage is set by:
The control loop is compensated using standard
compensation techniques.
Current is limited to a threshold of 2A (1V on R24). The duty
cycle is limited in this circuit to below 50% to prevent
transformer (T3) core saturation. The maximum duty cycle
limit of 45% is set using a threshold of V
the circuit in Figure 12 can be modified for voltage mode
operation by utilizing the slope current which appears on
pin 9 as show in figure 15 below.
The ramp amplitude appearing on pin 9 will be:
where R18 is the slope compensation resistor. Since this
circuit operates with a constant input voltage (as supplied
by the PFC section) voltage feed-forward is unnecessary.
385VDC
V
V
R
OUT
Figure 14. Two-Switch Forward Converter
I
D12
ML4819
R18
2
2 5 1
.
R V
( )
R
R
29
28
D11
T2
T2
REF
REV. 1.0 10/10/2000
/2 on pin 7.
Q2
Q3
T3
(21)
(22)