PRODUCT SPECIFICATION ML4812
REV. 1.0.4 5/31/01 11
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
OUT
is generally a good setpoint. 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
4
= 356K then R
5
can be calculated
as:
Choose 4.53kΩ, 1%. Note that R
1
, R
2
, R
4
and R
5
should be
tight tolerance resistors such as 1% or better.
Controller Shutdown
The ML4812 provides a shutdown pin which could be used
to shutdown the IC. Care should be taken when this pin is
used because power supply sequencing problems could arise
if another regulator with its own bootstrapping follows the
ML4812. In such a case a special circuit should be used to
allow for orderly start up. One way to accomplish this is by
using the reference voltage of the ML4812 to inhibit the
other controller IC or to shut down its bias supply current.
Off-line Start-up and Bias Supply Generation
The ML4812 can be started using a “bleed resistor” from the
high voltage bus. After the voltage on V
CC
exceeds 16V, the
IC starts up. The energy stored on the 330µF, C15, capacitor
supplies the IC with running power until the supplemental
winding on L1 can provide the power to sustain operation.
The values of the start-up resistor R10 and capacitor C15
may need to be optimized depending on the application. The
charging waveform for the secondary winding of L1 is an
inverted chopped sinusoid which reaches its peak when the
line voltage is at its minimum. In this example, C9 = 0.1µF,
C15 = 330µF, D8 = 1N4148, R10 = 39kΩ, 2W.
Enhancement Circuit
The power factor enhancement circuit shown in Figure 12 is
described in detail in Application Note 11. It improves the
power factor and lowers the input current harmonics. Note
that the circuit meets IEC 1000-3-2 specifications (with the
enhancement) on the harmonics by a large margin while cor-
recting the input power factor to better than 0.99 under most
steady state operating conditions.
Construction and Layout Tips
High frequency power circuits require special care during
breadboard construction and layout. Double sided printed
circuit boards with ground plane on one side are highly rec-
ommended. All critical switching leads (power FET, output
diode, IC output and ground leads, bypass capacitors) should
be kept as small as possible. This is to minimize both the
transmission and pick-up of switching noise.
There are two kinds of noise coupling; inductive and capaci-
tive. As the name implies inductive coupling is due to fast
changing (high di/dt) circulating switching currents. The
main source is the loop formed by Q1, D5, and C3–C4.
Therefore this loop should be as small as possible, and the
above capacitors should be good high frequency types.
The second form of noise coupling is due to fast changing
voltages (high dv/dt). The main source in this case is the
drain of the power FET. The radiated noise in this case can
be minimized by insulating the drain of the FET from the
heatsink and then tying the heatsink to the source of the FET
with a high frequency capacitor (C
H
in Figure 12).
The IC has two ground pins named PWR GND and Signal
GND. These two pins should be connected together with a
very short lead at the printed circuit board exit point. In
general grounding is very important and ground loops should
be avoided. Star grounding or ground plane techniques are
preferred.
Magnetics Tips
L1 — Main Inductor
As shown in Table 1, one of several toroidal cores can be
used for L1. The T184-40 core above is the most economi-
cal, but has lower inductance at high current. This would
yield higher ripple current and require more line EMI filter-
ing. The value for R
SC
(slope compensation resistor on
RAMP COMP) was calculated for the T225-8/90 and should
be recalculated for other inductor characteristics. The vari-
ous core manufacturers have a range of applications litera-
ture available. A gapped ferrite core can also be used in place
of the powdered iron core. One such core is a Philips Com-
ponents (Ferroxcube) core #4229PL00-3C8. This is an
ungapped core. Using 145 turns of #24 AWG wire, a total air
gap of 0.180" is required to give a total inductance of about
2mH. Since 1/2 of the gap will be on the outside of the core
and 1/2 the gap on the inside, putting a 0.09" spacer in the
center will yield a 0.180" total gap. To prevent leakage fields
R
5
V
REF
R
4
×
V
OVP
V
REF
–
----------------------------------
5V 356kΩ×
395V 5V–
------------------------------- 4.564kΩ===
(20)
Table 1. Toroidal Cores (L1)
Material Manufacturer Part # Turns (#24AWG)
Powdered Iron Micrometals T225-8/90 200
Powdered Iron Micrometals T184-40 120
Molypermalloy SPANG (Mag. Inc.) 58076-A2 (high flux) 180
