ML4813
APPLICATIONS
(Continued)
Note that V
IN
(q) = V
P
x sin(q) and that V
P
= 1.414 x V
RMS
.
The average value of the input triangular current is:
I
AVG
q =
05
t
ON
I
P
sin
q
2T
(7)
Several core materials are candidates for the inductor,
such as powder iron, gapped ferrites, moly permalloy, etc.
There are no particular restrictions on the inductor except
that the inductance is the correct value and the losses are
acceptable.
INPUT BYPASS CAPACITANCE
The triangular high frequency current is bypassed by an
input capacitor (C
IN
). This should be a high quality film
capacitor with low ESR value for minimum losses and
heating. Polyester, polypropylene or x-type (for line side)
are good candidates. Typical values, depending on the
power level, can range anywhere from 330nF to 1.5µF.
The next filtering stage of the RFI filter has an inductor as
an input to isolate C
IN
from the other capacitors which
may be present at the input circuit. Note that C
IN
can be
on either side of the bridge rectifier. The preferred
location for low crossover distortion is on the input side.
The ripple voltage across this capacitor is:
V
C
0
P
-
P
5
=
Where I
AVG
is the average value of the switch current (the
value of the current at the input of the regulator after
filtering), and T is the period of the switch cycle.
Substitution of (6) into (7) yields:
I
AVG
q =
05
L
I
P 2
sin
q
2828
T
V
RMS
.
(8)
Equation (8) clearly shows that the average value of the
switch current is sinusoidal and in phase with the input
voltage. The peak value of the average current is:
I
AVG(PEAK)
Also:
I
AVG(PEAK)
=
2
P
IN
V
RMS
(10)
L
I
P 2
=
sin
q
2828
T
V
RMS
.
(9)
D
C
f
IN
P
IN
2P
IN
-
L
f
C
IN
f
V
IN
(14)
Rearranging equations (9) and (10) to solve for P
IN
yields:
L
I
P 2
f
P
IN
=
4
(11)
Where V
C(P-P)
is the peak to peak worst case high
frequency capacitor voltage, and D is the switch duty
cycle. The RFI filter that follows C
IN
has to be able to
attenuate V
C(P-P)
to the levels set by the relevant
regulatory specifications.
INPUT TRANSIENT OVERVOLTAGE PROTECTION
Careful examination of the power circuits reveals that
there is no large capacitance at the input of the regulator.
The only capacitance present is that of the RFI filter
capacitors. These capacitors have a combined value in the
range of a few microfarads, and their ability to absorb and
minimize any line induced transients is almost
nonexistent. Transients can also occur under sudden load
removal. If the line impedance is inductive, hazardous
drain-source voltages may be generated leading to the
destruction of the power MOSFET. To keep this from
happening, a transient over-voltage protection device
should be installed such that enough safety margin is
allowed for the power MOSFET. A good rule of thumb is:
BV
DSS
>
V
CLAMP
+
V
OUT( OVP)
For optimum performance and the lowest inductor peak
currents, the inductor current should be at the verge of
continuity at the lowest operating voltage point and at full
load. This can be satisfied if:
I
P
V
IN
V
OUT
f
L
V
IN
+
V
OUT
1
6
(12)
Finally, (11) and (12) can be combined to derive an upper
bound for the inductor value that will guarantee that the
regulator always stays in the discontinuous mode of
operation. If the regulator were to operate in the
continuous mode the average input current would not be
sinusoidal.
V
V
L
!
2 f
P
1
V
IN
IN
OUT
IN
+
V
OUT
"#
6 #$
2
(15)
(13)
FLYBACK INDUCTOR CALCULATION
Equation (13) gives the upper bound for the inductor
value for any set of specified operating conditions.
Normally, a few iterations may be required for finalizing
the value to correct for second or third order effects. This
means that a good initial value for the inductor is
probably 10 to 20% lower than the value calculated by
the right hand side expression in (13).
Where BV
DSS
is the drain-source breakdown voltage for
the MOSFET, V
CLAMP
is the activation or clamping voltage
of the over-voltage transient protector, and V
OUT(OVP)
is
the maximum output voltage which is set by the OVP
function of the controller.
THE OUTPUT CIRCUIT
The output circuit for this topology, although non-
isolated, does not share the same ground with the power
circuit. Therefore connecting the two grounds with the
measuring leads of instruments should be avoided.
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
