Micrel
MIC2172/3172
Finally, recalculate the transformer turns ratio to insure
that it is less than the value earlier found in equation (9).
requiring a transformer (forward), the MIC2172/3172 is a
good choice.
A 12V to 5V step-down converter using transformer
isolation (forward) is shown in figure 14. Unlike the
isolated flyback converter which stores energy in the
primary inductance during the controller’s on-time and
releases it to the load during the off-time, the forward
converter transfers energy to the output during the on-
time, using the off-time to reset the transformer core. In
the application shown, the transformer core is reset by
the tertiary winding discharging T1’s peak magnetizing
current through D2.
LPRI
a ≤
Then:
a ≤
(12)
LSEC
−5
1.8×10
−5
2.54×10
a ≤ 0.84 Use 0.8 (same as 1:1.25).
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know
the primary peak current which must be withstood
without saturating the transformer core.
For most forward converters the duty cycle is limited to
50%, allowing the transformer flux to reset with only two
times the input voltage appearing across the power
switch. Although during normal operation this circuit’s
duty cycle is well below 50%, the MIC2172 (and
MIC3172) has a maximum duty cycle capability of 90%.
If 90% was required during operation (start-up and high
load currents), a complete reset of the transformer
during the off-time would require the voltage across the
power switch to be ten times the input voltage. This
would limit the input voltage to 6V or less for forward
converter applications.
V
T
IN(min) ON
I
=
PEAK(pri)
L
PRI
So:
−6
4.0×5.5×10
18µ8
I
=
(13)
PEAK(pri)
I
PEAK(pri) = 1.22A
Now find the minimum reverse voltage requirement for
the output rectifier. This rectifier must have an average
current rating greater than the maximum output current
of 0.25A.
To prevent core saturation, the application given here
uses a duty cycle limiter consisting of Q1, C4 and R3.
Whenever the MIC3172 exceeds a duty cycle of 50%,
T1’s reset winding current turns Q1 on. This action
reduces the duty cycle of the MIC3172 until T1 is able to
reset during each cycle.
V
+
(
V
a
)
IN(max)
OUT
V
≥
(14)
BR
F
a
BR
Where:
BR = output rectifier maximum peak reverse
Fluorescent Lamp Supply
V
An extremely useful application of the MIC3172 is
generating an ac voltage for fluorescent lamps used as
liquid crystal display back lighting in portable computers.
voltage rating
a = transformer turns ratio (0.8)
Figure 15 shows a complete power supply for lighting a
fluorescent lamp. Transistors Q1 and Q2 together with
capacitor C2 form a Royer oscillator. The Royer
oscillator generates a sine wave whose frequency is
determined by the series L/C circuit comprised of T1 and
C2. Assuming that the MIC3172 and L1 are absent, and
the transistors’ emitters are grounded, circuit operation is
described in “Oscillator Operation.”
F
BR = reverse voltage safety derating factor (0.8)
Then:
6.0 +
(
5.0×0.8
0.8×0.8
VBR ≥ 15.625V
)
VBR
≥
A 1N5817 will safely handle voltage and current
requirements in this example.
Oscillator Operation
Forward Converters
Resistor R2 provides initial base current that turns
transistor Q1 on and impresses the input voltage across
one half of T1’s primary winding (Pri 1). T1’s feedback
winding provides additional base drive (positive
feedback) to Q1 forcing it well into saturation for a period
determined by the Pri 1/C2 time constant. Once the
voltage across C2 has reached its maximum circuit
value, Q1’s collector current will no longer increase.
Since T1 is in series with Q1, this drop in primary current
causes the flux in T1 to change and because of the
Micrel’s MIC2172/3172 can be used in several circuit
configurations to generate an output voltage which is
less than the input voltage (buck or step-down topology).
Figure 13 shows the MIC3172 in a voltage step-down
application. Because of the internal architecture of these
devices, more external components are required to
implement a step-down regulator than with other devices
offered by Micrel (refer to the LM257x or LM457x family
of buck switchers). However, for step-down conversion
M9999-041806
(408) 955-1690
April 2006
17
MICREL [ MICREL SEMICONDUCTOR ]
