MIC2172/3172
Micrel, Inc.
50%,theMIC2172(andMIC3172)hasamaximumdutycycle
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.
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.
Fluorescent Lamp Supply
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.
Figure 15 shows a complete power supply for lighting a
fluorescent lamp. Transistors Q1 and Q2 together with ca-
pacitor C2 form a Royer oscillator. The Royer oscillator
generatesasinewavewhosefrequencyisdeterminedbythe
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 Opera-
tion.”
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
mutual coupling to the feedback winding further reduces
primary current eventually turning Q1 off. The primary wind-
ings now change state with the feedback winding forcing Q2
on repeating the alternate half cycle exactly as with Q1. This
action produces a sinusoidal voltage wave form; whose
amplitude is proportional to the input voltage, across T1’s
primarywindingwhichissteppedupandcapacitivelycoupled
to the lamp.
Lamp Current Regulation
Initial ionization (lighting) of the fluorescent lamp requires
several times the ac voltage across it than is required to
sustain current through the device. The current through the
lamp is sampled and regulated by the MIC3172 to achieve a
given intensity. The MIC3172 uses L1 to maintain a constant
average current through the transistor emitters. This current
controls the voltage amplitude of the Royer oscillator and
maintains the lamp current. During the negative half cycle,
lamp current is rectified by D3. During the positive half cycle,
lampcurrentisrectifiedbyD2throughR4andR5. R3andC5
filter the voltage dropped across R4 and R5 to the MIC3172’s
feedback pin. The MIC3172 maintains a constant lamp
current by adjusting its duty cycle to keep the feedback
voltage at 1.24V. The intensity of the lamp is adjusted using
potentiometer R5. The MIC3172 adjusts its duty cycle
accordingly to bring the average voltage across R4 and R5
back to 1.24V.
Oscillator Operation
Resistor R2 provides initial base current that turns transistor
Q1onandimpressestheinputvoltageacrossonehalfofT1’s
primary winding (Pri 1). T1’s feedback winding provides
additional base drive (positive feedback) to Q1 forcing it well
T1
D3
1:1:1
1N5819
L1 100µH
VOUT
VIN
5V, 1A
12V
R4
1%
D4
C5
3.74k
1N5819
470µF
R1*
C2*
D1*
VIN
Enable
EN
VSW
FB
Shutdown
MIC3172
C1
22µF
GND
D2
P1 P2 S COMP
1N5819
R5
1.24k
1%
R2
1k
†
†
Q1
R3
C3
†
C4
1µF
* Voltage clipper
†
Duty cycle limiter
Figure 14. 12V to 5V Forward Converter
April 2005
15
M9999-042205
MICREL [ MICREL SEMICONDUCTOR ]
