HP 3465B - Power Supply

Section IV
Model 3465B
r
Vb ±.
I
CR
♦ on
.Vo=-f^ VB
ton = TIME SWITCH 3 CLOSED
totf=TIME SWITCH S OPEN
34€5-B-4l7n
Figure 4-9. Basic Diagram, DC—to—DC Converter.
4-72. Power Supply.
4-73. This paragraph is a description of a simple dc-to-dc
converter. Figure 4-9 is a diagram of this converter. When
switch S is closed (ton), battery voltage VB is applied to the
inductor L. Since the battery voltage is constant, the
inductor current increases linearly with time. The inductor
current establishes a magnetic fi eld which stores energy
transferred from the battery. During ton, C supplies current
to the load, Rl- When switch S is opened (toff), the voltage
across the inductor inverts and forward biases diode CR.
For a constant Vo, the inductor current decreases linearly
with time and transfers energy to the capacitor and load
resistor, C and Rl - The energy transferred from the battery
to the load is controlled by switch S. By controlling switch
S the output voltage can be made greater than, equal to or
less than the battery voltage VB and can be regulated for
changes in the battery voltage (VB) and in the load (Rl)-
For a steady state condition, the duty-cycle of switch S is
|Vo
IVo
i+ VB
ton = tin® switch S closed.
T = period of one cycle.
The duty-cycle depends only on the output voltage (Vq)
and the battery voltage (VB).
r
V.-
V
ON
CONVERTER
SWITCH
/OFF
C25 R8I
—
I
(Q34 a R98)
Tl
4-74. A simplified diagram of the dc-to-dc converter is
shown in Figure 4-10. Refer to this diagram and the
converter of Figure 7-5 for this discussion. A negative
output is derived from a portion of the energy stored in the
primary winding inductance or magnetizing inductance of
Tl while the switch, Q33 is on. This output is obtained by
the same process described previously for the simple
switch-type dc—to—dc converter. A positive output is
developed by transformer-coupling a portion of the energy
stored in the primary winding inductance through the
secondary winding of Tl. This output is equal to the
turns-ratio times the voltage developed by the primary of
Tl across C34 when switch Q33 is off.
4-75. The following paragraphs describe the circuit opera
tion of the dc-to-dc converter. When the battery voltage
VB is applied to the circuit, Q33 saturates. A constant
voltage is applied across the primary of Tl. The collector
current determined by the constant battery voltage (VB)
and the primary winding inductance, increases linearly with
time until Q33 comes out of saturation (hpgl). As this
occurs, the collector of Q33 begins a negative transition.
This transition at pin 1 of transformer Tl causes pin 3 of
Tl to begin a positive transition. The positive transition is
applied to the base of Q33 by the feedback circuit R81 and
C25 and turns Q33 off. As Q33 turns off, the voltage across
CRI8
-K-
C24
-HIV
>Rli
V
Ns= 21 turns
i ^
f Np = 14 turns
CR32
^^C34 < Rl2
3465-B-425I
Figure 4-10. Simplified Diagram, DC—to—DC Converter.
4-10