Tektronix SPG 1000, Instruction Manual

Page: 103 / 282

SPG 1000
8–7
Figure 8–3: Power Inductor Operation
bias their respective diodes (note the polarity
dots). In this way, there is no current flowing in
the secondaries while current is flowing in the
primary.
The primary current builds as a linear ramp, stor-
ing the energy in T1 according to the relationship
E= 1/2 Li
2
, where L is the primary inductance
and i is the current flowing through it.
The current path is broken when Q638 is
switched off, so current stops flowing in the pri-
mary. The flyback action of T1 then causes the
voltages in the secondaries to reverse polarities,
and CR575 and CR320 to turn on.
The current in the secondaries linearly ramps
down as the energy that was stored in T1 primary
is delivered to the load, charging the output
capacitors.
When all of the energy that was stored in T1 dur-
ing the first half of this cycle is delivered to the
load, the current in the secondaries is at zero, and
the diodes turn off. There is no current flowing in
either the primary or the secondaries until Q638
is turned back on to start the next cycle. As there
is not a continuous flow of energy in T1, this is
called discontinuous flyback operation. At low
line voltages or high loads all the power in induc-
tor T1 may not be transferred to the load during
the second half of the cycle, in which case the
diodes will not be off when Q638 turns back on.
In that case there will also be some energy still
stored in T1 at the end of the cycle (at low line or
high load).
Load regulation is provided by sensing the +5 V
supply with a divider comprised by R314, R315,
and R415, and using U410 to convert this to an
error signal. This error signal is optically coupled
through U520 back to the Pulse Width Modulator,
U722. U722 uses the error signal to vary the
width of the pulse that drives Q638.
When the +5 V goes too high, U722 narrows the
pulse width. This reduces the amount of energy
stored in T1, and therefore the amount transferred
to the load, so the +5 V goes down. Contrariwise,
when the +5 V is too low, the pulse width is in-
creased, increasing the amount of energy stored
in T1 and then transferred to the load, so the volt-
age goes up.
Pulse Width Modulator and Error
Amplifier
The Pulse Width Modulator, U722, is a current-
mode controller. It uses inputs from the primary
circuit and from the +5 V output to vary the
width of the pulse that controls Q638, as
mentioned above. This regulates the secondary
voltages throughout variations such as input
voltage, output load, and temperature.