Tektronix SPG 1000, Instruction Manual

Page: 101 / 282

SPG 1000
8–5
Power Supply Board (A4)
The switching power supply generates 5 V for
TTL and ECL devices. A stable linear supply of
12 V is provided for powering the analog
circuitry.
This type of power supply is called a current-
mode-controlled, discontinuous, flyback, switch-
ing power supply. The current output is
distributed between the four supplies as follows:
+ 12 V 0.8 Amps max
+5 V 7 Amps max
–5 V 6 Amps max
–12 V 0.8 Amps max
The maximum power is limited by the maximum
current in the primary of T1. This is also the only
current limit for the
5 V supplies, as they have
no secondary current limit. The 12 V supplies
are current limited on the secondaries by the 12
V linear regulators, U150 and U152, and the sec-
ondary
14.5 pulse width modulator, U200.
The power inductor, T1, is driven by switching
the voltage to its primary winding on and off at a
rate of approximately 60 kHz. T1 is not used as a
transformer, but as an energy storage device; the
energy is stored in the primary during the first
half of the switching cycle, while voltage is being
applied. On the second half of the switching
cycle, voltage to the primary is switched off and
the energy stored in T1 is transferred to the sec-
ondaries. Regulation is accomplished by feedback
from the +5 V supply to the Pulse Width Modula-
tor controlling voltage to the primary. This varies
the length of time that voltage is applied to the
primary, causing it to store either more or less
energy.
There is also circuitry to provide for operation
from both 110 and 220 VAC supplies, over volt-
age protection (crowbar) on the +5 V supply, and
shutdown circuitry that forces a restart of the sup-
ply if it remains in current limit for more than a
short period of time < 200 ms).
WARNING
All primary voltages are referenced to a floating
ground, not chassis ground. An isolation
transformer or a differential amplifier is therefore
required for troubleshooting the circuitry in the
primary and the Pulse Width Modulator, and in
the supporting circuits.