Tektronix SPG 1000, Instruction Manual

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SPG 1000
8–9
voltage. This causes U722 to increase the pulse
width, and thus the current and power through
T1.
On the other hand, if the load decreases, the +5 V
increases momentarily. The output pulse width
then decreases along with the current in T1, and
less power is transferred to the secondaries. In
this way, the +5 V is kept constant through
changes in the load.
Current Limit
Current limit is provided for the primary circuit
by the internal circuitry of U722. If the ramp
voltage at U722 ever reaches 1V, the output drive
pulse ends. This shuts Q638 off, so no further
voltage is supplied. Thus, the maximum primary
current in T1 is limited to approximately 2.5
Amps, which corresponds to a maximum power
level of approximately 140 Watts.
As the supply goes into current limit, U615A and
Q717 come into play. U722-1 provides an indica-
tion of the peak current in T1. This voltage is fed
to the inverting input of comparator U615A and
compared to a fixed voltage set by divider R616,
R615, and R510. R510 is an output power adjust-
ment set so trip point will be approximately 115
Watts. If U722-1 goes high enough to trip
U615A-1 low, then C717 will start to charge. If
the current limit condition persists long enough
for the charge on C717 to reach 700 or 800 mV,
U615B trips and Q717 is turned on. This applies
the reference voltage from U722-8 directly to
U722-3, shutting down the supply and forcing a
kick start. The supply will then cycle through
kick start, current limit, and shutdown until the
problem is corrected. Jumper P720 is included for
troubleshooting; its removal will disable the cur-
rent limit shutdown circuits.
Base Driver and Snubber
Q638 is a high (1000V) blocking voltage power
transistor. To prevent transistor failure and ensure
proper operation, Q638-base must have a large
forward current during the on-time and a reverse
base-emitter current during turn-off. The pulse
from U722-6 turns FET Q648 on and current
flows through R746, CR649, and into the base of
Q638. The current level is set by R746. Thus
Q638 turns on and 320 volts appear across T1,
pins 1 and 2. When the pulse from U722-6
ends, Q648 turns off and the collector current of
Q638 flows out the base of Q638 through CR648,
and quickly turns Q638 off. CR640 forms a Bak-
er Clamp to keep Q638 out of hard saturation and
a slow-turn-off. When Q638 is turned off, a volt-
age spike appears at its collector. A combination
of reflected secondary voltages, input voltage,
and transformer leakage inductance can combine
to produce a spike of over a thousand volts. As
this can exceed the voltage rating of Q638, a
snubber circuit limits the spike to approximately
twice the rectified line voltage. CR500, C500,
and the winding on T1, pins 3 and 4, make up the
snubber circuit.
Secondary Circuits
The secondary circuits all work in the same
manner. As mentioned earlier, during the first
half of the cycle, CR575 and CR320 are reverse-
biased, so there is no current flow.
On the second half of the cycle, when Q638 is
shut off, the flyback action reverses the polarities
of the secondaries, and the diodes are forward
biased. This allows the energy stored within T1 to
charge up the capacitors in the secondaries.
The +5 V and the –5 V supplies use LC filters
from this point, to further smooth the voltage and
eliminate most of the ripple.
Generation of 12 Volts
The
±
12 V supplies are generated with a second-
ary PWM and regulator composed of U200,
Q250, T2, and the associated circuits. This
secondary PWM is free-running with respect to
the primary PWM at a frequency of approximate-
ly 60 kHz. The frequency is set by R105 and
C105. Q250 is controlled by U200 to 5 V
and –5 V (10 Volts total) across T2 during the