User manual

VETTA Theory of Operation………………Line 6 confidential ……………………………. Page 2 of 18
An inductor can store energy while an ideal transformer transfers energy but does not store it.
We won’t buck tradition so we’ll call it a flyback transformer).
The basic operation is to apply the input voltage across T2-A by turning on Q3, 15. Energy is
stored in T2 and all of the secondary diodes are reverse biased. When Q3, 15 turn off, the
stored energy is transferred to the outputs. As the voltage fly’s back, the secondary diodes are
forward biased. Voltage clamp D17, C22, R17, 18, 20 limits the voltage across Q3, 15 by
providing a path for primary current flow while the energy is being transferred from primary to
secondary. The secondary voltages will track each other quite well as long as a nominal load is
applied: Service note: This supply is not designed to be operated with no load, and this
condition may over stressed the output caps. Constructing a test fixture with the following
resistors will allow test bench operation: 390Ω 10W from J6.2 – J6.3; 390Ω 10W from J6.4 –
J6.5; 10Ω 2W from J6.5 – J6.6; 330Ω 2W from J2.1 – J2.2; 330Ω 2W from J2.3 – J2.2; 10Ω
10W from J2.4 – J2.5
A buck regulator consisting of IC U3 Inductor L6 and Capacitor C59 regulates the voltage
generated by winding T2_B. This generates an 8V regulated output. Also, the voltages
generated by the T2_D winding are stepped down to ±18 V by the linear regulators U8 and U9.
The Voltage Feedback circuit monitors secondary voltages (+8Pre, +45Pre, -45Pre and
PS_Valid). The PS_VALID signal is AC coupled into the feedback and does not play a part in
determining the DC output voltages. Transistors Q1, 4, 5, 8, 9, 10, 12 comprise a circuit that
does level shifting, voting and output voltage switching. The regulation scheme looks at +8Pre,
+45Pre, and -45Pre and decides which is at, or below, their nominal regulation point and
regulates this output. The other outputs are ignored and allowed to exceed their regulation
points.
This circuit controls the duty cycle of the primary switching through the opto-isolator U1 and
shunt regulator U4 on the secondary side. On the primary side, controller IC U2 (U2 is itself
supplied by the T2_E secondary winding) drives the gates of the main MOSFETs through
buffer transistors Q2, 11. This provides the voltage regulation feed back loop. A thermal
protection circuits monitors the temperature of the primary components through the thermistor
NTC2, and shut down the controller IC U2 if necessary.
The sequence of events at application of power is as follows: C1, 2 are charged and current
flows through R1, 2. The PWM chip U2 is in a low power mode and the main MOSFETs are
not being switched. When the voltage at +HK (house keeping) reaches about 17V, the PWM
chip starts running and the main MOSFETs start switching. The current supplied by R, 1 is not
sufficient to allow continued operation so the voltage at +HK drops. If the secondary voltages
ramp up properly, then winding T2-E will power +HK before it drops to the lower cut off point
of about 10V. If there is a short on the output or one of several other failures, the PWM will
shutoff at +HK=10V and +HK will start to charge again. This charge and discharge cycle will
continue at a rate of several Hz. Service note: Do not attempt to monitor primary voltages
with an oscilloscope. The safest method is to use an isolation transformer. Removing the
‘scope ground connection or “floating” the scope is potentially lethal for the technician or
others that may come in contact with the ‘scope.