TTI CPX200, Service Manual

Page: 9 / 30

the secondary. When Q1 and Q2 are turned off the voltage across the primary will reverse and
generally bring diodes D1 and D2 into conduction; D1, D2 provide clamp protection against
overvoltage caused by leakage inductance in the transformer, so protecting the FETs. At the
same time the secondary e.m.f. generated will cause current to flow through D3, D4 into C14 and
the load; as soon as this starts to happen current will cease to flow through D1 and D2 and the
energy stored in the primary is transferred to the secondary.
Zener diodes D7 and D8 protect the FETs in the event of a voltage surge and protect the drive
circuit in the event of a FET failure.
FET Drive Circuit
IC1 is an IR2110 drive IC which has the necessary on-chip isolation (500V) to be able to drive
both FETs. The FETs need 15V gate drive. The 15V supply for Q2 is generated from the auxiliary
mains transformer via BR2 and IC16; this rail also powers all the ground-referenced control
circuitry on the primary side, including the UC3846. Q1 needs an isolated supply which is
generated from an extra winding on the power transformer in phase with the secondary; D5 and
C23 rectify and smooth the transformer output, dropper resistor R15 and 15V zener D9 regulate
the rail.
Until the FETs start switching no 15V supply for Q1 is generated. To overcome this R21 is used to
charge C23 at start-up; when VBULKA reaches about 150V there is enough voltage on C23 to
turn on the high rail of IC1 and maintain the FET in an oscillating mode independent of output
loading. If for any reason this rail should drop below 8V, IC1 will switch off, thus maintaining a
good enhancement voltage across the gate at all times.
R8, C10 and R9, C11 are snubber networks for Q1 and Q2 respectively.
Output Filtering
Large switching spikes caused by the switching of FETS Q1, Q2 and rectifying diodes D3, D4 are
filtered by the output stage to bring both differential noise and common mode noise into
specification.
Snubber network R10, C13 reduces noise generated by the output diodes.
L5 and C16 comprise the main differential filter; R16 damps any ringing in the filter. R13, R17 and
R20 are a small pre-load, ensuring voltage loop control with no external load and providing some
load for the L5, C16 filter. The common mode filter L7, C15/C27 further reduces differential noise.
Common-mode noise is minimised by the low capacitance design of power transformer and is
attenuated by C17, C18; R19 provides a discharge path for these capacitors when the output
terminals are floated from ground. Solid ground connections at E1 and E2 are essential for good
common-mode noise performance.
The front panel mounted components LLA, CAA and CAB provide both differential and common-
mode attenuation at high frequencies (>2MHz); HF differential noise is attenuated by the leakage
inductance of LLA with CAB and HF common-mode noise is attenuated by LLA and CAA.
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