Fluke 2680A, Service Manual

Page: 53 / 254

Theory of Operation
Introduction
2
2-5
Main Power Supply
The power supply circuitry is contained on page 6 of the 2680-1004 schematic. The main
supply consists of 2 switching regulators, U301 and U303. U301 takes either a 9 V to 42
V dc source from P6 pins 1 and 2 or a 10:1 stepped down 50/60 Hz 95 V to 265 V ac line
voltage from P9 pins 3 and 4 as an input. It performs a push-pull buck dc-dc conversion,
through an N channel FET(Q304), a P channel FET(Q303) and a 50
µ
H inductor(L301),
to produce a reg5 V, 5 A supply. U301 uses a constant off time architecture,
because of this its operating frequency varies from approximately 42 KHz to 85 KHz,
depending on the input operating voltage. This is the main out-guard source from which
all other supplies, both in-guard and out-guard are produced. The second part of the main
supply is produced by U303, a 5 V to 3.3 V buck topology dc-dc converter. Its output,
+3.3 V at 1.25 A maximum, is referred to as Vcc throughout the out-guard system. F301
and F302 are surface mount thermistors that serve to protect the instrument from a dc
overvoltage, undervoltage, or a negative voltage applied to the dc power input jack on
pins 1 and 2 of P6.
The 268XA can take power from a 9 to 45 V battery or from mains ac. The mains power
is routed from the IEC line connector at the rear of the instrument to the mains
transformer. See Figure 2-2 for an ac power connection diagram. Internally, the mains ac
black wire goes through a fuse before reaching the mains transformer. The mains
transformer converts the ac input to ~13 V rms. The secondary power of the transformer
goes to the power section on the 4004 controller/power supply board. The secondary
power is routed through the mains power switch located on the back of the chassis. The
primary power is always connected to the mains power.
Secondary Supplies
The power source for the vacuum florescent display primarily consists of a 50KHz
differential clock (FIL_CLK and FIL_CLK*), 2 N channel FETs (Q301-2), a transformer
(T2) and an linear LDO (low drop out) regulator (U307). The biphase clock source is
driven from the FPGA U25 upon its release from configuration. Q301-2 and T2 combine
to form an inverting boost DC-DC converter that produces a -30V dc source from the
fullwave bridge rectifier(CR304-5) and a 5.3 V ac source that is used for the filament
drive. U307, in combination with C313 and CR312, taps off of the primary side of T2 to
produce a regulated –5 V dc supply, which is needed by the display controller.
The fan power supply is a boost dc-dc switching regulator (U30). It uses a constant
frequency pulse width modulated architecture switching at 1.2 MHz, with a 1 A
maximum peak current. Its output is +24 V at 100 mA, this supply was designed to only
drive the fan, other circuitry should not be driven from this supply.
Out-Guard Overview
The overall topology of the out-guard control system consists of 2 primary components,
the microprocessor (U6) and the fpga (U25). The fpga handles the digital side of the
guard crossing for all 6 module slots and the microprocessor handles all of dataflow from
the in-guard to the host computer and the PCMCIA slot, as well as calibration and other
miscellaneous tasks. There are 2 surface mount LEDs on the 268XA-4004 PCA. The
green one (DS7) is a power on light, it comes on when the microprocessor is released
from reset. The flashing red one (DS6) is a heartbeat indicator, that will begin flashing
after the firmware application has begun operating. The 4004 PCA contains a number of
test points and wire points. The test points (TPs) were for use in development and serve
no useful purpose for service. The wire points (WPs) that are not chassis connections are
connected to digital ground.