Fluke 43B Service Manual page 36

43B
Service Manual
Fly Back Converter
When the test tool is turned on, the D-ASIC makes the PWRONOFF line (P-ASIC pin
62) high. Then the self oscillating Fly Back Converter becomes active. It is started up
by the internal 100 kHz oscillator that is also used for the Charger/Converter circuit.
First the FLYGATE signal turns FET V554 on (see Figure 3-5), and an increasing
current flows in the primary transformer winding to ground, via sense resistor R551. If
the voltage FLYSENSP across this resistor exceeds a certain value, the P-ASIC turns
FET V554 off. Then a decreasing current flows in the secondary windings to ground. If
the windings are "empty" (all energy transferred), the voltage VCOIL sensed by the
P-ASIC (pin 52) is zero, and the FLYGATE signal will turn FET V554 on again.
The output voltage is regulated by feeding back a part of the +3V3A output voltage via
R552-R553-R554 to pin 54 (VSENS). This voltage is referred to a 1.23 V reference
voltage. Any deviation of the +3V3A voltage from the required 3.3V changes the
current level at which current FET V554 will be switched off. If the output voltage
increases, the current level at which V554 is switched off will become lower, and less
energy is transferred to the secondary winding. As a result the output voltage will
become lower.
An internal current source supplies a current to R559. The resulting voltage is a
reference for the maximum allowable primary current (IMAXFLY). The voltage across
the sense resistor (FLYSENSP) is compared to the IMAXFLY voltage. If the current
exceeds the set limit, FET V554 will be turned off.
Another internal current source supplies a current to R558. This resulting voltage is a
reference for the maximum allowable output voltage (VOUTHI). The -3V3A output
voltage (M3V3A) is attenuated and level shifted in the P-ASIC, and then compared to
the VOUTHI voltage. If the -3V3A voltage exceeds the set limit, FET V554 will be
turned off.
The FREQPS control signal is converted to appropriate voltage levels for the FET switch
V554 by the BOOST circuit. The voltage VBAT supplies the BOOST circuit power via
V553 and R561. The FREQPS signal is also supplied to the D-ASIC, in order to detect
if the Fly Back converter is running well.
V551 and C552 limit the voltage on the primary winding of T552 when the FET V554 is
turned of. The signal SNUB increases the FLYGATE high level to decreases
ON-resistance of V554 (less power dissipation in V554).
3-12
Primary current
Secondary current
FLYGATE SIGNAL
Figure 3-5. Fly-Back Converter Current and Control Voltage
V554 "ON"
V554 "OFF"