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EN 132
9.
FM23, FM24, FM33
supply is regulating, the current through the opto-coupler is
amplified and will deliver power to the IC.
Control is done in the usual way by a TL431 at the secondary
side. Voltage V_rs, a control signal coming from the display, is
mixed into the feedback voltage, using an additional TL431
(7011 at schematic P6). It influences the output of the V_s
supply. The output voltage of the V_s supply varies according
the following formula: V_s = 70 + (10 * V_rs). Via this
stabilisation circuitry for V_s, the output voltage is stabilised. If
necessary, it is possible to adjust the voltage via potmeter
R3026 (not for FM33).
The V_s is fed via a voltage divider to IC7010 (TL431). If the
voltage at pin 3 of IC7010 is higher than 2.5 V, a current will
flow from cathode to anode. This current also flows through the
secondary side of the optocoupler 7002. The voltage at pin 7 of
the MC34067, determines the output frequency. The higher
this voltage, the higher the output frequency. Thus, if the
voltage on pin 7 increases, the frequency increases and V_s
decreases.
When the output voltage rises, the voltage at the reference
IC7010 also rises, this causes the current through the diode of
the opto-coupler to rise. The transistor of the opto-coupler
conducts more, because of which the voltage at pin 7 of the
MC34067 increases.
Accurate Over Voltage Protection (OVP) is added, using a
TL431 (7304, diagram P3) as reference/comparator and an
additional optocoupler (7003) that acts on the fault input pin 10
of the MC34067P (see also "Power Supply Protections ").
The Controller
The MC34067P controller, is used for the following reasons:
•
Zero voltage switching.
•
Variable frequency oscillator (above 1 MHz).
•
Precision one-shot timer for the dead time.
•
5 V reference output.
•
Double, high current totem-pole output.
•
Soft start.
•
Wideband error amplifier.
•
Fault input (protection).
V CC
15
50k
7.0k
7.0k
Enable /
UVLO Adjust
9
50k
8.0V
OSC-CHARGE
Q1
1
OSC-RC
R3003
C2004
2
I OSC
One-Shot RC
R3004
16
One±Shot
C2005
Oscillator
Control Current
3.1V
3
I OSO
R3005
Error Amp
Clamp
Error Amp Output
6
8
Noninverting Input
9.0µA
Inverting Input
7
Error Amp
Soft-Start
11
Figure 9-6 MC34067
The oscillator circuit is build around an internal comparator with
two threshold-voltages: 4.9 and 3.6 V.
C2004 is first charged via transistor Q1. If the voltage across
C2004 is more then 4.9 V, the output of the upper comparator
becomes low, the NOR-port output will be high, and Q1 will be
blocked because the base will be shortened by Q2. C2004 will
be discharged via the resistors R3003 and the oscillator control
current (I_osc).
If the voltage across C2004 is below the lower threshold (3.6
V), transistor Q1 is conducting and the capacitor is charged
Circuit Descriptions, List of Abbreviations, and IC Data Sheets
MC34067
V ref
5.1V
Reference
V CC UVLO
V ref UVLO
V ref
4.2/4.0V
D1
Q2
Steering
Oscillator
Flip-Flop
Q
T
4.9V/3.6V
Q
R
4.9V/3.6V
R
Q
S
1.0V
Fault
Latch
OR3
CL 16532099_111.eps
260901
again. The oscillation frequency is modulated by the oscillator
control current.
The discharge current increases, when pin 3 is loaded even
more; thus the lower the voltage on pin 3, the higher the
oscillator control current and the higher the frequency. The
maximum frequency is reached when the output of the error
amp is minimal (0.1 V). Thus, R3005 determines the maximum
frequency.
The minimum frequency is reached, when I_osc current is
zero: C2004 then discharges only via the resistor R3003.
One Shot Timer
The one-shot timer is present, to de-activate both outputs
simultaneously, and to provide a dead time, so that only one
output will be activated.
The one-shot capacitor (C2005) is first charged by Q1. The
one-shot period begins when the oscillator comparator is
switched "off" by Q1. The one-shot capacitor is discharged via
the parallel resistance (R3004): if this voltage gets lower than
the lower threshold of 3.6 V, the comparator will be high and
controls the flip-flop, which makes one of both outputs high.
If Q1 is re-conducted through the oscillator comparator, the
one-shot capacitor is re-charged.
Fault Detector Input
At pin 10, there is a fault detector input. If this voltage reaches
1 V, the output of the OpAmp goes high, and both drive outputs
are switched "off".
In addition, the output of OR3 will be high via the "fault latch".
The output of OR3 drives Q1, so both the oscillator and the
one-shot-capacitor remain charged. Via OR3, the soft-start
capacitor is discharged.
Soft-start
Due to the soft-start circuit, the oscillator starts with maximum
frequency. The low voltage on the soft-start capacitor (C2027)
is buffered and keeps the error amplifier output low (if I_osc =
max then f_osc = max).
The capacitor is charged with a current of 9 uA, the output of
the buffer gets high, and the error amplifier input takes charge
of the oscillator control current.
Driver stage
The two secondary windings of the driver transformer control
the two switching MOSFETs. The primary winding of the driver
transformer is alternately controlled by the two totem-pole
V ref
5
outputs of the controller. Cross-conduction of both MOSFETs
is prevented by the dead time.
The gate of each MOSFET is controlled via diodes 6007/6008
and resistors 3014/3017.
Output A
14
The transistors 7007/7008 discharge the gate faster by
Power Ground
13
switching "off" situations.
The diodes 6017/6028 at the base-emitter of 7007/7008
Output B
12
prevent the zener effect of these transistors.
The zener diodes at the gate-source of 7005/7006 are for ESD
Fault Input
protection.
10
C2011 and C2014, form the capacity for the series resonant
circuit.
MOSFET switching
The total switching time can be distributed over 12 phases with
different current paths. Only four phases are discussed to
simplify the explanation:
Phase 1 (S1 closed, S2 open)
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