148 / 266 3CIRCUIT DESCRIPTION
The resetting of the first monoflop IC16 triggers a secondary pulse in the second monoflop IC16 which
represents the true actuation pulse for the output stage driver. The length of this actuation pulse is adjustable
using the trim potentiometer TP14.
The power module generator is therefore always only initiated at the start of a half oscillation through the
actuation pulse. Due to the oscillating circuit as load resistance, the oscillation is extended into a complete
sine wave.
For all further oscillations during an activation phase, the generator is fed back via the resulting sinus
signal. A part of the output voltage U_PRIM of the parallel circuit is directed to the comparator IC14, the
output pulses of which now trigger the monoflops IC16.
The power module generator therefore continues to oscillate as a feedback generator until the activation
signal HF_EIN is reaccepted by the processor.
In this way, it is ensured that the generator always oscillates exactly at the resonance frequency of the
oscillating circuit and the operating frequency need not be manually adapted to the production-related
tolerances of the oscillating circuit elements.
Synchronization of the QK output stage
The quasi-complementary output stage operates at a serial oscillating circuit (on the power module) to
produce the required sine-wave current. This serial circuit affects the current in the QK output stage like a
flywheel and forces the temporal characteristic of the current to a certain extent.
Due to the improved efficiency and to protect the QK output stage, the transistors should always be switched
over at the same time that the current in the serial circuit has just made a zero crossing. This then reduces
the losses in the transistors.
This requirement can best be met according to the principle of feedback. Such a circuit principle has
already been realized in the actuation of the power module. There too, the advantages of digital control
signals with their low electrical power consumption and its versatility could be combined with the feedback
for a generator of high efficiency.
The generator cannot start by itself; it needs a starting circuit. The digital oscillator IC18 is a free-wheeling
oscillator, the frequency of which should be as close as possible to the frequency of the serial circuit. Its
output signal is directed to the priority encoder IC11. As long as no current flows in the QK output stage,
the measurement signal of the current U_IHP is zero (measurement point MP1). The comparator IC9
detects no current at input 9, therefore its output signal is low and the monoflop IC13 produces no pulses.
Its output is then continuously low and thus also the selector S1 of the priority encoder IC11. As a result,
the inputs IA2 and IA3 are switched through to the output. In this way, the signal for the starting circui IC18
or actuating the QK output stage is switched at the start.
Once the QK output stage has “started up”, a current is produced there which, at sufficient level, exceeds
the threshold of the comparator IC9, Pin 10. Since the current in the QK output stage is half-wave-shaped,
a pulse results at the output of the comparator with the frequency of the QK output stage. This pulse
triggers the monoflop IC13 at input 4. The ON time has been selected somewhat longer than the period
length of the HF signal of the QK output stage, so that the monoflop does not drop out as long as pulses
from the generated HF retrigger this monoflop (principle of the watchdog circuit). The monoflop increases
the selector input of the priority encoder IC11 to high-level. In this way, the signal from the start generator
Control board
Slot J3
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