Mikhail Shustov, Tomsk
A simple driver circuit of bipolar pulses with separate and independent frequency adjustment and duty cycle is given.
Bipolar pulses are mainly used to switch transistors alternately in bridge and half-bridge voltage converters. It is well known that when such converters operate at higher frequencies, the inertial processes of cleanout of minor current carriers in the base circuits of power transistors begin to affect their operation. As a result, the daisy-chained transistors can simultaneously be in a conductive state, despite a pilot signal absence. In this regard, there may be high chances of failing expensive transistors due to the uncontrolled through current flowing through them [1-3].
In order to reduce the probability of flowing the through current between pulses, a pause is introduced, whose duration should slightly exceed the cleanout time of minority current carriers.
A method of obtaining a series of bipolar pulses from unipolar pulses sequence, using the impulser, a D-flip-flop, anticoincidence circuits, and an operating amplifier powered by a dual supply, was first described in the British press  and then duplicated in the Russian one .
(Fig. 1) The device inherits the output stages of driver circuits [4,5] and is distinguished by the possibility of smooth and independent frequency adjustment and width of bipolar pulses.
|Figure 1.||Bipolar Impulser|
The driving impulser is made on element DD1.1 CMOS of chip CD40106. The impulser operating frequency is determined by circuits RC: capacity of condenser C1 and the total resistance of resistors and potentiometers R1–R5. This frequency can be continuously adjusted with potentiometer R2 from 850 to 6000 Hz. The impulser frequency can be calculated from
f - in kHz,
R - in kOhm,
C - in ufd
Resistive chain R3–R5 is connected in parallel to the chain R1, R2. A sawtooth signal being formed during the charge-discharge of capacitor C1 is taken from the potentiometer slider of chain R4. This signal, inverted by element DD1.2 (Schmitt trigger, having a threshold switching effect), together with the signal taken from the output of the driving impulser, arrives both at element DD2.1 “OR” and the bipolar pulse driver (elements DD2.2, DD2. 3, of chip DA1), designed according to the previously known circuit [4, 5].
The adjustment of the potentiometer R4 allows, within a wide range, practically from 0 to 100%, changing the width of the output bipolar pulses without affecting the frequency of the impulser. Resistors R3, R5 are designed for the adjustment range of pulses width to negative and positive limiting.
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