A blocking generator is a signal generator with transformer positive feedback that generates short-term (usually from fractions of microseconds to milliseconds) electrical pulses that are repeated at long intervals relative to the pulse duration, i.e., having a large duty cycle.
They are used in radio engineering and in pulse technology devices . As an active element, a transistor or an electronic lamp is used .
A blocking generator is a relaxation circuit containing an amplifying element (for example, a transistor ) operating in a key mode and a transformer through which positive feedback is provided.
The advantages of blocking generators are comparative simplicity, the ability to connect the load through a transformer with galvanic isolation , the ability to form powerful pulses that are close in shape to rectangular.
Among the variety of cases of using blocking generators, four main ones can be distinguished:
- pulse shapers;
- Comparing devices - comparators ;
- pulsed oscillators ;
- frequency dividers.
When used as pulse shapers, blocking generators work in standby mode . Their most important characteristics are: sensitivity to start, the duration of the generated pulses and its stability, the maximum achievable response frequency.
In blocking generators with on-off windings (positive feedback), the number of turns of the base winding (or grid) of the transformer winding must exceed the number of turns of the collector (or anode) at least three times.
Content
Job Description
The circuit works thanks to the positive feedback through the transformer. During time T closed, the key is closed, during time T open, the key is open.
Key
When the key (is it a transistor or an electronic lamp) is closed, almost all the voltage of the power source V b is applied to the primary winding of the transformer. Moreover, due to the inductance of the winding, the magnetizing current I n = V 1 × t / L, where t is the time parameter increases approximately linearly.
This magnetizing current I n will follow the induced current of the secondary winding I 2 flowing into its load (for example, to the control input of the switch; the current of the first winding induced by the secondary winding = I 1 / N). A change in the primary current causes a change in the flux of the magnetic field passing through the transformer windings; this changing magnetic field induces a relatively constant voltage in the secondary winding V 2 = N × V b . In some circuits (as shown in the pictures), the voltage of the secondary winding V 2 is added to the input voltage of the source V b ; in this case, due to the fact that the voltage drop across the primary winding (so far the switch h) is approximately V b , V 2 = (N + 1) × V b . Or the key can get some of its control voltage or current directly from V b , and the rest will be from the induced V 2 . Therefore, the key control voltage is “in phase,” as it were, in the sense that it keeps the key closed and this (through the key) supports the input voltage drop of the primary winding.
In the case when the resistance of the primary winding or the key is small, the increase in the magnetizing current I n is linear, and is described by the formula in the first paragraph. If the resistance of the primary winding or the key or both (the total resistance is R, for example, the resistance of the primary winding + emitter resistance, the resistance of the field-effect transistor channel), the time constant L / R makes the magnetizing current a growing curve with a constantly decreasing slope. In any case, the magnetization current I n will overpower the total current of the primary winding (and key) I 1 . Without a limiter, it will grow forever.
- However, in the first case (low resistance), the key in the end will not be able to produce more and more current, which means that its output resistance will increase so much that the voltage drop across the key will become equal to the supply voltage; in this case, the key is called “saturated” (for example, this is determined by the gain of the transistor h fe or beta).
- In the second case (for example, the resistance of the primary winding and / or emitter is noticeable) (decreasing), the current slope decreases until the voltage induced in the secondary winding is already insufficient to keep the key open.
- In the third case, the magnetic core is saturated, which means that it can no longer allow further increases in its magnetic field; Under this condition, the induction of the primary winding into the second one ceases to work.
In any case, the slew rate of the magnetizing current of the primary winding (and therefore the magnetic flux), or directly the slew rate of the magnetic flux in the case of saturation of the magnetic core, drops to zero (or so). In the first two cases, even though the current continues to flow through the primary winding, it reaches a stable value equal to the supply voltage V b divided by the total resistance R of the primary winding circuit. In this case of limited current, the magnetic flux of the transformer will be constant. Only a changing magnetic flux causes induction EMF in the secondary winding, so that a constant magnetic flux will lead to the fact that this EMF will not be in the secondary winding. The voltage of the secondary winding will drop to zero. At time T unlocked, the key opens.
Key open
The magnetizing current of the primary winding is now I pulse, max. = V 1 × T closed / L. Energy U = ½ × L × I impulse, max 2 is stored in this magnetizing field created by I impulse, max . Now there is no longer the primary voltage (V b ) to withstand further increases in the magnetic field, or even at least the field in a stable state, the key opens, thereby removing the voltage from the primary winding. The magnetic field (flux) begins to collapse, and this collapse pushes the energy back into the circuit, creating current and voltage in the turns of the primary winding, secondary winding, or both. Induction in the primary winding will occur through its turns through which the lines of the magnetic field pass (represented by the inductance of the primary winding L); the compressing magnetic flux creates a voltage on the primary winding, causing the current to either continue to flow from the primary winding into the (now open) key or to flow into the load of the primary winding circuit such as an LED, a zener diode, etc. Induction into the secondary winding will occur through its turns, through which the mutual (connected) lines of the magnetic field pass; this induction causes a voltage to appear on the turns of the secondary winding, and if this voltage is not blocked (for example, by a diode or a very high resistance of the base of the field effect transistor), the current of the secondary winding will flow into the secondary winding (only in the opposite direction). In any case, if there is no one to consume current, the voltage on the key will jump very quickly. Without a load in the primary winding circuit or in the case of a very small secondary current, the voltage will be limited only by the stray capacitance of the windings (the so-called inter-turn capacitance), and it can destroy the key. When only the inter-turn capacitance and the smallest secondary load are present in the circuit, very high-frequency pulsations begin, and these “spurious pulsations” are a source of electromagnetic interference.
The secondary voltage is now negative as follows. A decreasing magnetic flux induces a current in the primary winding in such a way that it flows from the primary winding into a newly opened key, for example, to continue to flow in the same direction in which the leakage flow was maintained while the key was closed. So that the current flows from the end of the primary winding connected to the key, the voltage on the side of the key must be positive with respect to the opposite end, that is, to the side of the voltage source V b . But this represents the voltage of the primary winding opposite to the polarity that it was while the key was closed: during T it was closed , the side of the key of the primary winding was approximately zero and therefore negative relative to the side of the power source; now during T open it has become positive with respect to V b .
Due to the direction of the transformer windings, the voltage appearing on the secondary winding should now be negative . A negative base voltage will keep the key (for example, a bipolar NPN transistor or an N-channel field effect transistor) open , and this will continue until all the energy of the decreasing magnetic flux is absorbed (by something). When the absorber is a primary winding circuit, for example a zener diode (or LED) with a voltage of V s , connected back to the turns of the primary winding, the current shape will be a triangle with time T open , calculated by the formula I p = I pulse, max - V s × T open / L p , where I is the impulse, max is the current of the primary winding at the moment of opening the key. If the absorber is a capacitor, the voltage and current are a sine wave, and if the absorber is a capacitor together with a resistor, the voltage and current are in the form of a damped sine wave.
When finally the energy is wasted, the control circuit will become “unlocked”. The control voltage (or current) in the key can now freely "flow" into the control input and close the key. This is easier to see when the capacitor “switches” the control voltage or current; ripples transfer the control voltage or current from negative (key open) through 0 to positive (key closed).
See also
- Electric generator
- Electronic generator