I have been making some high voltage projects lately, such as a spark gap Tesla coil, slayer exciter, and a flyback transformer HV supply.
The flyback transformer uses a single transistor circuit, just like the slayer exciter circuit.
I know that solid state Tesla coils use a pair of transistors in order to create an AC signal to power the primary.
How can only 1 transistor create an AC signal to power the primary of a slayer exciter or flyback transformer?
Transformers don't require AC to operate. They require a changing current. This can be a changing DC current just as easily as an AC current.
Transistors can "generate AC", in fact with the right filters and switching you can get transistors to generate sinewaves (this is how most DC/AC inverters work).
In today's modern power supplies transistors are switched on and off rapidly to create changing current that can be passed through transformers. Many switched mode power supplies (SMPS) do this to convert AC/DC or vice versa.
This is typically done with some kind of feedback or control loop. The control loop either excites the voltage (like in the top diagram) or controls the voltage to a certain level (like when we want a constant 5V out of a 5V power supply). The transistor acts as an amplifier to provide positive feedback. In SMPS an analog or digital control loop is used to provide negative feedback.
how can only 1 transistor create an AC signal to power the primary
There are two things happening here.
The constant DC part will go straight through the transformer coil and create a constant magnetic field. This does not induce any voltage in the secondary. All it does is saturate the core a bit and increase losses.
However the AC part will still cause a varying magnetic field, which will induce a voltage on the transformer secondary.
In other words, transformers only work on AC but can ignore some amount of DC offset current on the primary. The transformer core still has a maximum field limit above which it will saturate, and this depends on instantaneous current, thus having a DC current on top of your AC current reduces the maximum AC current the transformer can handle before its core saturates.
When the transistor turns on, current rises in the primary. Since this is a transformer, this causes a corresponding rise in voltage on the secondary, but in a flyback transformer this part of the secondary waveform is not used. The voltage rises, but no current goes into the load, usually because there is a rectifying diode in the output.
When the transistor turns off, the primary is disconnected. But the magnetic energy in the core has not disappeared, which means current has to find somewhere to flow. If this was just an inductor, voltage would rise until something breaks and lets the current through, for example the transistor would avalanche. However this is not an inductor, but a transformer instead, it has two coils, so voltage rises very quickly on both primary and secondary until something breaks. On a flyback transformer, this is used to obtain high voltage on the secondary. The turns ratio of the transformer is chosen so magnetic energy stored in the core discharges through the secondary coil into the load before the transistor avalanches. Once the energy is dumped in the load, magnetic flux is back to zero and the cycle can restart.
So, in a flyback transformer, having DC offset on your AC is an intentional part of the design, it does not create problems like it would on a "normal" transformer. The goal is to ramp up primary current to store energy in the core, then dump it in the load via the secondary.
As long as the transistor stays on, the current and magnetic flux will keep increasing. This can't go on forever of course, first copper losses are in i^2 so they increase dramatically at high current, second at some value of flux the core will saturate. So, input voltage, ON-time, primary winding turns and copper gauge, and of course the core itself, should all be chosen together for best efficiency and lowest cost. Since a core with a higher saturation flux will be bigger and more expensive, peak primary current also influences cost.
I know that solid state tesla coils use a pair of transistors in order to create an AC signal to power the primary, so how can only 1 transistor create an AC signal to power the primary of a slayer exciter or flyback transformer?
If you want to more efficiently drive a tesla coil, then you would use a push-pull arrangement of transistors and "hard drive" the transistors as switches. This minimizes transistor losses and makes a more power efficient solution. However, you have to watch out for "killer" back emfs but, that's another story.
If you used a single transistor (as in your first circuit) it is likely that the transistor will operate in class A and be handling signals that are more akin to sinewaves. Because the transistor operates in class A, there will always be a standing collector current in the transformer primary and, the voltage on the primary will have an average value of zero because it will rise above the power rail as much as it falls below the power rail.
An example I have to hand is a Colpitts oscillator that uses an inductor in the collector but, this could equally be a transformer in the collector: -
Pictures from this Q and A.
As you can see, Vout (blue trace) rises equally above and below the 5 volt rail hence, the transistor remains in class A and, the average voltage across the inductor is largely zero. So, even though the supply is DC, the actual input voltage across the "primary" is virtually pure AC and, of course, with any inductor or transformer it has to be to avoid saturation.
These circuits use positive AC feedback to oscillate by pulsing DC into AC to generate the AC output. The 3rd coil on the primary side is a reliable method. The other methods rely on the flyback of proper polarity of magnetic and parasitic coupling to create positive feedback with gain >1.
DC positive feedback merely makes it a latch. but AC positive feedback with delay and proper polarity makes it an oscillator.
In order to make a great Tesla coil, Tesla did not use any transistors. because they weren't invented yet. (unlike @Andyaka 's)
So how did he maximize dV?
By minimizing dt into a small inductance and reasonably primary high arc voltage to create a low capacitance 2nd stage transformer winding with low turns ratio <100 but a primary step transformer with the highest step up ratio possible so the inductance won't affect the result using the purest 99.9% best insulation, Mica to couple the trigger to the 2ndary transformer and flyback the biggest > 1 MV arcs.
It is hard to simulate with Falstad's limited 2kB trace memory as aliasing occurs when the rise time is faster than 1/2 the sampling rate yet try to fit the repeated arc interval in one trace. But it looks something like this. Slow down the simulation just before it reaches 10.0 kV the trigger threshold of the gas tube I chose to trigger the arc. This will produce > 120 kV. Anything is possible in theory, but practise has physical limits of interwinding capacitance and L/R ratio of a 1kW high inductance primary transformer (huge).
V = L dI/dt as dt goes to zero you get the biggest arc voltage but with low L for low open circuit stage 2 secondary capacitance at some resonant frequency. .= LdI/V using an arc gap switch with the smallest dt arc rise time in xxx picoseconds.
HV arcs required a rapid negative resistance to collapse in voltage in picoseconds to nanoseconds to make a UHV flyback coil to generate an ultra-high voltage from the follow-on change in current.
The arc density determines how low the ESR becomes and how quickly the arc extinguishes to determine how fast dI/dt is the result. So by using a gas arc gap, you get far faster rise times than trying to force the current turn-off with transistor capacitance.
It is ultimately the lower capacitance and Q of the RLC resonance of the gas arc extinguishing that results in the hi V. But along with high Q is an envelope rise time so the peak voltage is not always on the 1st cycle but after the risetime of envelope as the primary discharge cap decays with a low negative ESR and the Tesla Coil DCR that affects the result.
Shown with 1ns sampling resolution you can see the rise of the output envelope with a reasonable L/R ratio of 32 uH/0.1 Ohm for Litz wire in a large air core and using a donut termination to reduce e-field gradient.
Now 175kV
Don't try this at home.
Your 3 versions have different operating principles. Also the transformer is used differently. The 1st and 2nd circuit use the transformer both to rise the voltage and to make feedback in the oscillator. Both circuit use a pulse oscillator principle where the transistor at first conducts, but gets suddenly closed to unconductive (=no Ic) state. The conduction starts again quite soon. How soon - that depends on many things, even parasitic capacitances affect. Well predictable (lower) operating frequencies can be got by inserting an explicit RC-time constant circuit instead relying on parasitic capacitances.
But the essential idea in getting high voltages in circuits 1 and 2 is to charge magnetic energy to the coil with growing DC current through the primary. Oscillator feedback is only a side function. When the primary current suddenly stops an inductive kickback pulse is generated. It's a circular electric field around the core in the transformer and so strong that at least one of the windings pushes current to somewhere. The voltage simply jumps as high as needed to allow gradual magnetic field decaying. Stepwise magnetic field change is as impossible as stopping a moving mass immediately from full speed to zero. It doesn't happen, every braking allows more or less motion because infinite forces do not exist. Read this old case for reference: https://electronics.stackexchange.com/questions/282053/how-does-the-inductor-really-induce-voltage?r=SearchResults&s=1|63.8725
The transistor stays hopefully unconductive at least as long as it's is needed to dissipate all of the magnetic energy stored in the transformer. Otherwise the current gradually cumulates and there's a short circuit.
version 2 differs from version 1 in one important area. In ver. 2 the transformer is loosely coupled. The secondary can well operate as resonant circuit (C=the stray capacitance) and oscillate some time after the inductive kickback. That's the same idea as in Tesla coil.
The 3rd circuit is different. Its output transformer is also loosely coupled and rings substantially after every input pulse. The mosfets feed alternating polarity pulses (=AC) to the primary. The primary isn't used to generate inductive kickbacks the transformer rises voltage more like a normal transformer. But the ringing due the resonance is essential to make the same high frequency content as ver 2.
A single transistor forward drive of a transformer with DC pulses. DC pulses work as well if there's an auxiliary flyback or "DC-reset" circuit which returns the magnetic energy from the transformer back to the power supply and thus prevents DC current cumulating and developing a short circuit.
Most several hundred watt power supplies I have seen in PCs use this principle which feeds the primary with DC pulses. The output in the secondary is in use at the same time as the transistor in the primary circuit conducts. The next Wikipedia image presents the principle:
The auxiliary winding returns the inductive kickback pulse to the DC input supply through D3.
For more examples search for "forward DC to DC converters". In flyback converters the output gets a new pulse when the primary circuit switch stops conducting. In forward converters the output gets a new pulse when the primary circuit switch is ON. Turning the switch off generates a pulse which is returned to the input DC voltage source.
how can only 1 transistor create an AC signal to power the primary of a slayer exciter or flyback transformer?
Its an AC signal + a DC bias. A transformer can have some DC bias without affecting its output, provided the core doesn't saturate.
Do you call a 5Vdc square wave "AC". No it is not. Inductors (coils) operate like this. Transistor is turned on via the R between the base and +5Vdc. The voltage rises across the half of the inductor the transistor is connected to (I'm using a center tapped transformer as the inductor, as it is easier to understand the cycle).The coil produces magnetic flux from the rising voltage, This flux is "expanding" and it expands as long as the voltage is increasing. At the same time, the flux causes the other half of the inductor to develop a voltage that is of the same polarity. Imagine a NPN bjt. It is on, so current/ voltage is passing to the emitter. The emitter is connected to the center tap. Therefore the voltage flows into the lower half of the induçtor and out the bottom which is connected to circuit ground. The polarity is + at the center tap and - at the bottom. The induced voltage in the top half has the same polarity,+ at the top and - at the center tap end. We connected a capacitor to the upper "+" end and then to the transistor base. This feeds more + voltage into the base which turns the transistor on more and more until it's all the way on. At this point, there is 5 Vdc across the lower half and since we are using a 5 Vdc supply that's all it can rise. As mentioned above the flux expands when the voltage across the coil is increasing. It stops expanding the same time the voltage stops increasing. Now it is referred to as "stationary flux". As it was expanding,the increasing voltage was supporting it as it increased it ( the flux). The voltage stopped increasing therefore what was supporting the flux is now gone. Like anything large and curved, kick the supports out and it collapses. Right. Kick them out fast and it collapses faster. On the upper end the situation is a little different.As the voltage peaks below so it does above.As the flux field is not expanding through the upper coil anymore that turning on voltage just disappears. This causes the transistor to start turning off. Cutting off the voltage to the lower half. The "supports" holding up the flux "disappear". More or less but it's easier to see ( in your head) the flux crashing down and collapsing back where it originated from. Now those flux lines are passing through the coils turns in the opposite direction. This generates a voltage with a reverse polarity i.e. positive at the bottom and negative at the top. The center tap leads effectively nowhere, so this induced voltaage is across the entire coil and because indictors are "magical" this voltage is much higher than 5 Volts. During all of this the flux reaches across the core and works it's magic on the secondary coil. Inducing voltage across it. Now to answer the question you posed. The output of the primary would be pulsed voltage from this setup. Hard to filter and not very efficient. By using a push - pull two transistor setup on the primary side. The secondary output would be more sinusoidal in form, more even and easier to filter. The circuits you mentioned seeing, that the described one transistor driver was part of. Well those circuits don't really care if the voltage waveform is smooth and conformal.They thrive on the huge pulses of energy from a flyback type. Won't do for a Tesla coil as it wants smooth delivery of huge pulses without all the generated noise disturbing the circuits resonant frequency. Hopefully all this has helped.
