Friday, 23 September 2011

Single-transistor flyback drivers.

keywords: single transistor flyback driver, single MOSFET flyback driver, simple flyback driver, one transistor flyback driver.

So far, i have dealt with several powerful flyback drivers such as ZVS and 555 timer. To be honest i m more interested in a simpler but powerful flyback driver, so i researched about drivers that utilise only one transistor.

The most popular one-transistor flyback driver is one that uses the ubiquitous 2N3055 NPN silicon power transistor. The schematic is as shown below. The number of turns of the primary is not critical but generally the trickler coil ( the one connected to the Base of the transistor) is having lower number of turns than that connected to the transistor's Collector.

My own variant of the circuit. This topology is also known as Armstrong oscillator, due to Edwin Armstrong who invented and applied this topology for his regenerative FM radio receiver. To recognise this topology is very easy; it has two coils, one having more turns than the other, and it has only one transistor ( or traditionally, a vacuum tube).

Surprisingly, when I substituted the transistor with MOSFET, the circuit is still working. I substituted the Collector, Base and Emitter of the transistor with the Drain, Gate and Source of the MOSFET, respectively. IRF840 and IRFP250N worked successfully, but it didnt work with IRFP450 ( dont know why, if u know why then u can teach me in the comment section). Below is the same circuit, but using MOSFET as an active component.


For both circuits, I found out that my laptop power supply that I m always been using ( 12 V, 3.16 A) cannot be used anymore for these circuits, since it baked the transistors and MOSFETs and if not, degraded them significantly, even though i ve used zener diodes at the base or gate of the devices. I inferred that might be due to large current (3.16 A) of the supply. Therefore I resorted to simpler supply which is the common 9V square battery. Both circuits produced less powerful arc than ZVS or 555 driver ( maybe due to lower power when using battery), and the arc was about 0.5 to 1 cm length. The winding direction of the trickler coils of both circuits can be reversed, and it still works successfully. I know that the circuit can be further improved so that it can fit larger power supply, but my superficial electronic knowledge and tight budget ( i am poor student, T_T) limit improvements and haunt me of more transistor burnings (so far I ve burnt between ten to twenty MOSFETs during three months of experimenting, and each of them worth 1pound. So count by yourself how much lost i ve been facing!). I really hope someone can tell me how the circuits above can be improved for larger power supply. :D

Below is the circuit that didnt work EVEN when using the battery power supply ( I think this circuit actually didnt work, it just damages your MOSFETs and BJT). I just posted it here cuz I am perplexed with most designs of Armstrong oscillator that utilises the following configuration. I dont know the reason behind that, but it seems that most of the designs are using higher power supply. (such as 50, 100 to 140 W, while the highest power supply i have which is the laptop power suppy is just 40 W, in fact, always lower than that!) I also hope that someone experts can explain why the following circuit didnt work.

This circuit doesnt work even when using 2N3055 transistor.

1 comment:

  1. Less than a month ago I knew Jack squat about electronix.. now I'm attempting to build circuits but so far no success. One of my biggest problems is I have no access to anywhere to buy specific componants.. i have just been harvesting the called for parts from Monitors and using the net for schematics and componants specifics. Even found an awesome site for resistor code and also for Identifying them. The hardest part I think I am having is transistors.. the most important piece. I want to try this first schematic you have posted. Can you recommend a site that lists good NPN transistors?

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