1
\$\begingroup\$

I'm fairly new to the world of electronics, and transistors are still a bit confusing to me. I'm trying to build a flyback transformer driver based on the schematics at http://www.powerlabs.org/images/schematic1.gif and https://youtube.com/watch?v=0jWpQ8HEYgk . I'm using a 2N3055 transistor rated for up to 15 A on the collector with power dissipation up to 115 W.

At 4 V, the system draws about 2 A and I get several thousand volts out of the flyback, as expected. The transistor gets a little warm, but not too hot to touch. As I slowly increase the supply to 6 V, the current draw rises to 3 A and then the flyback suddenly stops outputting any voltage. When I examine the transistor, it shows only a few ohms of resistance between any pair of pins, which I assume means I killed it. Replacing the transistor fixes the circuit. Hooking the circuit up to the 12 V, nominally 2 A laptop supply that I hope to use once I'm done prototyping also causes the transistor to instantly blow.

My question is, what would cause the transistor to die at so much less than its rated specifications? After reading something about voltage spikes caused by inductance in the transformer, I added a diode from ground to the feedback coil as shown in the video above, but it doesn't seem to help. These transistors are about $3 apiece, so I'd prefer to go through as few of them as possible. Any ideas?

\$\endgroup\$
  • 1
    \$\begingroup\$ How about adding a Heatsink to the transistor? \$\endgroup\$ – ammar.cma Aug 27 '16 at 14:52
  • 1
    \$\begingroup\$ It will generate about 7.98W heat at T(amb) @ 25C at 4V 2A . I think you should put a heatsink. \$\endgroup\$ – ammar.cma Aug 27 '16 at 15:03
  • \$\begingroup\$ @ammar.cma I did try adding a heatsink, which didn't change the situation at all. It was a fairly small one, though.... The 2N3055 is rated for up to 200°C, and it's not getting above 50°C, so I assumed that no heatsink was necessary. Is that wrong? \$\endgroup\$ – Matthew Schauer Aug 27 '16 at 15:06
  • \$\begingroup\$ Vceo= 60V max, Vbe=-7V max are absolute max ratings which are affected by V= L*di/dt on the primary switch. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 27 '16 at 15:26
  • 1
    \$\begingroup\$ 'the 2n3055 is rated for up to 200C' ?? Well, technically yes, but the rating at that temperature is zero dissipation. To actually use it, you need to be below that temperature by a large margin. I reckon a bare TO-3 would be good for a max of 2 watts. \$\endgroup\$ – Neil_UK Aug 27 '16 at 15:36
6
\$\begingroup\$

Unfortunately, a flyback requires ratings for a transistor waaaaay above what you would niavely expect.

The venerable old 2N3055 is specified to only 60v on the collector. The TIP35C mentioned in the video is 100V rating, but even that is rather low for the suggested configuration.

When you operate a flyback, the rise in collector voltage is determined by the energy stored in the primary inductance, and the capacitive load connected to it. It's not uncommon for TV flybacks, and auto ignition coils, to develop 300v on the primary, and therefore the collector of the driving transistor, when driven from a 12v supply.

It helps to measure the collector voltage with an oscilloscope, as the power supply voltage is raised.

There are several things you can do to avoid failure due to high collector voltage ...

a) Use a transistor with a higher voltage rating
b) Increase the load capacitance, to absorb the inductive energy at a lower voltage
c) Use an overvoltage clamp diode collector to emitter (inefficient)

... but it's all a bit hit and miss without an oscilloscope, so you can actually see what's happening.

You may get shown videos of 2n3055sTIP35Cs working in this configuration, but if their transformer is a bit soggier, or their transistor is a bit more robust, theirs could work OK, and yours fail.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the succinct explanation! I knew there was something more at play here, but wasn't sure what. Would (b) be as simple as adding a capacitor across the primary? What determines the appropriate capacitance and voltage rating? I've been thinking of buying an oscilloscope...maybe it's time to actually do it. \$\endgroup\$ – Matthew Schauer Aug 27 '16 at 18:02
3
\$\begingroup\$

Some factors to consider;

Saturation current and flyback slew rate for the transformer selected. These are determined by Core properties and winding resistance and number of turns.

Voltage rating and protection of switch device

When the switch turns off at max current, it's Vce will rise and the VI=P power product of this transition pulse will generate heat. Observe all device spec limits to avoid failure.

  • Vce(max), Vbe(max reverse) voltage when switch opens
  • 2N3055 Vceo= 60V max, Vbe=-7V max are absolute max ratings

    Suggestion to protect device. 1n400x diodes. enter image description here

Once arcing occurs - Safe Operating Area (SOA) for P vs time

If you can measure the voltage ratio with a small signal input and desire a 1" breakdown voltage or ~ 25kV ensure the primary voltage does not exceed 50Vce by reducing the number of primary turns and choose appropriate ratio for Vbe.

\$\endgroup\$
  • \$\begingroup\$ Transistor SOA is well under the limits @TonyStewart ; what could be the other reasons except for that? If the transistor is rated for much higher currents than the OP's conditions. \$\endgroup\$ – ammar.cma Aug 27 '16 at 15:10
  • \$\begingroup\$ Like I said, I'm fairly new to electronics, so I'm having a little trouble understanding parts of your answer! Keep in mind that the transistor is failing not when I disconnect power but when I increase it above 3 A or so. Would that be resolved by adding those diodes? \$\endgroup\$ – Matthew Schauer Aug 27 '16 at 18:22
  • \$\begingroup\$ Due to lack of details on your primary windings added, power supply and transformer, I can only guess why it failed. Maybe you reversed windings for Base and Collector? I found this might be helpful on other's experience blowing transistors on this. angelfire.com/80s/sixmhz/flyback.html and powerlabs.org/flybackdriver.htm#GOING%20FURTHER: \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 27 '16 at 18:45
  • \$\begingroup\$ @TonyStewart I've read a lot of articles online, but I didn't manage to find the angelfire one. I'm pretty sure the problem described in the "Circuit Operation" section (corroborated by Neil_UK's answer) is the one I'm having. I'll try some of the other transistors in that list. Thank you! \$\endgroup\$ – Matthew Schauer Aug 27 '16 at 20:11
  • \$\begingroup\$ It also could be your power resistors are Wire wound which raises the impedance (inductance ) and transient voltage causing damage to the 2N3055 Vbe. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 27 '16 at 21:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.