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Protecting a Signal Generator while Driving Transistors.

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cduma

Mon Jul 25 2011, 06:10PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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What I did when I was blowing up some 2N3055 transistors I just built a timer based on the NE555 with some high wattage passives and a socket for the 555. I did use another transistor on the output of the timer so ensure enough current.

Ash Small

Tue Jul 26 2011, 10:41AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

cduma wrote ...

What I did when I was blowing up some 2N3055 transistors I just built a timer based on the NE555 with some high wattage passives and a socket for the 555. I did use another transistor on the output of the timer so ensure enough current.

I'd still want similar protection if I went that route, Cduma.

I've been giving this some more thought, and come to the conclusion that any snubber circuit will still allow current to flow through the primary for an extended time, thus defeating the object of a fast 'switch off'. ( the shorter the switch off time, the greater the voltage spike, I think.)

I've realized that I'll have to measure the transient voltage spikes in order to know exactly what is going on, so I need to build a circuit (with some current limiting) and hope my 10 MHz 'scope is up to the job.

If not, it looks like it's time for a new 'scope.

Mattski

Wed Jul 27 2011, 03:08AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
I've been giving this some more thought, and come to the conclusion that any snubber circuit will still allow current to flow through the primary for an extended time, thus defeating the object of a fast 'switch off'. ( the shorter the switch off time, the greater the voltage spike, I think.)

A typical RCD snubber in a flyback converter is there only to absorb the voltage spikes caused by the leakage inductance of the transformer (which involves some current flow). The design has the antiparallel diode in series with a parallel RC duo. The idea is that the diode will charge up the capacitor from the leakage spike, which will then partially discharge through the resistor. Why partially? The snubber is only supposed to handle the leakage inductance spike and not the voltage impressed on the primary due to the output voltage, so keeping some voltage on the capacitor ensures that a big spike is needed for the diode to pull current, although this requires a somewhat steady state condition to have been reached. But this cannot easily be designed to work in the general case as your snubber values depend on many specific design variables.

It's also not always necessary to include the snubber. With a high enough voltage rating the spike might not even break down the transistor (although the 2n3055 has a pretty dismal breakdown voltage as I recall), and even if it does break down it just dissipate some extra energy from the leakage inductance spike heating it up a bit more, which isn't necessarily a problem as long as the transistor breakdown voltage is large enough not to break down from the voltage impressed on the primary by the output voltage.

I'm not familiar with the snubber design you've drawn it currently, but the capacitor will absorb some of the leakage inductance spike reducing the magnitude, and the diode will conduct some/all of this excess charge away by the next switching cycle. But even if the diode fully discharges the snubber cap there will be increased switching loss in the transistor because it has to charge up that capacitor in order to pull the base of the transformer to ground.

Also maybe I should have brought this up a while ago, but why even use the 2N3055? MOSFETs are generally far superior to BJT's in switching power circuits.

Ash Small

Wed Jul 27 2011, 09:41AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for your reply, Mattski.

I did some research on snubbers a while back, and found a few different combinations of RCD. The reason I chose this one for this application is, as I understand it, the diode takes time to start conducting, whereas the capacitor starts to charge immediately.

I'm going to have to build a circuit and see if I can see the voltage spikes on my 'scope, then try different snebber arrangements and see what happens.

There are several reasons for choosing the 2N3055, firstly, I have several, secondly, a lot of people use them, thirdly, they are cheap, fourthly, I should learn from this experience before going on to more powerful circuits.

I do agree that using a transistor/MOSFET with a higher voltage rating would solve some of the issues, but I wouldn't learn as much.

Ash Small

Sat Jul 30 2011, 10:17PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've been doing a bit of experimenting, and the circuit below doesn't work. It works fine without the capacitor, but when I add the capacitor, it doesn't work at all at 30 kHz, but works occasionally at 3 Hz.

The bulb only flashes briefly at 3 Hz, due to the capacitor blocking most of the pulse, it flashes a few times, then flashes occasionally.

I think this may be because there is no path for current to flow in the reverse or 'anti-clockwise' direction, so the capacitor cannot 're-charge' for the next forward or 'clockwise' cycle.

Any suggestions?

EDIT: Maybe a diode and 3.5k resistor from the ground rail (from the other terminal on the sig. gen.) to the resistor side of the capacitor, to allow the capacitor to re-charge during the negative half-cycle? I'll try it tomorrow.

Mattski

Sun Jul 31 2011, 12:25AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

That's a good observation Ash and I think you're right about the problem. You'll need to pull a continuous current to the right through the capacitor because of the base current needed while the transistor is on, but only a bit of charge flows in the opposite direction when the transistor is turning off.

Adding a resistor to ground as you mention would work, but it will need to be a very small resistor if you want it to work at high frequency. If you use a 3.5k shunt resistor it will take about 5RC to fully discharge, which is 19.25mS, so it would only work up to about 25Hz at 50% duty cycle. The problem is that the smaller you make that shunt resistor to improve that time constant the more of your drive current will be diverted through it instead of the transistor base. So perhaps what you could do instead is put a diode pointing towards the capacitor (cathode on ground, anode on the capacitor's right node) which would allow fast discharge of the capacitor, but wouldn't affect it while the transistor is on.

You might also want to do some rough calculations on minimum size of the capacitor, if it's pumping in a base current i_b, then over t seconds it's voltage will change by i_b*t/C, and you need to make sure that this is within the limits of your drive voltage with enough left over for the base-emitter and base resistor voltage drops. At high frequencies the minimum capacitor size decreases, but I'll leave you to run the numbers :)

Ash Small

Sun Jul 31 2011, 12:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks, Mattski. Other factors I'm thinking about are current available from the sig. gen, and trying to draw too much current from the sig. gen.. I'm still a relative 'newbie'.

I'll give it some more thought in the morning. Once again, thanks for the assistance.

EDIT: I also tried replacing the capacitor with a diode, thinking this would also give me the protection I want, but it didn't work at all. I don't understand this. I tested the diode, and it is good.

I can only assume it is to do with the voltage drop across the diode, most of the voltage being across the 3k5 resistor, so insufficient voltage to drive the diode.

Ash Small

Sun Jul 31 2011, 03:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Mattski wrote ...

So perhaps what you could do instead is put a diode pointing towards the capacitor (cathode on ground, anode on the capacitor's right node) which would allow fast discharge of the capacitor, but wouldn't affect it while the transistor is on.

You've made a mistake, Mattski. The diode goes round the other way. (This may be what you meant when you said 'pointing towards the capacitor'. Current comes out of the cathode on a diode, unlike batteries, etc. This used to confuse me) (EDIT: I still appreciate your input, though. Please read on, There are some things I don't understand about this circuit)

I've not yet put a resistor in series with the diode, but it works with a two volt output from the sig. gen., as opposed to the transistor starting to conduct at around 5V Pk-Pk, and isn't fully on until around 6 or 7V Pk-Pk with just the 3k5 resiator and no capacitor or diode.

This is because the capacitor gets fully charged on the 'off' half-cycle, then only partially discharges on the 'on' half cycle.

I should be able to get away with a smaller value capacitor, which will provide more DC blocking, and with putting a resistor in series with the diode to limit current drawn from the sig. gen. and increase the value of the 3k5 resistor and then increase the amplitude of the signal.

This is the circuit at present, which runs from a 2V Pk-Pk signal @ 30kHz :

EDIT: The base is still negative with regards to the emitter during the 'off' half-cycle, due to the voltage drop across the diode. (I think........well, it works, so it must be)

EDIT EDIT: I think this also possibly acts as a 'voltage doubler', but I'm still puzzling over it. (It isn't, according to my 'scope, so I don't really know what's going on, just that is is working much better than I expected, just by adding a diode. It looks like a doubler circuit, it works at a lower voltage than with just the resistor and no diode or capacitor, but my 'scope says it's not doubling the output from the sig. gen.)

Villard circuit:

EDIT EDIT EDIT: I think the next step is to put a resistor on each output of the sig. gen. and see what happens. (I'll try 100R each to start with.)

Mattski

Mon Aug 01 2011, 02:58AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
You've made a mistake, Mattski. The diode goes round the other way. (This may be what you meant when you said 'pointing towards the capacitor'. Current comes out of the cathode on a diode, unlike batteries, etc. This used to confuse me)

Oops, right you are, I did mix those up

wrote ...
EDIT: The base is still negative with regards to the emitter during the 'off' half-cycle, due to the voltage drop across the diode. (I think........well, it works, so it must be)

Yep, that's exactly what's happening. It's not a problem though as the base emitter junction is a diode so nothing bad will happen there. It might cause slightly slower turn-on if I remember my physics correctly, but it's been a while since I studied BJT transistors...

wrote ...
EDIT EDIT: I think this also possibly acts as a 'voltage doubler', but I'm still puzzling over it. (It isn't, according to my 'scope, so I don't really know what's going on, just that is is working much better than I expected, just by adding a diode. It looks like a doubler circuit, it works at a lower voltage than with just the resistor and no diode or capacitor, but my 'scope says it's not doubling the output from the sig. gen.)

It depends if your signal generator square wave just goes to 0V, or if it goes negative. In the circuit you posted it relies on the output of the transformer going negative, i.e. the bottom terminal becoming a higher voltage than the top terminal. That charges the capacitor through the diode so that it's 0V on the left and +Vpk on the right, and when polarity shifts back to top terminal positive it raises up the left terminal to +Vpk so the right terminal is now +2*Vpk since the right terminal is +Vpk higher.

Ash Small

Mon Aug 01 2011, 10:12AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Mattski wrote ...

Well, the manual for the sig. gen. is a bit vague (I posted a link earlier in the thread), but according to my 'scope the output does go negative.

I think I ought to tidy up the circuit a bit as it's just a 'bird's nest' at the moment, then I'll run some more tests.

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