Flyback Inverter for Fluorescent Lamp: Part 2, A Little Theory of Operation

In part 1 of this posting “Flyback Inverter for Fluorescent Lamp: Part 1, Making Repairs” a little careful and straightforward troubleshooting and repair brought my friend’s fluorescent lamp assembly back to life again. But a fluorescent lamp has quite a few unique requirements to get it to start up and stay illuminated. How does this flyback converter manage to do these things?

I had first looked around to see if I could find a schematic for this fluorescent lamp assembly, but nothing turned up for me. However, the parts count was low enough, and circuit board large enough, that it was a fairly simple matter to trace out and sketch the inverter’s schematic in fairly short order, as shown in Figure 1.

Figure 1: Fluorescent lamp single-ended flyback inverter circuit

When first powered up the switching transistor is biased on by the 812 ohm resistor, energizing transformer winding W1. This in turn applies positive feedback to the transistor through winding W2, driving it into saturation. There are two mechanisms in the flyback transformer that are critical for making this inverter work:

• First it has a gapped core. This allows it to store a substantial amount of energy in its magnetic field which in turn gets dumped over to the fluorescent tube through the secondary winding W3 when the transistor turns off and the transformer’s magnetic field collapses.  During this period the winding voltage continues to climb as the magnetic field collapses until the energy can find a place to discharge to, in this case into the fluorescent tube. The voltage is also further increased by the turns ratio of the transformer. This is the “flyback” effect that creates sufficiently high enough voltage to get the fluorescent tube to “strike” or ionize its gas to get it to start conducting and give off illumination, typically many hundreds of volts.
• As can be seen this inverter is a very simple circuit with a minimum of parts. A second mechanism in the transformer is it is designed to saturate in order to make the inverter oscillate. At the end of the transistor’s “on” period the transformer reaches its maximum magnetic flux at which point the transformer saturates. Winding voltage W2 drops to zero and then reverses driving the switching transistor into cutoff.  After the magnetic field has collapsed and energy discharged to the fluorescent tube the process repeats itself.

The switching transistor’s collector and base voltages during turn on are captured in the oscilloscope diagram shown in Figure 2.

Figure 2: Inverter switching transistor collector and base voltage waveforms

A number of interesting things can be observed in Figure 2.  The oscillation period is roughly 50 microseconds, or oscillation frequency of 20 kHz. It takes about 10 cycles, 500 microseconds, for the fluorescent tube to strike. During this initial phase the peak collector voltage is flying up to nearly 100 volts or about 8 times the DC input voltage being applied. Again, this voltage is being multiplied up by the turns ratio of windings W1 and W2 to bring this up in the vicinity of 600 volts or so needed to make the fluorescent tube to strike. Once the tube does strike and starts conducting its impedance drops. This causes the collector voltage to drop down to about 35 volts which is consistent with the proportion of drop in voltage needed for the fluorescent tube once it’s gas is ionized and is conducting. Note also the collector voltage pulse also widens as it takes a longer time for the energy in the transformer to be dumped when it’s at a lower voltage.

Although this inverter at first glance is a rather simple and minimum viable, minimum parts count circuit, with careful design it can be made to be very efficient. This is where the design of the transformer becomes as much art as science, knowing how the subtle characteristics of the magnetic material and inductive and capacitive parasitics can be used to advantage in contributing to and improving the overall performance of the design.

Anyway, what my friend really cared about is the lamp now works and he is able to put it to good use in his camper!

Flyback Inverter for Fluorescent Lamp: Part 1, Making Repairs

A friend of mine approached me a while ago asking for some help. The fluorescent lamp assembly for his VW Westfalia camper was dead and, knowing I knew more about electronic devices than he did, figured it was worth challenging me with it.  I was actually happy to do so. Being involved with DC power conversion of a variety of forms I was always a bit curious to learn about how fluorescent lamp assemblies that were powered from low voltage DC worked anyway.

“My lamp does not work; can you look at it for me?”

“I suppose. Did it just stop working? Did you try anything to get it working again?”

“Well, it really never worked for me. I messed around with it a little but it did not help. I may have hooked it up backwards.”

“Why do you think you hooked it up backwards?”

“Well, it did not work so I tried reversing the power connections. That didn’t make it work however.”

“You really should not do that with electronic things!”

I took the lamp home and later when I had chance to look at it carefully I visually identified several problems. Like many other things I have repaired, a lot of the times it is not the device itself but rather a previous owner unintentionally inflicts unnecessary damage on it when attempting to make repairs.  In my friend’s partial defense, someone previously had already made unsuccessful attempts at trying to make it work again, unwittingly making things worse.

Referring to Figure 1 I unanchored the inverter circuit board from the back of the lamp assembly for closer inspection. It was immediately obvious there were problems that would keep it from working:

• The connectors for the wiring to the fluorescent tube were not making contact.
• A portion of a circuit board trace where the power feeds in was blown away.

Figure 1: Fluorescent lamp inverter board had obvious problems

Clearly someone had let the smoke out of it that made it work!  After making repairs to these problems I then tried powering it up using a power supply with a current limit to keep things safe. As I expected I was not going to get off that easy. The power supply went right up to its current limit setting. The lamp still did not work. 

The next step was to probe around the circuit board with a DMM.  With the abuse this lamp assembly has been subjected to I suspected the switching transistor would be damaged and sure enough it was measuring shorted. However, after removing it, it seemed to check out good. Probing around on the board again, a diode adjacent to the transistor measured shorted as well. Upon its removal it fell in half as a result of being overheated. I found where the rest of the smoke that makes it work had come out!  I replaced the diode, reinstalled the transistor and remounted the circuit board. Upon applying power again the result was a bit different as shown in Figure 2. I managed to reinstall all the smoke back into it again!

Figure 2: Fluorescent lamp assembly back in working order

While I had a general idea of how it works, now that I had the fluorescent lamp assembly working again I had take the opportunity to make some measurements and study the finer aspects of how it works, which I will cover, coming up in part 2. Stay tuned!