Notes on using my CCFL driver board(s) as the basis for a PMT power supply. Loading the HV output with a capacitor that was supposed to simulate a CCFL lamp didn't really drop the peak voltage output all that much. Attempting to further drop the output by reducing the power supply voltage worked--to a point. The output voltage dropped a little but then went to zero, probably because the PWM chip quit working. This is problematic because the designs I have seen on the web just use their inverter's Vcc input as the "control voltage" in a feedback loop.
Looking at the transformer connections I saw what appeared to be a type of differential drive, with the center tap of the primary connected to the input Vcc and the "outside" pins connected to a switch IC that would alternatively connect the transformer pins to ground . This is similar to what's called the "Royer" oscillator, see
here . I thought that it should be possible to break the center tap's connection to Vcc and drive the center tap with a variable voltage to vary the output voltage over a much wider range. The rest of the inverter circuitry would be "happy" because it's getting the voltage it was designed for.
So I cut the trace connecting the center tap to Vcc and soldered a wire to to the trace that will be used to provide an external "gain control" for the inverter. I also thought it would be a good idea to add a bypass capacitor from the center tap to the local ground, to handle the inevitable inductive spikes that arise during switching.
This approach looks like it will work OK, with an interesting side effect. I noticed that the HVAC coming out of the inverter isn't nearly as "peaky", even when I connect the transformer's center tap to the inverter's Vcc (i.e. it's running full-bore). There must be some pretty hefty transients present on an un-modified inverter! I'm using a .47uF 200V ceramic capacitor to bypass the center tap and so far, so good.
While other flavors of CCFL inverters may use somewhat different circuits to do their job, if your inverter uses a transformer with a center-tapped primary you probably can use a similar approach to get better control over the HV output compared to simply varying the inverter's supply voltage. I had initially thought I would have to de-solder the transformer pins and lift them off the board but then I found a circuit trace I could cut to accomplish the same result.
Note that the classic Royer Oscillator uses a saturable transformer core, so dropping the voltage on the center tap _might_ be problematic once the current is too low to saturate the core. Inverters that use a PWM chip shouldn't care too much about that.