Looking to your circuit:
The transformer has so called leakage inductance. This causes the voltage spike during turn-off. To reduce this spike, you need to add a snubber (can be RC circuit, tranzorb or combination of RC and D, or a capacitor only when using zero voltage crossing).
With very careful design, you can create a so-called zero-voltage switching converter. This requires tweaking of the current limiting inductor and adding capacitance across drain source. Such circuit provide > 90% efficiency.
You are operating the CCFL transformer far from it's intended operation conditions. These transformers have an intentionally high leakage inductance to act as a discharge lamp ballast. To determine, if efficient operation is possible at all, one must know the intended output current.
The snubber networks are between source and drain (both mosfets), value depends on power and leakage inductance of transformer. Your assumption is not OK. Do a search on snubber networks to familiarize yourself with them. You can also use zener diodes with sufficient breakdown voltage (about35V), parallel with drain and source.
Given 20 V supply and 40..80 KHz (according the specification), rulls out the saturation issue in your 12 V application. Does the spec show this transformer in push-pull operation also?
I don't understand what you mean with "harmonic saturation".
You surely noticed the nominal output voltage of 900 V and the 1:67 windings ratio.What do you mean by far?
Hello,
There are two things:
Normal operation (when the capacitor is charged up to the required voltage) and start-up operation (when the capacitor is empty).
Looking to your circuit:
The transformer has so called leakage inductance. This causes the voltage spike during turn-off. To reduce this spike, you need to add a snubber (can be RC circuit, tranzorb or combination of RC and D, or a capacitor only when using zero voltage crossing).
You also need some current limitation means. This can be an inductor in series with the transformer's secondary. This limits the current rate of change, enabling you to use larger pulse width (up to about 48%). A larger pulse width means that more energy per cycle is transferred to the 170V storage capacitor. You have to figure out whether your transformer can stand the v*t product. If this product is to large (for your transformer), the transformer will saturate, leading to a strong increase in current (though this increase is not seen in the secondary side).
The inductor between transformer and rectifier will also provide current limitation during start-up (when the capacitor is empty). To save components, simulation can be very helpful.
With very careful design, you can create a so-called zero-voltage switching converter. This requires tweaking of the current limiting inductor and adding capacitance across drain source. Such circuit provide > 90% efficiency.
Regarding Royer oscillator. This is a simple and good means to generate high voltage with about 90% efficiency. It works perfectly with BJT. The primary side of the transformer is used as a resonant circuit with the parallel capacitor. So the output waveform is sinusoidal and the collector waveforms are rectangular.
A capacitor in series with the rectifier can be used as limiting means. Note that maximum CE voltage is somewhat more the 3 times supply voltage, so check Vcemax.
Certain snubbers can definitely improve efficiency. A dv/dt limiting RCD snubber can decrease turn off losses in the FETs by keeping drain voltage low while drain current is falling. If done right, the savings in the FET can be greater than the dissipation in the snubber. This generally doesn't apply to peak limiting snubbers like the classic RC or the RCD clamp snubber.I understand the basic theory behind the snubber network. But the purpose of it is to dissipate power from inductive flyback when the MOSFET turns off, right? I don't see how dissipating power is going to make the circuit more efficient. As an experiment I used a series snubber of 100 ohms and 0.1 uF. This did not improve the circuit efficiency. (These values weren't exact nor were they arbitrary, but seeing as how they did not remotely affect the overall efficiency I am not convinced a snubber network will solve this problem.)
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