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max power for a flyback drivers?

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Ash Small

Sat May 21 2011, 02:13AM

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

Antonio wrote ...

There is no free-wheeeling diode in a flyback converter. Use the "quasi resonant" configuration. The same used in most CRT TVs and monitors.

Are you able to elaborate on this Antonio?

I've previously asked about this here, but was referred to snubber circuits.

Wikipedea refers to 'flyback diodes' as 'freewheeling diodes' (I understand this is different to the 'freewheeling diode' that protects a transistor)

I assume the 'quasi resonant' configuration involves placing a capacitor across the primary such that the resonant frequency of the resulting tank circuit equals the operating frequency of the flyback circuit. (However, due to the fact that the current in the flyback primary is not siusoidal or even symmetrical, I don't fully understand whether this is correct)

I understand a snubber circuit, which also has a capacitor across the primary would prevent resonance. (It's purpose is to prevent ringing)

I'd appreciate it if you are able to throw any more light on this subject.

Steve Conner

Sat May 21 2011, 07:46AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The "freewheeling" diode is used in the usual TV line scan circuit. The scan coil current flows in it for the left-hand half of the screen.

However in a flyback converter it's not needed, except if you use the snubber capacitor to make it "quasi-resonant". When you turn the transistor off, the flyback pulse is a half cycle of a sine wave, and if the output is lightly loaded, it will try to shoot below zero. So a diode is needed to catch it and protect the transistor. MOSFETs come with them built in, but bear it in mind if using BJTs or IGBTs.

Quasi-resonant means that something in the circuit resonates, but not at the operating frequency. In the TV flyback circuit the resonance is much higher than the 15kHz operating frequency. The half cycle of resonance is used to reverse the scan coil current as quickly as possible, returning the beam to the left-hand side of the screen in a small fraction of the line period.

PS: If you use a forward converter, it's the turn-on time of the device that determines the rise time of the pulse: the slow turn-off of IGBTs doesn't matter.

Ash Small

Sat May 21 2011, 11:48AM

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

Steve McConner wrote ...

Quasi-resonant means that something in the circuit resonates, but not at the operating frequency. In the TV flyback circuit the resonance is much higher than the 15kHz operating frequency. The half cycle of resonance is used to reverse the scan coil current as quickly as possible, returning the beam to the left-hand side of the screen in a small fraction of the line period.

Thanks for your comprehensive reply Steve.

So does the capacitor in a quasi resonant circuit play any part in storing energy that would otherwise be wasted, and returning it to the circuit on the next cycle? or am I getting confused with something else?

I had understood that this was the main advantage of a quasi resonant circuit, but was having difficulty understanding the exact mechanism by which it improves efficiency.

Steve Conner

Sat May 21 2011, 12:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It improves efficiency in a TV because a huge amount of inductive energy is needed for the deflection coils. The quasi-resonant circuit recycles this energy, so the TV's power supply only has to provide the losses.

In a flyback converter used as a power supply, the inductive energy is supposed to be transferred to the load. It's harder to see what the capacitor does in this case. In fact it is just a snubber protecting the transistor, in exactly the way that the "condenser" stopped your old car's points from burning out. The energy that collects in the capacitor can't be recycled, it is dissipated in the transistor and flywheel diode.

When you take leakage inductance and stray capacitance of the transformer into account, the capacitor can help to tune things for energy transfer in the manner of a Tesla coil. This is probably what goes on in our flyback drivers and single switch SSTCs.

Pinky's Brain

Sat May 21 2011, 01:03PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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cedric wrote ...
I have a working prototype witch operate at 60 W giving me a max frequency of around 1khz. the load of the fly-back is a 1nf high voltage capacitor,in serie with a hv diode,after the pulse the voltage in the capacitor is 5kv.

Are you sure? 0.5*1e-9*5e3^2*1e3=12.5 Watt power to the load ... that would be a sadly pathetic efficiency.

PS. for capacitor charging a forward converter is a little trickier than flybacks BTW. At least if you want to charge it in a single cycle and without tons of copper losses. I used to think that a series inductor at the primary and secondary with LC primary being equal to LC secondary would do it, but I was wrong ... it's a little more complicated than that. US military has an excellent doc on how to achieve efficient resonant charge transfer with normal transformer action :

Antonio

Sat May 21 2011, 04:16PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Ash Small wrote ...

Are you able to elaborate on this Antonio?
I've previously asked about this here, but was referred to snubber circuits.
I'd appreciate it if you are able to throw any more light on this subject.

The simulation above shows a typical circuit. The capacitor across the primary coil resonates with the primary coil when the flyback pulse occurs (the curve below shows the drain voltage) and limits the maximum voltage over the mosfet. The diode in parallel with the mosfet not only protects it (if it already doesn't have a diode) but returns unused energy to the power supply. Note the current in the primary coil above. It rises as a ramp when the mosfet conducts, and reverts when the mosfet cuts the current. This inverted current flows through the diode and the coil, returning to the power supply. The circuit is very efficient. Essentially only the load dissipates energy.

Steve Conner

Sat May 21 2011, 05:29PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Antonio's explanation is better than mine

Pinky's Brain

Sat May 21 2011, 06:20PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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It's not really relevant for the application though ... a QR converter needs to be frequency controlled with a fixed duty cycle to stay in ZVS mode, which isn't really an option for a variable frequency pulse generator with fixed energy per pulse.

Antonio

Sat May 21 2011, 06:34PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Pinky's Brain wrote ...

It's not really relevant for the application though ... a QR converter needs to be frequency controlled with a fixed duty cycle to stay in ZVS mode, which isn't really an option for a variable frequency pulse generator with fixed energy per pulse.

Nor the frequency nor the duty cycle must be fixed. You just have to leave a time interval after each switching off of the transistor for the pulse and the energy return. The maximum duty clycle is a bit larger than 50%, but there is no minimum.

Ash Small

Sat May 21 2011, 06:37PM

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

Thanks for a very informative reply, Antonio.

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