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Old 16th March 2014, 02:00 AM   #1
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Default Flyback SMPS question.

Does the flyback smps transformer need a magnetising current ?

What I need to know is if I output 100KHz to the transformer is it ok to stop sending 100KHz once the output voltage is correct.
Or do I need to send out short pulses to keep the transformer magnetized ?
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Old 16th March 2014, 04:29 AM   #2
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Look up "burst mode" (I think)

(or possible discontinuous mode)

If there was no load and you kept putting out pulses the output voltage would rise...probably not what you want.

This is where some small SMPS have a minimum load requirement because they cannot go to 0% duty cycle.
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Old 16th March 2014, 08:14 AM   #3
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The flyback stores energy in the core and airgap during on-time of primary power-MOSFET. During off-time that energy is transferred to the load. Hence there is always some magnetizing current present.

To keep track with various load input energy must be controlled. This can be done by controlling primary peak current, for instance.

In contrary to forward or buck converters, the duty-cycle relates to input voltage and primary flyback voltage, i.e. reflected transformed secondary voltage.

Btw, I designed lots of flyback converters over the last years. But I would not go for them to power any audio-amp.
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Old 16th March 2014, 11:25 AM   #4
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It's probably easiest not to contemplate on magnetising current for DCM flyback, but rather think more about leakage inductance and how that contributes to waveforms after the main primary to secondary energy has transferred on a cycle by cycle basis.

Each cycle can effectively have a different period (frequency) with DCM. There will be a little energy still flowing in the (typically primary) ringing waveform after the energy dump, which modern controllers then assess for the valley portion to next turn on the main switch, so as to reduce switching loss. That's a cute part of modern controllers - that was not available in the early days.
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Old 16th March 2014, 12:36 PM   #5
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This is certainly a point.
All in all stray losses are a big issue with flyback transformers thus producing additional losses in the transformer and requiring a snubber circuit to dampen oscillations.

Power dissipation inside the snubber increases with power, adequate cooling it makes the converter bulky at medium power rates - this beeing one reason not to use flyback converters above power levels of let us say 50~100 watts.

Do not let you trick by the simplicity of the circuit - the flyback-converter is on of the most treacherous topologies.

Most of the design know-how resides in one part: The power transformer!
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Old 16th March 2014, 04:57 PM   #6
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Thank you for the replies.

I will try outputting a very short pulse when output voltage is correct and see how I get on.

I have also used a feedback signal for output voltage level but also one from the secondary to see when the output pulses subsides and so I can apply another pulse.
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Old 18th March 2014, 01:57 AM   #7
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In continuous mode the initial current is low.
In discontinuous mode the current is higher as the transformer has completely discharged its magnetic field.
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Old 18th March 2014, 04:07 AM   #8
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I'm unsure what aspect of flyback operation you are trying to get a better handle on. Is it the way the primary and secondary currents start and stop in DCM, or what level they start at and stop at in CCM? Or is it a switching transition aspect, such as losses or emi related aspects? Or is it output ripple voltage characteristic, or magnetic core B-H locus.

Certainly in CCM the change in primary current, and secondary current, can be a lot less than the DCM current change that transitions between zero and some controlled peak value. But the core is only working in one-half of its BH for either mode.
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Old 18th March 2014, 07:37 AM   #9
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Continuous mode implies hard switching turn-on needed to turn off the secondary rectifier - imho an ugly feature. In that case there is no way for quasi-resonant transition.
On the other hand core utilisation is better than with discontious mode thus reducing ferrite core size.
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Old 19th March 2014, 12:56 AM   #10
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I was intending to use the mode in between continuous and discontinuous.
I am monitoring the output voltage of the transformer so I know when continuous mode has finished its discharge cycle. There is a special name for this mode but cant remember its name.
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