Flyback converter

Why Flyback converter can' t be operated in open loop for certain duty ratios?

instability?

It can, but the output voltage regulation is poor.
Old CRT TV sets used an open loop flyback circuit to generate the high voltage for the tube.

crutschow said:

Old CRT TV sets used an open loop flyback circuit to generate the high voltage for the tube.

Click to expand...

It is something related to transformer saturation.

dknguyen said:

instability

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Yeah it is unstable but i want to know the reason?

Not counting CRT flybacy because it is resonant. PWM square wave flyback will cause the inductor to fly up untill it hits a load. With no load the voltage will increase untill something breaks.

ronsimpson said:

no load the voltage will increase

Click to expand...

why will the voltage increase?

The Led lights for my xmas decoration have an open loop flyback driving them.
Not a good design though, I threw it together as quick as I could as I dont really care for such things.
The only thing regulating the voltage is the leds and the energy stored in the inductor.

Flyback with transformer:
When the transistor is on, current flows from supply through the primary of T1 to ground. (no current in the diode....yet)
Energy is stored in T1.
When the transistor opens up the collector voltage jumps up above the supply.
You can put AC across a transformer (inductor) but you can not put DC across the transformer. The average voltage must be 0 volts. When the transistor was on, if you put (12 volts for 10uS) on the primary then when the transistor is off it will fly back above the supply for 120VuS. (12Vx10uS or 24Vx5uS or 120V for 1uS ...)
The secondary voltage will also flyback (upward) until it runs into the voltage on C.
The power stored in T1 will be moved to C. This charges up C. Rl discharges C a little.
If Rl is removed the voltage on C will increase each cycle, with no limit. (assuming there is no feedback to cause the dutycycle to reduce)

**broken link removed**

The red square is power into the transformer. The green squares is power going out. The red and green area must be equal. (volt time)
Unlike most type of power supplies, a flyback (under some conditions) will try to make infinite volts.

Understanding volt seconds is a critical part of magnetics.