Fuse selection in flyback converter

hi,

I want to know, how input stage fuse values are selected in a flyback converter? If Ipeak at primary side is 1 amp then what should be the value of fuse current and also voltage ratings for universal input range (90 -220 Vac) ?

Hi,

Is the 1A for 90VAC? If yes you can use 2A fuse or above (Current is maximum drawn at 90Vac).

thanks

Thanks for reply.

Is it fast or slow blowing type?

Fast blow: because in case of spikes appear at input it should not damage the converter.

https://www.doityourself.com/stry/slow-blow-fuse-vs-fast-blow-fuse

For fly back converter the Peak current should be I pk = 5.5*(Pout)/Vin minimum , Put slightly higher rates fuse
Fast blow fuse may workout

Thanks for reply.

I am working on a universal flyback converted. The output is 5v @2.5 amp.
Is it necessary to implement short circuit protection at output stage? And how?

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I also need help in winding the flyback transformer. I am confused about dot convebtion.

HI,

Short circuit /overcurrent implementation can be done by adding a sense resistor at the source of mosfet and setting the maximum peak current of the transformer. Always choose the current protection 75% of the saturation current of transformer. This also works with short circuit. How are providing the PWM to mosfet?

"I also need help in winding the flyback transformer. I am confused about dot convebtion."

Winding of transformer or winding the transformer?

Thanks for reply.
I want to know the direction of insulating wire for primary and secondary side .

Sorry i dont know much about the making flyback transformer.

Since in a flyback converter, the diode must be reverse biased when the primary Mosfet is turned on, and vice-versa also, then you require to know the relative phase of the secondary winding with respect to the primary one.

Physically, I like to see the "dot" as the start of the windings. As long as the windings are wound in the same direction, all the winding starts would be the respective "dots".

BTW, winding phase is used in many transformer applications. For instance: in 50/60 Hz "universal transformers" there will be two primary windings, which must be connected in parallel or series for 115/230 volt applications. Knowing the relative phase of one winding with respect to the other is of paramount importance.

dot notation refers to polarity...when one dot is positive , so will the other dot on the other coil.......in a mains transformer, the field sof pri and sec oppose each other....and when current flows INTO the dot on the primary, it flows OUT of the dot on the secondary................this signifies that the fields from the pri and sec oppose each other and cancel each other out...at least they would in an ideal transformer.

Its the same-ish with a flyback, but just rememeber in a flyback pri and sec put the field in the same direction in the core.....the pri builds the field up, and then the sec collapses the field back down...but in flyback field lines go in same direction in core no matter if pri or sec is conducting.

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...you do short circuit protection usually with the controller which has an inbuilt short circuit protection feature...in the topswitch ones, when the feedback saturates due to short circuit then it shuts the controller down automaticALLY after about 800ms....the UC3843 types dont have inbuilt s/c protection......some say you get s/c protection with the collapsing of the bias coil...but very often leakage L means the bias winding does not collapse when there is s/c in output.

So really to get s/c protection you need a controller with inbuilt s/c protection.

adnan012 said:

I want to know, how input stage fuse values are selected in a flyback converter? If Ipeak at primary side is 1 amp then what should be the value of fuse current and also voltage ratings for universal input range...

Click to expand...

1. Fuses respond to RMS values; so you need to consider the current determined by the overall power taken by the input side.

2. Common fuses have lousy tolerances; you need to consider +50% overrating and next round up to the nearest standard value.

3. For this application always use a slow blow type (never a fast one).

4. Use the max voltage seen across the fuse (after it has fused) as the voltage rating.

In this case, you will use a 1.5 to 2A slo-blo 220-250V fuse.

hi,
I have arranged some TOP258PN switches for fly-back converter. For 17.5 watt of output power which core size is suitable? Switching frequency is 66KHz. I have observed some chines cellphone chargers which have 20/10/6 EE core inside and rated for 5V @3.5 amp. PI Expert tool selects at least 25/13/7 core for 17.5 watt of power. I want to use smallest possible EPCOS EE core.

PI Expert will produce a design which, based on actual tolerance and real performance limits, will produce reliable results consistently and with margin.

If you want to employ a smaller core, just be aware which tolerance or performance limits you'll be exceeding, and make an informed business decision whether the lowered quality or reliability is worth it.

Hi c_mitra, I agree with you that the fuse is selected with rms current :wink:, but I do not understand why you recommend a slow blow type.

Could you possibly explain a bit more or suggest a source of information.

Thanks

Fast acting fuses are expected to blow in 10-20ms (that corresponds to one period in common line frequency; half to one full cycle). Basically a fuse is energy limiting: the power is V.I and the energy is V.I.t- if you are using the common glass shell fuses, The current is sufficient to heat and melt the fuse wire in 10 ms. For slow blow fuses, the heat capacity of the fuse wire is such that it will not melt with a large current (relatively) for a short time. SMPS supplies can take surges in current for short while and it is good to use a slo-blo fuse (else you may have to replace the fuse too often and/ or change it to a much higher rating).

hi, I have uploaded Feedback circuit and BodeE plot. I need explanation of the Bode Plot. The Switching frequency is 66Khz. What is the cross over frequency and how much phase boost is there? I need detailed explanation.

Regards

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is there any small tool available to plot bode plots of feedback loop?

adnan012 said:

I need explanation of the Bode Plot.

Click to expand...

1. When we refer or mention a particular frequency, we always mean a sinusoidal waveform: it is the one produced by a sine curve.

2. Any given point on the sine curve can be represented by a (x,y) pair but we often use a polar coordinate system: (r,theta) - they are always interconvertible.

3. For a sine curve, the polar plot is a circle (r=const, theta=t).

4. We often call r the magnitude and theta the phase. x=r.cos(theta) and y=r.sin(theta)

5. Two points on the same circle has the same amp but diff phase. We call the diff as the phase different.

6. We always need a reference signal to measure the phase.

7. You need to tell us which point you took the signal for measurement and where the reference signal was injected.

8. Rest is just a matter of detail.

hi, I need explanation of the equations 1 to 8 starting from page 7/42 in the attached document.

adnan012 said:

hi, I need explanation of the equations 1 to 8 starting from page 7/42 in the attached document.

Click to expand...

I work slowly. Let me try to explain the first: the equation number 1:

First the assumptions. You need to take a print out of the block diagram on page 1 and follow during this. The author says:

"At the equilibrium, if we neglect the base currents, the current mirror brought by Q 8 and Q 9 duplicates I 1
which also circulates in Q 7 and Q 6 . Because of the current mirror arrangement between Q 1 and Q 4 , the same
current but scaled up by a ratio of 1 to 3 flows in Q 4 as well"

Is it clear that Q6, Q7 and Q8 has some current (say) I1- we ignore the base current and in the steady state (ignore capacitors), the collector and emitter currents are equal.

Now see Q8 and Q9. Do you think both will have the same collector current? (why and how)- Therefore Q9 also carries the same current I1. Same current must go via Q1 and hence this symmetry.

Is it clear that Q4 and Q5 currents will not be equal (but 3:1) - see the resistors R2 and R3, they are in the same ratio. If Q5 current is x, Q4 current will be 3x. I hope it is clearer.

Now look at Q5 and Q1. How do we know they have the same current?

Now you can understand the equation 1. It uses several important concepts in electronic circuits- but I guess you these better than me!!

I shall be going home and see the next equation later.