# Transformer Ratings

#### jlawley1969

Joined Feb 22, 2021
97
Hi me again not understanding thing.
Ok so for a transformer like this https://www.we-online.com/catalog/datasheet/7491182012.pdf
the current saturation is 0.8A but it is rated at only 0.35A on the output for 12 v.
My question is if I am trying to use the transformer(in a flyback topology with TNY290) to generate only 5V does that mean I am able to get more current through the transformer? I know the wire thickness is what is limiting the current but my logic is that the power capability would be 4.2W so I could use 5v and 0.84A.

I just want someone to explain why I am wrong.
thanks

ps the reason I am wanting to use a transformer with a higher rated voltage is that I want a turns ratio that is closer to 10:1 instead of 20:1

#### crutschow

Joined Mar 14, 2008
28,204
The limit is determined by the saturation current of the transformer.
You cannot exceed that peak value for any operating conditions of the transformer.

#### jlawley1969

Joined Feb 22, 2021
97

#### jlawley1969

Joined Feb 22, 2021
97
Either way my question still stands about the power/current capabilities

#### Ian0

Joined Aug 7, 2020
3,761
Why do you want a 10:1 ratio? The ratio is the ratio of the output voltage to the flyback voltage, not to the primary voltage.
You should aim for a flyback voltage between 100V and 150V on a 230V AC supply (rectified to 325V DC).
The output current is probably the average output current, so it will give you a 2x 12V @ 0.35A power supply (8.4W)
The primary current is a sawtooth waveform with a pulse that lasts probably 1.5us.
On a 230V supply the current at the end of the pulse will be
$$I=\frac{Vt}{L}$$
which works out at 540mA
The energy stored will be
$$E=\frac{1}{2} I^2L$$
which works out at 133uJ
repeated 132000 times a second gives 17W for a hypothetical perfectly efficient transformer, enough headroom to produce the 8.4W output easily.

The main reason that the primary saturation current is so high is that flyback converters are usually designed with a much shorter ON time than OFF time.

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#### shortbus

Joined Sep 30, 2009
8,966
So what would it be called then? WE's website says that transformer as well as the others in the series are " Designed for Tiny Switch ICs from Power Integration and NCP101x or 105x of ON Semiconductor "

#### jlawley1969

Joined Feb 22, 2021
97
yes ive read that a few times and the selection guide, Im just a little dumb lolol

#### jlawley1969

Joined Feb 22, 2021
97
Why do you want a 10:1 ratio? The ratio is the ratio of the output voltage to the flyback voltage, not to the primary voltage.
You should aim for a flyback voltage between 100V and 150V on a 230V AC supply (rectified to 325V DC).
The output current is probably the average output current, so it will give you a 2x 12V @ 0.35A power supply (8.4W)
The primary current is a sawtooth waveform with a pulse that lasts probably 1.5us.
On a 230V supply the current at the end of the pulse will be
$$I=\frac{Vt}{L}$$
which works out at 540mA
The energy stored will be
$$E=\frac{1}{2} I^2L$$
which works out at 133uJ
repeated 132000 times a second gives 17W for a hypothetical perfectly efficient transformer, enough headroom to produce the 8.4W output easily.

The main reason that the primary saturation current is so high is that flyback converters are usually designed with a much shorter ON time than OFF time.
what do you mean by the "output voltage to the flyback voltage, not to the primary voltage" is the flyback voltage the voltage above the rectified voltage?(I saw a figure on this before but I cant find it right now) and if so why use that ratio?

#### Ian0

Joined Aug 7, 2020
3,761
what do you mean by the "output voltage to the flyback voltage, not to the primary voltage" is the flyback voltage the voltage above the rectified voltage?(I saw a figure on this before but I cant find it right now) and if so why use that ratio?
The flyback voltage will be the voltage on the output capacitor plus a diode drop, multiplied by the transformer turns ratio.
The fly back voltage multiplied by the length of the fly back pulse will be the same as the supply voltage multiplied by the MOSFET on time.

#### shortbus

Joined Sep 30, 2009
8,966
yes ive read that a few times and the selection guide, Im just a little dumb lolol
The gap in the transformer core is what makes them a flyback, without the stored(magnetic) energy in the gap it would just be a transformer, and not work in the circuit. Ian0's drawing shows the "ringing" from that energy being released into the circuit.

#### BobaMosfet

Joined Jul 1, 2009
1,898
Hi me again not understanding thing.
Ok so for a transformer like this https://www.we-online.com/catalog/datasheet/7491182012.pdf
the current saturation is 0.8A but it is rated at only 0.35A on the output for 12 v.
My question is if I am trying to use the transformer(in a flyback topology with TNY290) to generate only 5V does that mean I am able to get more current through the transformer? I know the wire thickness is what is limiting the current but my logic is that the power capability would be 4.2W so I could use 5v and 0.84A.

I just want someone to explain why I am wrong.
thanks

ps the reason I am wanting to use a transformer with a higher rated voltage is that I want a turns ratio that is closer to 10:1 instead of 20:1
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### Ian0

Joined Aug 7, 2020
3,761
The Würth part you quote in post #1 IS a flyback transformer. With no gap the inductance would be much larger than 900uH.
There are two lots of ringing.
The ringing at the point the MOSFET switches off is due to the resonance between the MOSFET capacitance and the transformer leakage inductance. This overshoot can kill your MOSFET. Ringing at this point should be clamped and reduced as much as possible.
The ringing at the end of the fly back pulse is the resonance between the primary inductance and the MOSFET, and its allowable (although there are integrated controller ICs which don’t like it going below 0V)
In fact, that ringing can be utilised to minimise switching losses by switching at the lowest point on the oscillation cycle, that makes a quasi resonant circuit, but that’s another story. . .

#### jlawley1969

Joined Feb 22, 2021
97
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3
Quit Bullying me!!lol

#### jlawley1969

Joined Feb 22, 2021
97
The Würth part you quote in post #1 IS a flyback transformer. With no gap the inductance would be much larger than 900uH.
There are two lots of ringing.
The ringing at the point the MOSFET switches off is due to the resonance between the MOSFET capacitance and the transformer leakage inductance. This overshoot can kill your MOSFET. Ringing at this point should be clamped and reduced as much as possible.
The ringing at the end of the fly back pulse is the resonance between the primary inductance and the MOSFET, and its allowable (although there are integrated controller ICs which don’t like it going below 0V)
In fact, that ringing can be utilised to minimise switching losses by switching at the lowest point on the oscillation cycle, that makes a quasi resonant circuit, but that’s another story. . .
Yeah I have a clamp with diode and TVS diode
but that is interesting about the quasi resonant thing, I saw that when I was looking into flyback but I am obviously already having a hard time with this....

#### Ian0

Joined Aug 7, 2020
3,761
I think TVS+diode is the best clamp - it doesn't waste any power if the pulse doesn't reach the TVS voltage. It's not great for EMC because of the sharp edge when it starts to conduct, but 10Ω in series reduces that.
Quasi resonant is great if you want to run it near its full output a lot of the time. Otherwise, it's no different to ordinary flyback. One problem is solves is that it just can't do subharmonic oscillation, which flyback can be prone to.

But sort out ordinary flyback first. What are you trying to achieve?

#### jlawley1969

Joined Feb 22, 2021
97
I think TVS+diode is the best clamp - it doesn't waste any power if the pulse doesn't reach the TVS voltage. It's not great for EMC because of the sharp edge when it starts to conduct, but 10Ω in series reduces that.
Quasi resonant is great if you want to run it near its full output a lot of the time. Otherwise, it's no different to ordinary flyback. One problem is solves is that it just can't do subharmonic oscillation, which flyback can be prone to.

But sort out ordinary flyback first. What are you trying to achieve?
I am trying to do 110VAC to 5VDC ~2A to power two USBs. I have got the voltage to work and charge my phone with 750871110 but only at like 200mA. I am waiting on some parts like Yclass capacitors and higher precision resistors before I continue testing.
The only issue I had with the transformer 750871110 was how big it is so thus why I was trying to figure out other transformers but I have since been able to squeeze 2 more mm of clearance out of the enclosure to make it fit but its still close.

I recently took apart on of those cheap usb wall outlet chargers and while it charged my phone at 1A it also only had 15 components and not even an opto isolator so I figure I am atleast close.lolol

#### Ian0

Joined Aug 7, 2020
3,761
I have used these
https://myrra.com/flyback/
The opto isolator can be eliminated by taking the feedback from the auxiliary winding. The Wurth part doesn't have an auxiliary winding.

#### shortbus

Joined Sep 30, 2009
8,966
I recently took apart on of those cheap usb wall outlet chargers and while it charged my phone at 1A
I might be missing something(not that unusual for me) But don't you think there's a reason it is only charging at that rate? Can a phone battery take a higher rate of charge? They do need to meet all of the standards set forth by governing bodies around the world. But I guess it would make some sort of viral video, setting a cell phone on fire just by charging it.