# Based on what characteristics is safe to say 2 transformers are alike?

#### Younes Thabet

Joined Jan 9, 2019
144
Hello,

I want to use an SMD transformer instead of the THT one in the original design (characterisctics in below image).

I found an SMD transformer (highlighted in below image) that is close to the original one in terms of that turn-ratio and higher or close enough for the other characteristics.

Can this transformer be used instead of the original one no matter what circuit?
Or should all the characterictics be similar to say they are alternatives to each other?

Regards,

#### WBahn

Joined Mar 31, 2012
30,243
Those characteristics don't look very much alike at all to me.

Upon what are you basing your evaluation that a given characteristic is "close enough"? Also, you seem to be under the impression that you can use a component if some characteristic is at least as great as the corresponding characteristic in the original -- nothing is further from the case. In most cases you can't be too much greater OR too much lower. In many other cases you can be higher but not much lower, while in lots of others you can be lower but not much higher.

In general, it's nearly impossible to say that Component A can ALWAYS be substituted for Component B in ANY circuit.

It all depends on what is important for the circuit in question.

#### Younes Thabet

Joined Jan 9, 2019
144
Those characteristics don't look very much alike at all to me.

Upon what are you basing your evaluation that a given characteristic is "close enough"? Also, you seem to be under the impression that you can use a component if some characteristic is at least as great as the corresponding characteristic in the original -- nothing is further from the case. In most cases you can't be too much greater OR too much lower. In many other cases you can be higher but not much lower, while in lots of others you can be lower but not much higher.

In general, it's nearly impossible to say that Component A can ALWAYS be substituted for Component B in ANY circuit.

It all depends on what is important for the circuit in question.
by "close enough" I meant from the given SMD transformer list that particular P/N is close enough to the original but if its characteristics are applicable or not? that I don't know?
I know a bigger ET wouldn't hurt!
losses and leakage are bigger but I can live with that, I guess!
Primary inductance is OK to be bigger! please correct me if I am wrong!
The DCR is much bigger, this would cause more heat I think?
Bigger Hi-POT is a good thing.

As you noticed my choice of this transformer is based solely on characteristics comparison I can't even tell what would be the outcome of my choice on the circuit TBH.

#### WBahn

Joined Mar 31, 2012
30,243
How can we possibly tell you whether having the primary inductance being bigger would be okay or not? That depends on the circuit it is being used in and what is important and what is not important for THAT circuit.

The same applies for the other characteristics.

Consider this -- if (to pick just a couple) losses and leakage being bigger don't matter in ANY circuit, then why would a manufacturer bother making parts that had lower losses and leakage? Simple -- because in some circuits, that matters and you can't, therefore, replace those components with ones that have higher losses and leakage and expect them to work properly.

#### Younes Thabet

Joined Jan 9, 2019
144
How can we possibly tell you whether having the primary inductance being bigger would be okay or not? That depends on the circuit it is being used in and what is important and what is not important for THAT circuit.

The same applies for the other characteristics.

Consider this -- if (to pick just a couple) losses and leakage being bigger don't matter in ANY circuit, then why would a manufacturer bother making parts that had lower losses and leakage? Simple -- because in some circuits, that matters and you can't, therefore, replace those components with ones that have higher losses and leakage and expect them to work properly.
The circuit is a power supply that will be used to provide +15/-5V to gate drivers output side to drive MOSFETs (see attached image)
This transformer has 2 similar secondaries that will isolate the 2 channels of the gate driver (UCC21520).

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#### crutschow

Joined Mar 14, 2008
34,690
Have you actually built that circuit as I don't see how it can work(?).
Where did you get it?

Q1 and Q2 are N-MOSFETs with their sources connected to (+) VCC, which means they will always conduct through their parasitic substrate diodes and can never be turned off (below).

Even reversing the MOSFETs or using P-MOSFETs will not work.
You need a driver for the transformer with an alternating plus and minus signal between the two primary leads.
This could be done with a 4-transistor bridge circuit driving both transformer terminals, or a 2-transistor push-pull driver with a large capacitor in series to block the DC component driving one terminal with the other grounded.

You could configure it as a flyback but that would only give a unipolar signal from the secondary, not the bipolar you show with two output rectifying diodes.

So it would appear to have a larger problem than how to substitute the transformer.

#### Younes Thabet

Joined Jan 9, 2019
144
Have you actually built that circuit as I don't see how it can work(?).
Where did you get it?

Q1 and Q2 are N-MOSFETs with their sources connected to (+) VCC, which means they will always conduct through their parasitic substrate diodes and can never be turned off (below).

Even reversing the MOSFETs or using P-MOSFETs will not work.
You need a driver for the transformer with an alternating plus and minus signal between the two primary leads.
This could be done with a 4-transistor bridge circuit driving both transformer terminals, or a 2-transistor push-pull driver with a large capacitor in series to block the DC component driving one terminal with the other grounded.

You could configure it as a flyback but that would only give a unipolar signal from the secondary, not the bipolar you show with two output rectifying diodes.

So it would appear to have a larger problem than how to substitute the transformer.

View attachment 287488
I am sorry this is not the updated schematic (see below)
the switching devices are actually pnp transistors. and the tl494 has 2 internal transistors forming a Darlington. with the 2 pnp external transistors it forms an H-bridge arrangement.
I want to keep the same circuit but change the transistor to an SMD one so that's why i am looking for what characteristics i should care for and what i can ignore because it will be hard to find a transformer with same characteristics as this one.

#### crutschow

Joined Mar 14, 2008
34,690
it will be hard to find a transformer with same characteristics as this one.
Okay I understand the circuit now.

The transformer replacement you reference will likely work, although with slightly worse efficiency since its winding resistance is much higher.
The higher ET and primary inductance should be okay as that just allows for a lower frequency of operation.
One possible problem is that the 10 times higher leakage inductance may cause high voltage spikes during the bridge switching.