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Protection diode needed across current sense transformer primary in SMPS?

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Flyback

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Hello,
In the following current mode Two Transistor Forward converter, is the Diode (D1) absolutely necessary across the current sense transformer primary in order to prevent overvoltage of the top switching FET of the converter? As you can see, the overvoltage is due to ringing of the leakage inductance of the current sense transformer.
Also, do you agree that the freewheeling diode, (D13) must be connected to the input capacitor end of the current sense transformer primary, and not the other side of the current sense transformer primary?

(Schematic and LTspice simulation attached.)
 

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ronsimpson

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1) It is common to put the sense resistor across the transformer. (put the 9.2 across the transformer) Then add the diode.
If you want the diode to be first then add D1+R2 so the transformer sees the same resistance in + and in -.
2) A K=.9 seem wrong. I have never seen a real transformer ring like that. K=0.99? The coil craft parts ring at 10mhz.
3) If the diodes are first, then the voltage drop is removed from the equation. (-0.7V) If the resistor is first then there is a 0.7V error which is usually not a problem.

upload_2014-5-24_11-45-21.png
 

Flyback

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I would have thought poor pri to sec coupling is common in current sense transformers, since one primary turn cannot possibly be physically near to all 100 of the secondary turns?
Are you sure your circuit allows for a reset v.dt equal to the v.dt when the fet is on?
 

ronsimpson

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Placing the R across the transformer will reset the transformer.
Using two RDs should do the same thing.
What you did with one diode on the primary and the other one the secondary is troubling.
 

Flyback

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if you remove the diode D1, then you can see why D1 is so necessary. Without D1, the voltage across the top fet gets very high, due to the ringing with the leakage inductance...you can see this in the simulation.
I must also confess that I have never seen a current sense transformer datasheet which shows the leakage or the coupling factor.
I think the manufacturers think that all SMPS current sense transformers will be used in full bridge type applications where the current is bipolar in them and any current in the current sense transformer leakage can always flow through the bridge mosfet parallel (intrinsic) diodes. However, this is not the case for the two transistor forward converter and the diode D1 is necessary...you agree?
 

ronsimpson

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I have used CTs in many different topologies. Often in (measure current on one direction) mode.
You really need to have a resistor around the transformer in both directions.
In a real CT, there can not be much TV on the transformer. The core will saturate. That is why there is usually only mili Volts across the primary.

If there was a spice function that allowed for only 5Volt micro seconds across the primary then your spice would act different.
 

Flyback

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The way to stop such a current sense transformer as the above from saturating , is to keep the voltage across the secondary as small as possible. The magnetising current in the secondary is what can saturate the current sense transformer.....it saturates it by di = (V.dt)/Lsec.
There is no literature stating that a resistor is needed across the secondary of the current sense transformer (or the primary)......it can be a zener instead, -as long as it resets the transformer.
 
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ronsimpson

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The way to stop such a current sense transformer as the above from saturating , is to keep the voltage across the secondary as small as possible.
YES
There is no literature stating that a resistor is needed across the secondary of the current sense transformer
In every data sheet example there is a resistor.
There is typically a recommended resistor value.
it can be a zener instead
You read books that I don't have. I have n0t seen a Zener in use.

The purpose of a CT is to measure current.
Take a 100:1 transformer.
A 100 ohm secondary resistor makes it look like there is a 0.01 resistor on the primary. A 0.01 ohm 10A resistor is hard to find. A 100 ohm 0.1A resistor is easy to find.
Looking at a data sheet:
input 10A, 0.1V
Output 100mA, 10V
VT=81VuS So it can hold that current (voltage) level for 8uS.

A CT should not be used with out a load. Load does not have to be a resistor, but I can't think why it would be anything else.

OK I do have an example. The wire going to the water pump in my well. I want to know it the motor is on. I used a 60hz 100:1 CT and soldered two LEDs across the transformer. (LEDs in parallel facing opposite directions) It takes 1.5V to to turn on the LEDs. There is 0.015 volts loss in the primary. The LED current is 1/100 of the motor current.
rn
 

Flyback

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I saw them use a zener in a 300w full bridge telecoms smps, in one of the worlds biggest telecoms co's (you will have heard of it)
Directly across the current sense transformer secondary, they actually had a 470opF cap. (the transformer reset was done with a zener and not a resistor.
The current sense transformer was a PA1005.100 NL
I don't see why it has to be a resistor, as long as the transformer resets.
LTspice shows it working without the resistor (as in the first post).
 

ronsimpson

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Here is the same part over at Coil Craft. They, in many places, show a resistor on the secondary.
Pulse Eng. data sheet show using it like you do. I think you will get large voltage spikes like your spice shows.

I think I see what you are doing. You want to have a large voltage spike on the CT while there is no current, to reset the CT.
I don't have any large reset spike and it works.
I guess we are using the CT in a different way.
 

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Flyback

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Yes I think that's it
the following does "appear" to suggest that a "burden" resistor is necessary, but I think the one after the diode performs that function in the sim in the top post here.
http://www.mmgca.com/apps/MMG-ctdesign.pdf

I must admit that I think that current sense transformers that simply have a straight wire going through a toroid wrapped with say 100 turns, must have a poor coupling factor.
I know in theory that a straight wire going through a toroid is a whole turn round the toroid.
However, I have multiple references that say that windings must be near each other for good coupling, and the "loop" of that wire that goes through the toroid certainly isn't near all the secondary turns, -its only near to them at that part of the primary loop that's going through the toroid.
 
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