TOP225Y not reach current limit.. Why?

TOPSwitch has a Drain cycle-by-cycle current limit. If the transformer primary inductance is too small, this limit will be reached before the output full load rated power is reached.

What is your input voltage and primary inductance?

Thanks a lot for quick reply!
Ok, the inductance of POL15020(my example) is 620 microhenry and input voltage is rectified 220 volt (i.e. app.300V). I understand that drain current limit can be achieved more early with small inductance. But, I used current transformer to control the drain current (ferrite ring with 50 turns loaded 50 Ohm resistor, and 1 turn (jumper)from trans to drain. And only 0,6V (may be 0,7V) amplitude (vertex of triangle) was riched, than TOP began restart. That is why I asked about possible control di/dt in TOP22x.
P.S. Sorry for my English, I have not enough practice.

Do you have a current probe to measure Drain current?


Is the 0.6V you see peak or peak-to-peak?
Is your current transformer AC or DC coupled? Does it only have a 50 ohm resistor as a load and no diode circuit? Can you show the oscilloscope trace?


What do you mean by "I used current transformer to control the Drain current "? Do you mean "to measure Drain current"?


TOP225 current limit is 2 A.

I don't understand what you mean by "di/dt" control.
It does have "leading edge blanking" in the current limit to ignore the initial Drain current spike at the turn-on edge, but if the spike is very large it will current limit prematurely.

See Fig 12 and 13, on page 14 of the datasheet:
http://www.powerint.com/sites/default/files/product-docs/top221-227.pdf

Well, lets me explain. Sure, I mean "to measure Drain current". My current trans has only resistor (loaded on windings) without diod circuit. Unfortunately, I can't show ooscilloscope trace (because I have old one, not digital) but oscillogram shape looks like at Fig.13 (without bend at the end - no saturation)/
Yes, 0.6V - peak (right edge current oscillogram, vertex of triangle, if not taking into account spike tleb).
What you mean by "current probe"?
I mean di/dt control because in table on page 11 self-protection current limit represented with remark, for example: TOP225Y - current limit - 1.8-2-2.2A at Tj=25C and di/dt=320mA/microsec.
So, I suppose that Ilimit may depend on di/dt.

Is 0.6V peak of the triangle, measured from the bottom to the top? If so that is only 0.6A, and not near the current limit spec of 2A. This assumes that the inductance of your current transformer is high. Have you measured the inductance? Or can you tell me the Al value of the core you used? Also can you tell me the cross sectional area of the core?

Have you checked that the initial current spike "envelope" is not violated? (no false triggering due to excessive initial current spike)

A "current probe" is like the voltage probe of an oscilloscope, but used for measuring current instead of voltage. An example is the Tektronix P6021 (AC couled) and TCP305 (DC coupled).

The di/dt spec is to show the current limit at that di/dt. The reason a di/dt is specified for the current limit is that the internal current limit comparator has a delay. The higher the di/dt of the Drain, the higher the Drain current limit is at turn-off, after the delay. So the current limit is specified at a given, typical di/dt that is seen in an application.

Yes, I told about it - only 0.6-0.7A and top began restart. But inductance was measured by Smart Tweezers (http://www.advancedevices.com/index.htm) and be approx. 620 microhenry (within accuracy of ST). Unfortunately I don't know Al value, I'll try to attach datasheet on used trans.
About this-"The higher the di/dt of the Drain, the higher the Drain current limit is at turn-off, after the delay." Do you mean "The higher the SPECIFIED (by datasheet) di/dt..." or PRESENTED in real? Becouse I see that at low inductance (high di/dt)TOP225 don't reach even 1A drain current, so I supposed that the higher di/dt (in reality)- the lower Drain current limit for TOP.
P.S. I put resistor 0.24Ohm in series with source pin to ground and measured V on it. I attach a printscreen of oscillogram maked by OWON PDS5022S.(Sorry, I'm not familiar with this scope, because recieve them only yesterday night :) ) This oscillogram just before TOP restart.
I don't understand what is the big negative spike at the end of current triangle?

Well, it really looks like the TOP have an internal current limit associated with di/dt. And the greater di/dt - the lower upper current limit, which TOP may achieve.

I mean internally the IC senses the Drain current then turns off the MOSFET. There is a delay between the time the Drain current limit is sensed, and the MOSFET is actually turned off. During this delay time, the Drain current continues to rise. So there is a small error, which becomes larger with higher di/dt. Our devices are trimmed to have a certain peak current limit in operation, with the specified di/dt. If your actual di/dt is greater, then you will see a higher peak current.

The negative spike at the end of your trapezoid is not real - it is an artifact of the slight inductance in your sense resistor. Likewise the spike on the start of the trapezoid is exaggerated.

You should not leave that inductance in place, it will affect the operation of the TOPSwitch, because the C pin bypass capacitor is no longer referenced to the S pin. The resistor is sitting in the way.

Thanks for your reply! About negative spike - ok. Already exclude resistor (it was only for measurement).
But it really looks like the TOP have an internal current limit associated with di/dt. And the greater di/dt - the lower upper current limit, which TOP may achieve. (I don't mean initial spike after TOP turn on, but Imax which TOP may achieve during on-state).
Is it intrinsic characteristic of the TOP? Or I do something wrong?

No there is no characteristic built into the TOP225 that reduces current limit with increased di/dt.

Thanks for reply!
But I really don't understand why TOP225 not reach if only Imax=1.0A.
What can I see, when I'm look in Idrain at the TOP turn on - the saw (with some spike at beginning), which acieved some Imax, isn't it? Well, but this Imax only 0.6A - then TOP restart. I'm really confused. Where this limit result from? Scheme from datasheet on POL transformer (but TOP225 instead of TOP226)

This was the mysteries of nature, TOP225 and failed current limit ^_^ :))

At much lower input voltage, what is the current limit that you see?

Thank you for your attention!
Unfortunately I can't see current at much lower input voltage because I only use line voltage, app.~220V. Well, I'll try to find some transformer 220-110V and repeat measurements.

Thanks a lot, dear PI-Tucker for helping!
I shall have to make measurements with transformer 220-110V, but! It was stupefaction, when I have bought a new TOP225 in another shop and scheme began working :-/
I'm so angry, a lot of time wasted, f..g seller - real fake -TOP225 (I have bought 2 pieces at his shop - both fake).
Thanks again!
With best wishes,
SNS
P.S. What type of chip I have bought instead of TOP 225? ^_^ Something like TOP222 ...

Let this incident be a warning about buying PI parts from certain types of vendors - you may get counterfeit parts.