Failing to clamp transient with schottky. What is going on?

[king.oslo]

Hello,

I wound a flyback transformer on a pretty big core. ETD59, I think.

I am switching it low side with an NMOS. As expected I was getting large voltages rising when I switch the FET off.

To my surprise I cannot clamp these transient voltages with a diode forward biased to VDD. The diode current doesn't go higher than 7mA before I reach MOSFET drain source breakdown voltage of 80V.

LTSpice predict that it should be able to clamp the transient. Why doesn't this doesn't happen? What can I try and do to nail down source of the problem?

Thank you for your time.

Kind regards,
Marius

[c4757p]

How solidly is the cathode connected to VDD? If there's a good bit of impedance between them the spike might not want to go anywhere - fast edges don't like inductance.

[Rerouter]

put 2 35V zeners cathode to cathode across the inductor instead, this will limit your voltage positive or negative to 40V,

[micro_freak]

Have you tried a TVS diode, they act fast and can dissipate a lot of energy.
It has to be selected so that it's breakdown voltage exceed your supply voltage, for the power factor you will have to look up data sheets to make sure you select one that can repeatedly dissipate enough energy without overheating.
regards

[qno]

Where does the charge go?
If you feed the circuit from a battery you are ok.
If you use a powersupply you need some clamping or large capacitors

[king.oslo]

Yes, charge goes through the MOSFET which dies after about a minute.

I will try the advice which has been given to me.

Thanks.M

[Marco]

I wound a flyback transformer on a pretty big core. ETD59, I think.

I am switching it low side with an NMOS. As expected I was getting large voltages rising when I switch the FET off.

To my surprise I cannot clamp these transient voltages with a diode forward biased to VDD.

Something is obviously just plain wrong in your circuit. (Getting 10s of Volt forward voltage across a functioning Schottky? Nope.) On top of that ... if the Schottky was connected and functioning properly your flyback transformer wouldn't work. A flyback transformer is still a transformer, when the secondary voltage becomes positive the primary voltage also has to be allowed to become positive (relative to Vdd in your circuit). If you clamp the primary with a Schottky you are also clamping the secondary.

[Neilm]

A flyback transformer is still a transformer

No it isn't. A "flyback transformer" in a SMPSU is a pair of closely coupled inductors. Current is conducted in the primary but the secondary does not conduct until the supply to the primary is removed. The energy that has been stored in the magnetic circuit is then transferred to the secondary. A subtle but important difference.

The rectifier diodes in a flyback topology should be as fast as you can get. The problem the OP has could be a symptom of the secondary rectifiers not being fast enough. It could also be due to poor layout. If there is a significant amount of stray inductance it will radically alter the behavior of the snubber network.

When this was simulated in LTSpice, were "real" values and components used or did you just go for ideal components? Also, did you measure the leakage inductance of the transformer. (measure the primary inductance with the secondary shorted). The coupling in the simulator can be set to ideal (value of 1) and a separate inductor can be put in series with the primary inductance to simulate it. This might show that the original simulation has greatly underestimated the energy in the turnoff spike.

For more information on snubber networks look at this app note from Fairchild.

Neil

[AndyC_772]

One beer says this is layout related. Can you post a photo of your setup? I bet there's significant inductance in series with your diode.

Do also consider trying a transient voltage suppressor rather than a Schottky diode - they're faster acting and designed to absorb significant energy. But the layout has to be right first.

[A Hellene]

A flyback transformer is still a transformer

No it isn't.
[...]

Correct.
That is because:

An inductor having both a primary and a secondary windings is not necessarily a transformer; it could either be a transformer or a flyback or just two coils magnetically coupled together (i.e. the SEPIC dual inductor). What makes the distinction is not how the primary is driven, but how the energy is collected from the secondary.

Inductors and their windings have a polarity. Assuming that the primary has a positive drive, we can collect the energy from the secondary either by using both the positive and the negative phase outputs, or, by the use of rectifiers, the positive phase output only or the negative phase output only.

In the first two cases we have a forward topology and the inductor can safely be called a transformer because the energy transfer is direct and immediate from the primary to the magnetically coupled secondary.

In the third case we have a flyback topology, and the inductor can not be called a transformer any more because there is not any direct energy transfer from the primary to the secondary windings; instead there is energy storage from the primary to the magnetic medium and energy harvesting from the magnetics by both the primary and the secondary windings after a delay determined by the driver and the inductor circuit resonance, where the stored energy will pick the easiest path to flow no matter if this path is through the primary or the secondary or through both the windings. The most simple examples of this topology are the boost/buck/buck-boost converters, where the inductors used do not have a second (a 'secondary') winding.

Of course, conventionally, we call every inductor with more than one windings, a transformer.

[...]
But chokes are not only used for filtering. For example, the DC-DC flyback converters are called 'ringing choke converters' because the flyback inductor is not really a transformer; it is a choke actually (we should better call it a coil), often with a primary and a secondary winding, or with extra auxiliary secondaries. Why the flyback coil cannot be called a transformer? Simply, because in transformers the energy transfer happens directly from the primary to the secondary windings through the magnetic core. In the flyback, instead, the primary does not transfer energy to the secondary but it charges the magnetic material of the inductor, while the magnetics stored energy is discharged by the use of the secondary winding of the flyback inductor.

Now, if we can use the same exactly inductor we have just discussed at the flyback example above, and we use it in forward topology, only then we could call this specific inductor a transformer; but we cannot do that because a transformer needs a magnetic medium of different properties than those of a flyback coil.

This is an excerpt of a piece I wrote about flyback converters:
"Actually, [at the Flyback Topology] we are not talking about a transformer because we do not collect the energy as we are pushing it to the primary winding; this is the Forward Topology. Instead, we collect the energy stored in the magnetic medium, which is the ferritic core. So, this is two coils coupled magnetically together using the same ferritic core, where the primary creates energy that is stored in the visible air-gap of the ferrite core (or in the invisible countless tiny gaps created by the bonding material of the ferritic powder the ferrite cores are made of), and we discharge the coil's energy by using the second coil (the "secondary"). This is the so-called Flyback Topology that energised the electron beam of the TVs, where it was firstly used commercially and, thus, became widely known; the electron beam that "flew back" to the next line of the field/frame during the blanking time after rendering the end of the previous line."

-George

[Marco]

No it isn't. A "flyback transformer" in a SMPSU is a pair of closely coupled inductors.

As I said, a transformer.

Current is conducted in the primary but the secondary does not conduct until the supply to the primary is removed.

Good thing transformer action is a voltage transformation then isn't it, so it doesn't actually preclude my statement.

The energy that has been stored in the magnetic circuit is then transferred to the secondary. A subtle but important difference.

In a normal transformer you generally want it to store as little energy as possible ... but that's not really my point. You generally still want the coupling in a flyback to be as high as possible (in the smaller ones it's something like 0.99) and with close coupling transformer action (ie. voltage transformation based on turns ratio) is in full effect. If you clamp the primary to some low voltage, you're also clamping the secondary.

A normal RCD snubber in a flyback topology like he has will allow the primary voltage on the switch to rise above Vdd+nVo.

[croberts]

My apologies if this has already been mentioned but when trying to measure transients like this very narrow spike, it is important to have a very short ground on the scope probe (1" or less) and make the measurement right across the diode. I think they make ground probes that snap right on the scope probe ground collar just behind the probe tip.

http://www.google.com/imgres?imgurl=http://www.cliftonlaboratories.com/userimages/Update202.jpg&imgrefurl=http://www.cliftonlaboratories.com/july_2007.htm&h=284&w=400&sz=11&tbnid=2KVfX9IRNlJiuM:&tbnh=110&tbnw=155&zoom=1&usg=__v_ByYVmcNbNh2BVahJrqDIaVgX4=&docid=GxR4oJqSXMWk4M&sa=X&ei=CohEUfmxHomi4APq5YCYAg&ved=0CHEQ9QEwCQ&dur=364

[Icarus]

Diodes have also forward recovery time. May be your diode is just too slow ?

[Marco]

Diodes have also forward recovery time.

It's almost never relevant (usually swamped by inductive effects). Also it doesn't apply to Schottkys.

[Icarus]

I see, but Jim Williams says something different about that. If I got that correct
http://cds.linear.com/docs/en/application-note/an122f.pdf

[Marco]

He didn't exactly say something different ...

He didn't say Schottky's suffer the effect (as far as I can see the negligible width of the Schottky barrier will prevent resistance modulation during forward conduction, aka forward recovery, just as much as it prevents reverse recovery).
He didn't say the overvoltage was large enough to overcome the inductive effects of a through hole component either.

Look at what he measured and compare it to the component the OP is using .... do you think something like a 750 mV overvoltage excursion would be relevant in a circuit with a 80 V MOSFET switch? As I said, rarely relevant ... not never.

[Icarus]

He also didn't say roses are red and more over he didn't say violets are blue.
It's only matter what he said

[Marco]

I was trying to show by example some things he should have said to contradict me, to make it obvious why he did not, but just for you here is a shortened version of my previous reply ...

No, he didn't contradict me. You didn't get that correct.

Better?