Kapanadze Cousin - DALLY FREE ENERGY

[Void]

Please help Itsu with his push-pull problem.  That will free us to do just that.

Hi Verpies. If I thought I knew what might be causing Itsu's MOSFET problem I would have
commented on it already. I am following this discussion of Itsu's problem closely. I think understanding
what is causing the problem will be helpful to us all here. Itsu, don't worry about discussing this problem
in this thread. It is useful information for anyone here who is experimenting with similar push pull circuits.

[Hoppy]

Itsu,

Don't expect a clean drain to source waveform with the TVS diodes fitted, as some of the smaller transients will not be fully clamped. It may be necessary to increase the voltage rating of the diodes if they run too hot. I have never been able to get a very nice waveform using the yoke transformer, or manage a smooth adjustment of duty cycle, as the energy in the transient spikes is so high. The lossless clamp given to us by Verpies produced my best results.

I agree - RC snubbers are pants in this application.

I fully agree with Void. We can so much from each other by discussing these various issues, especially with Verpies on board.

Regards
Hoppy

[AlienGrey]

Hi have a look at this coil the kacher ? this is from Sergey's device to me i can only count 9.5 turns of 10mm tape, does any one else have any info on this device ?

[verpies]

Releasing the secondary shorts, and measuring one side of the primary gives 58.8uH.
Shorting then the other primary gives 17.2uH

In an ideal transformer "shorting the other primary half" should give you a 0μH reading (a 100% decrease).
In your transformer the inductance reading decreases only by 71%.

Anyway, since I would like to understand it better.  Could you post a scopeshot like the one below but with a very small duty cycle and at 5μs/div ?

I am puzzled about the drain starting signal which starts high, then falls of to the normal 48V. What is causing that?

The high voltage level is your snubber clamping level, e.g. the TVS diode clamping level.
When both MOSFETs are OFF the current "wants" to continue flowing due to the high leakage inductance.  Thus a high voltage is developed across that winding in an attempt to continue this current flow until the magnetic energy stored in the core/leakage inductance is exhausted.  The easiest path for the current is to continue its flow by starting flowing through the TVS diode.  If that diode had not provided the path for that current, then the Drain-Source junction HV breakdown would be the next best path.

When the 2^nd MOSFET turns ON then an easier current path for the current is established and the voltage can fall. However it cannot fall lower than the 48V level because the 1:1 autotransformer action, caused by the current driven by the 2^nd MOSFET, adds the auto-induced 12V to the +12V from the power supply and you get the normal 2*V_CC or 48V level.

Also puzzled about the gate signals which seems to "lag" its drain signal, how can that be?

Take a look at the scopeshot attached below.  I have marked the times when both MOSFETs are OFF in a gray color. Let's call them "gray zones"
If the leakage inductance was 0μH then it would store no energy and the drain voltage could fall to 0V during the entire "gray zones" and the drain voltage would appear to follow the inverse of the gate voltage ...just like you'd expect.

But the leakage inductance in your transformer is huge and it stores a lot of energy. This energy "fills up" the "gray zones" creating an illusion that the drain voltage leads the gate signal ...which by principles of relativity is the same as: the gate signal lagging the drain voltage.

If you decrease the duty cycle a lot, then less energy will be stored in the leakage inductance.  Less energy will last for a shorter time, so as you shorten the gate pulse widths there will be a point where this energy becomes entirely exhausted/dissipated by your TVS diodes ...and then the voltage in the "gray zones" will fall down to zero (albeit not from the beginning of the "gray zones" because the stored energy needs time to be dissipated).  BTW: Loading the secondary windings helps to exhaust this stored energy even faster.

All of this would be easier to see if the current flowing in the windings and TVS diodes was visualized also ...because inductors are current devices and only current matters to them.  Voltage is just a shadow for inductors, albeit it matters for diode and MOSFET junctions.

P.S.
My lossless clamps decrease the effective leakage inductance and turn it around in order to make a friend out of the enemy.

[TinselKoala]

I'm not sure if I agree with that explanation completely. The "grey zones" are times when both mosfets _should be off_ since the Gate signals are both low at those times. But both mosfets aren't off. It looks like each mosfet is turning On _before_ its Gate signal goes high, and this is happening exactly when the other mosfet's gate signal goes low and turns its mosfet off. There doesn't seem to be any problem with each mosfet turning off at the right time when its Gate signal goes low, the problem is the mosfet turning ON when the opposite Gate goes low. I'm not sure if the leakage inductance explanation fits this sequence of events. What am I missing here?