Push-pull SMPS (12V to +-25V) question - Page 2 - diyAudio
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Old 2nd June 2010, 01:07 PM   #11
rajudp is offline rajudp  India
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try this site
TRANSFORMERS
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Old 4th June 2010, 10:10 AM   #12
Hitec is offline Hitec  Finland
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Thanks for the site. Wikipedia also explains those things quite thorough. Have to read more but the SMPS seems to work and the class-D amp that the SMPS feeds works too So nothing to fix yet. Maybe later

How does the overvoltage over drain-source affect the transistor? Does it break it instantly or as in diodes, only overheating caused by the current caused again by the over voltage breaks it???

Any tips/sites for winding the transformer well (to avoid the leakage inductance)?? It is just a bit difficult to remove the transformer for rewinding it once it has been soldered to its place The transformer is ETD39 with 3F3 core.
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Old 4th June 2010, 08:12 PM   #13
space is offline space  Norway
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If you constantly have over-voltage spikes, the FET's will most likely fail catastrophically at one point. When is hard to say.

Toroids are easier to wind in respect to having minimum leakage inductance.


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Old 7th June 2010, 07:45 AM   #14
Hitec is offline Hitec  Finland
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That sounds scary. Do you mean they could latch-up or something (I have fuses though )?

The transistor that I'm using is IRF1010N and it is "fully avalanche rated" and the max avalanche current is 43A and energy is only limited by the junction temperature. So I guess I will still try these transistors until they fail (hopefully not catastrophically ). I read that after avalanche the Vds is clamped. But I will report if my amplifier cathes fire or something
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Old 9th June 2010, 07:00 PM   #15
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When the voltage becomes to high between drain and source, the MOSFET will turn on. This happens when it is suppose to be off. This can cause alot of trouble, and catastrophic failure is one likely scenario. The IRF1010N is guarantied to sustain 55V between drain and source. How much over this limit that the true limit is, is unknown but it is newer a good idea to push it beyond what is stated in the data sheet. Normal and good practice is to stay quite a bit below.

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Old 10th June 2010, 05:36 AM   #16
Hitec is offline Hitec  Finland
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IRF app. note. On page 7 figure 7 shows a diode-like behavior on Vds - the current rises rapidly after certain voltage but the voltage remains the same. I sent them a question about this. It would be nice to know for sure however, since many sources suggests that the transistor really turns on due to positive feedback in the transistor structure. I have no idea which is true
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Old 10th June 2010, 07:34 AM   #17
Hitec is offline Hitec  Finland
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Another IRF document:

"Some designers do not allow for avalanche operation; instead, a voltage derating is maintained between rated BVDSS and VDD (typically 90% or
less). In such instances, however, it is not uncommon that greater than planned for voltage spikes can occur, so even the best designs may
encounter an infrequent avalanche event. One such example, a flyback converter, is shown in Figures 1-3.

During MOSFET operation of the Flyback Converter, energy is stored in the leakage inductor. If the inductor is not properly clamped, during
MOSFET turnoff the leakage inductance discharges through the primary switch and may cause avalanche operation as shown in the VDS, ID, and
VGS versus time waveforms in Figures 2 and 3.

In this application, built in avalanche capability is an additional Power MOSFET feature and safeguards against unexpected voltage over-stresses that may occur at the limits of circuit operation."


So now I am quite certain of the fact that the FETs are good for my device with leakage inductance etc. Now should have time to test it properly
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