Regulated SMPS strange mosfet waveforms :-(

[pergo]

hi all,

first of all, i wanna say congratulation for this amazing forum.

anyway,
i'm an italian electronic technician, and i like built car-audio electronic equipment like SMPS for pre, tubes, little amp.
I made several 12V SMPS with success, from +-24V to +-150V, always unregulated.

I tried to "mod" the project for regulation, with (maybe) success. The rail voltage are correct and regulated, but i have a strange waveforms on switching mosfet.
No noise at output, no ringing, no heat, low ripple (but more than unregulated..correct?)...no problem.

I see the same waveforms in several car-amplifier, like italian Audison, and american Phoenix Gold M series (the second).

I always use SG3525, zener+opto reference, +compensation at pin 9 of oscillator.

I think that waveforms are false triggering of o-scope, but i buy a Kicker XS100, with similar regulated SMPS, that not have strange waveforms on mosfets.

This photo come from an audison LRx1.400 (900Wrms @1ohm) provide +-53V regulated, but egual to my projects mosfet waveforms.
I always add a bjt buffer from oscillator out, to gate, with pull-down resistor from fast discharge.
I don't know if that Audison have bjt buffer, but open loop (5v at pin9..eheh), the switching is fine and i've +-63V @12Vin.

The amplifier is original, but i upgrade mosfet from 80n06 to irf3205. the waveforms doesn't change.

This is drain @10Vin:
http://img690.imageshack.us/img690/3995/10vdrain.jpg

Gate @10Vin
http://img200.imageshack.us/img200/7583/10vgate.jpg

similar to open loop.

but, drain @12Vin:
http://img709.imageshack.us/img709/6708/12vdrain.jpg

gate @12Vin:
http://img192.imageshack.us/img192/7397/12vgate.jpg

sinewave????? + square wave pwm

drain @14V:
http://img193.imageshack.us/img193/1791/14vdrain.jpg

gate @14V:
http://img109.imageshack.us/img109/4905/14vgate.jpg



for drain measure, 5V/div, 5us/div and AC coupled.
for gate meadure, same but DC coupled.


what a hell is that waveforms??
false triggering??

i've no problem at the output, no heat from mosfet. everything ok but a wanna understand "why"

thank you very much



ps: sorry for my english

[Sch3mat1c]

Circuit?

[pergo]

my projects are based on this:
Switchmode Power Supply For Car Audio

but with some changes.
- NON INV IN: Vref/2 by voltage divider from Vref 5V
- INV IN: 10K goes to optocoupler and to ground with 1K resistor.
- Opto is 4N35 with collector to Vref.
- COMP feedback resistor to INV IN is 33k.
- Compensation Cap from COMP is 1000pF to ground.
- OUT A and OUT B goes to BJT buffer totem pole MPSA06/56.

This changes are from Diamond Audio D7401 car-amp i've cloned.(by service manual)

Mosfet are STP50N06 or IRFZ44 in my projects, and STP80N06 in the original Audison with max 22ohm gate resistor (10ohm for STP80N06).

Audison exact circuit is unknown. Is difficult to "debug" it because is all SMD.

But Phoenix Gold M series have the same "strange waveforms" but they use the same regulation of my project with opto (like Diamond Audio i've cloned).

I tried to change COMP capacitor with other value but change a little bit (sometimes get worse and make noise).
I tried to rise deadtime but nothing..same waveforms


I don't understand why there is that sinewave, and why happens in several car amps i have (some never repaired or touched)

thank you

[pergo]

here is the circuit (used in my SMPS):
http://img121.imageshack.us/img121/8530/im1y.jpg

i use LC filter for +-Vrail with 2200uF per rail, and unknown L (stolen from ATX power supply)


Audison use 4000uF per rail, unknown L made by 1" toroid
Audison LRx 1.400 inside - Amp Guts

Phoenix Gold M25 with the same strange waveform (but only C output filter):
Phoenix Gold M25 inside - Amp Guts

[event horizon]

Originally Posted by pergo

I don't understand why there is that sinewave, and why happens in several car amps i have (some never repaired or touched)

Looks like a typical type of waveform to me considering you are looking at the voltage accross the transformer. Perhaps you have not taken into account what the snubbers are doing? Don't forget that they'll be discharging through the transformer when the mosfets switch off

[pergo]

Originally Posted by event horizon

Looks like a typical type of waveform to me considering you are looking at the voltage accross the transformer. Perhaps you have not taken into account what the snubbers are doing? Don't forget that they'll be discharging through the transformer when the mosfets switch off

thanks for the reply

you're right, but is it normal? (i measure voltage across mosfet drain)
can snubber store so much energy that produce that voltage amplitude?

solution?? (RCD snubber?)

thank you


ps: kicker XS100, regulated 1000Wrms @1ohm, have snubber but don't make this joke

[event horizon]

Originally Posted by pergo

thanks for the replyyou're right, but is it normal? (i measure voltage across mosfet drain)
can snubber store so much energy that produce that voltage amplitude?

If you are measuring the voltage at the mosfet drain (from ground) you are effectively measuring the voltage accross the transformer.

Yep, perfectly normal When i used to build, test & fix SMPS at a couple of firms i worked for that was the kind of waveform you'd see all the time once powered up (if working correctly).

You'll likely only need to modify the snubbers if when the PSU is fully loaded you get high voltage spikes that could threaten the mosfets breakdown voltage.

A full or half bridge & push pull SMPS as you have there will all display the same type of waveform when running correctly.

[pergo]

fiuuuuuuuuuuuuuuuuu

god bless you!
thank you thank you thank you!

i was going crazy for this unfairy joke

Everything work perfect at idle and full power, but i didn't understand why..now i understand!

Snubber are fine in my projects. Under heavy load (1kW) i have less than 1V overshoot.
I've rebuilt several car-amp SMPS but all unregulated
...and when regulated.......sineeee

again, thank you thank you thank you!

[Eva]

What you are seeing is transformer magnetizing inductance and output inductance resonating with parasitic capacitances.

Drain waveforms look ok (as long as they are symmetrical and consistent through consecutive cycles).

Gate waveforms are ok too (as long as parasitic turn on does not happen). The sine like part is produced by drain-gate capacitance feding drain waveform to the gate and a wide class B dead band from gate buffers and SG3525, probably +/-1V.

[pergo]

Originally Posted by Eva

What you are seeing is transformer magnetizing inductance and output inductance resonating with parasitic capacitances.

Drain waveforms look ok (as long as they are symmetrical and consistent through consecutive cycles).

Gate waveforms are ok too (as long as parasitic turn on does not happen). The sine like part is produced by drain-gate capacitance feding drain waveform to the gate and a wide class B dead band from gate buffers and SG3525, probably +/-1V.

thank you Eva!

a question:
push-pull SMPS that not have this "joke", how are they made?
i mean, Kicker XS100 (and others), square wave is perfect (without sine).
Better trafo?
Better windings?