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Old 12th May 2010, 11:19 PM   #1
Archion is offline Archion  Norway
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Default SMPS flyback, transistor problem

Hey,

Currently I am having a problem with the mosfet switching, the mosfet itself won't mirror the drive
signal from the PWM controller, only a distorted pulse appear. I am sure the PWM controller is able
to drive the mosfet without a problem as the input capacitance of the mosfet is max. 750 pF and
the allowed capacitance for the PWM controller is 1 nF.

I guess the "distorted" wave at 0 V AC doesn't tell much, but check the huge spike over the transistor @10 V AC, it's like 50 V, shouldn't be so high, it looks like the transistor is operating wrong and the spike from the 0 V screen gets amplified.
The voltage between drain and source shouldn't be higher than 10 V * sqrt(2) (+ possible spikes).

I've tried several transistors now, still this strange issue, the GS voltage and everything seems to be correct...

Input from mains: 230~ V AC -> 325~ DC
Switching frequency: 450 KHz
output voltage: 19 V
output current max.: 4.74 A

parts:
PWM controller: UC3842
Mosfet: 2SK2700

The PWM controller receives power from an external power supply.

Circuit schematic and signal captures from the circuit are attached.

In the signal captures the yellow line is the switching from the PWM controller, blue line is the drain-source voltage @ 0 V AC and 10 V AC

Comments appreciated
Attached Images
File Type: jpg IMAG0147.jpg (393.3 KB, 146 views)
File Type: jpg IMAG0146.jpg (424.3 KB, 118 views)
File Type: png circuit.png (71.6 KB, 138 views)
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Old 13th May 2010, 12:32 AM   #2
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R10 is WAY too small. Try 10k. I've never seen the UC3842 run over 200kHz, and with the large dV/dt, 100kHz would be typical. You do not mention/label what the transformer inductance is, so I don't know what it is actually supposed to run at.

I don't know what your oscilloscope shots are of, your English description doesn't make sense.

Why did you take tremendously large, uncropped photographs of an oscilloscope which has a screenshot function built in?

Tim
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Old 13th May 2010, 10:15 AM   #3
Archion is offline Archion  Norway
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The R10 resistor must be so small to run at the desired frequency, the PWM controller can provide operation frequencies up to 500 KHz.

Inductance is 585 uH, continuous conduction mode.

The two shots are from measuring across drain-source at 0 and 10 V AC (blue line). Yellow is the drive signal from the PWM controlller.

Yeah the oscilloscope shots are kinda huge, should have scaled them better, and this particular oscilloscope doesn't provide any medium that i can use(zip drive).
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Old 13th May 2010, 02:35 PM   #4
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So the blue trace is Vds? Why is it 500mV/div?? Or is it Vds only in the second shot? I'm still confused.

AFAIK, this controller needs slope compensation to work properly in CCM.

Total gate charge is 25nC at 10V, so it looks kind of like a 2.5nF capacitor. With "only" 150ns max output risetime, you're asking a lot for this frequency.

Hard switching (i.e. CCM) is expected to produce big turn-on peak currents.

Tim
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Old 13th May 2010, 04:13 PM   #5
AP2 is offline AP2  Italy
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Hi,
R4 2,2K?? 22r change.
change R10 - C11 as datasheet RC-Table, becouse this change DT.
for you frequency fixed c11 at 220pF and adj R10.

Regards
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