ATX KENTEK KT-PSK400b Schematic Diagram and Question

Thread Starter

N11778

Joined Dec 4, 2015
174
Caution: I drew the diagrams from the boards so there will be some Errors.

I have a question for you ATX Experts.
What is the Small winding on the Driver transformer for?

I Like the cheepo way they got the 3.3volts, It puts a big load on the 5v supply.
So, the Supply is only $25 new on EBAY.

The 1000uf filter Cap. C15 on the 5volts keeps going bad.
One board is 2 years newer and they changed Supervisor Chip.

Interesting fact: When C15 was going bad 200uf measured instead of 1000.
The symptom was: When I put the Computer to sleep It would lock or crash.
Worked fine the rest of the time. It would boot right back up OK.
 

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AlbertHall

Joined Jun 4, 2014
10,022
What is the Small winding on the Driver transformer for?
That transformer output provides an isolated supply for the supervisor IC.
There are two windings that feed back to Q1 and Q2 base to sustain oscillation and then Q1 and Q2 drive the primary of T1.
 

ebp

Joined Feb 8, 2018
2,332
T2 is a base driver transformer. It is what is used to provide the base current to control the main switching transistors Q1 and Q2, It provides the galvanic isolation that is required between the low voltage circuitry and the high voltage circuitry. The topology appears to be a two-switch forward converter, which has some worthwhile advantages at moderate power levels. The driver transformer itself is being driven push-pull, which common with open collector/drain outputs on the control IC.

The "small winding" appears to be for positive feedback for the drive of Q1, but I'm not really sure.

The IC called PWM Control and associated parts form a conventional low-power flyback converter for the 5 volt standby supply and the power for the main switcher IC.
 

Thread Starter

N11778

Joined Dec 4, 2015
174
The "small winding" appears to be for positive feedback for the drive of Q1, but I'm not really sure.
It's in most designs so how about Neg. feedback to prevent runaway at higher power.

I think I will put a 100 watt bulb is series with the input power and take some scope readings.
Then Jumper the small winding to see the difference.
For some reason typing this I'm getting a faint smell of electricity.
Sparky
 

Dodgydave

Joined Jun 22, 2012
9,205
Nope. T3 and the circuit round it does the +5V standby.
Ah yes a doh! moment there, the winding is to help the self oscillator startup and syncronising the main output together.

Also there are two versions here, one uses a TL431 for the 3v3 output ,on the 5V line.
 

ebp

Joined Feb 8, 2018
2,332
I did consider the possibility of negative feedback for the main switch as a means of cycle-by-cycle current limiting. I'd need to look more carefully at the circuit. I'm not even entirely certain which side of the winding the phasing dot belongs to. If it is on the bottom as it at least marginally appears to be, then the phasing is correct to aid switching - in phase with Q1 and out of phase with Q2. But I'm really not sure why one would want to do that. The MJE13007 doesn't have especially high gain, so perhaps the effect would unburden the circuit on the input side of the drive transformer while still achieving fast, hard switching. I've used drive transformers in several switcher designs, but for FETs. I've never done anything other that very low power switchers with bipolars so I've never paid much attention to bipolar base drive circuits. There may be something in one of my reference books, a couple of which are old enough that bipolars were still being used in switchers. They seem to still be common where pennies count.

The other thing that makes me think it isn't negative feedback is C3. With that cap the transformer primary can be saturated without causing disaster.

Do remember that the entire input side of the switcher must be considered to be connected directly to mains "hot" for safety purposes and probing without an isolation transformer is very hazardous.

Unless they just aren't shown, that switcher is remarkably devoid of EMI/RFI countermeasure components.
 
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Thread Starter

N11778

Joined Dec 4, 2015
174
that switcher is remarkably devoid of EMI/RFI countermeasure components.
$25 you get what you pay for some times less. Check out the missing components. It does work though.
Takes about 1 year for the 5v filter cap to go bad.
 

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Thread Starter

N11778

Joined Dec 4, 2015
174
the phasing is correct to aid switching - in phase with Q1 and out of phase with Q2
Thanks for the Help. Makes sense. Going to bag the scope testing.
The Board minus the Electrolytic caps is in an oven at 200 deg. F trying to get rid of the coating smell.
It gives me a severe head ache and Asthma.
 
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