SMPS PSU articles

[davmax]

Re: SMPS PSU articles

because the emitter of Q1 swings from the mid voltage point to the high voltage point at the switching rate.

I slipped up here. Should read: the emitter of Q1 voltage swing is from Q2 sat to the high voltage rail.

[starfury1]

Re: SMPS PSU articles

So that passage should read as below ?
Just checking and just for clarity
(now sub divided)

Yes T2 is the transistor pulse driving transformer that isolates the primary voltage from the secondaries and therefore no need of an opto anywhere. Yes the DC feedback voltage is there connected to pin 1 of TL494.

I might add here that I do not favour the pulse transformer drive system because the emitter of Q1 voltage swing is from Q2 sat to the high voltage rail.

With such a large voltage swing and at those frequencies the stray capacitance in the transformer has to be very low. Not easy. I prefer other high side drivers.

(orginal text :the emitter of Q1 swings from the mid voltage point to the high voltage point at the switching rate. )

this is covering a lot of ground in detail so I guess some error or
miss interpretations can and will creep in

Thanks
Cheers

[davmax]

Re: SMPS PSU articles

Correct Starfury1.


What has always bothered me about these drive transformers is that they see what is known as high dv/dt (fast voltage change) this is stressful in any circuit/component eg you can see max dv/dt specifications for thyristors and triacs because as dv/dt increases there is risk of triggering the device ON through internal capacitance.

So many things to be aware of.

[starfury1]

Re: SMPS PSU articles

there is risk of triggering the device ON through internal capacitance.

I've heard of this (not in relation to switcher transformer (pulse) more as you said triac etc)

so you learn some thing new everyday

Generally speaking
Anything that poses a threat like that, even if its somewhat rare to happen is still a risk and in all likely hood not a good thing to have happen at the wrong time!

Thanks Davmax and thank you for all your input too

Cheers Mate

[davmax]

Re: SMPS PSU articles

I can also see that the Overvoltage circuit and the Power On circuits can bypass the TL494 power to shut it down.

I have to admit to being very wrong in this circuit area quote. The Power ON and Over Voltage circuits actually carry out an OR function on the Dead Time Control input (pin4). If either pulls this input high (> 3.3V) the drive to the switching transistors is inhibited and the PS shutsdown.

This mode of operation does not fit well with modern power supplies under the ATX 12V specification in that the inhibit relies on the correct function of the PWM controller. Removing power to the PWM controller is used as a better method to meet current specification. I quote the relevant section of the specification:

3.4.1. Over-voltage Protection


The over-voltage sense circuitry and reference shall reside in packages that are separate and distinct from the regulator control circuitry and reference.

This can be read as just the external OV sense circuitry is seperate and the PWM may be used to finally terminate. However I believe the intent is to bypass any problem that may have occurred in the pwm controller that could have caused the high output error, therefore terminating pwm power is much more effective. This shutdown method is then easily linked to the Power ON function. I have certainly seen this power termination/control method being used.
I cannot stop this section being in bold type, strange!

[davmax]

Re: SMPS PSU articles

For the insight of others that might read this thread the circuit under discussion can be view at this link:

http://www.pavouk.org/hw/en_atxps.html

[starfury1]

Re: SMPS PSU articles

Just to add
that design would be old by todays standards it's 200W
if you look at the schematic.
from what I Found
(there is a manual (sort of) out there,not overly useful... no text at all )

Manufacturer DTK Model PTP 2038.

No year can I find at this point.

TL 494 was in fairly common use in the days of the XT psu's, so I suppose this design does go a fair way back to AT days
(lest in it roots of design)

Anyway suffice to say designs have moved on from this and now with APFC being common (forced) and more dedicated controllers being used
(By the looks of it) it may not meet all later specs, defiantly not latest I suspect.

still basic concepts of operation still apply here I guess, just the details of how its achieved will change.

Oh yeah dont worry Davmax...your computer probably just entered the Twilight Zone

(YT into here )

mine does flaky things at times for whatever reason

Thanks Davmax

Cheers

[gonzo0815]

Re: SMPS PSU articles

Most of the cheaper units can not deliver any more then 200w w/o going poof.
TL494 or compatible chips are probably the most common SNT controller chips still today.

Sure, most 5Vsb supplies are using some Topswitchers today, but the basic circuit of the main psu has not changed dramatically.