12V in switchmode power supplies

[Kilowatt]

I think I will read a book on switchmode power supplies before I start building, as I have never worked with SMPS's before. Thanks guys.

[blmn]

Killowatt,

As Beanz recommend, the Billing's book is a very good start. See the chapter about self oscillating supplies. It's an easier way to start learning to work with high frequency transformers.

Beanz,

Since you have the book, I think you might read the chapter to see what I'm saiyng about flux imbalance, specially the last two paragraphs (2.2.5, page 45).

regards

[e96mlo]

I'm not a DC blocking cap will work that well. Think of the stresses and the ripple it must take!

/Marcus

[Kilowatt]

Yeah and the only thing messier than a burnt resistor is a blown up electrolytic!

[BeanZ]

An electrolytic is not the proper choise for this application because of it polarization. A film cap should be fine in this application. It is placeed in series with the primary winding of the transformer. Dual primaries may also be desireble too. Make sure that your output filter caps are rated for the ripple current specified for the output filter inductor.

BeanZ

[BeanZ]

As for flux imbalance and saturation of the core: It is almost guaranteed using MOSFETs and no combatant for their different Rds(on) values. A slight difference in the Rds(on) of the transistors will lead to catastrophic failure. When the core gets into the non-linear saturation region, the core will no longer support flux and the transistors will be driving a low impedance (short circuit) load. The transistors will fail. The process is much slower however with MOSFETs which have an increasing "on" voltage with increasing temperature. This helps to minimize flux imbalance on a short-term pulse by pulse basis but over time the core will eventually walk up the hysteresis curve and make it to the non-linear region. A small primary resistance is often added in series with the power transformer primary in car audio SMPS. This will slow the process even more. But, a few billion cycles down the road...the MOSFETs will go. Unitrode (TI) has some current-mode PWM controllers. They also have PWM drivers which have internal high-current totem-pole outputs for driving the high capacitance gate charge of a MOSFET.

BeanZ

[Kilowatt]

"A small primary resistance is often added in series with the power transformer primary in car audio SMPS."

And a DC blocking capacitor would provide just that.

Anyway, I need these to be very reliable and last practically forever, so I need some way to combat this non-linear thing. I assume almost perfectly matched parts are a must here (I wonder if I could design a current mirror into the thing ). This is a push-pull SMPS, for those who don't know. Rod suggested using 6x0.4mm wires (what AWG is that?) for the primary, I think I'll use 7. Please describe how I can keep it from saturating.

Thanks

[Electro]

Rod suggested using 6x0.4mm wires (what AWG is that?) for the primary, I think I'll use 7.

7 AWG is too much or big. It is around 10 AWG.

0.4 mm is 26 AWG wire.

[BeanZ]

10AWG is much too big of a wire to use for the primary at 50kHz. This is a number to indicate the equivalent amount of copper cross sectional area needed to carry the primary current. Due to the skin effect at 50kHz, several smaller guage wires are used to achieve a total copper cross sectional area about that of a ten guage solid wire. There are plenty of sites on the web that will give you this information, but here is some:

10AWG=11740 circular mils or 58.13cm^2 x 10^-3
resistance=.9987 ohms/1000'
current capacity = 13840 mA@750 Circ.Mil /A
= 20768 mA@500 Circ. Mil/A


20AWG=1246 circular mils or 6.315 cm^2 x 10^-3
resistance=10.13 ohms /1000'
current capacity = 1365 mA@ 750 Circ. Mil/A
= 2050 mA@ 500 Circ. Mil/A

[Kilowatt]

Who said anything about 10AWG wire? That's huge, I'd never be able to wind it in a 1.5" toroid. I was talking about using 6 or 7 26AWG wires. I'm not sure where Electro got 10AWG from. By "I think I'll use 7" I meant 7x0.4mm wires, not 1x7AWG wire.