Half-Bridge SMPS transformer seems too large? 1

[grigson]

Hi,

I am designing an offline Half-Bridge SMPS at 350W and Vin = 90-265VAC.
(there is no PFC stage)

I am finding that I need to use an ETD44 core , but this seems awfully large.
-I am wondering if it is normal to allow the core to go into saturation (i.e. above 300mT during transients etc?)

In selecting the core, I simply picked the smallest one that would NOT go above 300mT with the maximum possible primary volt.seconds.

Is this the usual way?

ETD44 Core datasheet.

http://ferrite.ru/uploads/pdf/produc...d_44_22_15.pdf

(I am using N87 with no gap.)

The switching frequency (Bridge frequency) is 111KHz.
(obviously each fet switches at half of this)

The transformer must be sized such that flux density does not exceed 300mT.

Now the maximum voltage that will appear across the primary is 375/2 = 187.5 Volts, (since the rail-splitting capacitors divide the DC Bus down by two.)

The maximum time that any FET can be on for is = D(max) x T
= 0.8 * 9us = 7.2us.

Therefore the maximum magnetising current in the primary is di = (V.dt)/L = 241mA
(where: L(pri) = 5.6mH, V=187.5V, dt = 7.2us)

The saturation current (based on saturation = 300mT) is = I(sat) = B.A.N./L = 368mA

= 0.3 x 172e-6 x 40 / 0.0056
(where: Amin = 172mm^3 and N(pri) = 40 )

Therefore, this shows that ETD44 will not saturate at the maximum possible volt.seconds seen by the primary.

The next size down (ETD39) goes over 300mT at the maximum volt.seconds on the primary so I avoided it.

Am I being a bit over-conservative here?……is it normal to choose a cheaper , smaller core and just let it fly off into saturation during transients etc?

 

[VEBill]

I'm not a PS Design subject matter expert, but I have seen many power supplies in my time. That core looks to be a perfectly reasonable (expected, normal, not surprising) size (45mm wide) considering the 350 watts power and the frequency that you're using. I have seen higher power density PS bricks, but they use higher frequencies (MHz) and cost as much as moon rocks.

Standby for others to chime in.

 

[IRstuff]

A typical 250W PC power supply is about 5" x 4" x 3" and your transformer seems to be consistent with that and the higher power in your supply.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Comcokid]

Make sure you are calculating the maximum saturation correctly. Since the half-bridge is a variation on the push-pull, and you are using a ferrite (of some type), you can do +/-300 mT on the flux density for a full flux swing of 600mT. It's usually this full flux swing that is used

Now, as to saturation, to parapharase a quote from a seminar by Dr Ray Ridley at the last APEC show, 'there is no reason you should ever saturate the core of your inductor or transformer'. Now, if you think you might push your design to the edge, be sure you use current-mode control adjusted for pulse-by-pulse limiting (and not average limiting) to give you some protection.

Remember, if you design on the bench for the maximum you can get without saturation it will not be adequate when the ambient temperature rises as the charcteristics of the ferrite change with temperature.

 

[grigson]

Comcokid:
I see your point about peak current mode control.

The thing is, this is a half-bridge and so as long as the fet-on time has a defined maximum, (which it does), then we'll never saturate, because half-bridge only saturates on magnetising current.

Page 4-7 of the following...........

http://www.ti.com/lit/ml/slup126/slup126.pdf

...warns us of the dangers of peak current mode control for half-bridge SMPS........

-one cannot do it unless using a balancing wionding...extra expense.

 

[Comcokid]

Yes, with the half-bridge you avoid most of the problems of flux-walking of the core are avoided provided you provide the deadtime to reset the core. That is, assuming that the incomng AC power is clean (well balance) which it is not always, especially if the supply is powered from an inexpensive inverter. Not to mention that capacitors may age unequally and develop leakage, and that center-tap or dual windings by nature are never fully identical (one is always a little more to the inside of the core than the other, etc).

However, most of my designs are push-pull or full bridge, so you may have more detail information on the fine points of half-bridge design than I'm familiar with.

There are two major methodologies to the design of transformers - the area product approach (AP) and the Kg approach. It looks like this paper you referenced is using the AP. If you want to try the other approach just to see how the transformer does size-out use the "Transformer and Inductor Design" by McLyman which has many examples (some using either approach). Online, you can find a half-bridge design app note by going to

http://www.coremaster.com

and looking at their application note AN108 which was written by McLyman. This app note is for a toroid, but will be the same method for your design with a ETD44. Use the flux density for your ferrite, and not the huge flux of the nanocrystalline that coremaster $ell$ (probably around $25 each in low quantities for a core the size you need).

As to the flux swing to use, the various ferrites types out there ("R" PC95 3F3 etc.) do vary slightly in the flux swing available. The use of +/- 300mT is the generic safe number generally used for all, but some may allow for a little more flux, so you may be able to go the next lower core size depending upon the specific core material you use.

 

[grigson]

Thankyou Comcokid,

I'd love to delve into all that Kg and AP material.

-Unfortunately Though where i work we have a different approach....

"everything must be as cheap as possible"

...so we pick a family of cores that we get cheapest, then pick the one we think will do the job.......if we end up with more space than we need , and are outside saturation, then we pick the next size down, because its cheaper.