The news D-Amp DLS3000....

[fredos]

What do you mean by magnetic snubber? 0.1uH in serie with each power mosfet?

Fredos

www.d-amp.com

[Pafi]

5uH to 20uH

Isn't is too huge?

[Eva]

Originally posted by Pafi


Isn't is too huge?

Not as seen from the 340V primary side of SMPS switching transformers. For example, I got 16uH from a 3E25 material 36mm OD, 23mm ID, 15mm H toroid with a 44 turn primary (and a shorted 4 turn secondary) intended to operate at an absolute minimum of 45Khz (before saturation arises) and with a practical power rating somewhere around 1000VA. Primary current was taking one whole microsecond to be flipped from -10A to +10A with 320V applied.

He seems to be using similar transformers. Cores may be bigger and turn counts lower, thus yielding lower inductance figures, but the phenomena is still the same.

[fredos]

Sorry Eva...Try the 3F3 material, lot more suited for switching application, simulate 30 turn in primary....Lose is bit higger, but try it, you will see something interesting!

Fredos

www.d-amp.com

[Eva]

I know, 3E25 is a ferrite intended for signal transformers. Power materials ask for at least 30% less turns (thus halving leakage inductance) but I have a lot of these toroid cores that I bought very cheap some time ago and I play with them from time to time High frequency power materials are very expensive.

For serious designs in that power range I'm using E42/21/20 in N27 material instead. That produces 8uH in a 340V 30Khz transformer good up to around 800VA with a simple two layer sandwitching approach (far better than the toroids).

p.s. Concerning magnetic snubbers, you'll have to find out by yourself, and be careful because the most obvious class D applications are covered by patents that will still take some time to expire

[Jaka Racman]

Hi,

Fredos, If I were you I would seriously look if WO2004001960 patent application does not violate your intelectual property. From your description I would say it does. (phase shifted primary full bridge and synchronous rectification of resulting BD modulated secondary signal) Only difference is that patent application uses center tap secondary but full bridge is covered also.

BTW Eva, patent is based on a working circuit.

Best regards,

Jaka Racman

P.S. All this ampliverter stuff is really based on US4479175 from 1984.

[Pafi]

36mm OD, 23mm ID, 15mm H toroid

Not too big.

Let's estimate: cross-section of leakage field is max. 2 cm^2. Average lenght is about 8 cm. Al=0,0002/0,08*1,2*10^-6=3nH/turns^2, L=6 uH, maybe 50 % more. Am I wrong? Maybe your winding scheme was not enough homogenous.

But fredo's trafo operates on 125 kHz if I understand well. I think it can have only 12-15 turns, and maybe can have a tighter coupling too. I think less than 1 uH leakage can be achieved easily. With interleaved coils it can be even better. (There is such wire with thin isolator wich can withstand 3,5 kV, so it is possible.)

I agree, phenomena is the same, but 5 uH is exaggerating. 1 uH ruins efficiency as well.

OFF: Could you (are you allowed to) tell me what kind of current sensor do you use in your average current mode controller? I have a similar task, and the LTS25 current sensors I wanted to use are much too noisy. Do you have any idea with excellent CMRR, and high precision?

[Tekko]

Eva, you mean renewed patents ?

[Jaka Racman]

Not necesssarily. Cited patent application is really something new, since synchronous rectifiers change state when primary is effectively shortcircuited by phase shifted bridge. Leakage inductance of the transformer is not an issue here.

Best regards,

Jaka Racman

[Eva]

Tekko:

No, I mean the same stuff patented a dozen times with a dozen minor and not relevant changes. Particulatly, USA patent system seems to approve anything if you pay their gentle bill.

Originally posted by Pafi

Not too big.

Let's estimate: cross-section of leakage field is max. 2 cm^2. Average lenght is about 8 cm. Al=0,0002/0,08*1,2*10^-6=3nH/turns^2, L=6 uH, maybe 50 % more. Am I wrong? Maybe your winding scheme was not enough homogenous.

You forgot one very important fact. Leakage inductance is doubled because primary and secondary appear in series. When you apply a short to the secondary, the primary is not being coupled to a perfect short, but to a short in series with the leakage inductance of the secondary, and vice-versa.

Thus, 12uH comes very close to the value I measured (taking into account that in a toroid some flux always leaks though the gaps between turns, particularly if primary turns are too spaced and there are no secondary turns near (by the way, that's the reason why a bigger toroid with less turns does not solve the problem at all, but a EI core with sandwitching does).

But fredo's trafo operates on 125 kHz if I understand well. I think it can have only 12-15 turns, and maybe can have a tighter coupling too. I think less than 1 uH leakage can be achieved easily. With interleaved coils it can be even better. (There is such wire with thin isolator wich can withstand 3,5 kV, so it is possible.)

I agree, phenomena is the same, but 5 uH is exaggerating. 1 uH ruins efficiency as well.

It's really hard to get tight coupling with low turn counts in toroids, particularly when secondary turn count is not close to primary turn count.

OFF: Could you (are you allowed to) tell me what kind of current sensor do you use in your average current mode controller? I have a similar task, and the LTS25 current sensors I wanted to use are much too noisy. Do you have any idea with excellent CMRR, and high precision?

It may seem silly but I'm currently using plain .010 ohm and 0.020 ohm metal shunts and compensating leakage inductance with RC filtering applied to the sensed signal. I mean these:

http://eva.eslamejor.com/I_SHUNT0.JPG
http://eva.eslamejor.com/I_SHUNT1.JPG

Of course, there are much better places to measure current than at the noisy source of some corss-conducting MOSFET (namely at output inductor and capacitor).