F5 power amplifier

[GRollins]

Originally posted by Variac


Can you make a power supply with just chokes and resistors and no caps?

In theory, yes. In practice it's difficult because you need some form of energy storage and getting (even winding your own) big enough coils would be difficult and expensive. And heavy. And bulky.
The storage in an inductor is in the magnetic flux surrounding the coil. The bigger the coil, the more flux. It's when the flux lines collapse that the current flows (which is why you see diodes across relay coils--keeps the backlash from roasting parts), but it tends towards sharp spikes so you need an inductor to smooth the spike from the inductor to smooth the spike from the inductor to smooth the spike from the inductor...

Grey

[Nelson Pass]

Originally posted by Variac
Can you make a power supply with just chokes and resistors and no caps? That would be thematically correct!

Inductors would be ideal for amplifiers that want a forced constant
current draw instead of voltage. Just as conventional amplifiers want
a constant supply voltage but draw current variably, this amplifier
would want constant current and would create a variable voltage
at the supply input.

[jackinnj]

Here's a conceptual PCB design which will probably do a better job of diffusing the heat -- the MOSFET's are on the wings (and are mounted with pins coming from underneath the PCB). The PCB is 3.7" * 1.8".

[apassgear]

Originally posted by jackinnj
Here's a conceptual PCB design which will probably do a better job of diffusing the heat -- the MOSFET's are on the wings (and are mounted with pins coming from underneath the PCB). The PCB is 3.7" * 1.8".

Conceptually looks great really, you got my humble blessing.

I would put those power resistos flat over the board though, even if that means a bigger PCB

[EUVL]

> Here's a conceptual PCB design

IMHO :

1) I would use a foil resistor at the MOSFET source (R11), like e.g. MPC74 0R47, which has a similar footprint as wire wound ceramic resistors.

2) I would place TH1 directly on the MOSFETs for thermal compensation.

3) I would place R13 as close to the MOSFET gate as possible.

4) I would place R3, P1, R15 as close to R13 & R11 as possible.

5) R19, R17, R21 and the BJT can be placed further away, as they are only for protection.

In addition, the feedback path also want to be as short as possible, without saying.


Patrick

[SCD]

I like the idea of having the mosfets spaced far apart like you did. I also like to use the pins of the mosfets to "raise" the board away from the heatsinks.
Good for you

[moe29]

does FET spacing really matter? I'm sure more space can't hurt, but is
it something to fret about?

This ZEN amp has been running for years like this:

[SCD]

The thought I had was to do with trying to get the heatsink disapating the energy quickly and evenly as soon as possible. So rather than having one central hot spot space them out to use more of the heatsink surface. I am not sure if it is an issue or not, I guess that would depend on the heatsink.

[apassgear]

Originally posted by SCD

... I guess that would depend on the heatsink.

In the end thats the point.

If space allow I would spread the power transistors... but hen again my Aleph4 runs them at 1 1/8" pitch, there are 12 FET's per channel on this one and no problems.

BTW, EUVL points are to be considered on the detail design, no doubt about that

[EUVL]

> I guess that would depend on the heatsink

To be more precise, it is about your thermal management strategy.

Apart from wanting a short electrical path with low resistance and inductance between power FETs (i.e. assuming you get that one solved one way or another), you DO want to have all FETs thermally well coupled so that they have the same temperature at all times as much as possible, so that they drift together in the same manner.

On the other hand, you might NOT want all 200W or whatever that you are going to burn to be all concentrated at a 50x50mm patch if your heatsink is say 400x200mm, because you are then likely to get uneven temperature distribution on the heatsink and therefore not making the most effective use of that.

How you balance one against the other is art. And everyone has his own preferences.


Patrick