EMI and temperature measurements of several amps

[V-bro]

Hi, I'm doing some EMI measurements on the AMP7 right now.

Now I'm in no way familiar with the procedures that are normally used to do this kind of stuff so if you want to know something in particular or have any hints or guidelines please tell me...

I am using a Rhode&Schwarz FS300 spectrum analizer with a 9cm long antenna connected to the input. (I know the formula is roughly (300/f)/4 times 0,8 should give the length of the antenna in meters, but I think this length should pick up EMI well enough if there is any) I have set the center frequency as 800Khz and the band I'm measuring in reaches from 9Khz to 1,6Mhz.




I have tested the amp at idle and with 1Khz tone connected. At idle I get some readings (800Khz) when I place the antenna real close (within 15cm of the board) The board is not shielded at all, just playing openly on my desk. When I move the antenna outside the 15cm radius the signal disappears almost instantly...

Have a look:

The setup:

The amp: (with IRFP4228 and all parts chosen by me, the stock version will be somewhat different)

Pretty close up (aprox 20cm from the amp, you can see the speaker cable right under the antenna. We may conclude that the output filter works)

With the antenna poking right into the amp: (1cm away from the toroids, here we can see a clear 800Khz peak)

With the 1Khz tone at 10cm distance:

At 12cm running at idle:


I hope you guys find it interesting [^]

Next up is THD measurements...


------X-max. can be several meters on any driver, too bad it can be done only once...------

[krilli]

Yep, it is interesting

[mikechw]

I found it amazing....

[longitude]

V-bro,
I know you have several other 41hz amps in hand,
I hope you plan to do the same measurements with them too.
I really like to see the result.

[V-bro]

Hi guys, yes I sure will!

To make some people happy (and try them out myself)I'm planning research on designing aircore inductors that show no (or low) leakage...

Funny detail by the way that big yellow 200W dummyload resistor there on my desk was driven pretty hot while the amp (with IRFP4228) was hardly breaking a sweat (1khz sine wave, aprox 1A current draw per rail but driving just one channel...). The amp was playing without heatsink!

I have used a Fluke IR thermometer gun and measured the fets and chip being 33 degrees celsius for the fets, 40 degrees for the chip. 2,2ohm Rg resistors driving the gates, BYV27-400 discharge diodes.


------X-max. can be several meters on any driver, too bad it can be done only once...------

[col_s]

It's good that there is not much EMI radiated from the board itself, but I wonder if the EMI would increase if you had a heatsink attached, as heatsinks can act like antennas. Also things like chassis and wiring also create antennas. You need two things to create radiated EMI: some noise and an antenna (eg. a wire, a pcb trace, a heatsink).

[V-bro]

Thanks Colin, I'll try that!

------X-max. can be several meters on any driver, too bad it can be done only once...------

[djQUAN]

what's your supply voltage? my very old amp2 can't run for more than a few secs without a heatsink on the fets even when idle. ran it at +/-47V.

[ivo]

Just to let you know my experience with Truepath. I have two Truepath modules and one Redrocks SMPS in on full Aluminum Case (so lots of cables/antennas connected to it). When I switch on the amp my wireless mouse and keyboard loses connection (the cordless access point is just laying next to the amp). I actually have to move the access point very close (about 50cm) to the mouse and keyboard to make it work again. My understanding is that these cordless desktops work in the 2.4 GHz range. Could it be that there is much more EMI radiation in this frequency range? Maybe we should build our amps according to Tripath Application Note AN17 (DOS AND DONTS WHEN DESIGNING FOR EMI COMPLIANCE) to avoid this kind of problems?
Cheers
Ivo

[V-bro]

quote:Originally posted by djQUAN

what's your supply voltage? my very old amp2 can't run for more than a few secs without a heatsink on the fets even when idle. ran it at +/-47V.

+/-61Vdc

Get some other fets!