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F5 Turbo Builders Thread
F5 Turbo Builders Thread
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Old 7th April 2018, 11:13 PM   #5121
stereohifi is offline stereohifi  Canada
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Default F5 Turbo V2

Upon buying all parts , boards, cabinets, I've went almost to over 50% of this fantastic 511 pages thread and noticed some issues that other members came up to.

I was in fact wandering if a kind of items or revision that a new builder should look at before soldering his boards.

I tooked some noted and like to share ;

- Thermistors from post # 4885, insulation of thermitors to ground is very important.

- Oscillation problem, add some 3n9 or 4n7 small caps to R7 /R9.

- Soft start and DC offset protection circuits

-Bias R5/R6 resistor mod at 1,3Kohms.

Off course I might have missed some other considerations, but think that some experimented members could add some more information is quite welcome.
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Old 22nd April 2018, 11:43 AM   #5122
Zatopek is offline Zatopek  Germany
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Angry Unbalance of P-channel vs. N-channel

Hi all!
I have 90% completed my V2 and tested it by sound and am more than happy with that. One severe problem holds me from closing the box:
Bias so far is set to modest about 850...900mA, both channels. All 4 output boards have separate and identical heat sinks which should be OK for a bias of 1A easily. Touching the sink of the 2 P-channel boards, the temperature is lukewarm and I can touch it forever which is as expected at 800-900mA. Touching the N-channel boards they feel really hot and I have problems holding my hands there for such 5 sec's which NP is teaching us.
I have measured bias for all 4 boards separately directly at the 1Ohm resistors (rather than TP's) and they are all reading same current within few mA. Also supply voltage is identical to 0.1V both polarities and both channels. The devices are IRFP240/IRFP9240 from VISHAY. I have matched Vgs such, that Delta is around 0.1V on every board. Even the Delta beween P and N is <0.2V on each channel. DC-voltage at output is around 10mV, going up/down slightly over time.
So suspicion is, that the ON-resistance of the P- and N-devices is significantly different, heating up the N's more than the P's.

Anyone having had same symptoms?? Any suspicion about root cause? Any good advice of a therapy????
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Old 22nd April 2018, 11:52 AM   #5123
Zen Mod is offline Zen Mod  Serbia
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F5 Turbo Builders Thread
thermal interface between mosfets and heatsink must be uniform

what you're using ?
all screws tightened in same amount ?

find a way to measure temperature ; difference between mosfet case and heatsink just near by can't be higher than 10C

and yes , naive question - are positive and negative rails of same value ?
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Old 22nd April 2018, 12:09 PM   #5124
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
That doesn't sound right. If you have the same voltage and current on the P and N side, the power and thus heat dissipation should be the same. If the transistors are not mounted properly on the cool heatsink then those transistors themselves would likely burn up because the thermal resistance to the heatsink would need to be much higher for it to be that much cooler than the second channel. I would check the source resistors and make sure they are all the correct value. If the ones on the cool heatsink are much higher in value it would cause the difference you are seeing. Also verify that you have no open connections or short circuits on the boards.
On resistance difference of the FETs would not change the power dissipation but changes in the source resistors would because you are using the voltage drop across them as a means of measuring the current.
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Old 22nd April 2018, 12:18 PM   #5125
6L6 is offline 6L6  United States
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F5 Turbo Builders Thread
Zatopek - what is the DC voltage measured across the N- and P- channel source resistors?
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Old 22nd April 2018, 01:08 PM   #5126
Zatopek is offline Zatopek  Germany
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Default Unbalance of P-channel vs. N-channel

Thanks for supersonic return!
Answering questions first:
* Rail voltage is exactly + and -36.5V with load, pretty stable.
* All source resistors are 2x1Ohm ll = 0.5Ohm. This is measured yesterday evening, but only N-channel boards. I will check the P-channels later today making sure, they dont differ, causing the imbalance. The P-channels are really "cold" so this might be the reason, but why both P-channel boards??
* Heat sinks are 4 times 300x165x40mm each one for 1 board only! They are 0.31°C/W. Mounting situation slightly differs but shouldn't account for such drastic temp difference. If the devices would not be tightened properly, the FET's on the hot sink should be cool and the ones at the cool sink should be hot. That's not the case!
* DC-voltage measured at source resistors is around 420-450mV, about equal for all 4 boards and measured in 6 places. Will check the remaining 2 test points later today.
Thanks again.
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Old 22nd April 2018, 01:10 PM   #5127
Zen Mod is offline Zen Mod  Serbia
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Old 22nd April 2018, 01:14 PM   #5128
6L6 is offline 6L6  United States
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I think perhaps the diodes are starting to conduct on the N- channel side...? What diodes did you use?

And as Zen Mod asked, please post some well-lit, in-focus photos. Thanks!!
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Old 22nd April 2018, 01:29 PM   #5129
Zen Mod is offline Zen Mod  Serbia
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also exact measurements can reveal much

skin feel can lie sometimes

borrow any sort of temperature measurement gadget , preferably fun gun looking laaaaazer thingie
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Old 22nd April 2018, 01:31 PM   #5130
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