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mosfet amp transistors getting HOT!

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MrDEB

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using circuit posted by Audio Guru that should work as he is almost always right BUT? I wonder if a mis caculation on R9 and R11 are correct.
The two get very hot.
something isn't right?
I checked all my connections. Took the pic out then rechecked voltages. Seems like the transistors in question got hotter but ?
any suggestions before I replace Q7 and Q8. Q8 seemed the hottest.
 

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I pasted but cut to much out
for testing I have 9 v at R1/R3 junction (12v final design)
and output from a pic at R2 / 10k resistor
 
That H-Bridge is a disaster! As it switches, the shoot-through current from the positive supply through the PFET and the NFET to ground is on the order of 120A because the gates are driven from the same (slowly changing) voltage. The slew rate of the gate drive is limited by the gate capacitance of the the two FETs, and the whimpy current that can be sourced by the 1K pull-up resistor.

To do this right, go buy an H-Bridge driver IC. It solves the problems of making non-overlapping gate drive signals.
 

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You have the gate of Q1 not connected to the collector of Q7.

R9 has a current of only 1.2mA. Then it dissipates (1.2mA squared x 3300 ohms)= a whopping 0.0048W if it is turned on all the time. But since it is turned on only for half the time then it dissipates an average of only 0.0024W. How can it be hot?.

R11 has a current of 1.5mA. Then it dissipates a whopping (1.5mA squared x 3300 ohms)= 0.0074W. But since it is turned on for half the time then it dissipates an average of only 0.0037W. How can it be hot?

Mosfets oscillate at VHF frequencies which makes them hot if they are in a circuit built on a breadboard or do not have a series 10 ohms to 47 ohms resistor in series with the gate mounted very close to the gate pin.
Also, when switching, all Mosfets are turned on for a moment then they short circuit the power supply which makes them heat a little.

EDIT:
You will never get a shoot-through current of 120A from a whimpy little 9V battery or from a weak little wall-wart.
 
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I see where I missed another connection.
Hopefully the transistors are not destroyed? shouldn't hurt the Mosfets hopefully?
When I designed a board I missed several connections 1 ground and 1 Vdd
this is why they are proto types.
Will get back to you after making correction.
THANKS
 
...
You will never get a shoot-through current of 120A from a whimpy little 9V battery or from a weak little wall-wart.

What a poor excuse for not doing it right! How long do you think the whimpy 9V battery is going to last if you effectively short it twice during each switching cycle? How much current do you think a weak little wall-wart will put into a dead short?
 
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Q8 still getting HOT

fixed Q1 to Q7
replaced Q8
Q8 still getting HOT?
need to go to the bank etc will get back later.
Seems like every time I hit a wall, just STOP and come back later and all seems to go right
hopefully this will happen.
 

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Q8 has a 1k load. If the supply is 9V and Q8 saturates with a Vce of 0.2V then it dissipates (8.8V/1k) x 0.2V= 0.0018W if it is turned on continuously or 0.0009W since it is turned on for only half the time. It will also dissipate a little more because it is discharging the fairly high gate capacitances of the Mosfets.
 
my next plan of attack is to breadboard the amp and connect a 555 as input signal.
Could be something with my pcboard that I am failing to see. Won't be the first time.
I used LT spice as did Mike and don't know how he came up with such high current draw?
One thing I noticed is his R1 has 12v connected to it. supposed to be 5v
 
You should connect a voltmeter in place of the speaker. With the input connected to ground, note the DC voltage reading. It should be nearly the supply voltage, but either positive or negative depending on which way the meter is connected. Connect the input to +5v and the voltmeter should read almost the same voltage but opposite sign.
 
going to bread board but thinking maybe just maybe R1 may be shorted. that might cause Q8 to get hot?(all the resistors are smd.
still going to bread board and maybe find / learn something?
looked the pcboard over several times. Maybe a misplaced trace that only a blind guy can see , seeing how I can't find anything wrong??
will try the voltmeter after I breadboard.
 
definatly a shot as ohm meter indicates

now need to locate but nothing else connected to excess copper but ??
 

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The top end of both 1k resistors MUST be tied to the same voltage as the Source of the PFETs. If they are not, then the PFETs will stay on!

It matters not if my R1 (3.3K) is returned to 5V or 12V. At 12V it dissipates a max of E^2/R = 12*12/3300 = 44mW.

If you are messing with surface mount components, I'll bet you just mixed up the resistors.

When you simulated it, did you use some "real" parts (ones that have similar On resistances, threshold voltage, gate capacitance), or where you just using the default models. The default models are "tiny" devices, and will give unrealistic results.
 
Running traces between pads on surface mount parts is generally a bad idea, especially when the board will be soldered by a non-experienced person. The trace between the 22k pads at upper left could be a problem. Worse are the little slivers of ground plane left between some of the pads - R6, R5 and R10 are examples. A short to one side of R6 could be the problem. Jp4 seems not to be connected to anything.
 
just got back from camping w/ LOML.
Looking at board and referring to what Jon stated, I think he may have hit on my problem of little sliver of traces between smd.
next board I am not going to use a filled plane.
hopefully the short I indicated is why Q8 is getting hot!
 
Found a small trace that was shorting.
I connected a 200ma wal wart and tweeter sounded very weak but it was out putting some sound.
Connected to car and got magic smoke.
I got to looking and taking someones idea and found an IC that takes the place of the 4 mosfets.
https://www.electro-tech-online.com/custompdfs/2010/07/ZXMHC6A07T8.pdf
Wonder if this approach would be better for this app?
just one component verses 4. Looks like it will handle 1.8 A but just how much is this circuit handling. According to Audioguru only 1.8ma?
 
May have found MAJOR problem!

I STOPPED, STEPPED BACK and RECHECKED everything.
After looking over the pcboard layout then comparing the pin out with the data sheet I think I discovered a major problem with the pcboard software. Having never worked with Mosfets I am looking for a second opinion to recheck my conclusions.
Was also wondering if using a QFET (FQP7P06) P chn Mosfet and a UniFET( FDP55N06 N chn Mosfet) could be a problem?
pictured is Q1 with R3 connected to pins 2 & 3
 

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Putting the mosfet amp on trial

Using the CRITTER RIDDER while driving the highways in Idaho is a pretty good test.
NO DEER, birds can't fly away fast enought
BUT!!!
the mosfets connected to the 12v supply (the top two in schematic) still get warm to hot but only after running for 30 miniutes.
Someone mentioned an IC to do the same more efficiently or maybe add a heat sink.
only the P mosfets are getting warm to hot.
perhaps bolting the TO-220 case to a copper plane on the board or ?
need to determne if the tabs can be connected together to one long aluminium strap?
not much room in enclosure.
Want to get this solved as easy and quick as possable. I am friends with the Lemhi County sherrif and he wants to install on one of his deputies cars for testing.
Hoping to solve using bar stock as I am on vacation and want to try out again on way back home in two weeks.
 
You have a powerful transistor driving the gates of the Mosfets in one direction but a slow 1k resistor trying to charge or discharge the gate capacitances in the other direction. So there are moments when the top and bottom Mosfets are turned on a little (10A or more of current) making heat. The current spike is called "shoot-through".
If you speed up the switching by reducing the value of the 1k resistors then the heating is reduced. Try 330 ohms.

A heatsink has a large surface area usually by using many fins. A bar of metal does not have a high surface area so will not work well as a finned heatsink.

The tabs on the P-channel Mosfets are the active drains so they cannot be connected together.
 
will put a 1k resistor in parallel with the 1k 1206 smd resistor I have on the pcb. EASY FIX. Can't find an electronics store here that even caries smd resistors.
on the heat sink was thinking of cutting slots to creat fins. Was hoping the two P channel mosfets could be bound together with the TO-220 tab.
QUESTION, why dosn't the N channel mosfets seem to get warm?
another thought to aid cooling is to put some holes in each end of enclosure but ONLY as a last resort.
thanks for the advice. Audioguru comes through again
 
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