N-Channel Mosfet Linear amp Open Circuit Cross - Conduction

[Workhorse]

Hi ,
Recently I came across a Friend's Mosfet N-Channel Class-AB linear amp similar to Anthony's holton amp .
When the amplifier is fed with a signal sinewave or music , With Output not connected to any speaker or load i.e open circuit operation.At increasing the input signal from zero to a finite value the amp starts heating and consumes lot of current and dissipiates the energy in the form of heat without any load connected to its output.
Are these are signs of Cross-conduction or something related to biasing problem.OR not a Problem At All.
VCC=+- 56VDC
VGS=3.9VDC
I[drain quiescent current]=75mA

With Regards
AmPmAn

[hunter audio]

hi

i guess

1) it may be oscillating or

2) check the output in an scope make sure the pcb design is not arising any spurious hi frquency wave form at the input

3) constantly monitor quiscent current - see if it is not running away

i have yet to try out a Holton design though every one here says they are quite stable is this the the buz circuit in the audio vedio special - then the ntc is not of the correct value

any way take care

suranjan

[Workhorse]

Originally posted by hunter audio
hi

i guess

1) it may be oscillating or

2) check the output in an scope make sure the pcb design is not arising any spurious hi frquency wave form at the input

3) constantly monitor quiscent current - see if it is not running away

i have yet to try out a Holton design though every one here says they are quite stable is this the the buz circuit in the audio vedio special - then the ntc is not of the correct value

any way take care

suranjan

Hi HUNTER,
I think the Feedback factor is not adequate .
The biasing is very stable indeed.Nor there are any oscillations occuring in it.
Whereas, In my own professional design amplifiers I use a Anti parallel Diode with gate resistor to over come any cross- conduction between upper and lower mosfets.

With Regards
AmPmAn

[subwo1]

I think I noticed on many n-channel AB amps that since the gate pull-down resistor usually supplies less current than the transistor driven pull-up, as the input frequency is increased, the amp operates more like class A. I could be missing something.

[sajti]

If the heat caused by cross conduction, You have no heat if You drive the amp with low frequency sine wave - say 100Hz. And if You apply high frequency square wave (10-20kHz) You will get more heat. Try this test.
If the result is more heat with high frequency try to decrease the gate resistors (I used 22ohms with good result). If it doesn't help, increase the current of the driver stage.

sajti

[tschrama]

Hi,

I've had done some investigations in cross-conduction in a N-ch MOSFET JLH-amp and I agree with sajti.. cross-conduction should be freq dependend. However I could not decrease the gate-snoopers below 150 Ohm, so be carefull. Adjusting the gate-snoopers might help, but you need to monitor the currents in you circuit with a osci to do that propperly.


Goodluck,
Thijs


PS
Does anyone have the schematic of the N-channel amp of aussiesamps? I was availeble for public, bus hasn't been updated and is still missing.

[Workhorse]

At input sinewave 50 Hz , it also shows cross conduction.
Regards
Ampman

[tschrama]

At input sinewave 50 Hz , it also shows cross conduction.

hmmm then something is wrong. Could you post a schematic?

[Workhorse]

The circuit is similar to Holton's circuit.
The R20 is creating cross-conduction to an extent, when i eliminated it the cross conduction also lowers.
In my Professional circuits our N-channel amps[which are our own creations] use Antiparallel diode with gate resistors to overcome this cross-conduction.
With Regards
Ampman

[tschrama]

Hi,

Thx for the schematic. I would also think R20 is the key. This resistor ensures that both output halves are driven from the same inpedance. This is important as the output mosfet's capacitances have to be charged and dis-charged at equal speed. Well that what I think, and did when building my MOSFET JLH 10W Class A, which has a similar output stage.

Adjusting this R20, while observing the current through both output halves, amp driven with a square wave will make it easy to minimize cross-conduction.

Bye,
Thijs