Re: Benq 17" Q7T3 fix
I have been waiting for parts to complete the short circuit protection of this monitor by causing it to trip in 10uS or less.
Whilst waiting I happened to notice another circuit flaw (see previous posts for first flaw). Again reinforcing the previous view that a student designed the electronics.
Benq Q7T3 (FP737s) LCD monitor design flaw.
MOSFET drive circuit.
Refer to the attached diagram.
Inspection of the circuit shows that there is no current control through transistors Q753 & Q752. They are directly connected between the +15 volt rail and ground there is nothing to limit the current between the supply rails (only the ill defined current gain of Q753). With Q752 saturated ON drive to Q753 is about 4 mA, a current gain of 50 makes about 200mA Ic (gain of 100 = 400mA) and the voltage across this transistor is about 13 volts (between 260 and 520mW x duty cycle).
The same current flows through Q752 that has an absolute max current of 150mA.
Additionally the absolute max current for the Ic drive is 21mA.
Clearly these components have survived but it is a poor design without a current limiting resistor between supply rails.
This problem has been seen before where there is a focus on switching speed more than other parameters. Not a good practice for reliable design.
The design should guarantee absolute maximums are not exceeded.
A 100 ohm resistor inserted at the emitter of Q752 will ensure that 150mA is not exceeded and likewise the 21mA IC load is not exceeded.
How does this mod affect performance? Increasing the MOSFET gate source resistance will increase the turn ON time by about 150nS when switching 7.2amps (based on data sheet). However the circuit max operating current is 1 amp so the increase in turn on speed and switching loss is much less significant. The switch OFF speed will not be affected because the 100 ohm is not part of the switch off circuit.
To conclude the insertion of 100 ohm will not impact performance significantly but will make sure current through devices and across the supply rail is controlled.
I have been waiting for parts to complete the short circuit protection of this monitor by causing it to trip in 10uS or less.
Whilst waiting I happened to notice another circuit flaw (see previous posts for first flaw). Again reinforcing the previous view that a student designed the electronics.
Benq Q7T3 (FP737s) LCD monitor design flaw.
MOSFET drive circuit.
Refer to the attached diagram.
Inspection of the circuit shows that there is no current control through transistors Q753 & Q752. They are directly connected between the +15 volt rail and ground there is nothing to limit the current between the supply rails (only the ill defined current gain of Q753). With Q752 saturated ON drive to Q753 is about 4 mA, a current gain of 50 makes about 200mA Ic (gain of 100 = 400mA) and the voltage across this transistor is about 13 volts (between 260 and 520mW x duty cycle).
The same current flows through Q752 that has an absolute max current of 150mA.
Additionally the absolute max current for the Ic drive is 21mA.
Clearly these components have survived but it is a poor design without a current limiting resistor between supply rails.
This problem has been seen before where there is a focus on switching speed more than other parameters. Not a good practice for reliable design.
The design should guarantee absolute maximums are not exceeded.
A 100 ohm resistor inserted at the emitter of Q752 will ensure that 150mA is not exceeded and likewise the 21mA IC load is not exceeded.
How does this mod affect performance? Increasing the MOSFET gate source resistance will increase the turn ON time by about 150nS when switching 7.2amps (based on data sheet). However the circuit max operating current is 1 amp so the increase in turn on speed and switching loss is much less significant. The switch OFF speed will not be affected because the 100 ohm is not part of the switch off circuit.
To conclude the insertion of 100 ohm will not impact performance significantly but will make sure current through devices and across the supply rail is controlled.
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