Understanding Monitor B+ Field Effect Transistor Circuit 

I have quite a number of questions from my repair friends and students about a field effect transistor (fet) connected in the path of B+ circuit. That part is actually either a "buck" or "boost" circuit. If you look around the Fet, you will also find a coil, a diode and a filter capacitor. The coil i call it as B+ coil. Depending on how they are arranged, they can "boost" a b+ voltage or "buck" a voltage down. Most of the monitor that i repaired are from the boost circuit design! Tv usually do not have this circuit because they are running on a single resolution.

True case example: Nec monitor came in with high voltage shutdown, upon checking found the b+ line too high. How?-the filter capacitor is a 220 uf 160 volts and the line voltage measurement that i measured also around 160 volt! This can't be and the voltage should be lower than 160v. If i continue to switch on the monitor for few more times i guest the capacitor will blow due to the peak voltage that the capacitor cannot handle. The caused of the increase of the b+ line voltage was a bad electrolytic with high esr located at the feedback circuit. 

Some monitors start with a high b+ (160-200volt) and buck it down depending on the mode (640 x 480, 600 x 800 and etc) some manufacturers prefer to start with a low B+ voltage (60v) and boost it up. The higher the monitor resolution the higher the B+ voltage which is provided by the circuit. This circuit is located in between the switch mode power supply and the flyback transformer and horizontal output transistor (HOT) so it is very often gets damaged by a shorted HOT. Anytime if i found a shorted HOT i will automatically search for this circuit and check the components (FET,B+ coil,diode and filter capacitor). As for me the buck circuit is a little bit dangerous. Why? because whenever the fet developed a short circuit , the high voltage of 160v-200v cannot be pull down and send the voltage straight to the input of flyback transformer. This will cause the high voltage to suddenly shoot up to more than 30 Kilovolt and sometimes produced a loud 'BANG' before the monitor goes into shutdown mode. In few cases it might blow some of the components in the monitor circuit.

Conclusion-The working voltage of a capacitor is a good indicator to find out the voltages of a particular circuit. Though we can't get the exact voltage at least it save our time in the guessing game as what the output voltages should be.

Author By : Jestine Yong