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.
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.

Boost circuit is easy to handle, if you have any monitor shutdown problem, just remove the fet and see if it still shut down. If it ok suspect a wrong signal at the gate of the fet. If it still shutdown check the b+ voltage and the high voltage circuit which include the flyback transformer, safety capacitor and etc. Typical part n umber of fet used in the boost circuit are IRF630,IRF640,K1507,IRF740 and generally from the n-channel mosfet. The buck circuit fet are IRF9610,IRF9620,IRF9630,J306,J448,J512 and etc. They are generally the p-channel mosfet.

Get an electronic books and search for the buck and boost circuits to familiarize yourself with the components arrangement. Once you fully understand how this circuit work, then troubleshooting monitor will be a lot easier.