How to repair pulse power supply circuit using top254pn ?


Top254pn is a very popular power supply ic in pulse power supply circuit. Usually , we can see TOP254PN in power circuit of air conditioners, televisions, washing machines and other household devices. One of the most common errors of household device electronic circuit is can not start the machine when you supply input voltage 220VAC or 110VAC. There are many cases cause this problem, but the first step of solving this problem is checking pulse power supply circuit. Usually, you will see a TOP254PN in the pulse power supply circuit. So today we will study how TOP254PN works in pulse power supply circuit and how to repair a electronic circuit using TOP254PN.

The functions of TOP254PN pinout

DRAIN (D) Pin:
High-voltage power MOSFET DRAIN pin. The internal start-up bias current is drawn from this pin through a switched highvoltage current source. Internal current limit sense point for drain current

Error amplifier and feedback current input pin for duty cycle control. Internal shunt regulator connection to provide internal bias current during normal operation. It is also used as the connection point for the supply bypass and auto-restart/ compensation capacitor

MULTI-FUNCTION (M) Pin (P & G packages only):
This pin combines the functions of the VOLTAGE MONITOR (V) and EXTERNAL CURRENT LIMIT (X) pins of the Y package into one pin. Input pin for OV, UV, line feed forward with DCMAX reduction, output overvoltage protection (OVP), external current limit adjustment, remote ON/OFF and device reset. A connection to SOURCE pin disables all functions on this pin and makes TOPSwitch-HX operate in simple three terminal mode (like TOPSwitch-II)

Output MOSFET source connection for high voltage power return. Primary side control circuit common and reference point
How does TOP254PN work in a real pulse power supply circuit?

We use input voltage 95VAC to 265VAC, it suitable for all countries in the world. This case we use 220VAC inout voltage. The 220VAC input voltage goes through F1 fuse and RT1 thermistor, these two components protect circuit from input voltage increasing, short circuit input. Then 220VAC goes to input noise filter block using C3 capacitor, L1 coil, r1 r2 resistors. Now input voltage is safe and no noise pulse, it goes to bridge rectifier d1,d2,d3,d4 diodes. The ouput voltage after bridge rectifier is made flat by C4 capacitor and it's around 300VDC. This 300VDC goes to a pulse transformer has 23 coil, 34 coil, 56 coil at primary side, and has 7 11 coil, 11 9 coil at secondary side. At pin 4 of primary coil connects to D pin of TOP254PN, This D pin is also D pin of the Mosfet inside TOP254PN. inside TOP254PN it has a mosfet and a pulse generator. This pulse generator connect to G pin of the mosfet and create pulse with a certain frequency turn on and off mosfet, so that create a induction current on secondary coils. If you want to understand more please read these previous articels we wrote below.

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Output voltage is depended on frequency of pulse generator inside mosfet and voltage after bridge rectifier. you can see the principle at this diagram. 

Mosfet switches on and off at a high frequency also creates a opposite current at primary coils, this opposite current will harm Mosfet and maybe damage Mosfet so We must a RCD structure block to protect mosfet from opposite current. At secondary coils, it also create induction current and converted to DC voltage by D7 diode SB560 , D8 diode SB530 , then C13 C14 C15 C17 C18 capacitors to make output voltage 5VDC 12VDC flat. also we use PC817 opto to isolate output circuit from input high voltage. Let's study how this feedback block works

At output voltage we create a voltage divider circuit include R20 R21 R18 resistors, when the output DC voltages change, the voltage of R21 resistor will change. This voltage of R21 resistor connects TL431 IC. TL431 has a reference voltage 2.5VDC, It means when inpout volatge of TL431 higher than 2.5VDC TL431 will open, when input voltage of TL431 lower than 2.5VDC, TL431 closes. When output voltages 5VDC 12VDC increase, the voltage Vr21 will increase higher than 2.5VDC, TL431 opens, PS2501-1 opto works, then Voltage of C pin TOP254PN will increase, that makes pulse generator inside TOP254PN change frequency to Mosfet, that will make input voltage down back to 5VDC 12VDC stable. When Vr21 lower than 2.5VDC, TL431 closes, PS2501-1 doesnot work, voltage of C pin decrease, the pulse generator will create a suitable frequency to stable output voltages to be 5VDC 12VDC. 

How to check TOP254PN broken or not and what are some tips repairing pulse power supply circuit?

in a case r18,r20 or r21 resistor broken, the input voltage of TL431 will be always higher than 2.5VDC, so PS2501-1 opto always open, so input voltage of C pin TOP254PN always be high, so the pulse generator inside mosfet always create wrong frequency to Mosfet. 

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