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rev : 1.1 1 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p general description the n3856 is a low cost , high efficiency, full featured ,synchronous re ctificati on control l e r that specifically des igned for the synchronous rectification applications of the flyback ac/dc pwm mode switching power supply . the n3856 is included a totem pole output ideally suited for driving a synchronous rectifi- cation power mosfet . 2 way of the current detect provided for wide applications by any continuous or disco tinuous mode operating (patent pending).in cont inuous mode operatin- g, it included a timi ng control to adjust maximum turn-on duty to allow the designer to determine the optimal condition for the best efficiency by the external resistor and capacitor. in disconti nuous mode operating, it included a very sensitive current detector (about 20mv) to cont rol the synchronous rectification mosfet , avoid happening a large reverse current via the synchronous rectification mosfet to damage the device. fea t ures high ef ficiency operating. adjustable max. duty cycle designed. decrease areas of heatsink or pcb. solved heat dissipation. sensitive current detecter(about 20mv) suited for continuous and discontinuous mode operation. auto frequency tracking with pwm frequency. suited for fixed or variable frequency soic-8 and dip-8 package. application flyback ac/dc power supply device selection guide dip- 8 soic- 8 n3856pg N3856Vg absolute maximum rating parameters symbol l imits units vcc to gnd vcc -0.3 to 30 v surge voltage of vcc(below 200ns) vcc (surge) 45 v p-p drain to other pin 180 v power dissipation at ta = 25 j ,derate 8mw/ j for ta ? 25 j p d 725 mw operating junction temperature range tj -40 to +150 j storage temperature range t stg -65 to +150 j lead temperature (soldering) 10 sec t lead 300 j http://
rev : 1.1 2 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p electrical characteristics (t c = 25 c) limits parameter test conditions min typ max unit supply voltage vcc 7 12 30 v supply current - 4.5 - ma gate sink current 1 a gate source current 1 a rise time c l = 1.0nf 40 ns fall time c l = 1.0nf 40 ns delay time 200 300 ns max duty cycle 90 95 % block diagram vcc driver - + gate aout 1/3 vcc rt/ct osc comp. 10k detecter bias 20mv d gnd buffer drain current pgnd adj. slope
rev : 1.1 3 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p operation description ?discontinuous mode in discontinuous mode application, the n3856 is detecting the mosf et current to control the mosfet be on or off by a inte rnal current detecter , in disc ontinuous mode , the current of the secondary winding flows thr ough the source and drain of t he mosfet is changing from maximum level to zero , it cause a voltage drop of the mosfet from the maximum level down to zero , the voltage drop is compared with internal refe rence voltage 20mv, if voltage dropout is over 20mv ,will make the mosfet tu rn-on, if the voltage drop is under 20mv, will make the mosfet become turn-off. for the various mosfet applications , different r ds(on) will cause a different voltage drop between the mosfet,s source and drain , the bias pin provided the ?adjustable slope? function delaying turn-off the mosfet for bette r efficiency , bais pin connected a resistor with vcc used to adjust the gate output slope , change the resistance will get the different slope ,a higher resistance will get the smoother slope, and mosfet turn-off become slower , a smaller resistance will get the sharper slope , and mosfet turn-off become quicker . considering of bias setting, first, adjust output to heavy load but still in discontinuous mode , changing the bias resistor make the gate wa veform of mosfet is sloping to connect the resonant waveform of lc tank of the output circuit, second,adjust output to the minimum load and check the gate waveform is ok or not , to decrease the resistance until waveform is normal . ?continuous mode in continuous mode application , the current det ecter can not turn-off the mosfet directly, the n3856 provided two way to turn off the mo sfet in good time, one is the ?maximum duty control? function by rt/ct ,in low line ac i nput voltage and maximum loading to adjust rtor ct will determine the maximum duty and timely turn off the mosfet, another is using a synchronous signal from the primary side to c ontrol the aout pin , pull aout pin to low will turn off the output of gate driver . aout pi n can be used to increase gate drive capability when two parallel mosfet application , connected a resistor to vcc if need .
rev : 1.1 4 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p typical application circuit a. non-sy nchronous signal ty pe the non-synchronous signal type is designed for a low cost flyback power supply . the structure is suggested to design in discontinuous mode for high line input voltage and low line input voltage in continuous mode that can get better efficiency for full range input design . in this structure, to seclect small tollerence(1~2%) of some componets will be necessary, such as rt/ct of n3856 ,and the frequency componets rt / ct of the pwm controller . otherwise , it may cause q1and q2 turn-on at same time. dead time = t turn off time (min) x ( total tol.% ) ? t d(off) of s.r. mosfet total tol.% =(rt +ct + rt+ ct )tol.% q2 ----off q2 gate q2 --- on q1 gate q1--on q1-- off dead time delay time ? t d u t y max. duty t turn off time (min) q2 drain ? t duty : duty variation from the ac ripple volatage dead time don?t cross q1 duty s h ift this line
rev : 1.1 5 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p d3 sb3100 d2 + c5 ct 470p r9 c4 r8 u2 tl431 pwm controler rt 51k rt,ct are removed and pin7 of u1 connects to pin3 of u1 if make sure power is aways working in discontinuous mode . r4 r5 b. d1 1n4148 + c6 r10 c2 470p + c3 a. vo vcc u3 pc817 1 2 4 3 r1 16k u1 N3856V 1 2 3 6 4 7 8 5 gate pgnd gnd aout bias rt/ct vcc drain q2 r6 rt,ct is requested for determining the tur n on duty in continuous mode applications. t1 1 5 6 4 8 r7 q1 g d s schematic 1 non-sy nchronous signal ty pe w i th aux w i nding application schematic 1 is a non-synchronous signal ty pe with aux winding application , the aux winding is designed to provide the supply vo ltage about 10v ~15v , the supply voltage via the diode d1 to vcc of the u1 without any capacitor . in the continous mode, to change the values of the external resistor rt and capacitor ct that can set the fixed maximum turn- on duty for the synchronous rectification mosfet. in the discontinous mode , current detect pin 5 can sense the voltage drop of the mosfet , if it is under 20mv ,this ic will turn off the mosfet .
rev : 1.1 6 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p b. sy nchronous signal from primary side the structure is designed for a high power , high performerce flyback power supply , the synchronous signal is provided before the mosfet q2 turn on from the triangle wave of the pwm controller , it need to use a isolation transformer between the primary and the secondary for safety request .when the circuit is designed in continuous mode , the mosfet of the secondary side to be turned off by a synchronous signal from the pr imary side , if works in discontinuous mode that turned off by the current detect pin 5 to sence the voltage drop between the source and drain of the mosfet . pwm controler q1 g d s d3 1n4148 + c5 u1 N3856V 1 2 3 6 4 7 8 5 gate pgnd gnd aout bias rt/ct vcc drain + c3 q2 q4 2sa733 q5 2sc945 t1 1 5 6 4 8 c6 0.1u t2b r6 r10 d1 1n4148 vcc r1 16k vref r7 u3 pc817 1 2 4 3 r9 c3 u2 tl431 r3 1k r5 + c4 vo r8 t2a sync x'frm * c2 470p ct d4 q3 2sc945 * r4 d2 sb3100 schematic 2 sy nchronous signal ty pe w i th aux w i nding application schematic 2 is a synchronous signal type with aux winding application ,in this application , the pin 7 is connected to pin 3 of this ic to cancel maximum turn on duty function because the mosfet can get best control from the synchronous signal .
rev : 1.1 7 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p typical characterics pic.1 secondary w i nding output and gate voltage of mosfet w aveform at discontinuous mode condition . pic.2 secondary w i nding output and gate voltage of mosfet w aveform at continuous mode condition .
rev : 1.1 8 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p pic.3 secondary w i nding output and gate voltage of mosfet w a veform at no load condition .
rev : 1.1 9 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p pin configurations 2 bias vcc rt/ct 3 aout 4 drain 5 6 7 8 top view sop-8 & dip-8 1 gate pgnd gnd pin functions no. function description 1 gate this pin is the output pin to drive the gate of the power mosfet. 2 pgnd this pin is to power gnd for output drive circuit. 3 gnd this is signal gnd pin for the control logic signals. 4 bias this pin is bias voltage supply for the internal current detector. 5 drain input pin of the internal current detector, connect to drain of mosfet . 6 aout this pin is internal comp arator output to control ?pin1? output duty . 7 r t /ct this pin is connected an external resi stor and capacitor to set the fixed maximum turn-on duty. 8 vcc this pin is for supply voltage.
rev : 1.1 10 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p
rev : 1.1 11 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p soic-8 (d) mechanical data mm mm dimension min. typ. max. dimension min. typ. max. a 4.8 4.9 5.0 h 0.5 0.715 0.83 b 3.8 3.9 4.0 i 0.18 0.254 0.25 c 5.8 6.0 6.2 j 0.22 d 0.38 0.445 0.51 k 0 4 8 e 1.27 l f 1.35 1.55 1.75 m g 0.1 0.175 0.25 n
rev : 1.1 12 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p demo board description the schematic 3 presents a demo board for t he N3856V , this board replaces the output rectifier diode with synchronous rectification mo sfet in flyback power supply and includes all the components needs by the N3856V to operate, r3 and c1 can be adjusted for diffferent switching frequency and turn-on duty demand vcc r3 51k c2 470p r1 16k gnd gate c1 470p u1 N3856V 1 2 3 6 4 7 8 5 gate pgnd gnd aout bias rt/ct vcc drain d1 1n4148 drain . pic.4 demo board lay out----componects
rev : 1.1 13 dec-14-2004 synchronous rectification controller niko - sem N3856V n3856p pic.5 demo board lay out----soldering side

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