application note 1062 apr. 2011 rev. 1. 0 bcd semiconductor manufacturing limited 1 high voltage green mode pwm controller ap3105 prepared by wu qikun system engineering dept. 1. introduction the ap3105 is a low start-up current, current mode pwm controller with green-mode power-saving operation. different from ap3103, ap3105 pwm switching frequency at normal operation is fixed at 65khz dithering with a narrow range. the difference between ap3103/ap3105 is shown in table 1. the dithering of frequency will improve emi feature. when the load decreases, the frequency will reduce and when at a very low load, the ic will enter the ?burst mode? to minimize switching loss. a minimum 20khz frequency switching is to avoid the audible noise as well as reducing the standby loss. a so-called vcc maintain mode is applied under light load to realize a stable output and to reduce the loss on the start-up resistor. the standby power of the system using ap3105 can be reduced to 60mw at 230v input. ap3103 ap3105 frequency adjustable fixed at 65khz vfb resistor 4.5k ? 10k ? standby performance better best external protection na by ?ctrl? pin vcc ovp auto-recoverable latch /auto-recoverable olp & focp auto-recoverable latch /auto-recoverable table 1. the difference between ap3103/ap3105 the ap3105 integrates a lot of functions such as the lead edge blanking (leb) of the current sensing, internal slope compensation and several protection features which include cycl e-by-cycle current limit (ocp), fast ocp (focp), vcc over voltage protection, otp, olp protection. the ?ctrl? pin is designed for customers to add external protection functions such as ovp and otp. the ap3105 is specially designed for off-line ac-dc power supply, such as lcd monitors, notebook adapter and battery charger applications. it can offer the designers a cost effective solution while keeping versatile protection features. the ic uses sot-23-6 package type to realize its compact size. this application note includes detailed explanation of the ic?s major functions, some considerations about the pcb layout, and methods for reducing the standby power loss, and finally presents a demo design of a 12v 1a adaptor. 2. function description 2.1 ctrl pin for some applications, the system requires external programmable protection function. the ctrl pin has two kinds of modes to trigger the protection: high level trigger and low level trigger. the low threshold voltage is 0.5v and high threshold is 2.5v. when the ctrl pin voltage is lower than 0.5v or higher than 2.5v, latch or auto-restart protection will be triggered (different versions of 3105 offer different protection combination, which is shown in table 2). version vcc ovp olp& focp ctrl(low) ctrl(high) ap3105 auto- recoverable auto- recoverable latch auto- recoverable AP3105V latch auto- recoverable latch latch ap3105l latch latch latch latch ap3105r auto- recoverable auto- recoverable auto- recoverable latch table 2. version classification of ap3105 ctrl pin voltage maintains 1.6v if the pin is floating, so leave ctrl pin open if the designer does not need this function. once the latch protection is triggered, the bulk capacito r will provide the energy to the ic through start-up resistor to ensure the ic disable the output signal (latch mode). this mode will not be released until the ac input is shut off. therefore, the de-latch time is mainly depending on the value of hv startup bul k capacitor. if the system needs a short de-latch time, it is better for the startup resistor to take power from the point before the rectifier bridge. typical application of ctrl pin is shown in figure 1. note: 1. the sink current to the ctrl pin should be lower than 5ma by selecting a proper pull up resistor. 2. if the designer needs to apply a bypass capacitor on ctrl pin, the capacitor should not be more than 1nf.
application note 1062 apr. 2011 rev. 1. 0 bcd semiconductor manufacturing limited 2 ovp and otp ovp otp figure 1. ctrl pin application 2.2 fast ocp function when the load is short-circ uited, the power converter can be protected by olp protection. but if the output filter inductor and the secondary schottky is short-circuited, the transformer will be immediately saturated resulting in the breakdown of the mosfet due to high voltage stress. the ap3105 bears built-in fast ocp function to alleviate the saturation of the transformer and reduce the voltage stress of mosfet. the focp positio n and focp waveform are shown in figure 2 and figure 3. when the secondary schottky and the output filter inductor is short-circuited, the power converter can trigger latch or auto-restart immediately within several switching cycles with fast ocp. the focp threshold on fb pin is 1.8v. fast ocp figure 2. focp position diode shorten vcc fb sense figure 3. focp waveform 2.3 vcc maintain mode under ultra light load or load transient condition, v fb will drop to lower than 1.4v, thus the pwm drive signal will be stopped, and there is no more new energy transferred due to no switching. therefore, the ic supply voltage may drop to the uvlo (off) threshold and the system may enter the unexpected restart mode. to avoid this situation, the ap3105 holds a so-called vcc maintain mode which can supply energy to vcc. when vcc decreases to a setting threshold (10.1v), the vcc maintain comparator will output a drive signal to make the system switch and provide a proper energy to vcc pin. when vcc increases to 10.6v, the gate signal will be stopped. the vcc maintain function will cooperate with the pwm and the burst mode loop to make the output voltage variation be within the regulation. this mode is designed for reducing startup resistor loss and it will achieve a better standby performance with low value vcc capacitor and larger startup resistor. the vcc will not reduce to the uvlo (off) threshold during the startup process and under ultra light load or load transient condition. to avoid the ?vcc maintain mode? triggering in normal operating condition, it is suggested to design the vcc value higher than vcc maintain threshold under minimum load condition. the processing of vcc maintain mode is shown in figure 4. gate fb 10.1v 10.6v 1.4v load decreasing burst mode vcc maintain mode vcc figure 4. the process of vcc maintain mode 3. pcb layout consideration 3.1 high frequency loop consideration as shown in figure 5, there are four major high frequency current loops: 1. the current path from bu lk capacitor, transformer, mosfet, r cs returning to bulk capacitor 2. the path from gate pin, mosfet, r cs returning to the ground of ic
application note 1062 apr. 2011 rev. 1. 0 bcd semiconductor manufacturing limited 3 3. the rcd clamp circuit is a high frequency loop 4. transformer, r ectifier diode, and output capacitor is also a high frequency current loop the loops must be as short as possible to decrease the radiate area for a better emi, and if the mosfet and schottky diode have heat sink, the heat sink should be connected to their ground separately. figure 5. high current loop in addition, the ic should not be placed in the loop of switching power trace, and in some applications, the power ground could be crossed over by the control signal (low current and low voltage), but the switching power trace with pulsating high voltage should not be crossed over. 3.2 esd consideration electro-static discharge (esd) is an important testing item for switchi ng power supply. the system?s ability for bearing the test could be improved by designing a pa th to release the electric charge to the ground. as shown in figure 6, the red line represents the proposed path to release the charge. the copper tips for discharging should be placed between primary side and secondary side, but the distance between two copper tips should be consistent with the requirement of the safety specification . the input common mode filter and differential mode filter will affect the effect of transient discharging, so the copper tips should be added and their distance should be as short as possible. another way is placing a resistor paralleled with the inductor to replace the copper tip and the resistor?s value is about 1k ? to 5k ? . a smaller resistor is helpful to esd but has bad effect on lighting surge. ac cx1 l1 c1 r3 r6 ntc r8 r7 r9 r11 r14 r15 c8 r16 r17 r load c9 c2 bd1 c3 c7 c6 l3 c4 d1 d2 d3 d4 t u1 u2 u3 l2 r1 r2 r5 r10 r13 f1 rt1 c5 r12 fb sense gate gnd ctrl 6 1 3 4 ap3105 vcc 5 2 cy1 q1 figure 6. the path of release charge of esd
application note 1062 apr. 2011 rev. 1. 0 bcd semiconductor manufacturing limited 4 3.3 ground layout consideration a proper ?ground? layout is important to decrease unknown noise interference and emi issue in the switching power supply. a so-called ?star? connection is highly recommended for primary gnd. as shown in figure 7, the ground of mosfet, auxiliary winding, y-cap and control ic are separated and finally connected together on bulk capacitor ground. the width of these grounds should be kept as large as possible. the primary side of y-cap could also be connected to the high voltage pin of the transformer. bulk ground mosfet ground y-cap ground ic ground auxiliary winding ground figure 7. star connection of primary gnd 4. standby power loss reduction some methods are recommended here for reducing the standby power loss. 4.1 x-capacitor and x-resistor a good quality x-capacitor will be helpful to save the standby power, and a low value x-cap could also decrease the x-cap loss. acco rding to iec 60950, for the x-cap exceeding 0.1f, the voltage will be decayed to 37% of its or iginal value during an interval equal to one constant, and after calculating, the rc value is determined by the formula ?rc<1?. therefore, for a low valu e x-cap, a higher value x-resistor could be used, and the losses on x-resistor will be reduced. 4.2 current sampling resistor the value of current sampli ng resistor could affect the standby power. a lower value cs resistor is good for low standby power. but it also has effect on the olp result, a lower value cs resistor will make a larger olp point. 4.3 ?sense? pin rc value the value of ?sense? pin rc could also affect the standby power. a larger value of rc can make the i-peak sense signal and the voltage on ?fb? pin smaller. a smaller voltage on ?fb? pin will result in a lower operating frequency. it is good for achieving low standby power, but it will also make the olp point larger. 4.4 the output voltage dividing resistor the value of output voltage dividing resistor should be as high as possible, but the maximum value of the resistor connected to gnd (r17 in figure 6) should not exceed 15k ? . 4.5 primary rcd clamp circuit to get a better standby pow er, the rcd clamp circuit could be replaced by a transient voltage suppressor (tvs) and a diode (figure 8). the advantage of the tvs clamp is that it only conducts when necessary and it is independent of the switching frequency. compared to a rcd clamp, it reduces no-load power but increases costs and emi. besides, a lower value of rc is contributed to standby power, while the voltage stress on mosfet should be in the spec. figure 8. clamp circuit with tvs 4.6 secondary diode rc snubber a low value rc which parallel with schottky could be helpful to the low standby power. the value should be adjusted to make the voltage stress on shottky not exceed spec at turn-on. 5. demo design of 12v 1a adaptor 12v 1a demo using flyback topology is designed, and the system specification is as follows: ? output voltage and current: 12v/1a ? input voltage range: 90vac 265vac table 3 is the test result of the standby power, and table 4 shows the demo board components list. when the load is 12v/0a, the standby power is less than 60mw in the input voltage range under 230v. the power is measured by a power meter chroma 66202. figure 9 shows the application circuit schematic.
application note 1062 apr. 2011 rev. 1. 0 bcd semiconductor manufacturing limited 5 input voltage 90vac/60hz 115vac/60hz 180vac/50hz 230vac/50hz 264vac/50hz input power 0.032w 0.0345w 0.0463w 0.0605w 0.0742w table 3. test result r1,r2 res,1206,510k,j 2 r3 res,1206,10 ? ,j 1 r4 res,1206,0 ? ,j 1 r5 res,1206,1.5 ? ,f 1 r6 res,1206,10 ? ,f 1 r7,r10 res,1206,1.5m,j 2 r8 res,0603,1k,j 1 r9 res,0603,10k,j 1 r11 res,0805,10 ? ,j 1 r12 res,0603,24k,f 1 r13 res,0603,5.6k,j 1 r14 res,0603,100k,j 1 r15 res,0603,91k,f 1 r16 res,0805,0 ? ,j 1 r18 res,1206,3k,j 1 j1 base,3.96mm,3pin,3n2 1 j2 base,3.96mm,2pin 1 t1 x'fm,ef-20,102ts,1.6mh 1 q1 mosfet-n, 1a,600v,to-251,std1na60 1 c1 cap,y1,1000pf,250v 1 c2,c3 cap,al,10f,400v, 10mm, rubycon, yxa, 105c 2 c4 cap,cd,222m,1kv 1 c5,c6 cap,low esr, al,470f,16v, 8mm, capxon, lz 2 c7 cap,al,2.2f,50v, 6.3mm, rubycon,yxf 1 c8 cap,al,22f,50v, 6.3mm, rubycon,yxf 1 c9 cap,0805,102,100v,j 1 c11 cap,0603,151,50v,j 1 c13 cap,0603,683,50v,j 1 c14 cap,0603,222,50v,j 1 d1~d6 diode,1n4007,1a,1000v,do-41 6 d7,d8 diode,1n4148,0.15a,100v,ll-34 2 d9 diode,schottky,sb3100, 3a,100v,do-201 1 u1 ic,pwm,ap3105,sot-23-6,bcd 1 u2 ic,opto coupler,pc817b,dip-4 1 u3 ic,as431i,to-92,bcd 1 f1 fuse,cubiform,slow,1a,250v 1 l2 choke,fer,dr6*8,1.5mh, 0.3a, 6mm 1 l3 choke,2.2h 1 table 4. bom of demo board (12v/1a)
application note 1062 apr. 2011 rev. 1. 0 bcd semiconductor manufacturing limited 6 figure 9. application circuit schematic ac c2 r7 r4 r14 r11 r9 r5 r10 r17 r13 c13 r15 r12 r load c4 c8 d1~d4 c11 c6 c5 c14 d5 d6 d8 d9 t1 u1 u2 u3 l2 c7 fb sense vcc gate gnd ctrl 5 6 4 2 1 3 ap3105 r1 r8 d7 r18 f1 c9 r3 r6 r2 12v 1a l3 c3 r16 c1
|
