? semiconductor components industries, llc, 2001 april, 2001 rev. 3 1 publication order number: and8038/d and8038 implementing the ncp1200 in a 10 w ac/dc wall adapter prepared by: christophe basso on semiconductor introduction the ncp1200 implements a standard current mode architecture where the switchoff time is dictated by the peak primary current setpoint. by combining fixed frequency and skip cycle operation in a single integrated circuit, on semiconductor ncp1200 represents an excellent solution where cost and ease of implementation are premium: lowcost ac/dc adapters, auxiliary supplies, etc. furthermore, the device does not require any auxiliary winding to operate and thus offers a real breakthrough alternative to uc384x based supplies. this application note details how to build an efficient and rugged 10 w adapter. this adapter is designed to operate from a universal mains (90260 vac) while providing a good standby power at no load. the electrical schematic driving an external mosfet, the ncp1200p60, only requires a sense element and a vcc capacitor. working together with an internal highvoltage current source, this vcc capacitor provides the ncp1200 with an average dc level of 11 v typically while it also controls the shortcircuit time out. all these parameters are detailed in the application note and8023 available to download at www.onsemi.com . the electrical schematic appears in figure 1: 1 2 3 4 8 7 6 5 12 v @ 0.85 a d2 mbr360t3 + c2 47 f 400 v + c7 100 f/16 v universal input m1 mtp2n60e ic2 tl431 + c9 1 nf + + ground c6a 470 f 16 v c6b 470 f 16 v l2 22 h c5 2.2 nf y type c4 100 nf r6 1 k r5 3.9 k r3 560 r4 1.8 1 w c3 22 f 16 v t1 1:01 d3 mur160 lp 1.8 mh r7 22 k 2 w c8 10 nf 400 v r2 1 meg l1 2 x 27 mh cm schaffner rn114008/2 r1 10 adj fb cs gnd drv v cc hv figure 1. a 10 w ac/dc adapter built with the ncp1200 100 nf x2 c1 b1 smd http://onsemi.com application note
and8038 http://onsemi.com 2 as stated in and8023, the vcc capacitor needs to be evaluated taking into account the startup sequence (actually seen as a transient shortcircuit by the controller). an internal error flag is raised within the ncp1200 when an output overload occurs. if this error flag is still asserted when the vcc capacitor reaches uvlo low (around 10 v typical), then the ic goes into the latchoff phase: the output drive is locked and the internal consumption falls down to 350 m a typical. when another vcc breakpoint is reached (around 6.0 v), then the internal current source turns on again and the ic tries to restart. if the error is still present, the protection activates again. if the shortcircuit has gone, the ic resumes its operation and delivers its normal level. to check the correct value of the calculated vcc capacitor, you need to monitor both output voltage and vcc level on an oscilloscope. a shot as proposed by figure 2 confirms the validity of a 22 m f choice. we can see that the internal error flag goes high first but as soon as vout reaches its target level, the flag goes back to zero, confirming the normal controller behavior at the uvlo low checkpoint. this experiment should be carried in the worse case conditions, e.g. low mains and maximum output load. error flag figure 2. the startup sequence shows a v out establishment before uvlo low is reached ok, flag = 0 feedback loop in this application, a precise output voltage is obtained through the use of a tl431. since we target a 12 v output, you calculate the upper and lower voltage sense elements by applying the following formula: v out rupper rlower 1 vref depending on the tl431 type, vref can be 2.5 v or 1.25 v. with a 2.5 v reference, rupper (r5) = 3.9 k w and rlower (r6) = 1.0 k w . this network ensures a bridge current flow of 2.0 ma which is good for the noise immunity. as any closed loop systems, a compensation network needs to be tailored to stabilize the loop. in this aspect, the ncp1200 average spice model will save you a tremendous amount of time. the simulation template appears in figure 3 on the following page, showing how to wire the ncp1200 average model with intusoft's isspice4.
and8038 http://onsemi.com 3 r5 100 m x3 tl431 ncp1200 averaged in ctrl out gnd fb 24 6 1 + v in 126 12 7 15 9 14 120 11.8 127 12.2 12.2 12.2 12.2 2.38 0 x1 ncp1200_av fs = 66 k l = 1.8 m ri = 1.5 lol 1 kh col 1 kf + v stim ac = 1 x1 xfmr ratio = 0.1 d1 mbr140p r4 100 m v in out1 out2 r17 300 m c2 10 m f c5 470 m f c1 470 m f i out out1 out2 2.38 11.1 2.50 v out cf 100 nf r low 1 k r upp 3.9 k r15 560 11 5 10 13 r load 14 + rs 10 m l1 22 m f figure 3. the average model of the ncp1200 when used in ac analysis the loop is kept opened in ac thanks to lol which exhibits a fairly high value. however, during its bias point calculation, spice opens all capacitors and shorts all inductors. therefore, lol closes the loop in dc but blocks the ac stimuli to allow bode plot generation. figure 4 portrays the simulated results with a 100 nf feedback capacitor, while figure 5 offers the true measurement curves. 10 bw = 600 hz y = 45 /div y = 20 db/div 100 1 k 10 k 100 k 0 mag (db) 80.00 60.00 40.00 20.00 0.00 20.00 40.00 60.00 80.00 10 100 1 k 10 k 100 k phase (deg ) 180.00 135.00 90.00 45.00 0.00 45.00 90.00 135.00 180.00 phase gain figure 4. bode plot obtained using spice figure 5. confirmed by a network analyzer measurement
and8038 http://onsemi.com 4 as you can see, curves are in good agreement, despite the small dc gain error which predicts a slightly lower bandwidth in the case of spice. in both cases, the phase and gain margins confirm the good stability of the design, but also the validity of the spice model (based on bengurion university gsim approach). the ncp1200 fb pin being a high impedance path, a 1.0 nf placed between this pin and ground will prevent any noise picking during operation. transient results using the ncp1200 design aid spreadsheet lead us to a transformer offering the following specs: lprim = 1.8 mh, np:ns = 1:0.1, rm8 or e25 core. for ease of implementation, this transformer will be available from coilcraft, as referenced in the bill of material. the maximum peak current has been fixed to 600 ma. this value essentially defines the air gap requirement in the transformer but also the final potential transformer mechanical noise generated in standby. as explained, the ncp1200 skips switching cycles in standby operation. by default, skip cycle takes place at 1/3 rd of the maximum peak current: 200 ma in our case. because skip cycle frequency will naturally enter into the audible range, it is important that the skip current value does not engender noise. fortunately, if that would be the case, you could still wire a resistor bridge on pin 4 to fix a dc point different than the default one (1.4 v). as a result, you can force skip operation to happen at less than 1/3 rd of the maximum peak current. however, keep in mind that the highest peak currents in skip mode offer the best standby power. this is because of the switching cycles population within the bursts: less cycles mean less switching losses and better efficiency at no load. a quick method to assess the rms current in the mosfet consists in simulating the whole ac adapter with spice. this has already been presented in and8029 and the schematic will not be reproduced here. the simulated results are given below through figure 6 and figure 7 while the supply is delivering 10 w: 1.030 m 1.040 m 1.050 m 1.060 m 1.070 m figure 6. transient results obtained with isspice4 figure 7. compared to true measurements worse case conditions (low mains, maximum output current) gives an rms drain current of 230 ma. associated with a 6.5 w rds (on) @ tj = 100 c, the conduction losses grow up to 340 mw. using a to220 package for the mosfet, offers the ability to dissipate a given amount of power in freeair conditions (without a heatsink) of: pmax tj tamb r j a 1.3 w. further switching losses measurements confirm the ability to use this mosfet without any heatsink up to an ambient of 80 c.
and8038 http://onsemi.com 5 90 0.15 1.0 10.0 mhz 30.0 db m v en_v_qp 80 60 40 20 0 20 figure 8. the final composite qp plot carried over one line while the other is loaded (230 vac, p out = 10 w) conducting emi filtering the 10 w ncp1200 demo board is equipped with a front stage filter who lets you pass the cispr22 emi tests in both quasipeak and average detector methods. the method we used for calculating the filter is described in and8032 aconducted emi filter design for the ncp1200''. the front stage is made of a single common mode (cm) choke whose wiring method gives enough leakage inductance for differential mode (dm) filtering. figure 8 plots the final cm+dm noise component confirming the test passing. final performance we have carried some power tests on the 10 w adapters and the below numbers will confirm the pertinence of choosing on semiconductor's ncp1200 for your next designs: vin dc pout(w) pin(w) (%) 126 0 0.245 126 10.5 12.6 83.3 356 0 0.462 356 10.5 13.17 79.7 the standby power can be further reduced by implementing one of the method proposed in and8023 either through an additional diode or an auxiliary winding. thanks to its inherent protection circuitry, ncp1200 protects the power supply in presence of a permanent output short circuit. when shorted, the average output current was less than 500 ma.
and8038 http://onsemi.com 6 10 w demoboard, bill of material r1 10 w , 1 w through holes r2a r2b 2 times 560 k w smd in series r3 560 w smd r4 1.8 w , 1w smd or 1.8 w 1 w through holes r5 3.9 k w smd r6 1 k w , smd r7 22 k w , 2 w through holes l1 schaffner rn11408/2 l2 22 m h, 1 a m1 mtp2n60e, to220 through holes, on semiconductor ic1 sfh615a2, smd (optocoupler) ic2 tl431bc (to92), on semiconductor ic3 ncp1200p60, dip8, on semiconductor c1 100 nf x2/ 250 vac c2 47 m f/400 v, snapin vertical c3 22 m f/16 v, vertical c4 100 nf, smd c5 1.5 nf y1 type only c6a 470 m f/16 v, vertical c6b 470 m f/16 v, vertical c7 100 m f/16 v, vertical c8 10 nf/400 v d1 mur160, on semiconductor d2 mbrs360t3, on semiconductor b1 bridge 1 a/600 v, mini dip transformer available from coilcraft u.s, ref. : y8848a mains connector: schurter gsf1.1202.31 with fuse other available documents related to ncp1200: and8023/d, aimplementing the ncp1200 in lowcost ac/dc adapterso and8029/d, aramp compensation for the ncp1200o and8032/d, aconducted emi filter design for the ncp1200o pspice, isspice4 and microcap averaged and transient models available in readytouse templates at www.onsemi.com ncp1200 design aid spreadsheet with ebncp1200/d
and8038 http://onsemi.com 7 printed circuit board details figure 9. component side, silk screen, scale 1 figure 10. solder side, silk screen, scale 1 figure 11. copper traces, scale 1
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