1/4 AN1620 application note november 2002 introduction this note describes an off-line autoranging (85vac-264vac) battery charger designed to operate in three differ- ent functional modes: C constant voltage C constant current C constant power these requirements are typical of battery chargers for notebook computers. the resulting output characteristic of the system is shown in fig 1. the a-b portion of the v-i characteristic is a constant voltage mode. this situation happen when the battery pack is almost completely charged. vo has to be set at the final value of the battery pack. from point b to c the charger operates in constant power mode, reducing the output voltage while increasing the output current. the benefit of introducing the constant power mode is that the maximum output power is lower with respect to the power output at point e. this reduces the power managed by the primary circuitry and in consequence re- duces the total dissipation power. this is a useful improvement considering that the complete system is pack- aged in a plastic case with a very limited capability of dissipating heat. from point c to d the adapter works in constant current mode until the output voltage reaches 0v. in practice it is difficult to have a constant current characteristic below an output voltage of 2-3v because the primary auxiliary supply voltage drops at the controller turn-off threshold (internally fixed at 7.6v typ.) and the v-i characteristic exhibit a foldback to zero. if mandatory to operate in constant current mode till 0v, a secondary post-regular or other tricky solutions on the windings, primary or secondary, are required. fig 2 shows the v-i characteristic obtained with the circuit of fig 3. electrical specification: input voltage 85vac to 264vac regulated output voltage 18v maximum output power 25w switching frequency 100khz target efficiency @ full load >80% topology discontinuous current mode flyback fig 3 shows the electrical circuit. figure 1. v-i output characteristic figure 2. practical v-i characteristics abe c di v d96in437 0 0.5 1.0 1.5 2.0 i o 0 5 10 15 v o d96in439 v in =220v ac v in =110v ac by domenico arrigo 25w off-line autoranging battery charger with l5991a
AN1620 application note 2/4 figure 3. ac-dc converter electrical schematic c3 c0 fuse 85 264 vac b1 r28 2.7m c4 82 m f 400v r1 91k 0.5w r20 10k c1 50pf lf r3 10k 10 r6 10 13 r30 1k 12 c8 100pf 11 r29 0.5 c6 22 m f 9 8 14 r26 10 r10 36k r7 5.1k 2 3 4 c9 1 m f r05 10k c10 2.2nf 5 7 c11 10nf q1 stp4nl80z t1 d2 r4 20k d3 stth1l06 c13 2 x 470 m f r16 40m r19 13k d8 9v r21 12k in537_mod c18 1 m f r14 82 d6 d11 l5991a -+ thermistor d4 stth1l06 41t c5 120pf l1 470 m h q3 bc337 c7 22 m f d5 18v r9 5.1k 6t r8 5.1k v ref rct tcdt 1101gb ic2 6 comp ss 5 4 fb c12 1nf isen out v c v cc dis dcc sgnd pgnd 5t d7 stps8h100d l2 5 m h c14 330 m f 18v/2a 25w 3t r12 10k c15 22 m f q4 bc337 1 2 ic2 r18 4.7k r17 20k r13 240 c17 220nf r23 100k c18 2.2nf r25 2.2k c2 1 m f c16 220nf r24 8 2 6 4 1 5 3 7 tsm 101a r22 1k v ref crin vrin c cref csen output v cc gnd
3/4 AN1620 application note the realisation of the three different functional modes requires dedicated controls. the voltage and current controls are located on the secondary side of the transformer, and the error signal is transferred to the primary controller via an optocoupler in order to have an isolated feedback. the tsm101a a dual op/amp, with an internal 1.24v reference is used. one op/amp is dedicated to the voltage mode control and the second one to the constant current control. the two output are or-ed and the common point drives the optocoupler. the current signal is taken across the 40mohm current sense resistor, r16, while the r18 potentiometer ad- justs the output voltage. the constant power characteristic is easy to achieve with discontinuous mode operation, since constant primary peak current means constant output power. the value is programmed by adding a proper offset voltage, defined by r3 value (fig 4), to the current sense ramp at pin 13. figure 4. schematic diagram for output constant power. the value of this resistor is related to the desired output power pout by the below relationship: where l1 is the primary inductance, h is the expected efficiency, f is the switching frequency and rs is the cur- rent sense resistance. the l5991a introduces a delay, td @ 100ns, at turn-off, on the current loop, and due to this, the primary peak current value increases according to its slope. the consequence is that the regulated output power is a function of the input voltage. r28 is introduced in order to compensate this error. its value can be calculated with the following formula: a primary auxiliary winding is required to supply the ic after turn-on, and it has to be designed to generate a dc voltage within the limits of the ic supply voltage range. it should be loosely coupled with the secondary winding in order to minimise the reflected secondary-to-primary effects when the output voltage is going down towards short circuit. a second auxiliary winding, on the secondary side transformer, forward coupled with the principal primary wind- ing, has been added. when the system is in constant current mode, and the output voltage is reduced, this wind- ing provides the supply voltage for the tsm101a. the table a summarises the efficiency performance of the complete system, and table b shows the electrical system performance. l5991a 2.7m 10k 1k 0.5 w 4 10 13 in438_mod 10 100pf r 3 r 30 4 1 2 l 1 f h -------------------- ? ?? r s p out C ------------------------------------------------------------------------ = r 28 r 30 / /r 3 () l 1 t d r s ------------------------------------- - =
information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. the st logo is a registered trademark of stmicroelectronics ? 2002 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - brazil - canada - china - finland - france - germany - hong kong - india - israel - italy - japan -malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states. http://www.st.com 4/4 AN1620 application note table a. system efficiency. table b. system performance (pout limited to 25w) vin (vac) iout = 1a iout = 2a, pout = 36w iout = 1.8a, pout = 25w vout (v) h vout (v) h vout (v) h 85 110 220 264 17.97 17.94 17.89 17.89 83.9% 85.1% 84.4% 82.0% 17.96 17.96 17.91 17.89 82.2% 83.7% 83.5% 83.0% 13.82 13.86 14.31 14.41 82.6% 83.7% 83.8% 82.7% line regulation vin = 85 to 264 vac, iout = 1a 80mv load regulation iout = 0.5 to 1.8a, vin = 85v vin = 264v 10mv 10mv maximum efficiency vin = 180 vac, iout = 1.8a 85.2% output ripple vin = 85 to 264 vac, iout = 1.8a < 200mv
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