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1/13 AN1660 application note september 2003 1 circuit description the simplified schematic and operating waveforms are shown in fig.1 and fig.2. figure 1. simplified schematic this half-bridge converter consists of switching devices q 1 & q 2 , resonant inductor l 1 , magnetizing induc- tor of transformer l 2 , resonant capacitors c 1 & c 2 , transformer t, rectifier diodes d 1 & d 2 , output capac- itor cout. cq 1 & cq 2 are the parasitic capacitors of the q 1 & q 2 . d q1 & d q2 are the parasitic reverse diodes of q 1 & q 2 . switching devices q 1 & q 2 repeat on and off alternately, and the on- and off-times are the same. 50% duty cycle. l-1 l.2mag driver-h +bus v@cres vout time time q1 driver-l q2 d1 c1 c2 load d2 out d03in1425 by helen ding zvs resonant converter for consumer application using l6598 ic in this paper we study the details of the multi-resonant zero-current-switching converter which has smaller size and higher efficiency, and low noise operation. it will shown the design equations and demoboard results of a high-end tv power supply using st l6598. the small size, high efficiency, and low noise operation makes the half bridge resonant topology actrative for converters. it will shown the design equations and demoboard test results of a high-end tv power supply us- ing st l6598.
AN1660 application note 2/13 this circuit has three operation. 1. the operation of a resonant circuit between l 1 and c 1 +c 2 . which supplies power to the load. 2. the operation of a resonant circuit between l 1 +l 2 and c 1 +c 2 . which does not supply power with the load. 3. the operation of a resonant circuit between c q1 + c q2 , l1+l2 and c 1 +c 2 . which achieves the zvs of the mosfet. figure 2. operating waveforms t0~t1: in q 1 reverse current flows through parasitic diode d q1 . q 2 is off. the initial value of the resonant current between l 1 and c 1 +c 2 at t0 is -i2, which coincide with the current in l 2 . the current in l 2 will increases at the rate of nv out /l 2 (n = n1/n2, n2 = n3). at t0 c q1 is discharged. the voltage becomes zero. zvs is achieved. the voltage of c 2 decreases further. c 2 is discharging. t1~t2: q 1 is on and q 2 is off. the resonant current flows through q 1 and in the opposite direction of t0~t1. the resonant current increas- es sinusoidally and reaches the maximum value then deceases till coincides with the current in l 2 at t2. the difference between resonant current and current in l 2 flows through the primary winding n1 of the transformer. power is supplied to the load. t2~t3: q 1 is on and q 2 is off. the current i1 in l 1 coincides with the current in l 2 at t2. no current flows through the secondary winding of the transformer. in this mode l 1 +l 2 and c 1 +c 2 resonate. vdq1 0v 0v 0v 0v 0a 0v 0a 0a 0a vdq2 vq1 vq2 iq1 icq1 il1 vc2 id1 t0 t8 t1 t2 t3 t4 t6 t7 t8 t5 current in parasitic diode (idq1) i2 i1 -i2 vcmax vcmin d t -i1 il1pk d03in1426
3/13 AN1660 application note t3~t4: q 1 turns off at t3. both q 1 and q 2 are off. the charge stored in the parasitic capacitor c q2 of q 2 is discharged by means of the resonant current between l 1 +l 2 and c 1 +c 2 . whereas c q1 is charged. t4~t5: q 1 is off. the resonant current flows through the parasitic diode d q2 of q 2 . at t4 c q2 is discharged. the voltage becomes zero. zvs is achieved. the voltage of c 2 increases further. t5~t6: q 1 is off and q 2 is on. the resonant current flows through q 2 and in the opposite direction of t4~t5. the resonant current de- creases sinusoidally and reaches the minimum value then increases till coincides with the current in l 2 at t6. the difference between resonant current and current in l 2 flows through the primary winding n1 of the transformer. power is supplied to the load. t6~t7: q 1 is off and q 2 is on. the current -i2 in l 1 coincides with the current in l 2 at t6. no current flows through the secondary winding of the transformer. in this mode l 1 +l 2 and c 1 +c 2 resonate. t7~t8: q 2 turns off. q 1 and q 2 are off. the parasitic capacitor c q2 of q 2 is charged by means of the resonant current between l 1 +l 2 and c 1 +c 2 . whereas c q1 is discharged. here the circuit returns to the first mode and the cycle is repeated. a main advantage of this resonant converter is that there are no turn-on switching losses exist in the fet because its inverse diode carries current and the voltage across the mosfet is zero before the mosfet conducts forward current. there are still turn-off switching losses. but it can be erased by placing small snubber capacitors directly across the fet devices. and no discharge resistors are needed. this was because the capacitor is not discharged by turning the fet on but rather is discharged by turning-off the opposite fet. also the switching losses due to coss and crss is eliminated by the same reason mentioned before in the lossless snubbers. the energy stored in any capacitance directly across the device is returned to the dc source by virtue of the opposite fet turning off.
AN1660 application note 4/13 2 l6598 device description the l6598 is an integrated circuit realised in bcd off-line technology. able to drive power mos or igbt, in half bridge topology, the l6598 is provided with all the features (such as vco, soft-start, op-amp and enables) needed to implement and control properly a resonant smps with a minimum components count. even though the device is able to withstand high voltage (up to 600v), it can operate at low voltage starting from its operative supply. table 1. device pins description pin n name function 1c ss soft start timing capacitor. the capacitor css sets the soft start time, according to the rela- tions: t ss = k ss c ss (typ. k ss = 0.15 s/ m f). during t ss the capacitor is charged by means of a current which depends on r fstart value (i.e. on the difference between fstart and fmin). in this way tss is always set at k ss c ss (i.e. t ss depends only by c ss ). in steady state the voltage at pin 1 is 5v. 2r fstart maximum oscillation frequency setting. the resistance connected between this pin and ground sets the f start value, fixing the difference between f start and fmin (f start > f min ). the voltage at this pin is fixed at v ref = 2v, and so r fstart set the ifstart current equal to v ref /r fstart . the mini- mum r fstart value which can be connected to this pin is 25 kohm. 3c f oscillator frequency setting. the capacitor c f , along with to r fstart and r fmin , sets f start and fmin. in normal operation this pin shows a triangular wave. 4r fmin minimum oscillation frequency setting. the resistance connected between this pin and ground sets the f min value. the voltage at this pin is fixed at v ref = 2v, and so r fmin set the ifmin cur- rent equal to v ref /r fmin . the minimum r fmin value which can be connected to this pin is 25 kohm. 5op out out of the operational amplifier. to implement a feedback control loop this pin can be connected to the r fmin pin by means an appropriate circuitry. 6op on- inverting input of the operational amplifier. 7op on+ non inverting input of the operational amplifier. 8 en1 enable 1. this pin (active high), forces the device in a latched shutdown state (like in the under voltage conditions). there are two ways to resume normal operation. the first is to reduce the supply voltage below the undervoltage threshold and then increase it again until the valid supply is recognized. the second is activating en2 input. the enable 1 is especially designed for strong fault (e.g. in case of short circuit). 9 en2 enable 2. en2 input (active high) restarts the start-up procedure (soft-start sequence). 10 gnd ground 11 lvg low side driver output. this pin must be connected to the low side power mosfet gate of the half bridge. a resistor connected between this pin and the power mos gate can be used to reduce the peak current. 12 v s supply voltage. this pin, connected to the supply filter capacitor, is internally clamped (15.6v typical). 13 n.c. not connected. it ensures the insulation between the high voltage section and the low voltage one. 14 out high side driver floating reference. this pin must be connected close to the source of the high side power mos or igbt. 15 hvg high side driver output. this pin must be connected to the high side power mosfet gate of the half bridge. a resistor connected between this pin and the power mos gate can be used to reduce the peak current. 16 v boot bootstrapped supply voltage. between this pin and vs must be connected the bootstrap capac- itor. a patented integrated circuitry replaces the high voltage external diode. this features is achieved by means of a high voltage dmos, synchronously driven with the low side power mos- fet.
5/13 AN1660 application note 3 analysis and design equations following assumptions is for simplify the analysis. 1) all circuit components are ideal devices. 2) dt (fig.2) is very short compared to the resonant time. dt = 0, and i1 = i2. fig.3 is the schematic of l6598 resonant converter demoboard. figure 3. 180w half-bridge resonant with the l6598: electrical schematic the spec. of the power supply. input voltage: dc 370v ~ 420v (output of a pfc stage) output voltage: 110v/1.2a , 24v/2a output power : 180w 3.1 resonant frequency and switching frequency before the design, we need to decide several operation frequencies of converter. that include the start switching frequency - fstart, minimum switching frequency - fmin and resonant frequency - fr. in this application, we choose: fstart = 250khz ( by design rfstart of l6598) fmin = 68khz ( by design rfmin of l6598) fr = 84khz ( by design l1, c17, c18) to calculate rfstart and rfmin, using below equations: (1) (2) c10 100nf c15 220pf c19 2200 m f 50v c12 100nf r7 6.8k r6 27k c17 20nf c18 20nf c16 100nf c26 2200pf/3kv c9 1 m f c8 47 m f r5 5.6k r17 r4 110k en1 rfstart opin- opout en2 vs vboot out lvg opin+ hvg r2 160k/2w r3 10 r13 22 d6 1n4148 c11 1nf c13 220pf r9 10k r8 27k l1 90 m h t1 d7 stpr1010ct d8 byt03-400 d9 byt03-400 r15 22 r14 10k r24 3.3k r23 150k q1 stp8na50 q2 stp8na50 transformer r16 10k r10 20k r12 10k r19 0.2 r18 1k r11 100k rfmin gnd css cf c24 100nf 24v vin dc370v-420v c21 330 m f 160v c20 1000 m f c25 100nf r22 1k r21 3.6k r25 1k u2 4n25 u3 tl431 c22 c23 0.47 m f 115v c14 0.22 m f d03in1427 l6598 rfmin 1.41 fmincf --------------------- = rfstart 1.41 fstart fmin C () cf ------------------------------------------------- - =
AN1660 application note 6/13 choose cf = 220pf, then rfstart is 35.2kohm , this is larger than the minimum value of rfstart which is 25kohm. in this application, we use r6, r7 voltage divider to get a fixed voltage to pin6 (opin-) as the voltage of pin2 (rfstart) is fixed at vref = 2v. at the same time pin7(opin+) connected to primary current sense re- sister. voltage of pin7 (vopin+) will increase while output power increase. the output of the operational amplifier will be high when vopin+ tends to be higher than vopin-. then another voltage divider r8, r9 will send this high voltage to pin9 (en2). l6598 will stop the normal operation and restart as long as en2 is high. thus over-current-protection is achieved. if set r6 = 27kohm , r7 = 6.8kohm, then the input voltage for pin6 is 0.4v. now, rfstart = r6+r7 = 33.8kohm rfmin = r11 = 100kohm recalculated fstart & fmin. fmin is 64khz, fstart is 254khz. the design for resonant components will be finished in the followed step. 3.2 design transformer and resonant components first select core size. (3) for half-bridge converter, k = 0.165 with pout = 180w, h = 94%, d b = 0.4t, fmin = 64khz, calculated ap = 0.402 cm 4 choose core: ec39, with ae = 1.32cm2 (4) (5) rfmin 1.41 68 10 3 220 10 12 C ------------------------------------------------------ 94.2k w 100 ? k w == rfstart 1.41 250 68 C () 10 3 220 10 12 C --------------------------------------------------------------------------- 32.5k w == a p 11.1 pin kb d f ------------------------- ?? ?? 1.31 = npmin vin min () 10 4 2fsmin b d ae ------------------------------------------------ = npmin 370 10 4 26810 3 0.4 1.32 --------------------------------------------------------- 51.5 == n n p n s1 --------- - v inmax 2 ------------------ v o1 v f1 + -------------------------- 3 = n 420 2 --------- - 110 1 + ------------------- 3 1.9 = ns 2 v o2 v f2 + () n s1 v o1 v f1 + ----------------------------------------- - =
7/13 AN1660 application note we choose np = 52t, n = 2 then ns 110 = 26t. this is 110v winding. the turns of 24v winding is 6. the turns of vcc supply winding is 3. now design the resonant components. set normalize output voltage is m = 0.95 set normalize output current is j = 0.2 (6) (7) (8) cr = c17+c18, choose c17 = c18 = 20nf, then cr = 40nf recalculate lr under fr = 84khz. lr = l1 = 90uh choose primary inductance l2 as 5 ~ 10 times the resonant inductance. l2 = 500uh now the resonant frequency of total circuit is (9) 3.3 conditions for resonance between l2 and resonant circuit (l1, c17+c18) when the resonant between l2 and the resonant circuit starts, current il2 varies from -i2 to +i1 during t2- t1, and the voltage is nvout and constant. the operating waveforms are shown in fig. 4 figure 4. operating waveforms zo v inmax 2 ------------------ ?? ?? 2 jm vo io ------------------------------------------- - = zo 420 2 --------- - ?? ?? 2 0.2 0.95 110 1.2 () 24 2 () + ----------------------------------------------------- 46.55 == lr zo 2 p fr ----------- 56.55 2 p 84 10 3 ------------------------------- - 88.2 m h == = cr 1 2 p fr zo ----------------------- 1 2 p 84 10 3 46.55 --------------------------------------------------- 40.6nf == = fo 1 2 p l2 l1 + () cr ------------------------------------------------ 32.5khz == d03in1428 d t d vc2 vc2 i2 i1 il2 il2 il1 i il1 il1 i il1p vc max vc min l1c1
AN1660 application note 8/13 therefore. (10) (11) when assumed i 1 = i 2 , then (12) calculated value of i1 is 1.32a. 3.4 maximum voltage vcmax and minimum voltage vcmin of resonant capacitor the voltage of the resonant capacitor varies in accordance with the charge and discharge current in both the resonant circuits between l2 and c17+c18, and between l1+l2 and c17+c18. the following equation shows the relation between the maximum value vcmax and the minimum value vcmin of the resonant capacitor voltage. vc max = vc min + 2(vin - nvout - vc min) + d vc (13) nvout: primary voltage of transformer d vc: charge in value of resonant capacitor voltage in the resonant circuit between l1+l2 and c17+c18 where vc max + vc min = vin (14) the vcmax and vcmin are derived from the relation with the output current. and the output current iout is: (15) where il 1 and il 2 are as follows: (16) (17) q is the period of t0~t1. while assume d t = 0, and i1 = i2 nv out l 2 i 1 i 2 + tr 2 ----- - --------------- = t ' r tr 2 ----- - p l1cr == i 1 nv out t ' r 2l 2 -------------------------- - = i out n t ' r t d + ------------------- - il 1 il 2 C () t d 0 t ' r = il 1 vin nvout C vcmin C () cr l 1 ------ t l 1 cr ----------------- q C ?? ?? ?? sin = il 2 i 1 i 2 + t ' r --------------- ti 2 C =
9/13 AN1660 application note (18) according to this equation, the minimum voltage vcmin is obtained as follows: (19) the vcmax can be calculated according to equation (14). the maximum voltage applied to the resonant capacitor tends to increase as the output current increase. calculated vcmax in this application is 310v at maximum output power. 3.5 peak value il1pk of resonant current resonant current il1 is calculated according to equation (16). the peak value of il1 is (20) calculated i l1pk is 1.9a in this application. 3.6 over current protection and over voltage protection we set the over current protection point at i l1pk = 2.1a. as the inverting input of operational amplifier is 0.4v, the calculated rsense will be 0.4v/2.1a = 0.19ohm. choose 0.2ohm/1w resistor as rsense (r20). then the voltage on r20 will be 0.42v at expected ocp point. by choosing the suitable voltage divider r18 and r17, we can get the 0.4v voltage to opin+. in this application we choose r17 = 39kohm , r18 = 1kohm. over voltage protection is achieved by sense the supply voltage of l6598 --vs. r4 and r5 is a voltage divider which connected to pin8 (en1). when the voltage of pin8 is higher than the threshold 0.6v, it forc- es the device in a latched shut down state. by choose r4 = 110kohm and r5 = 5.6kohm, the ovp is trigged at vs = 12.4v. that is 10% higher than the normal operating voltage. 4 evaluation result of demoboard 4.1 operating waveforms see the operating waveform in fig.5 the measured value : i1 is 1.4a, vcmax is 300v, il1pk is 1.9a. that is coincide with the calculated value. iout 2n l 1 cr t ' r ------------------------ - v in nvout C vcmin C () 2 cr l 1 ------ i 1 2 C = vcmin vin nvout C i out t ' r 2n l 1 cr ---------------------------- - ?? ?? ?? 2 i 1 2 + cr l 1 ------ ?? ?? -------------------------------------------------------------- C = il 1pk vin nvout C vcmin C () cr l 1 ------ =
AN1660 application note 10/13 figure 5. operating waveform figure 6. ch1: v q2 100v/div ch3: v c2 100v/div ch4: i l1 1a/div input voltage vin = 420v output 110v/1.2a 24v/2a ch1: v q2 100v/div ch3: v c2 100v/div ch4: i l1 1a/div input voltage vin = 380v output 110v/1.2a 24v/2a
11/13 AN1660 application note 4.2 switching frequency characteristics figure 7. switching frequency vs input voltage figure 8. switching frequency vs output power 4.3 efficiency characteristics figure 9. efficiency vs output power figure 10. efficiency vs input voltage the maximum efficiency is 95% at maximum output power of 180w.
AN1660 application note 12/13 4.4 ocp and ovp the power supply will automatically pass in restart procedure when the output power exceed a certain val- ue. in experiment, set output current 1.8a to 110v & 2a to 24v, the voltage of pin 9 (en2 of l6598) will be higher than 1v and the ic restarts the start-up sequence. thus the ocp achieves. the maximum out- put power is slightly depend on the input voltage. in this demo board the maximum output power is around 240w. the over-current-protection will be active as well when 24v or 110v is under short circuit condition. the operating frequency tends to increase as the output load decrease. the maximum switching frequen- cy is limited by (r10+r12)//r11. using equation (1) we can calculate the fmax value. it is 278khz. when the maximum frequency is reached and the load reduced further, the output voltage will no longer be reg- ulated and tends to increase. so does the vs voltage of l6598. once vs rise to 12.2v, the over-voltage- protection will be triggered. in the demo board, the tested value is v s = 12.2v and v o1 = 134v, v o2 = 29v. so to keep the output voltage regulated, a minimum output power is request. the test result can be find in the following table. table 2. the maximum switching frequency can be designed to a higher value to reduce the minimum output power by reducing the value of (r10+r12). but the fmax has a upper limitation given by l6598. it can not exceed 350khz. 5 conclusion we could obtain high efficiency in this circuit. because both zvs, zcs can be achieved of turn-on and zvs can be achieved of turn-off. the major drawback of this kind of converter is the line regulation is not good. so it is better to use the converter after a pfc circuit. 6 reference [1] a high efficiency 150w dc/dc converter yasuhito furukawa, kouichi morite, taketoshi yoshikawa [2] a comparison of half-bridge resonant converter topologies robert l. steigerwald, ieee power electronics specialists conference record, 1987,135-144 [3] l6598 datasheet and an1673. [4] an628 & an996 input voltage (dc) 370v 420v output minimum power 4w 110v/30ma 24v/30ma 9.5w 110v/80ma 24v/30ma switching frequency 70khz 240khz input power 7.3w 13w
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