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october 2007 rev 1 1/13 AN2530 application note solution for 150 w half brid ge resonant dc-dc converter introduction this application note describes a 150 w half bridge resonant dc-dc converter. this type of smps is highly attractive due to its high achievable efficiency, very low noise and compact size. resonant converters are among the least common smps topologies. there are several reasons why they are not often used, but we will not discuss these reasons in this application note. however, it is worth noting that the resonant topologies have undeniable advantages over the "hard switching" topologies. the very high achievable efficiency of over 90% and up to 95% is very common, as well as their low generated noise due to zvs (zero voltage switching) and resonant energy transfer. other related advantages derived from these converters are their compact size due to their need for smaller power switches (power mosfets usually), smaller transformers, and less generated heat (the lower losses are a part of this). less heat means a smaller heat sink and a longer life for power components. if the necessary care is taken in the design phase, the results are very good and the typical issues normally associated with these topologies are avoided. st's l6598 half bridge driver has been chosen for this design. please refer to the l6598 datasheet for full sp ecifications and ca pabilities, or to other documentation, application notes and books where it is used, in order to have the best picture of this design. all references are provided in figure 7 . this application note concentrates only on the power aspects, because as already mentioned, there are excellent guides for the driver (aside from the datasheet) as well as application notes for smps in general, magnetics, topologies, etc. www.st.com
contents AN2530 2/13 contents 1 functional overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 operational frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 transformer and resonant co mponents . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1 transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 converter's protection schemes, overcu rrent, overvoltage . . . . . . . . . 7 5 full load, normal operation wavef orms . . . . . . . . . . . . . . . . . . . . . . . . . 9 6 conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
AN2530 functional overview 3/13 1 functional overview the simplest way of describing the functioning of a resonant converter is to compare it with a non - resonant type. typically a "normal" half bridge transformer is connected to the principal dc bus through a capacitive divider network that creates a "false" ground to feed one of the transformer's ends ( figure 1 ). in this way, the transformer is fed with a voltage that swings (from the transformer's point of view) from zero to negative, negative to zero, zero to positive, then back to zero (therefore repeating the cycle). the mains dc bus is connected as noted in figure 1 for 110 v ac or 220 v ac . the operation is quite straightforward alternating the turn-on of each transistor. figure 1. smps half bridge simplified schematic the resonant variation for this type of converter places an "external" inductor to cause a resonance between the capacitive divider network and the external inductor ( figure 2 ), which sums up to the already present leak age inductance of the main transformer. these components are the ones that require most of the care for this variation of the converter. nevertheless, remember that every aspect of the design stage has an impact on the overall behavior of the converter. figure 2. resonant smps half bridge simplified schematic ta bl e 1 gives the bom (bill of materials) for this co nverter. most of the capacitors do not have an operating voltage, as they operate in low voltage. as for the driver, any voltage c1 c2 q1 3 1 2 q2 3 1 2 c3 d2 d3 d4 d1 t1 1 5 6 4 8 d5 d6 + c4 220vac 110vac d2 c1 d6 q2 3 1 2 d1 q1 3 1 2 c2 + c4 d4 d5 t1 1 5 6 4 8 d3 l1 110vac 220vac
functional overview AN2530 4/13 greater than or equal to 16 v is acceptable. the construction details of l1 and tr1 are discussed later. refer to figure 3 for the full electrical schematic of this converter. table 1. bill of materials (bom) qty. ref. part qty. ref. part qty. ref. part 1 ac 220 v ac conn. 1 c22 0.47 r16 10 k ? 2 cac1 1 nf/400 v 2 c26b 4700 pf / 2 kv r18 10 k ? cac2 1 nf/400 v c26a 4700 pf / 2 kv r23 10 k ? 1 c1 220 f / 400 v 1 dc 24vdc 1 r10 20 k ? 1 c4 1 1 d1 w08g 1 r11 100 k ? 1 c8 47 f 1 d2 stps20h100ct 2 r13 15 3 c10 100 nf 1 d3 1n4148 r15 15 c12 100 nf 1 d4 18 v 1 r17 39 k ? c20 100 nf 1 l1 51 4 r19d 1 1 c11 1 nf 2 q2 stp8nm60n r19c 1 2 c15 220 p q1 stp8nm60n r19b 1 c13 220 p 1 r2 150 k ? /2 w r19a 1 1 c14 0.22 1 r3 10 1 r21 3.6 k ? 1 c16 100 n 1 r4 150 k ? 2 r25 1 k ? 3 c17 33 n 1 r5 7.5 k ? r22 1 k ? c18 33 n 2 r8 27 k ? 1 r24 1.2 k ? c23 33 n r6 27 k ? 1 tr1 transformer 3 c19c 470 1 r7 6.8 k ? 1 u1 l6598 c19b 470 6 r9 10 k ? 1 u2 pc817 c19a 470 r12 10 k ? 1 u3 tl431 1 c21 82 n r14 10 k ?
AN2530 functional overview 5/13 figure 3. converter?s full electrical schematic cac2 1n cac1 1n j p1 c onn pwr 2-h 1 2 -+ dac 1.5a bridge 1 2 3 4 + c1 220f/400v u1 l6598 5 6 7 4 2 3 9 8 10 11 14 15 16 12 1 opout opin- opin+ rf min rf start cf en2 en1 gnd lvg out hvg vboot vs css r13 15 q2 stp8nm60n 1 2 3 q1 stp8nm60n 1 2 3 r15 15 r16 10k u3 tl431 2 1 3 r14 10k c22 0.47 c21 82n r25 1k l1 51 r17 39k c12 100nf + c19c 470 r18 10k c15 220p r19a 1 u2 pc817 2 1 3 4 r24 1.2k c11 1nf r9 10k r8 27k dc 24vdc 1 3 1 2 r19b 1 r19c 1 r19d 1 r4 150k c16 100n r11 100k r12 10k r10 20k r5 7.5k c4 1 c26b 4700pf / 2kv r6 27k c14 0.22 r7 6.8k c13 220p c10 100nf + c8 47f r2 150k/2w r23 10k d4 18v 1 2 ma ind c 360vdc 1 3 1 2 r3 10 d1 1n4148 c23 33n + c19a 470 c20 100nf d4 stps20h100ct + c19b 470 c26a 4700pf / 2kv r22 1k r21 3.6k d2 1n4148 c18 22n d3 1n4148 c17 22n tr1 transf ormersmps 7 6 4 2 9 13 11 cp 100n * * this capacitor must be placed just below u1, directly connected to vs (pin 12) and gnd (pin 10). current sensing resistor network.
operational frequencies AN2530 6/13 2 operational frequencies figure 3 shows a gray area with a note "optional". this rectifying stage is not really necessary as it was done for testing and measuring purposes. more explanations and clarifications are provided as we go through this design. much of the basis for this application note was taken from another st application note, mainly an1660 (zvs resonant converter for consumer application using l6598 ic), which is a 180 w zvs resonant converter. as stated in an1660 (zvs resonant converter for consumer application using l6598 ic) you must "choose" some operational parameters that are recalculated after real component values have been chosen. only your experience with this kind of smps can guide you. for this case the following values have been chosen: f start = 300 khz f min = 70 khz f r = 35 khz the frequency values have been chosen keeping in mind that 300 khz (f start ) is quite close to the driver's maximum operational frequency. therefore, we leave the converter much "room" to change its operational frequency (via the feedback) so the regulation does not suffer because of a range that is too restrictive. the calculations for rf min (r11) and rf start (r6); c f is c13 (220 pf) in our case, are shown below: equation 1 equation 2 recalculating f min & f start with actual values of r fmin & rf start : equation 3 equation 4 rf min 1.41 f min c f ? ----------------------- - 91.56 k ? = = ( 100 k ?) rf start 1.41 f start f min ? () c f ? ------------------------------------------------- 27.27 k ? = = ( 27 k ?) f min 1.41 rf min c f ? -------------------------- 64.09 khz = = f start 1.41 rf start c f ? ---------------------------- - f min 237.4 khz = + =
AN2530 transformer and resonant components 7/13 3 transformer and resonant components in order to avoid the majority of the most difficult problems related to resonant converters, great care must be taken in the design of those components whose primary task is to transfer the energy from the rectified line to the load. these components are the transformer, external inductor, capacitor divider network and the power switches. several "methods" and approaches have been taken into account in order to calculate the power transformer and the external inductor (refer to section 7: references at the end of this application note). an1660 forms the bas is for this application note and provides calculations for this objective. the objective of this application note is to take a closer look at the power stage, so that just the final results for the transformer and the external inductor are shown. however, it is important to notice that the transformer's type (material, size and shape) plays one of the main roles in any converter. for reso nants, the coil type is important also. ta bl e 2 gives transformer and coil data. litz wires have been used. 3.1 transformer brand: epcos ty p e : e t d 3 4 material: n67 4 converter's protection schemes, overcurrent, overvoltage overcurrent and overvoltage protection features can be added easily thanks to the pins of the l6598 controller. in this section we show how to calculate these values according to the operational parameters chosen. again, refer to an1660 (zvs resonant converter for consumer application using l6598 ic) or use your own "method" to calculate the peak current. you should expect to be at the maximum at l1 (as well as transformer's primary) and take a safety margin (10% more for example). in this case, the maximum current should be 1.8 a, so we set the maximum current to 2 a. table 2. tr1 transformer?s windings details turns wires wire's diameter [mm] primary 50 14 0.2 secondary 14 38 0.2 aux. 3 1 0.2
converter's protection schemes, overcurrent, overvoltage AN2530 8/13 according to the l6598 datasheet there is a constant voltage of 2 v at pin 2 (rf start ), so this voltage can be used to set the opamp's inverting input (pin 6) to 0.4 v through the r6 & r7 divider network. the inverting input of internal opamp is set to 0.4 v, so 0.4 v/2 a = 0.2 ?. a set of 1 ? /0.25 w resistances was chosen to be readily available and by paralleling them we get 0.25 ? /1 w, which "generates" 0.25 ? *2 a = 0.5 v at maximum current. then, we have to choose the values for r17 & r18 (a resistor divider network) to get the 0.4 v at pin 7 (opin+), r17 = 39 k ? and r18 = 10 k ? in our case. concerning feedback, regulation is achieved by means of varying the driver's frequency. a heavier load determines a lower operational frequency and the contrary is true for a lighter load. frequency is changed by varying the current at pin 4 (rf min ). as previously stated, r11 defines the maximum operational frequency and r10, r12 and optocoupler's internal resistance (that varies according to the current supplied to the load) set the actual operating frequency.
AN2530 full load, normal operation waveforms 9/13 5 full load, normal operation waveforms figure 4 shows the normal full load operation waveforms for this converter. channels 1 and 2 are v g at q2 and q1 respectively. notice that the voltage level at q1 (upper mosfet), is up to 370 v due to the charge pump inside the driver. channel 3 is the resonant current flowing through l1, measured with a hall effect probe. there is a lack of symmetry probably caused by the hand-wound transformer and coil. the major contributor to this should be the non-symmetric primary winding for the transformer that imposes different loads as current flow changes direction at the primary. channel 4 is the resonant voltage at c18. figure 4. operating waveforms at full load measurement conditions ? vin = 355 v dc @ dc main bus ? vout = 23.7 v dc , 6.35a (~150 w) as previously stated in the introduction, this type of converter has some very good characteristics, one of which is the very high efficiency, typically over 90%, that is easily achieved with resonant, very low rf and emi produced due to zvs. figure 5 shows the efficiency curve against the output power and against input voltage ( figure 6 ). notice that there are two curves ( figure 5 ), the upper one is the efficiency curve for this converter, as you can see in the schematic in figure 3 . the other one is the same converter connected after a pfc circuit. this one uses st's l4981a as its primary driver, provides 355 v dc and up to 200 w. the application note an628 appears in section 7: references and is referred to in the conclusion.
full load, normal operation waveforms AN2530 10/13 figure 5. efficiency vs. pout figure 6. efficiency vs. vin figure 7. switching frequency vs output power 60 65 70 75 80 85 90 95 10 30 50 70 90 110 130 150 pout [w] efficiency [%] eff. eff. with pfc 90 90.4 90.8 91.2 91.6 92 92.4 350 360 370 380 390 400 vin [vdc] efficiency [%] 50 100 150 200 250 20 40 60 80 100 120 140 pout [w] switchinf freq. [khz]
AN2530 conclusion 11/13 figure 8. thermograph the converter is working at full load in the thermograph in figure 8 . notice that the hottest spot is near the rectifier double diode (d2). the hot lines (white ones) are the dc out filtering capacitors that are being heavily heated by d2 due to the board's position. the transformer is working "cool" as well as the power mosfet transistors. it is important to notice that q1 and q2 are working in the 50 oc range. originally the board was assembled with bigger transistors, therefore the smaller ones can be used in this application and gain in efficiency (almost 20% gain for light loads). they are easier to drive and cheaper which means that you don't have to "oversize" these (as is usually done in other converter topologies). 6 conclusion as already mentioned, a certain degree of attention must be exercised with resonant converters because energy transfer is directly related to this phenomenon. the benefits are substantial and include low emi and rf noise, high efficiency, overall cooler operation, no need for "over sized" power components to prevent failure from spikes, etc., as well as other advantages if designed carefully. as you can see, the load regulation of this one is very good. it is remarkable that all these measurements and tests have been performed without any forced ventilation. the heatsink provided for the power transistors is very modest, considering the smps's power, so it would be easy to avoid any heat sink by designing a suitable copper area for smd transistors (i.e. dpak). the designer will notice that since the power factor for this converter is not good, therefore it is better to connect the converter after a pfc, such as the one with l4981 that has been used to do some of these measurements. it is normal that the power factor is low due to the "spike" nature of this converter's drawn curren t. if observed with an oscilloscope, a series of spikes can be seen. oc d2 tr1 q1, q2
references AN2530 12/13 7 references high frequency switching power supplies, theory & design. closing the feedback loop. lloyd h. dixon jr. unitrode? transformer and inductor design for optimum circuit performance. lloyd h. dixon jr. unitrode? l6598 datasheet an1673, l6598 off-line controller for resonant converters. an1660, zvs resonant converter for consumer application using l6598 ic. an628, designing a high power factor switching preregulator with the l4981 continuous mode. 8 revision history table 3. document revision history date revision changes 25-oct-2007 1 initial release
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