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december 2011 doc id 022523 rev 1 1/17 AN4016 application note 2 kw ppa for ism applications introduction stmicoelectronics has recently introduced a new generation of high voltage dmos products housed in stac ? air cavity packages and capable of delivering an output power of up to 1.2 kw for industrial, scientific, and medical applications such as 1.5 t and 3 t magnetic resonance imaging (mri). this new air-cavity technology now enables lower thermal resistance, lower weight, and reduced cost compared to devices in ceramic packages. in this application note we report on the design of a 2 kw-100 v, 123 mhz class ab peak power amplifier (ppa) for 3 tesla mri applications. it almost doubles the output power of previous amplifiers using mosfet transistors in standard ceramic packages. the design techniques and construction practices are described in enough detail to permit duplication of the amplifier. the devices used in this amplifier are two stac4932b n-channel mosfets in a push-pull configuration capable of 1.2 kw each, under pulse conditions, and housed in the stac244b, a bolt-down air cavity package. the design goals for the amplifier are: frequency: 123 mhz supply voltage: 100 v pulse conditions: 1 msec ? 10% output power: > 2 kw gain: > 19 db efficiency: > 60% figure 1. steval-imr002v1 am10219v1 www.st.com
contents AN4016 2/17 doc id 022523 rev 1 contents 1 design choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 layout, parts list, and design considerations . . . . . . . . . . . . . . . . . . . . 7 4 mri board performance and application . . . . . . . . . . . . . . . . . . . . . . . . 14 5 conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
AN4016 design choices doc id 022523 rev 1 3/17 1 design choices the main objectives of the 2 kw power amplifier design are board compactness (100 x 150 mm), full smt technology, and to avoid the use of ferromagnetic components and coaxial transmission line transformers. in summary, see circuit diagram in figure 3 , the power amplifier uses double push-pull bolt- down devices, 2 x stac4932b (see figure 2 ) operate in class ab. the two stacs are driven in push-pull through the transformer t1 together with two in-phase power splitters: this choice seems to be the best topology layout in terms of circuit size and mechanical compactness. moreover, as the temperature coefficient of mosfet channel resistance is positive, this makes a short-circuit possible in each pair of stac4932b drains. figure 2. stac244b bolt-down package figure 3. steval-imr002v1 circuit diagram p1_rf input s ma_fem a le p2_rf output n_fem a le gnd gnd q 3 q4 q2 q1 gnd gnd c 3 1 c 3 2 gnd r1 3 c9 c14 gnd gnd + c6 gnd c21 c40 r4 c18 r6 r8 r10 r12 r14 r15 c41 gnd r19 r20 r21 r25 r26 r27 gnd vcc + c1 3 + c12 gnd gnd c2 3 c28 c 3 0 gnd r17 r22 r 3 vg1 c 3 4 c16 gnd + c 3 gnd gnd l2 c7 c10 gnd gnd + c15 gnd r7 c8 c11 gnd gnd c17 s tac49 3 2b s tac49 3 2b t1 t2 l14 l 3 r 3 0 r1 c57 gnd gnd c58 l5 r24 vg2 c48 + c47 gnd gnd l1 3 c49 c52 gnd gnd + c51 gnd r29 c50 c5 3 gnd gnd l15 gnd l10 l8 l16 l12 l4 gnd c2 gnd r11 r16 c24 gnd c20 c 3 6 c45 r 3 1 c55 gnd gnd gnd gnd gnd r28 r5 r9 gnd gnd c 33 c 3 7 c26 c25 rm (~5 v typ) (~5 v typ.) (+100 v) d1 led r 3 2 gnd r 33 gnd r 3 4 r 3 5 gnd c60 c59 c61 c6 3 c62 c64 vbia s _1 vbia s _1 vbia s _1 vbia s _2 vbia s _2 vbia s _2 gnd gnd rm gnd gnd c29 gnd gnd c42 j1 j2 j 3 gnd line_bridge gnd gnd 1 2 3 con 3 1 2 3 con 3 vg1 gnd gnd vg2 1 con1 1 con1 gnd inp u t bo a rd 3 l a yer o u tp u t bo a rd 2 l a yer c4 am10220v1
design choices AN4016 4/17 doc id 022523 rev 1 therefore, a compact design can be realized: only one rf output matching network, with one impedance transformer t2, and an rf input matching network that supports the phase and amplitude signals on each of the two gates stac4932b (electrical symmetry). the schematic incorporates the necessary input / output biasing networks for proper feed biasing on the gates and drains. finally, planar microstrip technology was the main choice for the design of rf circuits: in particular, the design of transformers t1 and t2 is fully embedded into the substrate (pcb) itself as rf planar structures, and allows easy assembly of the design.
AN4016 circuit description doc id 022523 rev 1 5/17 2 circuit description the input rf network must be carefully designed respecting the correct electrical symmetry, because it is affected by driving high level signals (pin ~ 20 w), and is made up of: 1. balun transformer t1, / 4-25 ohm transmission line type @ 123 mhz, needed to lower the 50 ohm rf input impedance to 12.5 ohm, and is realized in a stripline technique on a 2-layer substrate (roger 4350b, with a thickness of 20 +20 mils: see figure 5 ) and is fed by a suspended microstrip line ('line bridge' in figure 3 ). moreover, t1, being a quasi one-dimensional rf structure, can be mapped on the pcb without compromising the electrical symmetry. t1, finally, is loaded from r7 and r29 in order to dampen reflected waves from the gates and for stability purposes. 2. two in-phase power splitters (l4, l 8 , c16, c1 8 , c20) and (l12, l16, c36, c41, c45) simply decrease the impedance level (2 ohm), and more importantly, allow the gates of each stac4932 to be kept isolated. 3. rf decoupling filters, fed through the vg1 and vg2 connectors ( figure 3 ) need to bias each stac4932b gate. they are essentially lc multi-section filters with capacitors of several technologies (tantalum, ceramic) to improve effective broadband rf isolation. independent voltage dividers act on the 4 gates (r4, r32, r16, r33, r17, r34, r31, r35) to assure broadband rf stability, while the lower value series resistors (r6, r 8 , r10, ...) need to dampen mismatching reflections on the gate impedance and then mitigate any asymmetries on the gate impedance value. the output rf network acts on the dmos drains, in order to achieve optimal impedance by means of the rf transformer t2, and also to properly feed high dc current filtered at vd=100 v, through the output biasing network directly via the primary winding of t2. the transformer t2 (ratio 4:1) is designed on the top/bottom layers (see figure 7 ) using substrate roger 4350b of 60 mils thickness in suspended broadside coupled strips and acts as a composite transmission line transformer in balanced to unbalanced mode. the rf output (type n-female connector) is directly connected to the winding output strip of t2 (see top view in figure 7 ) through an air suspended microstrip-line (50 ohm): in this way, the current (differential) generated on the primary winding strip (on the top layer) between the two stacs is moved from t2 versus unbalanced rf output by the ground of the plate copper carrier (see figure 8 ) without further wave discontinuity, therefore avoiding losses and creating a reliable design to support very high rf output power. the transformer t2 has been designed using commercially available sw (ads, hfss) and continues the refinement between electromagnetic and circuit simulation: t2, in fact, uses a lamped capacitor (c25, c26, c23 caps group on winding top strip, and c37, c42 caps group on the bottom side strip) to tune the proper impedance for dmos drains. in particular, the output biasing network (acts through the center tap of the winding top strip of t2) uses several multilayer ceramic capacitors, and also adds the following electrical functions: 1. dampens voltage overshoot generated by each transient effected by pulsed rf modulation: that is the group l10, r13, c29, c30, c33. 2. two test points can be inserted between two calibrated rm resistors for current / voltage monitoring. 3. lamp led d1, for safety purposes.
circuit description AN4016 6/17 doc id 022523 rev 1 finally, the two bipole groups, consisting of l3-c37-r1/r5 and l14-c5 8 -r2 8 /r30, are inserted in the drain side of the amplifier and give more flexibility to the impedence, for example, it is used to improve low frequency stability, or to dominate harmonic impedance, or as broadband internal rf loads.
AN4016 layout, parts list, and design considerations doc id 022523 rev 1 7/17 3 layout, parts list, and design considerations as mentioned previously (see figure 3 ), the amplifier is built with separated input-output pcb cards: a) the input pcb, as shown in figure 5 , integrates the rf balun transformer, t1, together with the rf decoupling networks. b) the output pcb, see figure 7 , however, relates to the design of the rf transformer, t2, with the remaining biasing/filtering networks for the gates and drains of stac4932b. an image of the assembled board is shown in figure 1 , while ta b l e 1 gives the part list. figure 4 , shows the final assembled board on the copper carrier and heatsink: board robustness is an important factor in order to ensure electrical stability to manage very high rf power. the pcb cards are built in substrate roger 4350b, in order to reduce dielectric losses through the joule effect (50 to 100 w less when compared to fr4 @ 2 kw), and in particular to maintain thermal expansion compatibility with the copper carrier. another aspect of this rigid thermoset laminate allows the creation of a pcb with very good surface finish, planarity and roughness, which are compatible with copper carrier surfaces that support it. in fact, a carefully finished pcb surface is recommended: hal lf with tinned chemical deposition. moreover, accurate mounting procedures need to avoid bending/dirt that can compromise the planarity of pcb cards.
layout, parts list, and design considerations AN4016 8 /17 doc id 022523 rev 1 figure 4. 2 kw mri final assembled board figure 5 and figure 7 show the input and output pcb cards, top and bottom-side. a mechanical drawing of the base-plate is shown in figure 8 : in particular, the two counterbores housing the transformers t1 and t2, are designed to control the unwanted parasitic impedance (leakage) to ground. am10221v1
AN4016 layout, parts list, and design considerations doc id 022523 rev 1 9/17 figure 5. input pcb, and top and bottom view the new package technology (stac ? ) allows very low thermal impedance to be achieved, rtjc = 0.075 k / w (with t = 1 msec pulsed rf/duty = 10%), so that, in combination with a suitable heatsink (heatsink @ rca <0.2 k / w max.), it permits the junction temperature to be lower than the rating (tjmax = 200 degc): in fact, considering a 60% efficiency @ 2200 w, a deltatjc = 56 c, and deltatca <15 c, a tj=95 c max. junction temperature can be expected. the ability of stac ? to dissipate a high power pulse (see an3232) allows the possibility to reduce board dimension and external heatsinks; so that, using the flangeless package stac 244f (see figure 6 ), you can design a new board with the same electrical characteristics but with a dimension target of 8 0 x 100 mm. figure 6. stac244b am10222v1
layout, parts list, and design considerations AN4016 10/17 doc id 022523 rev 1 figure 7. output pcb, top and bottom view am1022 3 v1 table 1. 2 kw mri part list component id value manufacturer part code c12, c13, c6 1000 f, 100 v panasonic eca2am102 c10, c11, c52, c53 100 nf murata gcm1 88 r71e104ka57d c2 8 4.7 f, 100 v tdk ckg57nx7r1e226m c29 15 f, 100 v murata krm552r72a156m c15, c51 10 f, 35 v kemet t494d106k035at c3,c47 100 f, 20 v kemet t491x107k020at c4, c7, c4 8 , c49 22 f, 25 v murata grm32er61e226me15 c1 8 , c41 300 pf atc atc100b301fwn200xc c16, c20, c36, c45 6 8 pf atc atc 8 00a6 8 0jtn250x
AN4016 layout, parts list, and design considerations doc id 022523 rev 1 11/17 c30 1000 pf atc atc100b102fwn300xc c57, c5 8 3.3 pf atc atc100b3r3bw1500xt c 8 , c31, c32, c50 470 pf atc atc 100b 471fwn200xc c2, c17, c24, c34, c40, c55, c59, c60, c61, c62, c63, c64 2000 pf atc atc 200a202ktn50c c9, c14 1 f, 100 v avx 22201c105kat2a c33 1 f, 100 v avx 12101c105k4z2a c21 1 8 pf atc atc100b1 8 0fwn1500xt c25 75 pf atc atc100b750fwn1500xc c26, c23 100 pf atc atc100b101fwn1500xc c37, c42 56 pf atc atc100b560fwn1500xc l10 700 nh coilcraft cp-k0376-a l2, l5, l13, l15 8 2 nh coilcraft 1515sq- 8 2njeb l3, l14 110 nh coilcraft 132-10smj l4, l 8 , l12, l16 5.4 nh coilcraft 0906-5jlb r1, r5, r2 8 , r30 50 , 100 w anaren c100n50z4 r9 5600 tyco electronics smf25k6jt r13 22 tyco electronics smw222rjt r7, r29 100 panasonic erjp14j101u r11, r22 4.7 vishay 4.7 ohm -1206 r3, r24 43 panasonic erjp14j430u r32, r33, r34, r35 27 panasonic erjp14j270u r4, r16, r17, r31 20 panasonic erjp14j200u r6, r 8 , r10, r12, r14, r15, r19, r20, r21,r25,r26, r27 1 phycomp 23227111110 8 rm x 2 0.001 m tyco electronics tl3a r001 1% 1p_j3 1 double pole wieland 25.700.0153.0 spacer_j3 spacer wieland 07.300.2753.0 3p_j1,j2 3 poles phoenix contact 1725669 p2 n_female telegartner j01021a10 8 4 p1 sma_female radiall r124.510.000w (q1-q2)/(q3-q4) stac4932b stmicroelectronics stac4932b d1 led kingbright kp-160 8 surc table 1. 2 kw mri part list (continued) component id value manufacturer part code
layout, parts list, and design considerations AN4016 12/17 doc id 022523 rev 1 table 2. materials part list component description line bridge roger 4350b, three layers, 20+20 mils, 1 oz cu on top-mid-bottom layers, finit. metal hal lf; total tk=1.2 mm max., top screen printing component, tin chemical surface deposition. board input roger 4350b, three layers, 20+20 mils, 1 oz cu on top-mid-bottom layers, finit. metal hal lf; total tk=1.2 mm max., top screen printing comp., tin chemical surface deposition. fin fixing roger 4350b, two layers, tk=60 mils, 1 oz cu on top- bottom layers, finit. metal hal lf; total tk=1.6 mm max., top screen printing comp., tin chemical surface deposition. board output roger 4350b, two layers, tk=60 mils, 1 oz cu on top-bottom layers, finit. metal hal lf; total tk=1.6 mm max., top screen printing comp., tin chemical surface deposition. mechanical plate ppamri_002-rev b
AN4016 layout, parts list, and design considerations doc id 022523 rev 1 13/17 figure 8. base-plate copper carrier a m 1 0 2 2 4 v 2
mri board performance and application AN4016 14/17 doc id 022523 rev 1 4 mri board performance and application the power amplifier has been measured on two different rf scalar test benches: rf power lab. in stm catania (italy) and rf power lab. in stm quakertown (usa): the measurements are in good agreement (+ / -0.15 db max. error). the test includes a 2 kw/cw attenuator and a pulsed rf generator with high power amplifier driver to manage large signals at rf input (20 w min.) with good harmonic rejection (-30 dbc). figure 9. gain and irl frequency response figure 9 shows the large signal gain frequency response of the amplifier, as well as the input return loss, while figure 10 shows the gain compression curve and the drain efficiency curve vs output power at 123 mhz (idq = 200 ma and vds = 100 v) and rf pulse width=1 msec, duty cycle=10%. the maximum efficiency is 60% @ 2.2 kw of output power. -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 5 7 9 11 13 15 17 19 21 23 25 105 110 115 120 125 130 135 irl(db ) gain(db) freq(mhz ) stac4932b - gain & irl vs freq @ pin=43 dbm pulsed width= 1msec, dutycycle =10% vds=100 vdc, idq=2 x 100 ma gain irl stac244b am10225v1
AN4016 conclusion doc id 022523 rev 1 15/17 figure 10. gain and drain efficiency vs. output power for ims applications, two or more boards can be embedded to realize high power rf chains (4 kw or more): e.g., 10 kw rf power can be obtained by linking six rf basic units, properly using a gysel power combiner, and integrated with the appropriate / 4 transmission lines to improve electrical stability, together with a control/monitoring card to support global safety. 5 conclusion a pulsed rf high power amplifier (> 2 kw) has been described as a guideline-design, oriented to new high voltage dmos devices at vd = 100 v: stac4932b. in particular, the amplifier combines excellent high frequency response with an efficient use of dc power and allows a very compact design and robustness, in conjunction with smt technology and joined to the fully planar microstrip design (rf transformers). this amplifier can be understood as the basic unit for high power rf chains to achieve very high power for an rf pulse generator in the rf systems for medical magnetic resonance imaging (3t-fmri). 6 references rf and microwave power amplifier design, by andrei grebennikov - mc graw hill, 2005. essentials of rf and microwave grounding, by eric holzman - artech house, 2006 an3232 application note. 0 10 20 30 40 50 60 70 80 90 100 5 7 9 11 13 15 17 19 21 23 25 0 500 1000 1500 2000 2500 drain eff.(%) gain(db) pout(w) stac4932b - gain & eff. vs pout @ 123 mhz pulsed width= 1msec, dutycycle =10% vds=100 vdc, idq=2 x 100 ma gain eff. am10226v1
revision history AN4016 16/17 doc id 022523 rev 1 7 revision history table 3. document revision history date revision changes 23-dec-2011 1 initial release.
AN4016 doc id 022523 rev 1 17/17 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a particular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by two authorized st representatives, st products are not recommended, authorized or warranted for use in military, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or register ed trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2011 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - philippines - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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