ST AN4016 Application note

Introduction

AM10219V1

AN4016

Application note

2 kW PPA for ISM applications

STMicoelectronics has recently introduced a new generation of high voltage DMOS products housed in STAC 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

air cavity packages and capable of delivering an output power of

December 2011 Doc ID 022523 Rev 1 1/17

www.st.com

Contents AN4016

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

2/17 Doc ID 022523 Rev 1

AN4016 Design choices

P1_RF INPUT

SMA_Female

P2_RF OUTPUT

N_Female

GND

C31

C32

GND

R13

C9C14

GNDGND

GND

C21

C40

C18

R10

R12

R14

R15

C41

GND

R19

R20

R21

R25

R26

R27

GND

VCC

C13+C12

GNDGND

C23

C28

C30

GND

R17

R22

Vg1

C34

C16

GND

GNDGND

C10

GNDGND

C15

GND

C11

GNDGND

C17

STAC4932B

L14

R30

C57

GND

C58

R24

Vg2

C48+C47

GNDGND

L13

C49

C52

GNDGND

C51

GND

R29

C50C53

GNDGND

L15

GND

L10

L16

L12

GND

R11

R16C24

GND

C20

C36

C45

R31

C55

GND

R28

R9 GND

GND

C33

C37

C26C25

(~5 V typ)

(~5 V typ.)

(+100 V)

LED

R32

GND

R33

GND

R34

R35

GND

C60

C59

C61

C63

C62

C64

VBIAS_1

VBIAS_2

GND

C29

GND

C42

GND

Line_Bridge

GND

CON3

Vg1

GND

Vg2

CON11CON1

GND

Input Board 3 layer

Output Board 2 layer

AM10220V1

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 boltdown 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

Doc ID 022523 Rev 1 3/17

Design choices AN4016

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.

4/17 Doc ID 022523 Rev 1

AN4016 Circuit description

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, λ / 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, L8, C16, C18, 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, R8, 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.

Doc ID 022523 Rev 1 5/17

Circuit description AN4016

Finally, the two bipole groups, consisting of L3-C37-R1/R5 and L14-C58-R28/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.

6/17 Doc ID 022523 Rev 1

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