ST AN1224 Application note

AN1224

Application note

Evaluation board using SD57045 LDMOS

RF transistor for FM broadcast application

Introduction

LDMOS technology allows the manufacturing of high efficiency and high gain amplifiers for FM transmitters. LDMOS has proven advantages against bipolar devices in terms of higher gain, efficiency, linearity, and biasing simpleness that lower the overall system cost and make them attractive for high volume businesses demanding low cost RF power transistor solutions. Thanks to these advantages, LDMOS RF power transistors are the proven mainstay in the power amplifier business of the cellular base station today. The device used for the present characterization, SD57045, an STMicroelectronics product, is a lateral current, double diffused MOS transistor that delivers 45 W under 28 V supply. It is unmatched from DC to 1 Ghz making it eligible for a variety of applications, especially for high performance, low cost FM driver applications. This application note documents the feasibility of a low cost 900 MHz cellular device as a commercial FM driver. The key advantages of LDMOS technology are improved thermal resistance and reduced source output inductance. The wire-bonded connections to the external circuitry (DMOS config.) are no longer required because the source at the chip surface is connected to the substrate by the diffusion of a highly doped p-type region. Consequently, LDMOS has excellent high frequency response because of its high f capacitance and reduced source inductance. An additional advantage of the LDMOS structure is that beryllium oxide (BeO), a toxic electrical insulator required to isolate the drain with DMOS transistors, is no longer needed. Hence, not only the thermal resistance is improved, but package cost and environmental impact are significantly reduced. Finally, in an LDMOS, the parasitic bipolar has been nullified guaranteeing good ruggedness, efficiency and high current handling capability.

and superior gain due to the low feedback

October 2007 Rev 3 1/10

www.st.com

Contents AN1224

Contents

1 Circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Description and consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Characterization results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2/10

AN1224 List of figures

List of figures

Figure 1. Broadband 4:1 transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 2. Broadband power amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Layout for broadband power amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Drain current vs. gate-source voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Gate-source voltage vs. case temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Output power and efficiency vs. input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 7. Power gain and efficiency vs. output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 8. Class A safe operating area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3/10

Circuit design AN1224

1 Circuit design

1.1 Description and consideration

Input and output impedances for the SD57045 are shown in Table 1 below:

Table 1. Input and output impedances

Frequency (MHz) Z input Z output

88 10.8-j7.60 7.5-j0.15

95 10.6-j8.36 7.8-j0.34

108 10.5-j9.87 8.1-j0.61

With respect to these impedances, two 4:1 transmission line auto transformers were designed using a 25 Ω, 1/8 wavelength, semi rigid coaxial cable. To achieve this transformation across the band, a capacitor was added to the low impedance port of each transformer to cancel the leakage inductance. The frequency response is shown in Figure 1. Simple L-sections were utilized to make the final transformation from the low impedance port of the transformers (12.5 Ω) to the measured impedances of the device (see Tab l e 1). This design uses printed series inductors on a 30 mil glass teflon board. The gain of any power FET is extremely high from DC throughout the low HF frequency band. A feedback network is necessary to suppress the low frequency gain, as well as give a nominal amount of gain at the frequency of interest. This feedback also helps to increase the input impedance. Since LDMOS has such a high gain at low frequencies, a low value, high power, flange mount resistor must be comprised in the design. The capacitor in the feedback path (C3) provides negative feedback at low frequencies. This component was designed to be self-resonant. Far below the FM band, at 100 MHz, the capacitor looks slightly inductive, reducing the amount of feedback in the band of interest.

Figure 1. Broadband 4:1 transformer

S11

0dB

-30dB

-60dB

80MHz 90MHz

Unbalanced transformers offer an efficient matching method from 50 W to low impedance. Besides, auto transformers have a zero impedance point over a broad bandwidth, offering an ideal DC feeding point to the gate and drain circuits. In order to prevent high frequency oscillations, a bypass capacitor is used at the zero impedance point of the transformer. The capacitor value must be selected so that its own resonant frequency is above the frequency

4/10

100MHz

110MHz

AN1224 Circuit design

of interest. Depending on the application, additional low frequency bypass capacitors isolated with lossy elements (ferrite beads) may be required to prevent power supply noise affecting gate and drain circuits. Circuit schematic is given in Figure 2, and layout in Figure 3 with component values in Table 2 .

Table 2. Bill of material

Reference Description

L1, L3, L4, L7 50 Ω transmission line

C1,C13 1000 pF chip capacitor

C2 39000 pF chip capacitor

C3 36 pF chip capacitor

R1 1 kΩ resistor

C4, C6, C10 10000 pF chip capacitor

R2 1.2 kΩ resistor

C5, C12 10 µF, 50V electrolytic capacitor

R3 240 Ω / 40 W resistor

C9, C11 1200 pF chip capacitor

C8 33 pF chip capacitor

C7 25-115 pF variable cap-arco trimmer

L2, L6 4:1 transformers, 10.7", 25 Ω.

Board 30mils, 2 ounces of copper, ε

Figure 2. Broadband power amplifier

r= 2.55

5/10

Characterization results AN1224

Figure 3. Layout for broadband power amplifier

2 Characterization results

● T

Table 3. Absolute maximum ratings

Table 4. Thermal data

= 25 °C

case

Symbol Parameter Value Unit

(BR)DSS

DGR

DISS

JMax

STG

Drain-source voltage 65 V

Drain-gate voltage (RGS = 1 MΩ) 65 V

Gate-source voltage +/-20 V

Drain current 5 A

Power dissipation (at TC=70°C) 93 W

Operating junction temperature 200 °C

Storage temperature -65 to 200 °C

Symbol Parameter Value Unit

th(j-c)

Junction-case thermal resistance 1.4 °C/W

6/10

AN1224 Characterization results

Figure 4. Drain current vs. gate-source voltage

3.5

2.5

1.5

ID, DRAIN CURRENT (A)

0.5

2.5 3 3.5 4 4.5 5

VGS, GATE-SOURCE VOLTAGE (VOLTS)

Figure 5. Gate-source voltage vs. case temperature

1.04

1.02

0.98

0.96

-25 0 25 50 75

VGS, GATE-SOURCE VOLTAGE (NORMALIZED)

Tcase, CASE TEMPERATURE (˚C)

Id=250mA

Id=3 A

Id=2 A

Id=1.5A

Id=1 A

7/10

Characterization results AN1224

Figure 6. Output power and efficiency vs. input power

Output Power (W)

0.1 0.2 0.3 0.4 0.5 0.6

Input Power (W)

Pout

Freq=95 MHz Idq=250 mA Vdd=28V

Figure 7. Power gain and efficiency vs. output power

Gain

Gain (dB)

Freq=95 MHz Idq=250 mA Vdd=28V

15 30 45 60

Pout (W)

Eff

Efficiency (%)

Eff

Efficiency(%)

Figure 8. Class A safe operating area

Tj=200˚C

ID, DRAIN CURRENT (A)

1 10 100

VDS, DRAIN-SOURCE VOLTAGE (VOLTS)

8/10

Tc= 70 ˚C

Tc=100˚C

AN1224 Conclusion

3 Conclusion

In this application note we have demonstrated the feasibility of a low cost, 900 MHz cellular device as a commercial FM driver. One can conclude that ST LDMOS technology offers viable solutions for power amplifiers at frequencies covering the high HF throughout the high UHF bands. More information about these devices can be found at http://www.st.com/rf.

4 Revision history

Table 5. Document revision history

Date Revision Changes

13-Sep-2007 2 No content change

26-Oct-2007 3

– Document reformatted no content change – Modified: title

9/10

AN1224

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 herein 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 assumes 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. If 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 whatsoever 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 AN AUTHORIZED ST REPRESENTATIVE, 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 whatsoever, any liability of ST.

ST and the ST logo are trademarks or registered 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.

STMicroelectronics group of companies

Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan -

Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America