By Melanie M Daniels

Power modules, as discussed in ihis article, are the output stage of the RF (radio frequency) amplification chain in a mobile telephone (Fig. 1). Some telephones use integrated circuits as power solutions, but for output power greater t han one watt a discrete device is usually used. A power module uses networks to match the discrete stages in a hybrid amplifier.

This article describes a power module designed for a GSM (Global System for Mobile Communications) handheld telephone. GSM telephones transmit in the frequency range of 880 to 915 MHz. The peak transmitter carrier power for power class 4 is 3.5 watts at 1/8 duty cycle. Unlike other TD MA (time division multiple access) systems, it is possible to run a GSM power module close to compression because the amplitude is constant using GMSK (Gaussian minimum phase shift keying) modulation. The pulse width of the transmission burst: is 577 microseconds, and the rise time of the power module must be less than 2 (is. It is necessary to supply full output power at a supply voltage of 5.4 volts (five NiCad cells at end of life) with 40% efficiency and 0-dBm input power. This is a requirement of the customer for the phone to be competitive. Future generations of phones may use only four NiCad cells or other batteiy types and voltages. Of course, a handheld phone must be inexpensive and small and have long talk time ( i.e., efficiency) and this dictates the specifications for tire power module.

The design goals called for the power module to be small, inexpensive, user friendly, efficient, and manufact ttrable in volume, and to supply full output power.

Silicon bipolar devices were chosen over GaAs FET devices for this product because of their cost advantages and the fact that IIP had developed new silicon power devices that met (he stringent requirements or applications in which GaAs had traditionally been used (i.e., low device voltages and excellent efficiency).

Fig. 2 is a photograph of the power module. Tire schematic diagram, Fig. -3, shows the electrical design of the power module. The bias circuits must be simple and fast because of the pulsed nature oft be GSM modulation. Because of the

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Fig. 2 is a photograph of the power module. Tire schematic diagram, Fig. -3, shows the electrical design of the power module. The bias circuits must be simple and fast because of the pulsed nature oft be GSM modulation. Because of the

Speaker

Antenna

Micro phone

Micro phone

Fig. 1. Block diagram of a typical handheld digital teléj jlrote

Original

Version

Original

Version

Current Version

Fig. 3. GSM power module fvolutjon-

low voltage requirements, proprietary silicon power bipolar devices were developed. The collector of each stage simply has an RF choke to the 5.4V minimum supply voltage. Vl:c., and a bypass capacitor to ground. 'lite base voltage supply is used to turn the amplifier on and off and to control the output power level of the module. The control voltage. V',., is pulsed at 1/8 duty cycle with a square wave from OV when the module is off t o 4.0V when die module supplies full output power. The base of each stage has a series resistor to the control voltage. This resistor is adjusted to compensate for each transistor's current gain, [i. This is done using active laser trimming and will be discussed as a separate topic. Since the power control voltage supplied by the phone does not have I he capability of supplying the base current of each stage, a low-frequency n-p-n transistor, Q4, is used to buffer the control voltage. The collector of Q4 is biased by the supply voltage, Vcc, The base of Q4 is driven by the power control voltage and the emitter supplies the necessary voltage and current to the base of each RF stag«".

The fiF design uses lumped elements for input, interstage, and output matching. The design requires three si ages to achieve the gain requirements. The fust stage is a driver stage that is c!ass-A biased. The second and third stages are class-AB biased for efficiency.

The third-stage transistor also has some internal matching widtin the package. The input impedance of the silicon power transistor chip is about 1.5 ohms. This must be transformed up to a larger impedance by a marching network that is physically as close to the chip as possible. This is achieved using aO.OQl-mch-diameter bond wire as a series inductor from the base of the chip to a shunt MOS cajjacitor at the input of the transistor package (Fig. 4). This configuration makes a very higli-Q input matching network. The exact value of capacitor and the length of bond wire had to be empirically optimized to achieve the maximum transformation within the transistor package.

The ti lost critical and sensitive part of the matching networks is the output of the final stage. High-Q lumped-element components are used in the output matching network to achieve the low losses necessary to meet the efficiency requirements.

Since the design has more than 45 dB of small-signal gain in a 1 -inch-by-0..5-inch package, stability :md isolation were quite challenging. The placement and values of the RF chokes and decoupling capacitors were critical. Large-value capacitors could not be placed on the base bias network, since this would slow down the pulse response of the module.

Mechanical Design

As previously mentioned, some of the primary design goals were (1) low cost because this is a commercial product, (2) small size lo allow phone manufacturers to design smaller products for portability (also a competitive advantage for HI'], and (II) compatibility with high-volume manufacturing. In addition, the power module component had to be supplied as a surface mount component in 1 ape-;uid-reel form. The mechanical design of the power module turned out to be one Of the most challenging parts of the development project. At the time the project was started, most competitors were using soft board for the substrate material and surface mount lumped components for matching. This material definitely

(Laser Trimmed!

(Laser Trimmed!

I Laser Trimmed)

I Laser Trimmed)

Tig, :), Si'henuitii Ii;igrutu of the i ISM jxiwer module

Fig. 4, Packaged output stage transistor.

Top View

Four Single Cells In Parallel

Side View

Top View

Four Single Cells In Parallel

Side View

Fig. 4, Packaged output stage transistor.

meets the cosl criteria, but there were thermal, RF matching, and laser trimming limitations. Thick-film alumina ceramic technology was chosen for the board material. Even though the material is more expensive, this is offset by the fact that the RF matching networks are more compact because of the high dielectric constant er = 9.1. Also, the resistors and inductors can he printed on the board, thus reducing the part count. Ceramic has superior thermal conductivity compared to soft boards. The most persuasive reason for ceramic substrates is that they rlo not require a metal carrier to be surface mounted. The vias in a ceramic board can be filled with metal paste so components can be placed directly on top of the via This reduces the emitter-to-ground inductance for the transistors and gives superior gain and efficiency performance. This factor also reduces the size of the module to

I inch by 0.4 inch. Standard surface mount components on PdAg traces are used for lumped-element matching and custom surface mount packages are used for the RF transistors.

The inputs and outputs of the power module use wraparound edge vias. This is commonly referred to as an LC.'C ('leadless chip carrier) for surface mount component manufacturers.

II is inexpensive because no leadframes need to be attached. The metal thick film used in the vias must pass all solder-ability tests.

Volume Assembly

Fig. 5 shows the process flow for manufacturing t he power modules. Modules are built in array form with 40 modules per 4-uich-by-4-inch array. More modules per array reduces the ceramic substrate cost and the surface mount assembly cost but also increases the complexity of substrate manufacturing. Tfie boards are populated by a subcontractor with standard pick-and-place equipment, then reflowed at a peak temperature of 220:C using SN96 solder paste. The high-temperature reflow was chosen to prevent a secondary re-flow by the customer when the power modide is surface mounted into the telephone. Developing the reflow profiles with lite chosen thick-film paste and high-temperature solder was not a trivial task.

The populated arrays are then actively laser trimmed. Each base bias resistor (three per module) must be trimmed with

Fig. 5. Volume assembly protest; flow

the laser to set the transistor collector current. This is to compensate for (i variation in device fabrication. The simpler bias scheme was chosen because we couldn't afford the cost and efficiency loss of an activ e bias circuit. Developing the laser trim process was another challenging aspect of this product.

Two of the transistors are buised off (have the bases grounded) while the third is being trimmed. This is necessary to avoid oscillations caused by the high gain and the difficulty of adequate grounding while the modules are in array form. Extensive development was required to obtain good RF grounding on the backside of the module and in the bias probes while laser trimming. A grounding gasket made of silver impregnated niblier is used with a vacuum to achieve backside grounding. High-frequency probes with good 50-ohm loads ar e used on all inputs and outputs to avoid oscillations.

Special software was developed for the laser trimmer. Algorithms were wrilten to compensate for the current drawn through the gronnded-base transistors. In addition, the resistors and transistors heat up during the trim process and this has to be compensated. The trim process hits to be done in a pulsed bias mode, since the module cannot be run in CW mode without thermal runaway. Finally, the output power cannot reach saturation before the maximum control voltage is reached, since this impacts the customer's power control loop. To resolve this issue, I he modules are ttinuned at a control voltage of 3.2V maximum.

After laser trimming the lids are attached in array form. The array is then separated into individual units, which are tested using automated test equipment. All of the problems addressed for the laser trimming were also present for the automated test process. The module grounding is absolutely critical fo testing RF parameters. Developing test fixtures that could tesl hundreds of modules per hour, three shifts a day, and si ill retain a good RF ground was critical to the success of the product, The average test time per module is 20 seconds. The automated lest programs are complex be cause of the number of" tests thai have lo be performed and the fact that they all have to be done in pulsed mode,

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