Anatomy of RF Amplifiers – Failure Mechanics and Maintenance Strategies

Tuesday 28 May, 2019

 

Article by M S Duhan, Deputy Director General (Engineering), AIR & Doordarshan, India

 

RF Amplifiers are the key component in any TV and Radio Transmitter. They use costly transistors and these are cascaded to provide higher powers. So, the main concern of broadcasters is to minimise the failure of these components. The cooling systems, alignments, skills of operating and maintenance personnel are key for minimising the failures and optimal performance of the transmitters. Heat management and stability of the gain of RF amplifiers can be greatly improved by improving cooling systems.

This article provides an anatomy of failures of RF Amplifiers and improving their optimal performance through various maintenance strategies.

 

1. Introduction

The RF power amplifier is a pivotal component in broadcasting systems because its performance & power consumption dominates the other parts in the system. The purpose of the RF PA is to amplify the radio signal to a desired power level. The RF output Power, efficiency and linearity in amplification process is of the essence. 50V LDMOS devices have dramatically increased power density, efficiency and reliability. The most prominent stresses on transistors are temperature, voltage, vibration and moisture etc. The number of Transistors used in some of the air cooled and liquid cooled TV Transmitters of Doordarshan’s Terrestrial Network are detailed in Table 1.

Table 1: Transistors in TV HPTs

 

 

 

 

 

 

 

 

 

 

 

The reliability of semiconductor devices depends on their resistance to various stresses applied to the devices and possible mechanisms to avert or minimise such stresses. The liquid cooling systems provide controlled temperature design of amplifier, reduces their size. The other important aspect of the maintenance strategy includes optimisation of the RF chain alignment, use of efficient jigs & soldering techniques and instilling skills among the O&M personnel.

 

2. Anatomy of Failures

LDMOS technology development is an ongoing process offering significant improvements. One of the major improvements is Idq drift reduction in LDMOST devices. A MOSFET dissipates little energy during its steady on or off states, but it dissipates considerable energy during times of transition. It is therefore desirable to switch as quickly as possible to minimise power dissipated. A high rate of voltage change over time (dv/dt) between the drain and source of the MOSFET may cause permanent damage or false switching. Carriers that are injected into the depletion layer are accelerated by the high field, and some of them gain enough energy to cause impact ionization. These carriers have higher energy than thermal energy and are called hot carriers. In power transistors, failure is often associated to instability, which causes one or more device parameters (current, voltage and/or temperature) to diverge (runaway). RF Transistor failures results mainly 50% parameter Change, 40% shorted, 10% open. More displacement current is built up with greater dv/ dt. The displacement current is added to avalanche current and the device becomes more vulnerable to failure. A device experiences avalanche current and displacement current at same time. Therefore Vgs, Vds, Id must be within limits. There is also a power limit given by the maximum junction temperature. Basic values for the upper maxima of these parameters are given in the safe operating area (SOA) graph in the transistor datasheet.

New 50V LDMOS devices are introduced that dramatically increase power density, efficiency and reliability. The BLF861A transistor demonstrates exceptional performance for UHF transmitters, which can withstand source/load failures under full power conditions for a short period of time with no thermal damage. Bad handling of FETs can lead to damage by electrostatic discharge. The results of resistance tests on FETs are generally not as easy to interpret as in bipolar transistors. The only effective test for IGFETs, is by substituting a known good device. A component genuinely is another key issue as substandard components are also available in market.

Some of the basic causes of failure of amplifiers and other broadcast equipment are temperature, humidity, vibrations, dust, static electricity stress, VSWR & thermal fatigue. So if we take care of these causes then the failure can be minimised. Of the above, heat or temperature is the biggest element. It is due to rise of junction temperature and the type of cooling system employed.

It is important to note that every 10°C rise in temperature over the rated junction temperature (Tj), reduces the life by 50%. The junction temperature is related with the output power and cooling system employed.

The various observed stress mechanics of Transistors are explained as below:

 

     2.1 Temperature:

The temperature affects the life of semiconductors. When a rapid or gradual change occurs to a device, the characteristics of the device may be deteriorated, and finally the device may malfunction. The power loss of the device turns into heat and increases the junction temperature. This degrades device characteristics and reduces its life span. It also affects the gain and linearity of the Transmitter. It is very important to lower the junction temperature by discharging heat from the chip junction. The bias adjustment is critical in this regard. The excessive RF power may cause a rise of junction temperature, and the rise of junction temperature may raise the failure rate. So, the electric current should be lowered to the permissible limits.

     2.2 Humidity & Dust:

The surface of each IC chip is covered with a surface protective film. For this reason, IC chips are not easily affected by humidity, but resin moulded devices are water permeable. If it is expected that the device is operated under severe conditions of humidity, it should be operated particularly carefully. Dust creates a thermal barrier. The dust factor, combined with, is fatal and needs to be taken care of most.

     2.3 Mechanical Stress/Vibrations:

If the device is strongly vibrated during transportation, loose foundation of racks, loose fan or, if an extremely strong force is applied to a device during installation or O&M, the device may be mechanically damaged. In addition, moisture or a contaminant may enter the device through the damaged area, and may cause deterioration of the device.

     2.4 Static Electricity:

Electrostatic charges can damage equipment. Recently, plastic is generally used for the casing and the structure of equipment. To prevent electrostatic charge through the human bodies during repairs it is necessary to take preventive measures, such as the use of anti-static wrist bands against static charge.

     2.5 Thermal Fatigue:

When a heating device is repeatedly turned on and off, thermal distortion of the material may be repeated, and fatigue may occur in the material and between different types of materials. As a result, the device may malfunction. Thermal fatigue typically occurs when the thermal resistance is increased by deterioration of die bonding material. Safe Operating Areas (SOA) defines the maximum value of the drain-source voltage and drain current to guarantee safe operation when a device in a forward bias condition.

 

    2.6 VSWR:

The mismatch of RF output with the antenna system is another reason of MOSFET Failures. It may cause very high heat generation and various components may be blown, as shown in Figure 2. The circulators reject loads, combiners, capacitors, transistors and PCB track etc. are main failures with high VSWR.

 

3. Tryst with Cooling System for RF Amplifiers

The number-one factor determining the reliability of solid state devices is temperature. Heat management and stability of the gain of RF amplifiers can be greatly improved by introducing an effective cooling system. Air cooling systems may cause dust accumulation on the heatsinks etc., which act as heat insulation. Figure 1. shows the provision of bias adjustment to prevent overdrive of RF Amplifier. Further the fans are probably the most unreliable components, having moving parts and therefore a limited life. They are also susceptible to shock and vibrations. Generally about 13°C rise in temperature may occur between inlet and outlet duct. The optimal air pressure is another vital requirement.

Some observations related to cooling system etc. are summarised below:

     3.1 Experiments with air Cooling:

Normally the air cooled transmitters use air from outside the building. It is filtered and dried by an electrostatic source and then a blower directs the air to the transmitter through an inlet duct. The air is then passed through narrow ducts over the heatsinks of the PAs. The outlet duct passes the air out of building.

Modifications were carried out at many Transmitters, Vadodara, Surat etc. for closed air cooling. There the outlet duct is routed in the blower room where the same air is cooled and recycled to the Transmitter. The advantage is the absence of dust and humidity. Further experiments were carried out at HPT Ajmer and Varanasi where the outlet duct was opened in the top of the transmitter. The air route from transmitter to blower room enabled further cooling and reduced the PA temperature by about 6°C. Various cooling systems are shown in Figure 3.

 

     3.2 The planting of trees alongside the Transmitter building improved the cooling system at HPT Surat and LPT Khetri, where the efficiency and life of the air-conditioning units were also improved.

     3.3 Glazed white tiles were installed on the roof of the transmitter at HPT Vadodara and, by reflecting the heat, this achieved about 2°C drop within the transmitter hall.

     3.4 Liquid cooling of the amplifier’s solid-state transistors has a number of advantages. The various advantages of liquid cooling over air cooling are; controlled temperature design of amplifiers, reducing the size of the amplifier due to absence of large metal heat sinks, reduction of the heat load in the amplifier room and its resulting HVAC requirements, it also allows for fewer fans, which makes the amplifier audibly quieter. Humidity and dust/dirt control is excellent in liquid cooled systems. Thus operating costs can be reduced.

 

4. Observations, Field Experiments & Experiences of Transistor Failures

The field unit of DD Kendra carried out repairs and maintenance of the transmitters. Technical workshops were also organised at various Kendra’s. A few experiments, experiences and observations on the repair of various Amplifiers and modulators proved educative for the maintenance engineers and are expounded as below:

     4.1 Liquid cooled Transmitters have far fewer failures of RF Transistors. A 20 kW liquid cooled NEC Make PCU-1320 SSPQ/1 TV Transmitter was commissioned at HPT Dharamshala w.e.f 31.12.2007. It uses 18 BLF 861A MOSFETs in each PAs(12 Visual and 3 Aural Pas).There has been no failure of any MOSFET so far. In comparison, from 2011 to 2018, 185 2SK1543M transistors failed in the 10 kW air cooled TV Transmitter NEC PCN1610SSPH/1, with 12 Vision PAs and 2 Aural PAs, at HPT Kanpur.

 

     4.2 There are two 10 kW HPTs at Vadodara – Thomcast (VHF) and NEC (UHF). During two years of observations between June 2006 to June 2008, the monthly cleaning, alignments of bias & AGC and measurements/ calibrations etc. were carried out under direct control of only two expert engineers during night time, after close down of Transmission. The closed cooling system was kept optimal. There was no failure of transistors (BLF 278 and 2SK2396) during that period.

 

     4.3 Soldering Irons with temperature controlled tips should be used and peak soldering temperature should not exceed 260°C for 4 seconds. The pre-heat & Soak temperature gradient should be 2°C/s and   cool down temperature gradient should be -2°C or less. Anti-static wrist bands should be worn by engineers during repair of Transistors.

 

     4.4 Heat dissipation of transistors: Heat dissipated by Transistors need to be checked a shown in the Figure 5. The required alignment or replacement of components needs to be carried out within limits of  parameters for optimal functioning.

 

4.5 For electronic equipment, the most prominent stress is temperature. The importance of observing the limits cannot be overstressed, since the failure rate of the components will double for a 10°C increase in temperature. Decreasing the size of a unit without increasing its efficiency will make it hotter, and therefore less reliable. Two examples for relieving thermal stress are given below:                                            

i. Vision modulator in R&S Transmitters, NH7100V, 10 KW a Transistor Amplifier ACB 3303 (gain 20 dB) in Modulator unit was failing frequently. The circuit detail of it shown in Figure 6. It was observed that heat accumulates in the cabinet. A fan was mounted on top of cabinet to extract the heat (Figure 5). The failure reduced significantly.               

ii. One typical fault was occurring in the exciter of 6kW DTT transmitter. It was analyzed & repaired. It showed RED indication in front screen as “RF MUTE”. The TCU showed “power low” and its upconverter voltage Vdc showed 0 V instead of 20 V. The Exciter output showed 1 mW peak instead of 100 mW. GaAs MMIC U 61 AM 012535MM in UP/DOWN Convertor was faulty. It was replaced. Its channel temperature range is 175°C and its gain is 20 dB. After replacing U61 the drain currents were adjusted by R202 and R299. The position of IC61 is shown in Figure 7. To prevent re-occurrence of such faults, a fan was mounted at the back of module chamber to extract the heat from the area

 

   

 

      

5. Modern Transmitters use various firmware so, backup of flash storage cards etc., used in the transmitter, must be kept safely. The flash storage card used for display of TCU in 6 kW DTT Transmitters at HPT Jalandhar and Pitampura. Copier/duplicators for for flash storage are also available. Such a copier is shown in Figure 8..

       

 

 

 

 

 

6. Test Jigs

Some test jigs were prepared for repair of DC Power Supplies in RF Amplifiers. Two of such jigs are as shown in Figure 9 and Figure 10. Figure 9 shows the 3 Phase supply was given through a test board consisting of series lamps in each phase to check for any shorting etc. If there is no shorting, then direct supply is given. The 3 Phase AC Supply is then fed directly through the test board. In order to switch ON the DC supply a command is to be given from PA. For this the required command is given using the shorting jig as shown in the Figure 10. Thus, PA PRESENCE is activated. To protect the DC supply from electrical transients the MOV should be replaced every 5 years.

 

7. Technical Workshops

Technical workshops were held at various HPTs of DD Network. Two such workshops were held, at HPT Varanasi and HPT Kupwara. Useful handouts were prepared to develop technical skills among staff, and tutorials were provided. Hands-on training was given on various equipment repairs, like DC power supplies, power amplifiers, exciter modules, control units etc.

 

 

 

 

 

 

 

8. Innovative drawings and Workshop Tables

The architecture of PAs , control circuits, alignment process etc. have been displayed in transmitter halls at various Kendra’s like Kasauli, Karnal, Agra , Vadodara, Surat etc., for better understanding of technical personnel. The Workshop Tables were prepared with provision of various DC supplies, ammeters, voltmeters, CRO, signal generators, waveform monitors, power meters etc. at various transmitter sites like HPT & DMC Vadodara, LPTs at Godhara, Chottaudaipur, Dahod, HPT Amritsar and others. These provisions improved the confidence of staff and enriched the working environs.

9. Conclusions

The reliability of semiconductor devices depends on their resistance to stresses applied to the devices, such as electric stress, thermal stress, mechanical stress, and external stress like humidity, etc. The vital requirement is to overcome these stresses by proper cooling system, optimal drive, matched loads, selection of genuine transistors, trained personnel for periodic alignments and cleaning, awareness of maintenance strategies etc. The core team of engineers needs to strive for any modifications to weak circuits or systems. The drawings & workshops are also vital features for repairs and maintenance. Eventuality study of the anatomy of Transistor failures and maintenance strategies leverages the repair & operational costs of transmitters.

 

10. References & Acknowledgements

i. Technical Manuals of various TV Transmitters.

ii. Field experiments and observations at various HPTs, LPTs etc.

iii. Technical Workshops organized by O/o ADG (DDM) (NZ) AIR & DD New Delhi.

iv. OEM data of various semiconductors.

v. Tutorial by Philips on Semiconductors.

vi. Tutorial by Panasonic on Failure Mechanics of Semiconductor Devices.

vii. Technical discussions with DD Engineers e.g. Sh. A. K Singh ADG(E), Satyapal DDE , Amritpal Singh DDE, , Dharmendra Kumar EA et al., and Engineers at field units of AIR and DD.

 

 

 

About the Author

This item was contributed by M. S. Duhan DDG(E), DDI, entrained vision for the system design and maintenance of all broadcasting organs encompassing Studios, Earth Stations and Transmitters of AIR and Doordarshan. He has over three decades of experience in broadcasting sector with a predominant focus on digital terrestrial technologies, eco-system and solutions. He possesses degree in Electrical Engineering, MBA and Master of Mass Communication. Sh. Duhan is a participant at various industry forums, events, Journals and TV Talk shows. He is a recipient of numerous honors and awards conferred by ABU, BES and DDAwards.

He has published many papers/articles on issues related to terrestrial TV and Radio Transmission and usage of RF Spectrum in National and International Journals. He has contributed significantly in the implementation of DVB T2 Transmitters in India and finalization of BIS Standard for DVB T2- HDTV STB and iDTV. He was resource person for “AIBD/RTM in-country Workshop” organised at Kuala Lumpur by ABU in May 2015 on DVB-T2 technology. He is currently posted in theZonal office of AIR & Doordarshan at New Delhi and is responsible for maintenance of Doordarshan TV Network in North Zone. With his penchant for perfection and pursuit of excellence, he is anchoring new initiatives in repairs, alignments and measurement methodology etc. for optimal functioning of TV Network.

Email: msduhan.de@gmail.com

 

 

Country India
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