MRF300 RF Power LDMOS Transistors

 

MRF300 RF Power LDMOS Transistors

MRF series is very well suited for high-power telecommunication applications in high frequency and very high-frequency ranges. The advantage of these multiple frequency operation devices is a single design solution. The frequency can be changed only using the inductors. No separate PCB layouts are needed to change the designs. The transistor comes in a standard TO-247 package, eliminating the need for a heat sinking solution and allowing room for various mounting options. The device is best suited for VSWR, broadcasting, medical, scientific, and ISM applications.

Specifications:

·         Handles 65:1 VSWR

·         Frequency: 1.8 to 250 MHz

·         Power: 54.77 dBm

·         Power(W): 300 W

·         Gain: 20.4 to 28.2 dB

·         Supply Voltage: 50 V

·         Drain Efficiency: 75.5 to 80.6%

·         Package: TO-247

 

 

The device comes in two mirror configurations for easy push and has enhanced C class operation due to the integrated ESD protection. The amplifier is designed to work even from a typical 50 Ohm impedance. The main structural goals were to achieve a reasonable loss of input return since most Amateur Radio's transceivers need to operate on those loads.

Efficiency and Gain Chart:

The typical RF performance of the device at the slandered operation of 50 VDC is demonstrated below. This gives a good estimation when working on the application-specific designs.

Frequency	Signal	Pout	Gain (dB)	Efficiency %
13.56	CW	320	28.1	79.7
27		330	27.4	80.0
40.68		330	28.2	79.0
50		320	27.3	73.0
81.36		325	25.1	77.5
144		320	23.0	73.0

 

MRF 300 Series Circuit Board Designs Parameters for Various Frequencies:

MRF300ANBN-13MHz

The 13.56MHz Board performance parameters in reference to the 50 ohms standard system

 (with V = 50 V_DC, I_DQ = 50mA, P_IN = 0.5W) is as below:

Frequency: 13.56MHz

P_out: 320W

G_dB: 28.1dB

Efficiency: 79.7%

The 2x3 inches circuit board design has a reference impendence input and output load to match the 50 ohms impedance.

Input Impedance Z_Source: 12.0 + j5.2

Output Impedance Z_Load: 5.1 – j1.0

MRF300ANBN-27MHz

The 27MHz Board performance parameters in reference to the 50 ohms standard system (with V = 50 V_DC, I_DQ = 50mA, P_IN = 0.6W) is as below:

Frequency: 27MHz

P_out: 330W

G_dB: 27.4dB

Efficiency: 80%

The 2x3 inches circuit board design has a reference impendence input and output load to match the 50 ohms impedance.

Input Impedance Z_Source: 32.13 + j11.22

Output Impedance Z_Load: 4.47 + j0.45

MRF300ANBN-40MHz

The 40.68MHz Board performance parameters in reference to the 50 ohms standard system 

(with V = 50 V_DC, I_DQ = 50mA, P_IN = 50W) is as below:

Frequency: 40.68MHz

P_out: 330W

G_dB: 28.2dB

Efficiency: 79.0%

The 2x3 inches circuit board design has a reference impendence input and output load to match the 50

 ohms impedance.

Input Impedance Z_Source: 7.83 + j13.51

Output Impedance Z_Load: 5.34 + j1.03

MRF300ANBN-50MHz

The 50MHz Board performance parameters in reference to the 50 ohms standard system 

(with V = 50 V_DC, I_DQ = 100mA, P_IN = 0.6W) is as below:

Frequency: 50MHz

P_out: 320W

G_dB: 27.3dB

Efficiency: 73.0%

The 2x3 inches circuit board design has a reference impendence input and output load to match the 50 ohms impedance.

Input Impedance Z_Source: 6.44 + j12.27

Output Impedance Z_Load: 5.05 + j1.36

MRF300ANBN-81MHz

The 81.36MHz Board performance parameters in reference to the 50 ohms standard system

 (with V = 50 V_DC, I_DQ = 100mA, P_IN = 1W) is as below:

Frequency: 81.36MHz

P_out: 325W

G_dB: 25.1dB

Efficiency: 77.5%

The 2x3 inches circuit board design has a reference impendence input and output load to match the 50

ohms impedance.

Input Impedance Z_Source: 3.86 + j7.90

Output Impedance Z_Load: 4.45 + j3.53

MRF300ANBN-144MHz

The 144MHz Board performance parameters in reference to the 50 ohms standard system 

(with V = 50 V_DC, I_DQ = 100mA, P_IN = 1.6W) is as below:

Frequency: 144MHz

P_out: 320W

G_dB: 23.0dB

Efficiency: 73.0%

The 2x3 inches circuit board design has a reference impendence input and output load to match the 50 

ohms impedance.

Input Impedance Z_Source: 1.62 + j6.44

Output Impedance Z_Load: 4.32 + j2.06

Testing And Measurements:

To test the LDMOS using a multimeter, we check the resistance between the drain, gate, and source points, respectively. We run the resistance tests between source-drain and gate-drain.

Resistance Outputs:

Source-Drain: High

Gate-Drain: High

Gate-Source: Infinite

If the resistance is high, this indicates the points are functional, and the device is in good condition. If the gate and source resistance is very low, this indicates a short and the device has run bad and needs to be replaced.
Although this is not a conventional way to run a test, it gives a good idea and a fast test to identify the issue. One more thing that needs to keep in mind is that there are two units in a single chip, and both need to be checked if they are functional.

Heatsinking Solution:

Since there is a lot of switching involved in transmission and these devices need to switch rapidly, the result is the tremendous heat produced. One of the efficient design possibilities is to use a copper plate between the aluminum heatsink and the LDMOS unit. The standard mean temperature to maintain is 65 degrees Celsius, and the datasheet mentions the junction temperature of 175 degrees Celsius at a current rating of 8.7A.

What a copper plate does is that it conducts the heat faster by being a good conductor, which makes the heat dissipation even faster, thus helping the LDMOS stay at a lower temperature. Since the transistor comes into a package, which already dissipates some heat and the transistor can be installed in many ways. If installed directly onto the copper plate also decreases the heat.

Amplifier Application:

One of the major applications is amplifiers. The output power is increased by certain folds as compared to input while maintaining the input impedance. This, however, limits the output of the amplifier, which can be calculated using the equation below:

Pout < (𝑉𝑏𝑟 – 𝑉𝑘)2/8𝑍𝑜

Where Zo is to match the loading impedance which are 50 ohms for most of the designs.

A600 Broadband 600W Linear Amplifier with MRF300 Transistors:

The MRF300 transistor-based amplifier provides high output power and a steady idle current of 300mA at a voltage of about 3V when not in use. The design uses a heatsink solution with the MRF300 transistors in the feedback mechanism. When the temperature increases, the feedback mechanism helps decrease the gate voltage when the temperature rises, and thus, the current is also limited, which in turn stabilizes the idle current.

The major challenge n this design is to match the impedance load. Using a low voltage supply with a transformation ratio of 1:9 will result in higher output power. Still, the impedance is hard to match, whereas 1:4 transformation ratio provides better impedance matching but lower output power.

The other thing that affects the amplifier's efficiency is the substrate used, FR4 has proven to generate better results than the copper boards. The size of the board also matters. The SMD components are used in the design to reduce the harmonic effects, significantly reducing the need for the harmonic’s contents.

Source:

https://qrpblog.com/2019/10/a-600w-broadband-hf-amplifier-using-affordable-ldmos-devices/ 

Oscilloscope Test:

The oscilloscope can be used to run the bandwidth and roll-off test on the amplifier. For an amplifier of 600W power, the setup should have a 3.7MHz frequency, but as the power increases, the signals are distorted with increased harmonics, resulting in lower bandwidths.

To run the harmonics test on the oscilloscope, follow the steps below:

1.       Turn on the oscilloscope and switch it to harmonics/spectrum analyzer mode.
2.       Set the vertical scale to 10db or 20db roll-off.
3.      Place the probe on the output point.
4.       Now change the input voltages and measure the output at different frequencies.