George Smart - M1GEO

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180W PA KIT CONSTRUCTION

Posted: 22 Jul 2020 05:31 PM PDT
https://www.george-smart.co.uk/2020/...t-construction



This page is about the Chinese RF_PA_250_3_HV_V201 by ZGJ 2018-01-25,
version 201, commonly for sale on eBay, AliExpress, etc.




Sellers, you are welcome to link to this page in your listings.




This page is a mix of information from my own investigation as well as
information found online (from several sources). It is useful for those
purchasing kits for such amplifiers.



Bill of Materials



ReferenceQuantityPartC1, C2, C8, C11, C19, C20, C21, C22, C24, C25,
C261110nF (103)C3, C9, C10, C13, C15, C16, C18, C468100nF (104)R29110KΩR9,
R10, R11, R1245.6ΩC5, C122680pF Mica capacitorC471100uF 25VC7, C1421000uF
16VL4, L52220uH Color ring inductance D91Red 2.54mm LEDD1019.1V Zener diode
(use with 24V power supply)R301560Ω 3W
(use with 24V power supply) R7, R82220Ω 3W Power resistance 120Ω 3W Power
resistanceRV1, RV225KΩ or 10KΩ preset potL1113mm NXO100 magnetism ring (2
cores)T11135 NXO magnetism ringT21Copper pipe 2pcs, 0.75 square mm wire
(60cm long), 18mm NXO magnetism rings (14 cores)Q1, Q32IRFP250U11LM78L06 or
LM78L09Insulating spacers2 0.8mm enameled wire1 0.75mm high temperature
cable1



PCB Dimensions







Schematic




Click image to enlarge. For transformer winding information, see below.







Build Information



Version History



V100 First editionV101 Second edition: Output transformer cores reduced
to 14.V201 Third edition: Power supply voltage is raised to 24V.



What you will need



13.8V/24V 40A (or higher) power supply. It is better to have the function
of current�limiting protection. 6 square-mm (or more) wires for connecting
the power to the amplifier board.A signal source that is capable of
outputting a 7 or 14 MHz signal at 10W.A 50Ω dummy load rated for 200W
(must be able to withstand continuous dissipation).A heatsink suitable to
dissipate the power of Q1 and Q3. (Recommended size: no less than
150x100x60mm).A multi-meter that includes a 10A scale.An oscilloscope
capable of at least 20 MHz (or a spectrum analyser).



Before you start soldering



Wind the inductor (L1) and transformers (T1 and T2) in accordance with the
information further on in this page.Bend the legs on Q1 and Q3 (TO247
package) upwards, see the illustration below. Do not mount it to the top
side of the board. Do not shorten the leads.Tap the holes for Q1 and Q3.
Screw should be M3 (3mm screw). Clean the heatsink, and remove any metal
chips to avoid a short circuit.







Soldering



Start with smaller components first, working up towards larger components
and finally plugs.SMT parts can be easily soldered with an iron by adding a
small amount of solder to one pad, and using tweezers to push the SMT part
into the molten solder on the pad. Once cooled, add a small amount of
solder to the other pads.L1 and C5/C12 are not fitted at this stage.



Preparation for Powering



Check for any solder splashes, and poor or missing solder joints.Check the
DC power supply resistance to ground no short circuits. If you have not
fitted L1 yet, test from the other side of L1 pad.



(Note: in the V201 version, there are 14 cores in the output transformer,
not 16 as shown here)



Check the LM78L06 regulator output resistance to ground no short circuits.







Check the bias-set variable resistors. Rotate them as shown in the
following diagram. Be careful, to rotate them to the correct end-stop. If
you get this wrong, you will destroy the IRFP250N power MOSFETs. You are
aiming for an initial bias voltage of 0V.







Mount the input transformer secondary load resistor (10Ω, 3W). Solder in Q1
and Q3 and affix to the heatsink. Flow solder on the PCB trace between the
MOSFET and the output transformer. This increases the current capacity of
the track. See below.







Mount L1 as shown below.







Set bias currents




The aim of this section is to adjust the bias current to 100mA for each of
the two transistors. When making adjustments, you must act slowly, and with
great care the current will do nothing for much of the adjustment range
and then rise sharply. The transistors must be bolted to a heatsink during
adjustment.



Double-check that the variable resistors are zeroed as described above,
such that when power is initially applied, there is no bias voltage
present. Connect a current meter in series with the positive power supply
cable of the amplifier. Apply power.Adjust the upper MOSFET quiescent
(static) current using the upper variable resistor to cause an increase in
current of 100mA (0.1A).As before, now adjust the quiescent current of the
lower MOSFET to further increase the current another 100mA. (A total
increase of 200mA between both transistors.)Solder in choke inductor L1 and
mica capacitor C5/C12 if you have not already done so the bias adjustment
is complete.



Signal test



Connect a 50Ω dummy load to J2. The load must be capable of handling
200W.Use an oscilloscope on a suitable range (or spectrum analyser with
suitable attenuation) to monitor the signal at the load.Connect the power
supply and monitor the supply current for a moment. If the current is
gradually increasing, the power must be cut immediately and check for
suitable thermal connection between the power transistors Q1 and Q3 and the
heatsink.With the amplifier powered and no input, check the oscilloscope
for signals. If there signals, immediately power off and debug the cause of
self oscillation.Input a small signal, gradually increasing the input
signal power.Observe the output waveform and the DC input current. In
general, 100Vpp output across the output load corresponds to a power output
of 25W into 50Ω. A load voltage of 141Vpp is 50W output, 180Vpp load
voltage gives an output power of 80W, and 200Vpp at the load is a power
output of 100W. Using an efficiency of 55% as an approximation, the
expected DC power input can be calculated.Check the temperature of the
heatsink. If it is too hot to hold, then you will need to use a fan to cool
the amplifier.Check the output power is stable over time, and that there
are no large fluctuations in output power for a fixed input power.



Finishing



Use a flux remover to clean any solder flux residue and tidy any poor
solder joints.Mount the amplifier into a box or case with suitable TX/RX
switching.Accompany the amplifier with a suitable low-pass filter board.



Transformer & Coil Winding




In the following diagrams process, please note:



To avoid scraping the enamelled wire, use needle nose pliers to smooth the
edge of the ferrites. Hole edges may be sharp.A turn on the coil is
regarded as wire passing through the centre.



Winding T1




Transformer T1 primary should be 6 turns (black lines). The secondary of T1
should be 2 turns (red lines). The turns ratio is important, since if there
are too many turns, the voltage on the gates of the MOSFETs will exceed the
breakdown voltage and the parts will be destroyed.
















Winding T2




Transformer T2 primary should be 1 turn made from the two end PCBs and
copper pipe. The secondary of T2 should be 5 turns of high temperature wire.




In version 201 of the kit, the number of ferrite rings is reduced from 16
to 14. You will also need 2 ferrites for winding L1 (see below).







Winding L1




L1 is a high frequency RFC choke. The 7-10 turns should be wound around two
ferrite rings as used in T2. I chose 10 turns as this provides the largest
choke inductance.