A switched mode TWT Power supply
( up to 1KW DC... or more !!! )

Surplus TWTs, available on the amateur markets, seem to appear in much greater number than their power supplies. The following design is a switch mode power supply for TWT tubes. It operates directly from the sector and is capable of delivering up to 1KW DC (Proper resizing can have the same design running about 3kW DC). This design was done for medium power TWT's but all circuits are versatile enough to accommodate up to 10kV helix and up to 8kV collector voltages. The design description is done in a way that it can be assembled for any specific TWT you may have, providing the necessary modifications.

See at the end, notes on improvements and sugestions, and how to run very high power tubes.

The original article was presented in DUBUS 1/99.
Here it is the english version, you can download 1kW(dc) TWT power suply design.(227K)
The diagrams of the main board, Helix and Collector boards.(372K)

My prototype running an AEG tube for 6GHz 35W. / Some transformers.


My 5.7GHz system that goes into the dish feed
with all the stuff, including the TWT supply.

Email group.

I've started an email group dedicated TWT tubes, amplifiers and power supplies.
The group is dedicated to the discussion on TWTs and their power supplies. This group is essentially targeted to amateur applications such as EME and microwave DX.

Feel free to join.

Post message: [email protected]
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List owner: [email protected]

Usefull Links for this project:

For the UC3846 data sheet look at Unitrode
For the power transistors look at International Rectifier
For the vacuum relays (grid switching) look at RF Parts

Design comments and feedback from builders.

Comment from Hubert ON6JZ.
( with Geert PA3CSG )

Hi Luis
We did some modifications on the psu. First I made the regulator voltage a litle bit higher, then between the pins 13 and 15 I add a resistor of 220 ohm and from pin 13 to ground add a capacitor of 47uF. The zener D11 is changed to 24 volt.
A slow start on the regulator ic was fit. A capacitor of 1 uF from pin 6 to pin 15 was added so and now the psu starts up nice.
At the firts trials, with a twt connected, the protection of te helix voltage tripped imediatly due to the capacitance of the cable to the twt. This probblem was cured with a capacitor of 470nF in parallel to R14.
I also did some modifications on the helix regulator around Q3 Q4
I divide R21 in two resistors of 4700 ohm each, and from their middle point to the ground I placed a 12 volt zener diode. This modification made the helix regulation more stable.
I changed also the resitors R6 R7 R8 to 1Mohm and R5 to 47Kohm for a litle more current in te voltage divider for better regulation

Best regards.
Hubert ON6JZ

Power transistors... (dmk)

The transistors recomended for up to 600W are the IRFPC50, and up to 1KW the IRFPC60.
Other devices are usable also. Select other power MOS transistors observing the following key parameters:
- The transistor must be fast enougth for 40KHZ switching at < 20% dead time.
- Maximum drain current Idd, must be enougth for the power required.
(note that you must account for a duty cycle of about 40 % per transistor when calculating the current.)
- The resistance when 'on' Rdson taht must be low 0.8 Ohm or better.
(calculate for the maximum power dissipation the transistor can handle)

For low power there are many devices available...
For maximum power, not many devices are available,
so here it goes a small list of possible devices for the 600W to the 1KW range at 110Vac sector: *BUK437-400A, BUK457-400A, BUK637-400A, BUK657-400A, BUK437-600A, BUZ330, BUZ331/2, BUZ334, *BUZ338/9, BUZ353, BUZ354, *BUZ382/3/4/5, IRFP340, IRFP440, IRFP448, *IRFP450, *IRFP460, IRFPC50, *IRFPC60.
list of possible devices for the 600W to the 1KW range at 220Vac sector: ?BUK437-600A, BUZ355/6, *BUZ376/7, BUZ380/1, IRFPC50, *IRFPC60.
( * work well above 1kW dc ).

Thesr are just sugestions as I've only made tests with IRFPC50 and IRFPC60 at 110Vac and with BUZ355A at 220Vac.

73 de CT1DMK.

Sugestion: How to run HIGH POWER TWT's, >> 1kW... (dmk)

Use the presented design to supply the Heater of the tube, that for big tubes is quite a respectable power. And to supply a stabilized voltage for the helix and grid 2.
The power handling at the helix circuits is about 30 to 50W that makes a 10kV 30ma supply possible.
The colector voltages, that must provide nearly all the power to the tube can be obtained from a classical high voltage supply (as for a big triode amplifier).

73 de CT1DMK.

Part List from Rainer DF6NA.

Hi Louis,
It is not complete yet - but a good start. (Part List)

73, Rainer

More comments to be inserted here...
Let me know...




PCB and Cores available:

Luis Cupido