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Switched power supply 2x 35V 350W

     This switching power supply works in the single switch forward topology. In the article about switching power supplies it is II.A. Operating frequency is cca 80-90kHz. As a control circuit UC3844 is used. It is similar to the UC3842, but duty cycle is limited to less than 50%. The transformer Tr1 was made rewinding the transformer of SMPS ATX, who also worked in the forward topology. The primary inductance of such transformers is around 6.4 mH. The core is without air gap. Primary is divided into two halves; Wind first half, then the other winding starting secondaries, and finally the second half. It is also possible to use the original bottom half of the primary and not rewind it. This power supply is suitable for power amplifiers. It has protection against short circuit and overload. The output voltage is stabilized. Feedback is introduced through the optocoupler.
     The table shows various MOSFETs rated for voltages 800 - 1000V usable as T1. For the 350W power, typical resistance in the conductive state (Rds on typ.) should not exceed 0.8 R. MOSFETs can be connected in parallel to reduce resistance. Of course - the smaller the resistance, the better (lower conductive losses). Voltage UDS should be 900-1000V, in worst case you can use 800V. I also wondered about using IGBT, but you can not use those rated 600V. The 1200V ones tend to be too slow. Experiments with IGBT BUP213 have not been successful, is has far greater losses than MOSFETs. Very hopeful is SPP17N80C3 MOSFET with resistance only 0.25 R. Note: The data in the table may get out od date. More appropriate MOSFETs may appear. The coming 900V IGBTs may be good.

Type Voltage Current at 25°C Resistance at 25°C Gate capacity Max loss at 25°C times (ns) Note
UDS ID Rdson typ. (max) CGS PTOT td(on) tr td(off) tf
2SK2648 800V 9A 1,28R (1,5R) 1200pF 150W 30 120 95 60 salvaged from ATX
STW11NM80 800V 11A 0,35R (0,4R) 1630pF 150W 22 17 46 15 .
STW12NK90Z 900V 11A 0,72R (0,88R) 3500pF 230W 31 20 88 55 salvaged from ATX
2SK1363 900V 8A 1,1R (1,4R) 1300pF 90W 25 40 20 100 .
2SK2611 900V 9A 1,1R (1,4R) 2040pF 150W 25 60 20 95 .
IRFPE50 800V 7,8A ----- (1,2R) 3100pF 190W 19 38 120 39 .
SPP17N80C3 800V 17A 0,25R (0,29R) 2320pF 208W 25 15 72 6 .
STW11NK100Z 1000V 8,3A 1,1R (1,38R) 3500pF 230W 27 18 98 55 .
STW18NK80Z 800V 19A 0,34R (0,38R) 6100pF 350W 46 32 140 32 .
STP10NK80Z 800V 9A 0,78R (0,9R) 2180pF 160W 30 20 65 17 .
FQA7N80 800V 7,2A 1,2R (1,5R) 1420pF 198W 35 80 95 55 .
IRFBE30 800V 4,1A ----- (3R) 1300pF 125W 12 33 82 30 only in parallel
IRFPE40 800V 5,4A ----- (2R) 1900pF 150W 16 36 100 32 only in parallel
2SK2397 800V 5A 1,7R (2,3R) 800pF 50W 25 100 130 70 salvaged, only in parallel
SPP06N80C3 800V 6A 0,78R (0,9R) 785pF 83W 25 15 72 8 .

     Warning! Switching supply is not for beginners, because most of its circuits are connected to fatal mains voltage. If poorly designed, the mains voltage can reach the output! Capacitors can remain charged to dangerous voltage even after disconnected from mains. Everything you do at your own risk, for any injury to health or property I do not take responsibility.

Schematic of the Switched power supply 2x 35V 350W with protections for audio amplifier

Added: 2008