Switch-mode Power Supply for a Tube Car Amplifier

(c) klausmobile 2001

Design goals

Heater supply and timers

To minimize heat loss on any transistor, I use two pass FETs. In the car, input voltage is either 11.5-12.5V (engine off) or 13.5-14.5V (engine on). In the first case, T1 and T2 are fully ON, no need for heatsink. In the second case - T1 is OFF, T2 ON, all voltage and power excess is released across R5, no heatsink necessary for FETs. However, when input is in between (13...14V) - T1, in a linear regulator mode, may dissipate up to 3W (for a 3A load current).

Parts choice/substitutes:

Plate regulator

I use UC3825 controller chip because it needs no external gate driver, minimizing parts count. Current-mode control is not necessary: in 12V networks, the transistors have at least 5x current margin, so in case of a bad short they will survive before the fuse blows. UC3825 is clocked at 180kHz (transformer-dependent). Filter is a ladder CRCRC (no L's!), voltage feedback is drawn from the second C in this chain. Such arrangement, together with a low error amplifier gain, guarantees stability.R120-T1-D107 current varies within 0-4mA range, so this network itself is an additional shunt regulator (with very narrow dynamic range).

Parts choice/substitutes:


PCB size is 260*80mm, component footprint 250*60mm. Blue snowflakes mark mounting holes for the three heat-sinked FETs - they are soldered under the board. Total heatsink requirement is under 10W. All HV network is surface-mounted on the upper side (no vias or through-holes). T101,102 drains are soldered to 8*15mm contact pads on the upper side. Hot leads of transformer primary and snubber R's are soldered to this pads too. Note that C105, 106 and matching ceramic caps (1uF, SMD type) are placed between T101,102 drains and transformer primary center tap - to minimize AC current loop. The converter works fine with generic (standard ESR) capacitors. The last capacitor (actually - capacitor bank) is not on this board, but on amplifier PCB.

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