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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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SMPS oddities
Hi all,
not exactly reapir related but, I think, interesting. I've been studying SMPSs for a while out of personal interest, and I stumbled upon an old schematic for a discrete component-based SMPS: http://bitman3.altervista.org/SMPS.png As I understand it, this is a self-oscillating SMPS operating in current mode. T302 is the main switcher, Th301 is the turn-off SCR, T301 acts as the voltage error amplifier whose output controls the maximum current on the primary coil. In particular, the cathode of Th301 is subject to a sawtooth negative voltage due to the primary coil current flowing through R317, while the gate voltage is controlled by the error voltage amplifier T301; the main switcher is turned off on a cycle-by-cycle basis when Vgk on Th301 reaches the turn-on threshold, and this determes the duration of Ton. The general principle of current mode operation is that a lower than expected voltage on the secondary should result in a longer Ton. My problem is that in this case it seems that the opposite is true! In fact, the secondary rectified voltage drives the base of T301: a higher secondary voltage results in a lower current on R304 and thus in a lower Vg on Th301. This, in turn, means that a higher magnitude of primary coil current is needed on R317 to turn on Th301, resulting in a longer Ton. In short: the higher the secondary voltage the longer the Ton, positive feedback, BOOM! I haven't got the real thing on hand so I cannot check the waveforms. I assume it's been built and it works, so I'm obviously missing something here. Can anyone shed some light? Thanks in advance, -- bitman |
#2
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SMPS oddities
Bitman wrote in message
... Hi all, not exactly reapir related but, I think, interesting. I've been studying SMPSs for a while out of personal interest, and I stumbled upon an old schematic for a discrete component-based SMPS: http://bitman3.altervista.org/SMPS.png As I understand it, this is a self-oscillating SMPS operating in current mode. T302 is the main switcher, Th301 is the turn-off SCR, T301 acts as the voltage error amplifier whose output controls the maximum current on the primary coil. In particular, the cathode of Th301 is subject to a sawtooth negative voltage due to the primary coil current flowing through R317, while the gate voltage is controlled by the error voltage amplifier T301; the main switcher is turned off on a cycle-by-cycle basis when Vgk on Th301 reaches the turn-on threshold, and this determes the duration of Ton. The general principle of current mode operation is that a lower than expected voltage on the secondary should result in a longer Ton. My problem is that in this case it seems that the opposite is true! In fact, the secondary rectified voltage drives the base of T301: a higher secondary voltage results in a lower current on R304 and thus in a lower Vg on Th301. This, in turn, means that a higher magnitude of primary coil current is needed on R317 to turn on Th301, resulting in a longer Ton. In short: the higher the secondary voltage the longer the Ton, positive feedback, BOOM! I haven't got the real thing on hand so I cannot check the waveforms. I assume it's been built and it works, so I'm obviously missing something here. Can anyone shed some light? Thanks in advance, -- bitman Try sci.electronics.design. I got some useful advice , posting about a SMPS problem a week or two back -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
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