:On Sat, 28 Feb 2009 03:06:38 GMT, Ross Herbert : wrote: : :>
:>I pulled a psu from a defunct Epson inkjet printer and it uses a similar setup :>to your description. The mains input primary side switcher uses a mosfet :>controlled by bjt's and the secondary side uses a L4962EA 1.5A smps controller :>to produce a 42V output. :>
:>As for the OP's query, my initial reading is that the digital isolator cannot :>be applied to smps feedback control. The output control of an smps relies on :>
: :I don't think so.
:Once the analog error signal gets to a PWM (or other), the error :signal crosses into digital land. :This works when the PWM is on the secondary side.
Take a look at the data sheet for a typical controller IC commonly used in SMPS, eg. UC3842.
"Pin 2 - Voltage feedback - This is the inverting input of the Error Amplifier. It is normally connected to the switching power supply output through a resistor divider."
That says the feedback signal is analog, not digital.
In SMPS which don't use a PWM controller IC, the feedback signal is always analog.
:>sampling an analog voltage against a reference and producing an analog :>feedback signal which is applied to the primary side switching control usually :>via an opto-isolator. Using a digital isolator (which is quite complex in :>itself) would : :Digital isolators are easy to apply and are easy to model. :My digital isolator model uses a buffer with only 5 parameters Td Tr :Tf Vh Vl.. :I tried modelling a 6N135 linear opto, what a pita.
6N135 is not a good choice for SMPS feedback control imo. Why not use a device which is designed and recognised for this application?
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Another commonly used OC for f/b control in SMPS is the Sharp PC817 series. There are others as well.
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