Re: The whole Two Transistor Thing
This is the way I understand it.
The regulation loop controls the ON time of the chopper transistor during the forward period to control how much energy is sent through the transformer during the following flyback period. The small control transistor only takes notice of the opto transistor's conduction state during the forward period.
When ripple develops at the 5V output because of the capacitor going high ESR, the ripple troughs correspond to the forward period, diode reversed biased and the capacitor only supplying load current.
The ripple peaks correspond to the flyback period, diode forward biased and charging the capacitor.
So what tends to happen is the regulation loop regulates the ripple troughs to about 5V, but ignores the high ripple peaks during flyback. This causes the average voltage to be much higher than 5V, although the 431 is conducting heavily through the opto's LED during flyback.
The high ripple peaks also get reflected back to the chopper transistor's Collector/Drain, causing voltage stress and possible failure.
This is the way I understand it.
The regulation loop controls the ON time of the chopper transistor during the forward period to control how much energy is sent through the transformer during the following flyback period. The small control transistor only takes notice of the opto transistor's conduction state during the forward period.
When ripple develops at the 5V output because of the capacitor going high ESR, the ripple troughs correspond to the forward period, diode reversed biased and the capacitor only supplying load current.
The ripple peaks correspond to the flyback period, diode forward biased and charging the capacitor.
So what tends to happen is the regulation loop regulates the ripple troughs to about 5V, but ignores the high ripple peaks during flyback. This causes the average voltage to be much higher than 5V, although the 431 is conducting heavily through the opto's LED during flyback.
The high ripple peaks also get reflected back to the chopper transistor's Collector/Drain, causing voltage stress and possible failure.
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