22-Jun-2006
EP-93 – 30 V, 1.07 A, 2.7 A (peak), Universal Input Supply
4.3 Output Rectification and Filtering
The secondary of the transformer is rectified and filtered by D8 and C12. As the peak
load condition is of short duration, the output capacitor ripple current rating is appropriate
for the continuous output current. As capacitor lifetime is a function of temperature rise,
this can be used to determine if the capacitor rating (ESR and ripple current specification)
is acceptable. Resistor R8 and capacitor C11 are fitted to reduce high frequency EMI.
4.4 Output Feedback
Diodes D9 and VR2, along with the forward drop of the LED of optocoupler U2, set the
output voltage of the power supply. Resistor R13 provides a bias current through D9 and
VR2 to improve regulation by operating VR2 closer to its knee and test current. Resistor
R12 sets the overall gain of the feedback loop while capacitor C15 boosts high frequency
loop gain to reduce pulse grouping. A high gain (300-600%) optocoupler U2 is used to
reduce control loop delays.
4.5 Output Protection
Components Q1, Q2, R9 to R11, R14, C13, C16, D10, and VR3 are used for latching
overvoltage and overcurrent protection in conjunction with the smart AC sensing feature,
to shut down the supply in a fault condition. If either an output overvoltage (e.g. opto-
coupler failure), or overcurrent (e.g. motor stall) fault occurs, SCR Q2 is fired, shorting the
output winding. The SCR is connected directly to the secondary winding to allow a lower
current rating and lower cost device to be used, as the SCR does not have to discharge
the output capacitor.
The value of VR3 is selected to give the desired overvoltage trigger threshold. For
overcurrent protection, the value of R9 is selected to turn on Q1 at the desired
overcurrent threshold while R10 and C13 provide a time constant, to prevent short
duration (~200 ms) transient loads from triggering shutdown.
The shutdown condition can be reset by briefly removing AC power for ~3 seconds
(maximum), the time required for C7 to discharge and the current into the EN/UV pin to
fall below 25 µA.
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