EP-93 – 30 V, 1.07 A, 2.7 A (peak), Universal Input Supply
22-Jun-2006
4 Circuit Description
4.1 Input EMI Filtering
Components C1, C2, C3, C10, C17, C19, R15, L1, and L2 provide common mode and
differential mode EMI filtering. The use of two Y capacitors (C10 and C19) together with
an output common choke (L2) and the frequency jitter feature of PeakSwitch allows the
supply to meet EN55022B conducted EMI limits even with the output connected directly
to safety earth ground. On the PCB layout C19 is placed so that the primary side is
connected as close to the bulk capacitor as possible to route surge currents away from
U1. Resistors R1 and R2 discharge C3 when AC power is removed.
4.2 PeakSwitch Primary
Components D5, C7, and R5, R6 provide AC line and under voltage sensing for
PeakSwitch U1. At startup, switching is inhibited until the input voltage is above the
under-voltage threshold, determined when a current >25 µA flows into the EN/UV pin.
Once the threshold is exceeded, the under-voltage status is not checked until auto-restart
is triggered (no feedback for 30 ms). This allows the supply to continue to operate even
below the under-voltage threshold as long as the output remains in regulation,
maximizing hold-up time.
The separate AC sense network of D5 and C7 allows the PeakSwitch to determine the
cause of loss of regulation. If the input voltage is above the under-voltage threshold, then
a fault condition is assumed. In this case PeakSwitch will latch off. If the input voltage is
below the under-voltage threshold then loss of regulation was due to a low line condition
and PeakSwitch will stop switching (but not latch off) until the under-voltage threshold is
exceeded again.
Once latched off, the supply can be reset by removing the AC input such that C7
discharges and the current into the EN/UV pin falls below 25 µA. The under-voltage
function can be disabled by removing R6. Resistor R16 provides a small amount of bias
to the U1 EN/UV pin to keep the under-voltage lockout function activated during brownout
conditions when C7 may discharge.
Diode D7, C6, C8, and R7 provide bias power and decoupling to U1.
Diode D6, C5, R3, R4, and VR1 clamp the U1 drain voltage to safe levels. Use of a
moderately slow diode (tRR ≤ 500 ns) for D6 increases power supply efficiency.
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