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EPR-00015 Datasheet(PDF) 5 Page - Power Integrations, Inc.

Part No. EPR-00015
Description  3W, Universal Input, Single Output, Isolated Converter with TNY254 (EP-15)
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Maker  POWERINT [Power Integrations, Inc.]
Homepage  http://www.powerint.com
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EPR-00015 Datasheet(HTML) 5 Page - Power Integrations, Inc.

 
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Engineering Prototype Report
EPR-00015
Page 5 of 24
4.0 Circuit Description
This circuit was designed for emergency lighting battery charging applications.
The unit stops charging when the mains voltage drops below ~175Vac (in a 230Vac system), or
~100Vac (in a 120Vac or universal system). The voltage threshold is set/programmable with the
voltage divider R5, R6 and R7. Two ¼ W resistors (R5, R6) are connected in series for voltage
rating and board layout flexibility.
For 100Vac threshold R5=470K, R6=510K and R7=39K.
For 175Vac threshold R5=820K, R6=910K and R7=39K.
The threshold accuracy is determined by the resistor value tolerance and is temperature sensitive
as the Vbe of Q1.
The EMI standard is met with a low cost transformer (only shield winding, no need for flux band)
and low cost input filter (no common mode choke). The R4, C3 snubber reduces the drain dV/dt
of U1 (slows the switching speed), reducing the EMI.
In this application, the AC input is rectified and filtered by D1-D4, C1 and C2 to create a high
voltage DC bus which is connected to T1. Inductor L1 forms a pi-filter in conjunction with C1 and
C2. The resistor R2 damps resonance in inductor L1. The operating mode of TNY254 allows the
unit to meet worldwide conducted EMI standards using a simple pi-filter in combination with a
small value Y1-capacitor C5 and a proper PCB layout. R4 and C3 form a snubber circuit that
limits the turn-off voltage spike to a safe level on the TNY254 DRAIN pin.
The secondary winding is rectified and filtered by D5, C6 with additional filtering provided by L2,
C7 to give the 12Vdc output. The output voltage is determined by the sum of the voltage drops
across the opto-coupler U2 and the Zener diode VR1 at the bias point. The optocoupler voltage
drop is minimum (<1V) at the current required for the TinySwitch control pin and varies with the
optocoupler part number. With a 11V 5% zener the output voltage could be as low as 7% off the
nominal 12Vdc. For better nominal voltage accuracy a 2% zener should be used.
Resistor R8 sets the bias current for VR1 and improves the optocoupler U2 response time. If
LED1 is not used, R8 value can be decreased such that VR1 pre-loading maintains the no-load
output regulation.
The primary-to-secondary isolation is assured by using parts/materials (opto/transformer
insulation) with the correct level of isolation and creepage distances (opto slot/transformer
bobbin).
The 12Vdc monitoring light emitting diode (LED1) and R9 are optional, and have been included in
this circuit for troubleshooting convenience. R9 dissipates approximately 20mW and helps the no-
load output regulation.
Test points TP1 (U1 SOURCE) and TP2 (U1 DRAIN) are provided for ease of monitoring Vds.


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