EPR-12 Datasheet, PDF (10/40 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP-12 - 145 W PC Forward Converter with TOP247 and 10 W 5 V Output Standby Flyback with TNY266

EPR-12 – PC Forward Converter with Standby Flyback Power Supply
01-Feb-05
4 Circuit Description
With line feed forward, duty factor reduction, programmable primary current limit, line-
sense for input UV and OV, and soft-start function for smooth start-up, the
TOPSwitch−GX family has all the needed functions to operate in an off-line, single-
ended forward converter configuration. Also the TOPSwitch−GX family has a sufficient
power capability to bring PC main applications easily within its reach.
In the present design the LINE-SENSE pin (L pin, please refer to TOPSwitch−GX data
sheet) senses the rectified DC input voltage through the combination of R3, R5, and R6
and inhibits the supply from switching until the minimum voltage of 80 VAC (doubled
mains) or 160 VAC is reached. Upon reaching this voltage and activation of switching,
current sourced from R8 will immediately establish a maximum duty factor limit by
injecting the appropriate amount of current into the LINE-SENSE pin (adjusting maximum
duty factor, please refer to TOPSwitch−GX data sheet). The input from R8 is a quasi-
integrated, DC-rectified forward voltage sourced from the bias winding and will restrict
the duty factor to a greater degree as the line voltage is increased. This is a very
significant function to ensure that the transformer will not saturate, even in extreme
transient load conditions.
A TOP247 was selected for this 145 W application and its primary current limit was
adjusted to limit at approximately 2.5 A by R12 when U3 is on. This allows approximately
170 W of peak output power.
Lowering the input voltage will cause the converter to shut off by means of the under-
voltage lockout circuit around Q1. When input voltage is low enough to bias on Q1, the
collector of Q1 will pull up the X pin of the TOP247 via R39 and shut off the main
converter.
This design uses a Zener primary clamp (D3, D4, D5) with a capacitor (C4) in parallel
that is coupled to the drain of the TOPSwitch−GX through a diode (D1). This is a very
efficient snubber as it allows the maximum flyback voltage to develop during the off time
which returns a significant amount of energy back to the transformer during the reverse
time recovery of the diode D1. The total dissipation of the primary snubber clamp circuit
was measured to be only 0.8 W at maximum load.
It is necessary to use voltage mode control in the regulation loop when using
TOPSwitch−GX. As the data will show, the transient response is very good and there
appears to be no difficulty in compensating the voltage mode control loop for optimal
performance.
The remote ON/OFF function is implemented by using a very simple circuit around Q3.
When the ON line (green wire in output cable) is grounded to secondary return, Q3 is
turned on and drives the LED of U3 on, which will ground R12 on the primary side and
Power Integrations
Tel: [phone] Fax: [phone]
www.powerint.com
Page 10 of 40