Because RFL™ and Hubbell® have a policy of continuous product improvement, we reserve the right to change designs and specifications without notice.
9.2.3 THEORY OF OPERATION (9547-840)
The topology used in the front end is that of a "FLYBACK" converter. The method of conversion uses
the transformer core for primary energy storage. The energy is transferred to the secondary upon
demand. It is a preferred topology due to its simplicity and reduced number of parts. Only one
switching device is required, and since the transformer acts as a common mode choke, additional
inductors and filtering are not required on the secondary. Another benefit is its ability to regulate from a
feedback winding which does not require isolation from the primary circuit. The feedback winding
powers the front end for greater efficiency and is monitored by the Pulse Width Modulator (PWM). The
PWM varies the duty cycle of the switching device to compensate for changes in line and load. The
duty cycle is varied proportionally with load and inversely with line. To provide better cross regulation
between outputs, linear regulators are used to provide line and load regulation.
9.2.3.1 INPUT CIRCUIT
Input power is first applied to fuses F1 and F2 which provide primary overcurrent protection. For
safety, F2 on the 120VAC unit is replaced with a short so that neutral leg is always connected. For
protection against line transients in the transverse mode, CR1 limits the overvoltage to a manageable
level. In the case of line transients that are Common Mode, an L-C filter created by L1, L2, C1, & C2
provide immunity. Transverse voltage spikes are limited by C3 and CR2. To prevent power switch and
fuse failures, inrush current limiting is provided by thermistor RT1. Reverse line voltage protection is
provided by the bridge rectifier circuit CR3 through CR6. Application of any polarity provides normal
operation on the 125V & 250V units. To increase efficiency on the 48V supply, CR5 and CR6 are
removed and replaced with wire jumpers. Reverse input voltage will force the input fuses to clear on the
48V power supply as a result of removing CR5 and CR6. Lastly, the input voltage is filtered by
capacitor C5 which serves as a reservoir for the primary current of the transformer.
9.2.3.2 START-UP CIRCUIT
For increased efficiency, the PWM circuit obtains power from a feedback winding. In order to activate
the PWM, current is drawn from the input supply through the startup circuit. When power is first
applied, current flows through R2 and zener diode CR7 which pulls Q1's base to 15V. The transistor
then turns on and current flows to the V+ bus and starts the PWM controller U1. The peak current is
limited by R3 to within the safe operating area of Q1. When the control loop stabilizes, the V+ bus is
bootstrapped to approximately 17V, which then applies a reverse bias to the transistor and cuts off the
current.
M-DACS-T1 RFL Electronics Inc.
August 7, 2012 9-3 (973) 334-3100
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