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11-Feb-13
RDR-91 12 V, 1 A, Universal Input Supply
4 Circuit Description
This flyback power supply was designed around the TNY278PG (U1 in Figure 2). The
output voltage is sensed and fed back to U1 through optocoupler U2. That feedback is
used by U1 to maintain constant voltage (CV) regulation of the output.
4.1 Input Rectification and Filtering
Diodes D1–D4 rectify the AC input. Capacitors C1 and C2 filter the rectified DC. Inductor
L1, C1 and C2 form a pi filter that attenuates differential mode conducted EMI.
4.2 TNY278PG Operation
The TNY278PG device (U1) integrates an oscillator, a switch controller, startup and
protection circuitry, and a power MOSFET, all on one monolithic IC.
One side of the power transformer (T1) primary winding is connected to the positive leg
of C2, and the other side is connected to the DRAIN (D) pin of U1. At the start of a
switching cycle, the controller turns the power MOSFET on, and current ramps up in the
primary winding, which stores energy in the core of the transformer. When that current
reaches the limit threshold, the controller turns the power MOSFET off. Due to the
phasing of the transformer windings and the orientation of the output diode, the stored
energy then induces a voltage across the secondary winding, which forward biases the
output diode, and the stored energy is delivered to the output capacitor. When the power
MOSFET turns off, the leakage inductance of the transformer induces a voltage spike on
the drain node. The amplitude of that spike is limited by an RCD clamp network that
consists of D5, C4 and R2. Resistor R1 and VR1 provide hard clamping of the drain
voltage, only conducting during output overload. Resistor R2 also limits the reverse
current that flows through D5 when the power MOSFET turns on. This allows a slow, low-
cost, glass passivated diode (with a recovery time of ? 2 ? s.) to be used for D5, which
improves conducted EMI and efficiency.
Using ON/OFF control, U1 skips switching cycles to regulate the output voltage, based
on feedback to its ENABLE/UNDERVOLTAGE (EN/UV) pin. The EN/UV pin current is
sampled, just prior to each switching cycle, to determine if that switching cycle should be
enabled or disabled. If the EN/UV pin current is <115 ? A, the next switching cycle begins,
and is terminated when the current through the MOSFET reaches the internal current
limit threshold. To evenly spread switching cycles, preventing group pulsing, the EN/UV
pin threshold current is modulated between 115 ? A and 60 ? A based on the state during
the previous cycle. A state-machine within the controller adjusts the MOSFET current
limit threshold to one of four levels, depending on the load being demanded from the
supply. As the load on the supply drops, the current limit threshold is reduced. This
ensures that the effective switching frequency stays above the audible range until the
transformer flux density is low. When the standard production technique of dip varnishing
is used for the transformer, audible noise is practically eliminated.
Power Integrations
Page 7 of 35
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