RDK-138 Power Integrations, RDK-138 Datasheet - Page 6

• Manufacturer Part Number: RDK-138
• Description: KIT REF DESIGN 1.2W PS TN FAMILY
• Manufacturer: Power Integrations

Specifications of RDK-138

• Mfg Application Notes: LinkSwitch-TN Design Guide, AN-37
• Main Purpose: AC/DC, Non-Isolated
• Outputs And Type: 2, Non-Isolated
• Power - Output: 1.2W
• Voltage - Output: 12V, 5V
• Current - Output: 80mA, 50mA
• Voltage - Input: 85 ~ 265VAC
• Regulator Topology: Buck
• Frequency - Switching: 66kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LNK304
• Product: Accessories & Kits
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Other names: 596-1197

4 Circuit Description
The power supply described has a jumper that allows for the supply to operate using
either the buck topology–enabling the two outputs to be configured as positive outputs
with respect to AC neutral–or a buck-derived topology–enabling one of the two outputs to
be configured as a negative output with respect to AC neutral. Shorting pins 1 and 2 of
JP1 will operate the supply in a buck-derived topology, while shorting pins 2 and 3 will
operate the supply in the buck topology. See Figure 1 for an illustration of the two jumper
positions.
While in the buck topology, the power supply has a 12 V output capable of delivering 80
mA and a 5 V output capable of 50 mA. If the supply is in the buck-derived topology, it
provides a –5 V output when referenced to return and can provide 50 mA of current. The
other output can be used to supply 80 mA and a 7 V output when referenced to return, or
it can be configured as a floating12 V output.
The data presented in the report were taken using the buck topology with both outputs
configured as positive outputs.
4.1 AC Input Rectification and EMI Filtering
The AC input is half-wave rectified by fast recovery diode D1 and general purpose diode
D4. The fast recovery diode helps to improve differential mode conducted EMI. A second
diode, D4, is placed in the return conductor of the supply to provide EMI noise gating.
This helps minimize common mode noise and reduce EMI.
Included in the path of the AC input is a fusible flameproof resistor, RF1. In the event of
a catastrophic failure, the failure mechanism of this resistor is open circuit, with no flame
or smoke emitted. Additionally, this resistor is wire wound to prevent premature failure
due to the inrush currents when AC is applied.
The input stage is completed by capacitors C1 and C2, which provide bulk energy
storage. Inductor L1 forms a π-filter with the bulk capacitors, which helps to minimize
differential mode conducted EMI.
4.2 LinkSwitch-TN Device
LinkSwitch-TN is a low-cost, full-featured monolithic IC integrating both the control
circuitry and high voltage MOSFET switch into the same device. Some of the most
prominent features of the device include an On/Off type controller, a maximum switching
frequency of 66 kHz, which includes ±4 kHz of frequency jitter to help reduce EMI and a
700 V MOSFET. The LinkSwitch-TN device also includes a number of protection
features, including output short-circuit protection, open loop fault protection, MOSFET
current limit protection and thermal shutdown with hysteresis. The device is powered
from the drain pin with local supply decoupling provided by the 100 nF capacitor C3
connected to the bypass pin (BP) of the LinkSwitch-TN device, U1.
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