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The RDK-877 is an isolated flyback converter designed to provide a nominal output voltage of 12 V at 0.5 A load from a wide input voltage range of 90 VAC to 305 VAC, as well as an isolated zero crossing detection (ZCD) signal. This power supply utilizes the LNK3306D from the LinkSwitch-TNZ family of ICs. This circuit is configured as a flyback topology power supply utilizing the LNK3306D. Secondary-side constant voltage (CV) regulation is accomplished through optocoupler feedback with a Zener reference.
Input Rectifier and Filter
Input fuse F1 provides safety protection from component failures. Varistor RV1 acts as a voltage clamp that limits the voltage spike across the DC bus during line transient surge events. The AC input voltage is rectified by input bridge BR1. The rectified DC is then filtered by the bulk storage capacitors C1 and C2. Inductor L1, C1 and C2 form an input pi filter, which attenuates differential mode conducted EMI.
LNK3306 Operation
The LNK3306D device U1 integrates the power switching device, oscillator, control, startup, and protection functions. The rectified and filtered input voltage is applied to the primary winding of T1. 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, delivering energy from bulk capacitor to 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 is delivered to the output capacitor during off time. 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 D2, C3, R2 and R3. Resistor R2 and R3 not only damp the high frequency leakage ring that occurs when the power MOSFET turns off, but also limit the reverse current through D1 when the power MOSFET turns on. This allows a slow, lowcost, glass passivated diode (with a recovery time of 2 s.) to be used for D2. The slow diode also improves conducted EMI and efficiency. Using ON/OFF control, U1 skips switching cycles to regulate the output voltage, based on feedback to FB pin. The FB pin current is sampled, just prior to each switching cycle, to determine if that switching cycle should be enabled or disabled. If the FB pin current is <49 A, the next switching cycle begins, and is terminated when the current through the power MOSFET reaches the internal current limit threshold.
Output Rectification and Filtering
Output rectification is provided by D1. Low ESR capacitor C5 achieves minimum output voltage ripple and noise in a small can size for the rated ripple current specification.
Feedback and Output Voltage Regulation
The supply’s output voltage regulation set point is set by the voltage that develops across Zener diode VR2, R5 and the LED in optocoupler U2. The value of R6 was calculated to bias VR2 to about 0.5 mA when it goes into reverse avalanche conduction. This ensures that it is operating close to its rated knee current. Resistor R5 limits the maximum current during load transients. The values of R5 and R6 can both be varied slightly to fine-tune the output regulation set point. When the output voltage rises above the set point, the LED in U2 becomes forward biased. On the primary-side, the phototransistor of U2 turns on and injects current into the FB pin of U1. Just before the start of each switching cycle, the controller checks the FB pin current. If the current flowing out of the EN/UV pin is greater than 49 A, that switching cycle will be disabled. As switching cycles are enabled and disabled, the output voltage is kept very close to the regulation set point.
Output Overvoltage Shutdown
PI’s proprietary primary overvoltage detection eliminates the use additional optocoupler and enables to low voltage rated output. It is accomplished by sensing the switching bias winding voltage during power MOSFET off time. When the power MOSFET is off, the reflected voltage on the bias winding is proportional to the output voltage by a factor determined by the bias and output turns ratio. When this voltage exceeds the sum of VR5, forward voltage of D4, and the BYPASS (BP) pin voltage, an overvoltage condition occurs and current begins to flow into the BYPASS pin. When this current exceeds 6 mA the internal shutdown circuit in U1 is activated. Reset is accomplished by allowing the BYPASS pin voltage to drop below 2 V. Resistor R15 can be used to fine tune the overvoltage limit.
Zero Crossing Detection
During normal operation when AC input is present, the Z1 and Z2 pins draw 22 A as its supply current. The AC Line node is directly sensed by resistor R8, and resistors R9 and R14 are returned to primary ground. The supply current then goes thru the input bridge BR1, before returning to the AC Neutral node. Due to the presence of BR1, Z1 and Z2 pins can only conduct and draw supply current during the positive half of the AC cycle. The diode in BR1 will be reverse biased during the negative half cycle. When Z1-Z2 conducts during the AC positive half-cycle, MOSFET Q1 is turned on by the voltage across R9. Zener diode VR3 protects Q1 gate-source from excessive high-voltage during transients. MOSFET Q1 then switches optocoupler U3 and current-limiting resistor R10. On the secondary-side, the ZCD output is pulled low by the transistor in U3. During the AC negative half-cycle, Z1-Z2 is prevented from conducting by bridge BR1, so both MOSFET Q1 and optocoupler U3 are also turned off. At the secondary-side, pull-up resistor R11 and Zener diode VR4 clamp the ZCD output to 5 V.
EMI Design Aspects
In addition to the simple input Π filter (C1, L1 and C2) for differential mode EMI, this design makes use of shielding techniques in the transformer to reduce common mode EMI displacement currents. Resistor R3 and capacitor C3 are added to act as damping network to reduce high frequency transformer ringing. These techniques combined with the frequency jitter of LNK3306D gives excellent conducted EMI performance.
Note: I have added the handheld oscilloscope to aid in testing the RDK-877. This review should not cover the handheld oscilloscope
Begin enrollment: | Dec 8 2023 |
End enrollment: | Jan 10 2024 |
Select roadtesters: | Jan 11 2024 |
Ship unit: | Jan 17 2024 |
Begin Roadtesting: | Jan 24 2024 |
Element14 follow up: | Feb 24 2024 |
Post Reviews by: | Mar 24 2024 |
Power Integrations, Inc. is a leading innovator in semiconductor technologies for high-voltage power-conversion. The company’s products are key building blocks in the clean-power ecosystem, enabling the generation of renewable energy as well as the efficient transmission and consumption of power in applications ranging from milliwatts to megawatts.
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Top Comments
The Schematic link above is the best to check the evaluation board.
I applied for the Road Test.
The scope is a great incentive for this road test. In my workflow it would sit at my computer workstation, where I program microcontroller modules, but often need to do quick electrical tests to check…