Reference design DER-736 from Power Integrations describes a non-isolated flyback converter designed to provide dual output of 5V at 200mA and 12V at 500mA from a wide input voltage range of 85- to 440-Vac. This adapter utilizes the LNK3696P from the LinkSwitch-XT2 900V family of devices.
Targeting high-efficiency isolated and non-isolated flyback power supplies up to 8W, the 900V versions of the LinkSwitch-XT2 IC family enable designs to easily meet the EU’s Energy-Related Products (ErP) Directive. Less than 30mW no-load consumption and high conversion efficiency across the load range make the new ICs suited for IoT and home and building automation (HBA) systems.
The LinkSwitch-XT2 900V family of devices integrates a high-voltage (900V rated) power MOSFET with an internal oscillator and ON/OFF controller inside a single monolithic IC. Unlike conventional pulse width modulation (pwm) controllers, LinkSwitchXT2 900V devices utilize a simple ON/OFF control scheme combined with an internal current limit circuitry to regulate the output voltage.
The ac input is rectified by bridge rectifier BR1 and filtered by the bulk storage capacitors C2 and C3. Resistor RF1 is a flameproof, fusible, wire wound type and functions as a fuse, inrush current limiter and, together with the filter formed by C1, C2, C3, L1 and L2, differential mode noise attenuator. Resistors R13, R14, R15 and R16 function to balance the voltage between the bulk capacitors in series.
LinkSwitch-XT2 Primary Side
The rectified and filtered input voltage is applied to the primary winding of flyback transformer T1. T1 is switched and connected to the IC via the DRAIN (D) pin. The D pin provides internal operating current for both start-up and steady-state operating conditions.
During the power MOSFET turn on time, current ramps up in the primary winding, storing energy inside the transformer core, while the two secondaries remain cut-off due to reverse biasing of the diodes. The primary winding current eventually exceeds the internal threshold (ILIMIT), causing the power MOSFET to turn off for the remainder of the switching cycle.
At the beginning of the next and all succeeding switching cycles, the IC decides whether to turn ON the power MOSFET or let it remain turned OFF. ON/OFF control is performed by comparing the output voltage to the reference voltage VFB, resulting to the power MOSFET being enabled or disabled.
Through the use of ON/OFF control, voltage regulation is maintained by skipping cycles without using an error amplifier and ramp generator, as in traditional power supply controllers.
Primary RCD Clamp and Output Rectification
A low-cost RCD clamp is connected across the primary winding of transformer T1. This is composed of resistors R20 and R13, capacitor C13 and diode D5. The clamp helps in dissipating the energy stored in the leakage inductance of T1.
Transformer T1 has two secondary windings on its core – one for each output voltage. For both secondaries, the secondary switching voltage is rectified by Schottky diodes D3 and D4, and then filtered by super low ESR type capacitors C6, C7, C8 and C9. For each Schottky diode, a snubber network is connected in parallel using resistors R4 and R8, and capacitors C5 and C10. The snubber network helps limit the peak inverse voltage spikes seen by the diode. Pre-load resistors R23 and R23 are connected on each output to improve cross-regulation performance.
The two outputs 12 V and 5 V are sensed through resistor dividers R5, R6 and R7. The sensed voltage is fed back to U1 through the FEEDBACK (FB) pin. This voltage VFB must be accurately maintained at 2V, so the sensing resistors must have 1% tolerance.
A weighted feedback circuit using resistors R5, R6 and R7 are connected to both output voltages and into the FB pin. The magnitude of the weights are determined by the relative contributions of current delivered into the FB pin, which then determine the values of R5 and R7. In the schematic, a larger current is contributed by the 5V output, due to the larger current through R5 as compared to R7. Effectively, the 5V output voltage has better feedback sensitivity and output regulation.
Output Overvoltage Protection
The BYPASS (BP/M) pin has multiple functions: it provides a connection point for an external bypass capacitor as well current limit value selector via the capacitance value, and it provides a shutdown function. Similar to the FB pin, when the current delivered into the BP/M pin exceeds the internal threshold (IBP(SD)) for a time equal to 2 to 3 cycles of the switching frequency, the device enters auto-restart. By designing a circuit that injects current into the BP/M pin due to a fault, the device can be equipped with both output overvoltage and line overvoltage protection.
For the 5V output, a simple clamp circuit using Zener diode VR3 and resistor R22 is used for overvoltage protection. For the 12V output, Zener diode VR2, diode D6 and resistor R7 comprise the overvoltage protection circuit. A voltage at the 12V output the exceeds the sum of the VR2 voltage rating, D2 diode drop, and the bypass voltage will cause a current in excess of IBP(SD) to be injected into the BP/M pin, which will trigger the autorestart and protect the power supply from overvoltage.
Line Overvoltage Protection
The device indirectly senses the dc bus voltage by sampling the secondary voltage during the power MOSFET on-time. During this time, the voltage across the secondary winding is proportional to the voltage across the primary winding. When the dc bus voltage exceeds the limit, Zener diode VR1 and diode D2 are turned on causing current to flow through transistor Q1 and into the FB pin. The device will go into auto-restart mode if the FB pin current limit is exceeded for at least 2 consecutive switching cycles.