12V-7A AC to DC Flyback Switching Power Supply

May 29, 2024

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12V-7A AC to DC Flyback Switching Power Supply.

The most essential part of any electronic device is the power supply unit. Any instability or malfunction in this part causes the device to stop its duty or show weird behavior. In this article/video, I introduced an AC-to-DC flyback switching power supply that converts 180V-260VAC to 12VDC, which can be used in various applications.

The maximum power delivery of this power supply is 84W, which means it can handle 7A continuously at 12V output. The transformer can deliver higher currents, however, the educational purpose (hand winding of the transformer) and thermal stress on the switching controller were considered (the peak current of the controller’s MOSFET is around 2.3A, at 7A). The controller chip is KA1M0565, which uses a 470K startup resistor and a small auxiliary winding on the transformer. The transformer’s ferrite core is ER28-17-11, and the switching frequency is 67KHz. A potentiometer allows the user to adjust the output voltage precisely and set it to 12V.

To design the schematic and PCB, I used Altium Designer 24 and the Octopart website to quickly gather the necessary component information and generate the BOM. I used the Siglent SDL1020X-E DC Load and Siglent SDS2102X Plus oscilloscope to test the board for voltage drop, current delivery, and output noise. The output voltage drop is only around 40mV at the maximum output current.

Circuit Analysis

Figure 1 shows the schematic diagram of the device. The heart of the circuit and the switching controller is the KA1M0565 chip¹.

Figure 1: Schematic diagram of the 12V-7A AC to DC Flyback Switching Power Supply. — Figure 1: Schematic diagram of the 12V-7A AC to DC Flyback Switching Power Supply

P1 is the AC input terminal (KF127). F1 is a 1A 250V-AC fuse to protect the circuit. R1 is the 10D516K varistor² to protect the circuit against transients and voltage spikes. C2 and C3 are 100nF-275VAC³ capacitors, and T1 is a 10mH common mode choke to reduce the AC voltage noise. BR1 Is the DB107G bridge to rectify the AC voltage⁴, and C1 is a 100uF-400V capacitor to reduce the voltage ripple.

D3, R2, and C5 build a snubber circuit to dampen the primary winding voltage spikes. D3 should be a slow diode such as FR207⁵. R4 provides the initial (startup) current for IC1. After startup, the auxiliary winding of the transformer powers the IC1. D2, R5, and C10 rectify the voltage of the auxiliary winding, and D4 protects the IC1 against any overvoltage.

IC1 is the KA1M0565 switching controller. According to the datasheet: “The Fairchild Power Switch (FPS) product family is specially designed for an off-line SMPS with minimal external components. The Fairchild Power Switch (FPS) consists of high voltage power SenseFET and current mode PWM controller IC. PWM controller features an integrated fixed oscillator, under voltage lock out, leading edge blanking, optimized gate turn-on/turn-off driver, thermal shut down protection, over-voltage protection, temperature compensated precision current sources for loop compensation, and fault protection circuit. compared to discrete MOSFET and controller or RCC switching converter solution, a Fairchild Power Switch (FPS) can reduce total component count, design size, and weight while increasing & efficiency, productivity, and system reliability. It has a basic platform well suited for cost-effective design in either a flyback or forward converter.“

OP1 is the PC817 optocoupler⁶ that provides galvanic isolation between the primary and output ground points and a feedback path for IC1 to sense the output voltage (using Reg1) and regulate it. D1 is the 2-Pin MBR20100⁷ Schottky diode to rectify the output voltage. C6 … C9 and L1 are filtering components to reduce the output voltage. R3 is a 25mA dummy load to stabilize the output voltage. D5 is a 5mm yellow LED to indicate the existence of the output voltage, which confirms the true operation of the power supply.

Transformer

You should prepare the materials and wind the transformer using the following instructions:

A. Ferrite Core: ER28-17-11 (for instance: B66433 core, TDK)
B. Bobbin: 6+6, Horizontal (Figure 2) [8]
C. Primary Winding (Pin-1 to Pin-3): 56 turns of a 0.5mm * 1 wire (inductance: 568uH)
D. Auxiliary Winding (Pin-5 to Pin-6): 9 Turns of a 0.3mm * 1 wire
E. Output Winding: (Pin-7,8,9 to Pin-10,11,12): 5 Turns of 0.5mm * 10 wires (10 wires in parallel)

Figure 2: The selected bobbin for the ER28-17-11 ferrite core. — Figure 2: The selected bobbin for the ER28-17-11 ferrite core

Start the procedure by winding the primary, clockwise or counterclockwise. Then put the ferrite cores in place (in the bobbin) and measure the inductance of the primary winding using an LCR meter. If you can select the measurement frequency on your LCR meter, the best frequency is 67KHz (switching frequency of the transformer), otherwise, set it to 40KHz. Grind the middle leg of the ferrite core (Figure 3) and measure the inductance of the primary winding till you measure a value close to 568uH. A small tolerance is acceptable and does not cause problems (for example 580uH or 550uH).

Figure 3: A gap in the middle leg of the ferrite core. — Figure 3: A gap in the middle leg of the ferrite core

The rest of the job is easy. Follow Figure 4 and wind the auxiliary and the secondary windows. The rotation direction of the auxiliary and secondary windings must follow the primary winding. Pay attention to the dots or starting points as well.

Figure 4: Transformer and winding directions. — Figure 4: Transformer and winding directions

L1 Inductor

The ferrite core of the L1 inductor is T80-26B, Micrometals (Figure 5). The core’s outer diameter is 20.3mm, the inner diameter is 12.6mm, and the height of the ring is 6.3mm. You should use two 0.8mm wires in parallel (0.8mm * 2) and wind them for 20 turns, however, the number of turns is not critical. This inductor is for noise reduction, so you can use other cores and wires as long as the core fits on the board and does not get saturated at the maximum current.

Figure 5: The T80-26 Micrometals, Yellow-White Iron Powder Core. — Figure 5: The T80-26 Micrometals, Yellow-White Iron Powder Core

PCB layout

Figure 6 shows the PCB layout of the design. It’s a single-layer PCB board, and all components are through-hole. The components that are labeled as “J” are zero-ohm resistors.

Assembly and Test

Figure 7 shows the assembled PCB board. I used a DC load to test the voltage regulation and output voltage drop. First, I tested the output voltage without any load and second with the 7A maximum load (Figure 8). For detailed testing, please watch the YouTube video. The controller chip has an embedded soft starter circuit, introducing a spark-free power supply startup.

Figure 7: Assembled PCB board of the 12V-7A AC to DC Flyback Switching Power Supply. — Figure 7: Assembled PCB board of the 12V-7A AC to DC Flyback Switching Power Supply

Figure 8: Power supply regulation and output voltage drop. — Figure 8: Power supply regulation and output voltage drop

More Improvements

One of the effective methods to reduce the leakage inductance of the primary winding is the sandwich method. Wind the first half of the primary from Pin-1 to Pin-2, then full auxiliary winding, then full secondary winding, and finally the second half from Pin-2 to Pin-3.

To further reduce the output noise and impedance of the ground path, I added another layer on the top of the PCB board just to the ground (figure 9).