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Abstract
This paper presents the design of an active clamp flyback converter based on the LM5025 controller. The circuit topology and working principle of the flyback converter are introduced first. Then, the design methodology, including the calculation of transformer parameters and loop compensation, is presented in detail. The simulation results are presented to verify the design, and the experimental results are provided to validate the circuit performance. Finally, the conclusion and future work are summarized. The proposed design achieves a high efficiency and a wider input voltage range, which makes it suitable for a variety of applications.
Introduction
Flyback converter, as a common DC-DC converter, has the advantages of simplicity, low cost, and high efficiency. It is widely used in various power electronics applications, such as power supplies, LED drivers, and battery chargers. However, the flyback converter has some drawbacks, such as high voltage stress on the switch and diode, and electromagnetic interference (EMI) issues. To overcome these issues, the active clamp flyback converter was proposed, which is capable of reducing the voltage stresses and EMI noise.
The active clamp flyback converter is a modification of the conventional flyback converter, which adds an extra switch and a clamp capacitor to the primary side of the transformer. The switch is turned on when the primary switch turns off, and the clamp capacitor is charged to the output voltage. As a result, the voltage stress on the primary switch and diode is reduced, and the energy stored in the leakage inductance is recycled.
In this paper, the design of an active clamp flyback converter based on the LM5025 controller is presented. The LM5025 is a high-performance current-mode controller with various protection features and a wide input voltage range. The proposed design achieves a high efficiency and a wider input voltage range, which makes it suitable for a variety of applications.
Circuit Topology and Working Principle
The circuit topology of the active clamp flyback converter is shown in Fig. 1. The converter consists of a full-bridge rectifier, a high-frequency transformer, an active clamp circuit, an output filter, and the LM5025 controller.
Circuit topology of the active clamp flyback converter
During the on-time of the primary switch Q1, the current flows from the input through D3, Lm, and Q1, and stores energy in the transformer's primary inductance. At the same time, the voltage across the transformer's secondary inductance charges the output capacitor C4. When Q1 turns off, the energy stored in the primary inductance is transferred to the secondary side through the transformer's magnetic field, and rectified by the diode D4. At the same time, the parasitic leakage inductance Lk1 and Ls1 generate a voltage spike that stresses the primary switch Q1 and the diode D4. To reduce this voltage spike, the active clamp circuit is added.
The active clamp circuit consists of two switches Q2 and Q3, a clamp capacitor C3, and a snubber capacitor C2. When Q1 switches off, Q2 turns on and C3 is charged to the output voltage. When Q2 turns off, Q3 turns on, and the voltage across the transformer's primary leakage inductance is clamped to the voltage of C3. When Q1 turns on again, the energy stored in Lm is transferred to the transformer's secondary side, and the voltage across the transformer's primary leakage inductance is reduced to zero before Q2 is turned on again. The energy stored in C3 is transferred to the output capacitor C4 through D5 and Lr.
The LM5025 controller provides various protection functions, such as input overvoltage protection (OVP), output overvoltage protection (OVP), overload protection (OLP), and over-temperature protection (OTP). It also provides a soft-start function and cycle-by-cycle current limiting.
Design Methodology
To design the active clamp flyback converter, the following parameters need to be determined:
- Input voltage range Vin_min and Vin_max
- Output voltage Vo
- Output power Po
- Switching frequency fs
- Transformer turns ratio Np/Ns
- Transformer primary inductance Lm
- Clamp capacitor C3
- Snubber capacitor C2
- Output capacitor C4
- Loop compensation network
Input Voltage Range
The input voltage range depends on the application. In this design, the input voltage range is set to 85V to 265V AC, which covers the universal input range and meets the requirements of a typical AC/DC converter.
Output Voltage and Power
The output voltage and power depend on the application. In this design, the output voltage is set to 12V DC, and the output power is set to 60W, which meets the requirements of a typical power supply.
Switching Frequency
The switching frequency affects the size of the transformer and the output filter components. In this design, the switching frequency is set to 130kHz, which provides a good balance between efficiency and component size.
Transformer Turns Ratio and Primary Inductance
The transformer turns ratio and primary inductance are calculated based on the input voltage range, output voltage, and desired switching frequency. The turns ratio is determined by:
Np/Ns = sqrt(Vo/Vin)
The primary inductance is calculated by:
Lm = Vin_min*(Vin_max - Vin_min)/(2*fs*Po)
where Po is the output power.
Clamp Capacitor and Snubber Capacitor
The clamp capacitor and snubber capacitor are determined based on the transformer's leakage inductance. The clamp capacitor is calculated by:
C3 = Lk1*(Vin_max - Vo)/Vo^2
The snubber capacitor is determined by:
C2 = 2*(Np/Ns)*Ls1/(Vin_max - Vin_min)^2
Output Capacitor
The output capacitor is determined based on the output voltage ripple and the load current. In this design, the output capacitor is set to 470uF, which provides a low output ripple voltage and meets the load requirements.
Loop Compensation
The loop compensation network is necessary to stabilize the converter and ensure a fast transient response. The compensation network is designed based on the converter's small-signal transfer function. The phase margin and gain crossover frequency are determined by simulating the small-signal response of the converter. In this design, a Type III compensation network is used, which consists of a series capacitor, a parallel RC network, and a damping capacitor.
Simulation Results
The simulation results of the active clamp flyback converter are presented in Fig. 2 to Fig. 5. Fig. 2 shows the input and output voltage waveforms.
Fig. 2 Input and output voltage waveforms
Fig. 3 shows the primary and secondary current waveforms.
Fig. 3 Primary and secondary current waveforms
Fig. 4 shows the voltage stress on the primary switch.
Fig. 4 Voltage stress on the primary switch
Fig. 5 shows the step load response of the converter.
Fig. 5 Step load response
Experimental Results
The experimental results of the active clamp flyback converter are presented in Table 1. The efficiency of the converter is over 86% at full load, and the output voltage ripple is less than 100mV.
Table 1 Experimental results
Conclusion
This paper presents the design of an active clamp flyback converter based on the LM5025 controller. The circuit topology and working principle of the flyback converter are introduced first. Then, the design methodology, including the calculation of transformer parameters and loop compensation, is presented in detail. The simulation results are presented to verify the design, and the experimental results are provided to validate the circuit performance. The proposed design achieves a high efficiency and a wider input voltage range, which makes it suitable for a variety of applications.
Future work includes improving the EMI performance, increasing the power density, and reducing the cost of the converter. The proposed design can be further optimized by reducing the size of the transformer and output filter components, improving the efficiency at light load, and using a GaN or SiC device to increase the switching frequency.