LM5025 Isolated Active Clamp Forward Converter w/ 30-60V in, 15V out @ 1.5A Reference Design
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This circuit uses the LM5025 voltage mode PWM controller with complementary gate drives to implement a high-efficiency single-ended active clamp forward converter.
MOSFET Q10 is an n-channel main primary side FET while Q13 a P-channel clamp FET. Circuitsaround the LM5025 implements various necessary functions: R10 and C14 implement start-up at pin 1; R14 and C20 implement a voltage ramp with input voltage feedforward for the duty cycle comparator at pin 2; R31, R29 and C21 implement over current protection at pin 4; C23 implements soft-start at pin 12 and R30 sets the switching frequency at pin 14.
Self driven synchronous rectifiers Q11 and Q12 are used on the secondary side to improve efficiency. But because the voltages off the main secondary winding would be too great for the gates of the synchronous rectifiers, an auxiliary, lower voltage center-tapped secondary winding is used to drive the gates of these FETs. The feedback control circuit for regulation of the output voltage consists of the LM4041 reference/error amplifier REF10 with the opto-coupler U12 to transmit output voltage information to the primary side. The LM5025 has a direct opto-coupler interface at pin 13 that simplifies the feedback interface.
- Internal Start-up Bias Regulator
- 3A Compound Main Gate Driver
- Programmable Line Under-Voltage Lockout (UVLO) with Adjustable Hysteresis
- Voltage Mode Control with Feed-Forward
- Adjustable Dual Mode Over-Current Protection
- Programmable Overlap or Deadtime between the Main and Active Clamp Outputs
- Volt x Second Clamp
- Programmable Soft-start
- Leading Edge Blanking
- Single Resistor Programmable Oscillator
- Oscillator UP / DOWN Sync Capability
- Precision 5V Reference
- Thermal Shutdown