30W Active Clamp Forward Converter
Created: Jun 09, 2016
No description available.
Nowadays, switched-mode power supplies (SMPS) are widely used due to its efficiency compared to linear power supplies. Essentially switched mode power supplies or SMPS act as DC to DC converters. The forward converter comes from the buck topology of DC to DC converters. The main difference between the two topologies is that the transformer employed in the forward topology provides input-output ground isolation as well as a step-down or step-up function. The transformer in a forward topology does not inherently reset each switching cycle as do symmetrical topologies (push-pull, half-bridge, and full-bridge).
This reference design is a forward converter with an active clamp circuit, which is composed of MOSFETs, and a clamp capacitor. This clamp circuit works by alternately turning on/off the MOSFETs. At first, Q1 is on and the reset switch Q2 is off. The clamp capacitor C7 will then be charged with a duration equal to the switching period. After that, Q1 is turned off and the reset switch Q2 is turned on. The magnetizing current flows through the clamp capacitor and Q2 instead of through Q1. Since the clamp capacitor voltage is greater than VIN, the voltage across the transformer primary is now reversed. The current will continue to build in the opposite direction as the clamp capacitor returns the energy that it had previously captured. Steady state conditions require the clamp capacitor voltage to return to the starting potential and the magnetizing current at the end of the reset time to reach the same magnitude (opposite polarity) of the current at the beginning of this reset time. The switching period is determined by the oscillator period of LM5026MT. The reset switch is turned off, stopping the flow of current from the clamp capacitor.
The benefits of active clamp involve switching loss reduction and lower voltage stress on rectifiers. This isolated active clamp forward converter accepts a voltage range of 24-36V and delivers a 12V output at 2.5A. This design uses the LM5026 active clamp current-mode controller. Output voltage regulation is maintained using an LMV431 shunt regulator and opto-coupler feedback. The schottky rectifiers allow for holdup of 2.5ms using the stored charge of the output capacitors. The schottky rectifier used in this design is the MBR10200UPS, which is capable of up to 200V of voltage and 10A of current. The isolation voltage is 2250VDC that meets basic insulation requirements.