Open source project of power pre-regulator built to provide all necessary power for the final stage (post-regulator) of a variable bench power supply. The project is initiated by need to replace “off-the-shelf” AC/DC off-line converter as a primary stage of efficient hybrid bench variable/programmable power supply.

The main challenge was to make a safe, robust and reduced noise AC/DC conversion that can operate with wide input voltage range and provide wide output voltage range controlled by external programming input (i.e. that comes from power supply post-regulator). Additionally, since variable power supply control circuits require some bias power inputs, it was desirable that converter also provide two or more fixed (low power) outputs.
For power part of the converter a so-called CF-DIC topology (Current fed, dual inductor converter) has been chosen combined with HV sync buck input stage to provide wide output voltage that can be used efficiently for variable power supply with 0 to 50 V, 5 A output.

Bias power supply, a QR (quasi-resonant) flyback topology has been selected that provide all necessary power for primary and secondary side of the CF-DIC stage, and connected post-regulator. Choosen VIPer35 controller comes with built-in various protections for improved safety.

The converter comes with built-in various protection against overheating (OTP), short circuit and over-voltage (OVP). Additionally, an Active Duty limiter is employed to guard it against fast changes of output voltage programming input that could result in dramatic increase of PWM Duty (for HV buck stage) that could be fatal with high input voltage (i.e. 230 Vac).
The synchronous rectification is used instead of classical diode full wave rectifications to improve converter efficiency.

In summary, the converter combine functionality of three important ingredients of a hybrid bench variable/programmable power supply:

• AC/DC power conversion (fixed DC output),
• AC/DC bias supply and
• DC/DC pre-regulator.

This converter employ few interesting components:

• SiC MOSFETs and freewheel SiC diode for improved efficiency and lower EMI of the HV buck stage
• High performance VAC Vitroperm 500F core material with high permeability and high saturation induction that power transformer can be much smaller that one with standard ferrite materials

Converter's BOM is available on the GitHub, and all used components can be sourced from the major distributors. Only two components: flyback and power transformer are custom made (that is common case for AC/DC converters) and can be sourced from Feryster (Poland). Shown enclosure is provided by Varisom (Portugal).

Basic facts:

• Wide range input: 85 – 265 Vac, 120 – 380 Vdc
• Wide range output: 2.5 – 52.5 Vdc
• Output current up to 3 A continuously without additional cooling (e.g external fan) or 5 A with additional cooling (max. power up to ~260 W)
• Output voltage controlled by tracker circuit or optionally with multiturn trimpot if fixed output voltage is required
• Cascaded PWM Controller LM5041B (sync buck + push-pull stage)
• Isolated HV power ground and signal ground for improved noise immunity
• HV buck section with high performance SiC MOSFET switches
• High performance VAC Vitroperm 500F core material for power transformer
• Synchronous rectifier with low Rds, on MOSFET switches
• Active Duty cycle limiter
• Over-voltage (OVP), short circuit protection and over-temperature protection (OTP) with latched shutdown and opto-isolated open-collector "Fault" output
• Over-current protection (OCP)
• QR flyback as bias power supply (VIPer35)
• Four bias power supply outputs: +6.5 V, -8.5 V and "floating" +12 V / -12 V that can be further regulated with LDOs
• Compact size (155 x 90 x 45 mm)
• Weight (with enclosure): 490 g