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UC3843: controller for a 2kW adjustable-output step-down converter (DC chopper)

Part Number: UC3843

I need support about the choice of the right controller for a high power (2kW) step-down converter with adjustable output voltage. The application is DC Variac (chopper).

- Topology is:  non-synchronous buck

- The goal is to control the output power

- Inductor current and output voltage are available as feedback

- Power loop is performed by a microprocessor (based on I and V feedback)

- The microprocessor gives a set to the controller

- V and I feedback can also be used to close loops on the controller

- Current limiting function is needed (cycle by cycle)

I was thinking about using a UC3843 (current mode) or a UC35705 (voltage mode with current limiting).
What do you think about ? Any recommendations ? Or any other suggestions ?

Can anyone show me a draft of schematics ?

Thank you

  • Hi Davide,

    Thanks for connecting with us through E2E. I can appreciate your thoughts about PWM controllers: UC3843, UC35795 are simple, legacy, single output PWM controllers that seem to offer the basic features you mentioned. However, I recommend you take a step back and reconsider the best power stage topology first and then search out the best controller for that power stage. A non-sync buck at 2 kW - let's think about the peak currents, the diode conduction time, high side switch loss and switch node voltage for your converter VIN, VOUT, POUT parameters? It sounds as if you do not require isolation but 2 kW is most likely going to require 2-4 primary discrete switches and possibly SR or parallel SRs on the secondary. The best topology depends on your power conversion ratio, output power and input voltage range. Have you tried TI Power Designer? This is a great and simple simulation tool for evaluating power stage topologies - you can get a feel for losses, peak current and voltage stress and this should help guide you. You can also play with designing the control loop using the "Loop Calculator" option from the main menu...give it a try and good luck with your design.

    Regards,

    Steve M

  • Thank you for your answer Steven. For this applicatoin the basic non-isolated non-synchronous buck topology is ideal.

    Input voltage is 300V, typical load is 40ohm. Therefore maximum power corresponds to 7A output current and will be delivered at duty cycle close to 100%. The lower the duty cycle, the lower the power delivered. In all conditions a simple power stage made with one SiC fet, one SiC diode and one 400uH inductor driven in standard hard switching can easily perform efficiencies as high as 99%, making synchronous rectification absolutely unnecessary (just to know, the power stage has already been tested in open loop and its performance validated).

    That said, my concern is about choosing the right controller for the application, and how to apply it.

  • Davide,

    Understood, I'll assume the power stage is validated but 300V is low-voltage for SiC. You want to drive a high-side switch and this is the first problem to overcome with either a gate drive transformer and a level shifted, floating gate driver IC. There are other issues with driving SiC you need to overcome. Driving a SiC MOSFET requires high-voltage gate drive (15V-20V) and a compatible UVLO from the controller or some way to hold off switching until VGS is at an acceptable amplitude for your SiC. Since your SiC is floating you also need a gate drive with a strong pull down or perhaps even negative turn-off bias to assure OFF is off. The next issue you need to consider for a high-voltage, high power buck is current sensing. For peak CMC a CS xfmr should be considered but not at 99% duty cycle (xfmr will saturate). Unless you can resolve the CS issue, you might consider VMC or hysteretic mode control? TI does not have a reference design close to these parameters I can point you towards. You can check out TIDA-0155 to get an idea of driving the SiC but this is a low-side SiC in a flyback, yours is high-side Buck. You can also search the TI Reference Design Library and maybe gather some ideas (VMC, hysteretic, SiC, current sensing) from other designs we've done but I believe you are going to have get creative with partial solutions coming from various sources. 

    Regards,

    Steve M

  • Steven,

    as already said the power stage has already been tested and validated, and it includes the isolated driver for the high-side fet wit its own ULVO protection and and its own floating power supply for driving the gate at +15V/-3V.

    Current measurement is performed by an hall-effect sensor (LEM) placed in series with the inductor, as required from average CMC: no saturation problems at any dury cycle level.

    Please let me know your thoughts

    Regards, Davide

  • Davide,

    Since you have identified and worked out all the issues I've highlighted, then yes, any single ended general purpose PWM such suffice. You mentioned UC3843 (current mode) or a UC35705 (voltage mode with current limiting) as two possible choices depending on your preferred control method.

    Steve