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DC/DC Switching Converters: Is it possible to run a current-mode converter in voltage-mode only? If so, is there an example using a buck converter?

In particular, I'd like to use UC3843 (LT1243) in LTspice as a buck converter in plain-old voltage-mode regulation (disabling current-mode regulation if possible). I've successfully run LT1243 as a SEPIC converter in the past, but I'd like to have run it as a stand-alone buck (step-down) for it's higher efficiency and single winding inductor. I cannot do a flyback (Texas Inst WeBench) for this application.

I apologize if this is a fundamental question. I'm a mechanical engineer by trade and have come a long way learning LTspice, routing PCBs, and programming MCUs. I've tried grounding the Current Sense (SENSE) pin, or changing the compensation, or changing the load type from a resistor to a fixed current load and it always settles at a particular Vout with respect to Vin, as if Vfeedback has no affect.

The UC384X and LT124X chips are current-mode PWM controller legacy chips.

Since I have used this Si7370DP N-channel Mosfet in the SEPIC LT1243 circuit using this UC3843/LT1243 controller, the controller has an internal totem-pole driver. I don't necessarily agree with the responses to this post: UC3843 buck converter

Thanks in advance!

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You can certainly turn a current-mode-controlled (CM) converter into a voltage-mode-controlled (VM) converter by increasing the compensation ramp and reducing the inductor current contribution. However, doing so changes the transfer function (and the good features inherent to CM) and a new control strategy has to be thought. Besides, the natural cycle-by-cycle peak current limit offered by CM is lost in VM and an additional comparator must be added to perform this function.

Regarding the UC384x, as you know, it is not very well suited to control a high-side switch as in a buck converter. You need to resort to a bootstrapped driver (as a possible solution) and you can build one with discrete components. Furthermore, if you want to turn the chip into a VM controller, an additional circuitry is necessary to generate the PWM ramp. Nothing insurmountable and it is a good exercise to build and tune such a board. I would not recommend to make an industrial version out it though as good specialized control circuits do that well for decades.

A possible implementation of the UC384x in a buck is shown below. It is excerpted from an APEC seminar I taught in 2019 which covers the buck converter structure:

enter image description here

This was for teaching purposes as I could use the same arrangement for CM and VM operations in two different boards. The high-side drive is using a bootstrapped driver described by Monsieur Balogh in an application note he wrote that is now a classic. See figure 23 for the schematic diagram of that portion. I tested it and it gives excellent results:

enter image description here

The seminar teaches how to compensate the board and it worked well. Good luck with your endeavor!

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  • \$\begingroup\$ First of all. Thank you for this descriptive response! Yes, I found that the current sense comparator has to be biased (voltage divider) using the RtCt ramp to never trip and that a Type III compensator needs (RC in parallel to Top Feedback Resistor) for UC3843. I've put the updated Type 3 compensation circuit into TINA-TI and LTspice and starts up nicely and sometimes reaches target voltages. However, I was lost why final output voltage could not be reached, until you clearly explained this chip needs a bootstrap for its driver. \$\endgroup\$ – user20200922 Sep 24 '20 at 19:22
  • \$\begingroup\$ Thank you and you've certainly been extremely helpful! At least paying it forward, I will post the 2 circuits I've successfully run in voltage-mode UC3843. Your seminar presentation is detailed and beyond my understanding unfortunately. In respect of your time, a quick closing question. Would a voltage-mode bootstrapped UC3843 reach typical efficiencies of non-synchronous buck (~84%)? I've attached UC3715 complementary drivers to make a synchronous buck converter but simulations in TINA-TI and LTspice have difficulty reaching output voltage. Is this the performance limit of this chip? \$\endgroup\$ – user20200922 Sep 24 '20 at 19:22
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    \$\begingroup\$ Glad if I could help you. I don't see a reason why an UC384x could not compete with other controllers. There are modern versions of the chip which consume less current than the venerable bipolar controllers from the 80s and efficiency should be ok. If you want to experiment sync rect, please add a bit of dead time between transitions to avoid shoot through currents which, at high frequency, will deteriorate efficiency big time. \$\endgroup\$ – Verbal Kint Sep 24 '20 at 19:29

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