Controller for a 2kW adjustable-output step-down converter (DC chopper)

Thread Starter

dado1973

Joined Jan 16, 2021
6
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

Thank you
 

Papabravo

Joined Feb 24, 2006
15,464
Would you mind telling us the input voltage range and the output voltage range. 2kw adjustable isn't really very informative. Ready made schematics are a tall ask for so little in the way of requirements. I could throw anything at all at you and it would take you a while to decide it met your requirements or not, but I don't know what they are so how could I be in a position to help you.
 

Thread Starter

dado1973

Joined Jan 16, 2021
6
Thank you for your answer Papabravo

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.
A basic non-isolated non-synchronous buck topology 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% at 100kHz in all conditions, without the need to to implement synchronous rectification or more sophisticated topologies (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 (note that the high side fet is driven by an isolated driver with its own floating power supply - UCC23513). If you need further informations please let me know.
 

Papabravo

Joined Feb 24, 2006
15,464
Thank you for your answer Papabravo

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.
A basic non-isolated non-synchronous buck topology 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% at 100kHz in all conditions, without the need to to implement synchronous rectification or more sophisticated topologies (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 (note that the high side fet is driven by an isolated driver with its own floating power supply - UCC23513). If you need further informations please let me know.
Your input voltage is 300 v but what is the output voltage? 2KW @ 7 amps implies the output is 285 Volts. Is that correct? Why are you being so stingy with information?
Next problem is that buck converters don't typically run at duty cycles close to 100% So I don't even know what to tell you.
 

Thread Starter

dado1973

Joined Jan 16, 2021
6
As already explained:

1. the application is a DC-Variac (also called chopper), that is a converter with adjustable output. The duty cycle can span from 0% to 100% just to change the output voltage, that in turn can span from about zero to almost 300V.

2. the high side fet is driven by an isolated driver (UCC23513) with its own floating power supply, therefore the converter can safely run at any duty cycle, included 100%.

3. the power stage has already been tested in open loop and its performance validated, my concern is about the controller choice

If you need further informations please let me know.
 

Papabravo

Joined Feb 24, 2006
15,464
As already explained:

1. the application is a DC-Variac (also called chopper), that is a converter with adjustable output. The duty cycle can span from 0% to 100% just to change the output voltage, that in turn can span from about zero to almost 300V.

2. the high side fet is driven by an isolated driver (UCC23513) with its own floating power supply, therefore the converter can safely run at any duty cycle, included 100%.

3. the power stage has already been tested in open loop and its performance validated, my concern is about the controller choice

If you need further informations please let me know.
If you have limited experience with controllers then you might be unaware of the stability problem associated with sub-harmonic oscillation as the duty cycle rises above 50%.. The usual method of slope compensation will effectively prevent operating at duty cycles over 50%. So if you want to do that, make sure you choose a controller that doesn't have that feature. One more thing, I expect you to be unhappy with the transient response and the ability to regulate to a voltage with a small ripple current. Maybe that is a don't care for you.

EDIT: So now I'm confused because the datasheet for the TL3843 says that you want "slope compensation" for duty cycles over 50% but there is only a vague schematic and no explanation of what it is doing. (See figure 6.)

https://www.ti.com/lit/ds/symlink/t...=https%3A%2F%2Fwww.ti.com%2Fproduct%2FTL3843B

As I understood what was happening you reset the SR flip-flop when the sense current gets to Imax before half the switching period. Besides preventing sub-harmonic oscillation it is supposed to limit the ripple.
 
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Thread Starter

dado1973

Joined Jan 16, 2021
6
If you have limited experience with controllers then you might be unaware of the stability problem associated with sub-harmonic oscillation as the duty cycle rises above 50%.. The usual method of slope compensation will effectively prevent operating at duty cycles over 50%. So if you want to do that, make sure you choose a controller that doesn't have that feature. One more thing, I expect you to be unhappy with the transient response and the ability to regulate to a voltage with a small ripple current. Maybe that is a don't care for you.
I have no experience with controllers, that's why I asked the question.

In my first post I expressed my doubts about the choice between a current mode controller like the UC3843 and a voltage mode controller with current limiting features like the UC35705 .

I just need someone to support me make the best choice and implement the correct circuit.
 

Papabravo

Joined Feb 24, 2006
15,464
I have no experience with controllers, that's why I asked the question.

In my first post I expressed my doubts about the choice between a current mode controller like the UC3843 and a voltage mode controller with current limiting features like the UC35705 .

I just need someone to support me make the best choice and implement the correct circuit.
Well like everything else there are compromises. Voltage mode control is generally not fast enough to respond to changes in the conditions that motivate us to introduce regulation and current mode control is fast enough and reduces the order of the control system by one degree, If you are using current control at near 100% the output capacitor does not have sufficient time to discharge before the next cycle, so how on Earth could you expect it to work correctly, unless you intend to skip cycles altogether. I think you might want to do some simulations or better yet hire someone who knows what they are doing. This is not a good place for amateurs.
 

Thread Starter

dado1973

Joined Jan 16, 2021
6
Sorry Papabravo but I really do not understand what you are saying.
By the way, although I'm not experienced with analog SMPS control IC's, I'm not an amateur: I am a design engineer with 20 years of experience in the field of power electronics.
 

Papabravo

Joined Feb 24, 2006
15,464
Sorry Papabravo but I really do not understand what you are saying.
By the way, although I'm not experienced with analog SMPS control IC's, I'm not an amateur: I am a design engineer with 20 years of experience in the field of power electronics.
And yet you know very little about SMPS control ICs. That's OK, I have no idea how to explain it better. Maybe someone else can give you better advice.
 

Ian0

Joined Aug 7, 2020
1,567
First, if it is non-synchronous with a FET and a diode, why switch the high side? If you switch the low side, then you can achieve 100% duty cycle without problems with the charge-pump that supplies the high side device.

I assume when you say "variac" you wish to control the output power manually, so you just need a variable duty cycle oscillator, which you can make with a 555.

What switching frequency are you intending to use?
 

Orson_Cart

Joined Jan 1, 2020
43
hello there - you really need input from an experienced power electronics design engineer -- they can provide a schematic and build details that will work, e.g. pwrtrnx . com
 

Papabravo

Joined Feb 24, 2006
15,464
First, if it is non-synchronous with a FET and a diode, why switch the high side? If you switch the low side, then you can achieve 100% duty cycle without problems with the charge-pump that supplies the high side device.

I assume when you say "variac" you wish to control the output power manually, so you just need a variable duty cycle oscillator, which you can make with a 555.

What switching frequency are you intending to use?
I think he wants to get to 100% duty cycle because he will allow the switch to be on all the time as the input drops to it's minimum value. In this case there is virtually no regulation at all and the ripple current will be whatever it is.
 

Papabravo

Joined Feb 24, 2006
15,464
assuming the 300vdc source is rectified mains the ripple will be 100/120 Hz at 100% pwm ...
That's the way I understand it. At that point the DC-DC converter is as useless as a screen door on a submarine.
It's not much better at 99% since the inductor has no time to discharge, before it changes violently again.
 

Orson_Cart

Joined Jan 1, 2020
43
That's the way I understand it. At that point the DC-DC converter is as useless as a screen door on a submarine.
It's not much better at 99% since the inductor has no time to discharge, before it changes violently again.
Terribly sorry - but you are completely wrong there.
 

Orson_Cart

Joined Jan 1, 2020
43
OK So explain how a buck converter that is not switching is doing anything useful.
for starters it is passing max available power to the output - as soon as the input volts go up - it can react to that and keep the output current / voltage bounded, and if the input volts fall too low the control can turn the series pass buck element ( mosfet ) off and protect the energy source ...
 

Orson_Cart

Joined Jan 1, 2020
43
the inductor always has a current path through the load and the free-wheel diode - so switched or not-switched - really makes no difference to the buck choke ...
 

Papabravo

Joined Feb 24, 2006
15,464
for starters it is passing max available power to the output - as soon as the input volts go up - it can react to that and keep the output current / voltage bounded, and if the input volts fall too low the control can turn the series pass buck element ( mosfet ) off and protect the energy source ...
What I meant was in line with your mention of the ripple current from the DC source when the duty cycle is fixed at 100%. There nothing much that can be done to regulate output voltage or current. Yes It can do something when input voltage rises a little but at a duty cycle of 99% for example with an SR flipflop controlled by a clock there is not a lot of time from the RESET pulse to the next SET pulse starting the cycle again, I don't se how it can do much of anything at a high switching frequency. In this kind of setup is the maximum switching frequency limited to allow for things to happen with a very short off time.
2nd question: how do you avoid subharmonic oscillation at duty cycles from 50% upto to 100% where the voltage and current can wander all over the place.
 
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