# Help current mode control and flipflops

Discussion in 'Power Electronics' started by Reino, Oct 3, 2020.

1. ### Reino

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Mar 26, 2017
I am a mech engineer and i am building or at least trying to understand current mode control.

The stuff i know is pid, statespace and transfer functions (i remember very little). And current control does not seem like any of these and more of a Oscillating pwm around a set point.

And a flip flop with a clock signal. I want to split up the pwm signals. Every second pwm cycle i want on one “channel” and every first cycle on another “channel”. Of i understand flipflops with clock input i can do this?

before you help me let me get my sharpest crayon .

2. ### Harald KappModeratorModerator

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Nov 17, 2011
You're mixing up a few things.
Current mode control takes its name from the fact that the controlled parameter of the circuit is the current (voltage follows current depending on the load). This is opposed to voltage mode control where the controlled parameter is the voltage (current follow voltage depending on the load). Both control methods can be used using a variety of mechanisms.
PWM is one such mechanism. Instead of adjusting the controlled parameter (either current of voltage) in a linear way, the parameter is turned on and off in rapid succession with varying duty cycle (ratio of on time to total cycle time).

Please explain in more detail what you want to do, preferably add a timing diagram (a picture is worth a thousand words).

3. ### Reino

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Mar 26, 2017
Oo ok, i thought current mode control was a specific method like PID i was not aware of. the controller i want to use is a uc3823an. Want to be able to make the current control board of a DC welder controlling a H bridge at a set frequency. the welder i am reverse engineering has a pmw frequency of about 20khz.

Let me quickly draw what i a talking about regarding the flip flop. The one on the machine is a tc4013bp and i think it is used to do what i was referring to.

4. ### Reino

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Mar 26, 2017

I hope this makes sense. just imagine that is a 50% duty cycle pwm signal

5. ### Alec_t

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Jul 7, 2015
The output duty cycle on either channel as shown is only half the input duty cycle. Is that what you intend?

6. ### Reino

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Mar 26, 2017
no, i need to split and alternate the incoming pwm signal. both switching halves of the H bridge cant be switched on at the same time. the incoming pwm signal can be anything between 0 and 100. the output will be the same but with alternating switching, but as a result the max duty cycle will be 50%. This is an effect, i just need to alternate switching. As I understand it i can do that with a flip flop, but i am not sure.

7. ### Alec_t

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772
Jul 7, 2015
Show us how you are driving your H-bridge. You would normally have one driving signal the inverse of another driving signal, but with a certain dead-time between them to prevent 'shoot-through'.

8. ### Harald KappModeratorModerator

10,342
2,247
Nov 17, 2011
No need for a flip-flop. A little bit of logic will suffice:
out_a = PWM_IN AND clock
out_b = PWM_IN AND NOT(clock)

9. ### Reino

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Mar 26, 2017
at the moment i am not driving anything. i am reverse engineering how a constant current is maintained in a welding machine. Attached is the current controller on the existing board. in the future i want be able to make my own but using uc3823an instead.

the current controller on the board

Last edited: Oct 3, 2020
10. ### Reino

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Mar 26, 2017
I think that what this is but i am not sure.

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11. ### Alec_t

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772
Jul 7, 2015
If you use a 3823 to generate the PWM signal and use that signal as the clock input to a flip-flop you can then use the complementary flip-flop outputs to do the logic Harald specified.

12. ### Reino

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Mar 26, 2017
Aa thanks, i have a few IC i ordered and will be playing with them. I can currently adjust amps on the current board (like most welders) by inputting a Voltage that goes to a comparator with another input form the current sense resistor (E/A comparator on block diagram).

When i understand more about this stuff i will start my design for the current controller board.