What is the difference between PWM current mode and voltage mode ??

Hello everybody and sorry for asking too many questions , sometimes it is very necessary .

As stated above what is the difference and what is the best way to monitor each mode's signal modulation ?

It all depends in what context?
There are servo drives that are operated either, but it depends on what is the application.
What signal modulation?
Max.

MaxHeadRoom78 said:

It all depends in what context?
There are servo drives that are operated either, but it depends on what is the application.
What signal modulation?
Max.

Click to expand...

I mean PWM signal .

Concerning the application it is a SMPS welder using UC3845 ic.

The main goal is how the output current is controlled with the different modes ?
In this case it is a current mode IC .

I'm presuming this is related to the SMPS repair you posted about recently?
My understanding (and we'll see how good that is) is the following:

It refers to what is the input to the "primary" control loop. That is, does the feedback loop of your regulated power supply monitor current or voltage?
Once you have a "primary" control loop, a second control loop can be added on top to regulate the actual output of the power supply in the way that you want - that is to say, "current mode regulation" does not imply that the complete SMPS will have a constant-current output.

For example, the IC you've mentioned is a common controller for flyback converters. It implements its control by monitoring the current flow through the transfomer and switching the power transistor off once this current exceeds a certain threshold, which is set by a voltage applied to an input pin. The remainder of the fixed-period cycle is spent in this off state and, by the end of the cycle, (for the sake of this example) the current has fallen to zero. Therefore, the average current through the transformer (and therefore out of the power supply) varies in proportion to the "threshhold" current at which the controller trips into the off state, and therefore also in proportion to the voltage applied to the control pin.
We therefore have a power supply whose current can be programmed by a voltage suppled to a pin on the controller IC. This is our "primary" control loop.

Assuming we actually wanted a power supply to produce a steady voltage, a second control loop can now be added whose job is to increase the current whenever the output voltage is low (ie, below where it should be), and reduce the current if the output is high.

I hope this makes sense, it's a simple concept but not necessarily one that's easy to explain... It would probably make more sense with a diagram. I also may not have used all the right terminology here - sure someone will clarify if not.

tomizett said:

I'm presuming this is related to the SMPS repair you posted about recently?

Click to expand...

Yes.

tomizett said:

We therefore have a power supply whose current can be programmed by a voltage suppled to a pin on the controller IC. This is our "primary" control loop.

Click to expand...

That's exactly what i was confused about .

Now , as i mentioned before in my previous thread , i can see the fixed 25 % duty cycle for the fixed output voltage .

But as you mentioned , when i change the voltage at the current control pin, can i see the current (I) signal changing using a digital scope ? because i only have a CRO now .

Its the type of feedback and control used.
In a smps the o/p voltage is monitored, and the diffrence between the required voltage and actual voltage (error signal) is amplified and fed to the output comparator.
In voltage mode the comparator simply compares the reference with the o/p voltage and increases or decreases the amount of time (pwm) the main switch is on for.
In current mode the comparator compares the error signal with the current through the main switch, ie the error is converted to the current in the primary for a smps, so a large error results in a large current increase/decrease in the primary.
So current mode controls the current in the primary depending on the output voltage error for a constant voltage supply.
Current mode supplies can be easire to design and have very good transient response.
I like the uc384x series ic's they are easy to use, the diffrent versions are max duty cycle and undervoltage lockout ratings.
One thing I've found with some uc384x supplies is that they often have a startup circuit, a cap and resistor from the 320v dc rail, one of these can go open and the circuit will not start, the other thing I;ve found is the reference in the chip fails and goes to ground.

In the servo world, Voltage control is a the varying voltage input controls the voltage output, in a current (torque) mode the varying voltage in controls the current in direct proportion in the drive intput.
IOW a trans-conductance amplifier.
Max.

Sorry I misunderstood, I thought you meant power supplies.

for examples of voltage mode control and current mode control, see the attached.
They are ltspice simulations, one in voltage mode, one on current mode. You will see the difference.

Basically one has a ramp from the sense signal, the other has the ramp from an artificial ramp generator.

BGAmodz said:

Yes.
That's exactly what i was confused about .

Now , as i mentioned before in my previous thread , i can see the fixed 25 % duty cycle for the fixed output voltage .
But as you mentioned , when i change the voltage at the current control pin, can i see the current (I) signal changing using a digital scope ? because i only have a CRO now .

Click to expand...

Regarding how to test the UC3845 in circuit, I'd like to make the following suggestions.
Firstly, have a look at whether your circuit is using the op-amp provided on the UC3845. The op-amp is provided for implementing a control loop for the output voltage but is not always used - some designers will fit an external op-amp of superior performance, or eliminate it altogether. Measuring the Comp pin will show you what signal is coming from the op-amp - check on the data sheet, but I *think* it's acceptable to drive a voltage onto this pin in order to override the op-amp. As the datasheet shows, 1/3rd of the voltage on Comp pin appears at the current comparator's inverting input, with the Current Sense pin driving the non-inverting input. If all is well, with the FETs removed so the PSU is generating no output voltage, the internal op-amp should be railed in the positive direction - in this case the internal zener will be clamping the current comparator's inverting input to 1V. If this is the case, you should be able to apply a signal to the Current Sense pin (probably by lifting a resistor somewhere) and alter the output PWM signal. If you apply DC to the Current Sense pin, you should see no pulse when the applied voltage is above 1V, and 50% duty cycle as soon as the voltage drops below 1V.

To be honest, I can't see why you would ever see 25% duty cycle with the feedback loop disconnected.
Any old scope should do - the waveform should be periodic.

Hope this helps.