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Current mode & voltage mode SMPS question

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ljy4468

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current mode smps

Hi

There are current mode control & voltage mode control in SMPS design.

and In current mode control,
Since the error amplifier is now used to command an output current rather than voltage, the effect of output inductor is minimized. -> single pole
(the book said)

I have a question that why 'the error amplifier is now used to command an output current rather than voltage'??

give me advice.

Thanks.
Regards.
 

current mode voltage mode

i don't undrestand your problem?
in current mode smps you need to sample the current and compare it with your reference
the current must be throgh from the primary inductor
 

votage mode control in smps

Either of two control mode have advantages and disadvantages.

Advantages of current mode SMPS:
fast transient response, simple compensation.
Disadvantages of current mode SMPS:
need linear current sensor for all current range.

Advantages of voltage mode SMPS:
no linear current sensor, simpler circuit.
Disadvantages of voltage mode SMPS:
slow transient response, complicated compensation.

Current mode is better for low switching frequencies, voltage mode is simpler for high switching frequencies.
 

high current control in smps

What do you mean 'command an output current ',
do you mean adjust the inductor current?
If that, i think Voltage Mode adjust the inductor current, too.
What is different is the input of the controller.
 

voltage mode control of smps

In voltage mode you get second order control.

you can change the output only after the LC responce. so it is a slow complex response. In Peak Current mode as in the flyback example you can control the input current peak (energy) before the output voltage changes. it is a fast pulse by pulse response each pule the duty cycle is changed according to the current peak.

In current mode you get a combination of input current and output voltage. it is not just current. and it is the best way to control flyback converters.

You can read more in books and not just in the applications so you will have more mathematical approach to the design.

Hope I helped
 

current mode control voltage mode control slow

Thanks for all your questions.

But, my question is,

Voltage mode is two pole system consist of output LC.
but, in current mode, L seems to be disappear, so it is considered to be 1 pole system.

So why L seems to be disappear in current mode??

Regards.
 

because in a current mode control , at low ferquencies the control feedback is the measured voltage across a small resistor connected in series with the inductor. thus the inductor becomes a voltage controlled current source, thus it ccan be replaced at dc inputs with low frequencies.
 

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