How to converter a PWM signal to a DC current souce signal

[bhl777]

Hi All, I want to converter a PWM signal with specific duty cycle to a DC signal (ideally I want it to be a current souce). I want this source is proportional to the duty cycle.
Currently I can only use CMOS technology, so no inductors can be used here.
Could anyone give me some advise how can I implement this? Thank you very much!

 

[umesh49]

What is your signal frequency? for a higher frequency more than 10Khz, it may be feasible to use a switched capacitor topology to convert it to DC voltage and then use a voltage to current converter topology.

 

[nitishn5]

I had done this for a project.

I filtered the PWM signal using a second order filter and then used an opamp based voltage to current converter to convert the voltage into a current.
The problem would be getting a 0mA current for the o% duty cycle. Otherwise it is quite linear. Mine was a microcontroller based project so I also put in a feedback technique to change the pwm duty cycle to regulate the current.

 

[umesh49]

If you use a switched capacitor proceed by a first order filter at output you can achieve this. You might need to use a inverter before mosfet gate to achieve desired relationship between input duty and output voltage.

 

[nitishn5]

Is there any advantage of having a switched capacitor before the filter?

 

[bhl777]

Hi umesh49, the frequency can be designed to more than 10kHz. Could you give me some example topology for SC to realize this function? Thank you very much!

What is your signal frequency? for a higher frequency more than 10Khz, it may be feasible to use a switched capacitor topology to convert it to DC voltage and then use a voltage to current converter topology.

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Hi nitishn5, could you tell me what kind of filter you are using? Active filter or RC filter? Thank you!

I had done this for a project.

I filtered the PWM signal using a second order filter and then used an opamp based voltage to current converter to convert the voltage into a current.
The problem would be getting a 0mA current for the o% duty cycle. Otherwise it is quite linear. Mine was a microcontroller based project so I also put in a feedback technique to change the pwm duty cycle to regulate the current.

 

[BradtheRad]

Here is the concept for a capacitor (as an RC filter or integrator) to provide an averaged DC output from incoming pulses.



Since the supply voltage is 10V, and duty cycle is 70 percent, then the output is 7V.

 

[nitishn5]

I used an active Bessel filter using two opamps. It was a 4th order filter not 2nd order as I have mentioned before.
My PWM signal was coming from the microcontroller.

If the load to the filtered output is not much, you can as well go for an passive filter. You can save some space and cost.

 

[umesh49]

Hi umesh49, the frequency can be designed to more than 10kHz. Could you give me some example topology for SC to realize this function? Thank you very much!


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Hi,
You can refer a simple analysis in attachment. you can tune the values and see if getting desired linearity and ripple. If output is having more ripple then you can add one more filter.