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delay PWM signal with a dead time

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yassin.kraouch

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Hi, i have a microcontroller who output one PWM signal, i would like to drive two N-MOSFET as a switch, so the problem is with this PWM signal i like that the two signal are with dead time in order to drive the two mosfet and also inverted,
 

andre_teprom

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If you are concernig to Full-Brigge topology, dead-time is mandatory, to avoid totem-pole mosfets conducting simultaneolusly.

+++
 

jpanhalt

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From what I understand of your question, you plan to use one of the N-channel mosfets to source current, i.e., source is connected to load. That can be done, but you then need to increase the gate voltage to approximately 10 V greater than the drain voltage, which you cannot do directly with a microcontroller. You will need a separate high-side gate driver.

A simple solution is to use a dedicated PWM driver that provides both high-side and low-side gate voltages. Many, if not all of those drivers require only a single PMW signal to create both top and bottom drives. They also provide for dead time to prevent "shoot-though" and some have other feedback controls for safety.

If you need suggestions for specific devices, let us know.

John
 

ard

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You havent specified which microcontrollers you are using, but Microchip have some with what they call Enhanced PWM; 16F887 is the one I've used, though not for this feature. Do look it up, I think it would fit the bill perfectly.

Regards,

Anand Dhuru
 
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KerimF

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Hi, i have a microcontroller who output one PWM signal, i would like to drive two N-MOSFET as a switch, so the problem is with this PWM signal i like that the two signal are with dead time in order to drive the two mosfet and also inverted,

Do you think you can use a XOR gate IC?
The circuit will need 2 gates, 2 small diodes, 2 resistors and two small capacitors.
Are you interested?

Kerim

---------- Post added at 16:33 ---------- Previous post was at 16:16 ----------

 

yassin.kraouch

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thank you KerimF, but you didn't give me any information about your circuit, please can you explain ?
 

KerimF

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You mean... I didn't give you the component values or how the circuit functions?
 

yassin.kraouch

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yes you didn't give me the component value, and also how the circuit function, i am a beginner :)
 

KerimF

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The XOR gate here has 2 inputs.
If they are both low or both high the output will be low.
If they are different, the output will become high.

Let us look at the upper gate U1. One input is made high always so...
If the other input is also high the output will be low.
If the other input is low the output will be high.

In other word, we made U1 to act as an inverter: input high/low => output low/high
 
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yassin.kraouch

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yes i know this information :), the total circuit isn't clear, i simulate the circuit and i did not obtain the result

---------- Post added at 16:33 ---------- Previous post was at 16:31 ----------

yes i simulate the circuit and i did not oblain a dead time between the two signal
 

KerimF

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Which values you used in your simulation... But first could you tell me the frequency of your PWM?
 

yassin.kraouch

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the frequency of the PWM is 1khz, the value used in the resistor is 1k and the cap is 0.1uF
 

andre_teprom

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Some microcontrollers have built-in PWM modules with 2 outpits, wich can performs that feature with no need of external components.

+++
 

KerimF

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I will try to simulate it too...
It is better to increase the value of R and decrease the value of C so that the diode can be more effective to make a difference between the rising and falling edges.

First, I like to tell you you are right... the circuit needs a modification... I am working on it... though you may find it before I do :)

Vout1 needs to be inverted ... that is all ;-)

Oh.... It was right from the beginning but the simulator wasn't set properly...
 
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yassin.kraouch

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Some microcontrollers have built-in PWM modules with 2 outpits, wich can performs that feature with no need of external components.

+++

yes i know but the problem is that i have also choosen the micro SP430F2011IPW it has only two CCR ( CCR0 and CCR1 )
 

KerimF

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The circuit is right ... Do you have LTspice?
 

yassin.kraouch

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no i used proteus 7.7, not LTspice, with the value of R=10k and C=50n, and V=5V, the circuit is right but the microcontroller will give voltage between 2.5 and 3.6 V, what do you think ?
 

KerimF

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As I remember CMOS gates may work at lower VCC than 5 ... do you need me to check about this? I mean will you be able to get a CMOS IC?

It seems 3.3V is ok for CD4070.
 

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