Sycnhronous converter design in Ltspice

Thread Starter

Adam21

Joined Mar 18, 2021
5
Dear all,


I am trying to simulate my synchronous buck converter and I managed to implement complementary signals to driver two different transistor.



The issue is as follows:

1- I inserted dead-time (green circle) but the problem is that at the start of the period two signals overlap together which will cause a short circuit. (The yellow circle).

2- Could you please let me know how I can generate that signals with the required frequency (200kHz) = 5u seconds (period).

Also, I might change it to a different frequency probably higher.



Q1: Could you please help me with that? Thanks



Q2: Do you know where I can references to simulate synchronous non-inverting buck-boost converter?

deadtime.png

Thanks I appreciate your help.
 

Thread Starter

Adam21

Joined Mar 18, 2021
5
Hi this is the buck mode of my converter.
This is supposed to step down the voltage from variable input (10-63 volts) to alwayes 12v.

The frequency intended here is 200kHz. The second transistor is used instead of the traditional diode.

Therefore, there is a need to have a dead-time where the gate signal for both of the transistors are low to avoid a short circuit when the two transistors are conducting at the same time.

Now, my problem here is that the dead-time (the green circle) is implmented in my screenshot for the control signals for the two transistors but there is an overlap in the yellow circle (see the first post - screenshot).
How to fix that to get complementary signals for the two transistors?
 

crutschow

Joined Mar 14, 2008
27,912
If M1 is to stay an N-MOSFET, then the gate voltage must be at least 5V higher than the drain voltage (or 68V) to fully turn on.
If you use a P-MOSFET than the gate voltage must go to 63V to fully turn off.
In either case the maximum gate-drain voltage of ±20V must not be exceeded.

For non-overlap you adjust the relative pulses so the turn-on time of M1 must occur after M2 turns off, and its turn-off time must occur before M2 turns on.
Hint: The two pulses cannot have the same pulse-width.
 

Thread Starter

Adam21

Joined Mar 18, 2021
5
Is this a school/college assignment?

At present you don't have one. You need to change the pulse voltage and delay parameters.
Thanks for your replay. Please look at the first post #1 where the dead-time is there look at the green circle for the two complementary signals with the same frequency.
 

Thread Starter

Adam21

Joined Mar 18, 2021
5
If M1 is to stay an N-MOSFET, then the gate voltage must be at least 5V higher than the drain voltage (or 68V) to fully turn on.
If you use a P-MOSFET than the gate voltage must go to 63V to fully turn off.
In either case the maximum gate-drain voltage of ±20V must not be exceeded.

For non-overlap you adjust the relative pulses so the turn-on time of M1 must occur after M2 turns off, and its turn-off time must occur before M2 turns on.
Hint: The two pulses cannot have the same pulse-width.
Thanks. I actually I did that but I used n-mosfet only. The complementary signals are included in my first post where there is a dead-time implemented indicated by the green circle but there is an overlap indicated by the yellow circle (I wanna fix that the yellow signal).

Also, as you said the gate-source signal here is 5 volt higher than the source for the high side mosfet (the blue signal).

The driving signal for the low side (red signal).

(please look at the screenshot attached).
deadtime.PNG
 
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