HELP! How to trigger MOSFET in PSpice

[Zandy Jackson]

I'm trying to design a buck converter in Pspice. The circuit works when I connect the mosfet to the supply (Image1). But when I try it with Vpulse, I'm stuck (Image2). What am I doing wrong? Please help me out guys. Been on this all day!! :-(



Vpulse attributes
V1=20
V2=0
TD=0
TR=0
TF=0
PW=10us
PER=20us

(Tried interchanging V1&V2. TD,TR,TF=1ns too)

 

[FvM]

Remember that a MOSFET is controlled by gate-source voltage, actually applied between gate and source. That's particularly easy in a simulation.

 

[crutschow]

You are using an N-MOSFET so the gate voltage must be more positive than the source (Vgs) by at least 10V (from data sheet) to fully turn on (the transistor is acting as a source follower to the load). Thus the voltage at the gate must be at least 80V for a 70V supply voltage since, when the transistor is ON, the source voltage will be 70V.

 

[Zandy Jackson]

Yeah tried connecting between gate and source too. Here's the img



What parameters of Vpulse do I need to change. Here's what I got for V1=80 and V2=0



I'm doing my project in soft switching of Buck Converter with coupled inductor. This is the circuit for hard switching. I followed the one here >> **broken link removed**

 

[FvM]

The latest when you're going to design real circuits, you have to care for component's maximum ratings.

IRF150 has e.g. a maximum Vgs rating of +/- 20 V. Means that the circuits with ground referred pulse source are likely to destroy the MOSFET. 0/+20V is already at the edge, +12 or +15V is a usual choice for gate driving voltage. Gate driver circuits must be designed to safely keep the voltage limits.

 

[crutschow]

View attachment 99569

What parameters of Vpulse do I need to change. Here's what I got for V1=80 and V2=0

Why do you think you need to change the parameters? :-?

 

[Zandy Jackson]

But I don't the results for Vpulse between G and S with V1=20 V2=0. The input V seem to appear across the load.

 

[crutschow]

The latest when you're going to design real circuits, you have to care for component's maximum ratings.

IRF150 has e.g. a maximum Vgs rating of +/- 20 V. Means that the circuits with ground referred pulse source are likely to destroy the MOSFET. 0/+20V is already at the edge, +12 or +15V is a usual choice for gate driving voltage. Gate driver circuits must be designed to safely keep the voltage limits.

In this case the source is operating as a source follower so the gate-source voltage (which is the critical value) would never be greater than 10V with a 70V supply and an 80V gate pulse.

 

[Zandy Jackson]

The latest when you're going to design real circuits, you have to care for component's maximum ratings.

IRF150 has e.g. a maximum Vgs rating of +/- 20 V. Means that the circuits with ground referred pulse source are likely to destroy the MOSFET. 0/+20V is already at the edge, +12 or +15V is a usual choice for gate driving voltage. Gate driver circuits must be designed to safely keep the voltage limits.

But I don't the results for Vpulse between G and S with V1=20 V2=0. The input V seem to appear across the load.

 

[crutschow]

But I don't the results for Vpulse between G and S with V1=20 V2=0. The input V seem to appear across the load.

The voltage across the load depends upon the frequency and duty cycle of the pulse, which you do not show.

 

[Zandy Jackson]

Why do you think you need to change the parameters? :-?

The voltage at the gate must be at least 80V. How do I do that without varying the attributes.

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The voltage across the load depends upon the frequency and duty cycle of the pulse, which you do not show.

 

[FvM]

In this case the source is operating as a source follower so the gate source voltage (which is the critical value) would never be greater than 10V with a 70V supply and an 80V gate pulse.

Theoretically, yes. But there's still a good chance to destroy the MOSFET by exceeding the maximum ratings during switching transients. Nobody would design a switch driver like this without additional protection means.

But I don't the results for Vpulse between G and S with V1=20 V2=0.

Please translate. What do you expect?

The input V seem to appear across the load.

You are showing a DC bias point. Setting V1=20 makes SPICE calculate an initial transient solution with transistor switched on. You get out what you put in.

 

[godfreyl]

You need to do a transient simulation and look at the waveforms.

 

[Zandy Jackson]

You need to do a transient simulation and look at the waveforms.

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You are showing a DC bias point. Setting V1=20 makes SPICE calculate an initial transient solution with transistor switched on. You get out what you put in.

I'm designing a converter with Vin=70V Vo=36V f=50KHz T=20us Ton=10us. The L and C values were calculated with these. I'm sorry I'm relatively new to this.

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Is there any way I can post the schematic file?

 

[FvM]

You already know that the circuit doesn't work with 20V ground referenced gate control voltage...

 

[Zandy Jackson]

You already know that the circuit doesn't work with 20V ground referenced gate control voltage...

It doesn't if I connect it across G and S too. What am I doing wrong?

 

[godfreyl]

V3 should be connected between gate and source, not between gate and ground, then it should work better.

To post a file, click on "Manage Attachments".

 

[Zandy Jackson]

V3 should be connected between gate and source, not between gate and ground, then it should work better.

To post a file, click on "Manage Attachments".

Doesn't accept .sch files

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I've uploaded the schematic file guys.. Please help out >> www.megafileupload.com/en/file/476886/HardSwitching-sch.html

 

[godfreyl]

Doesn't accept .sch files

Does accept zip files.

Anyway, the simulation seems to work fine now.
If you rum it for longer (maybe 100uS or 200uS), the output voltage will settle down to what it should be.

It started high because your V3 pulses started high, i.e. you set V1=20, V2=0 in the settings for V3.

If you swap those settings, then the output voltage will start at 0 and rise to the correct value.

 

[Zandy Jackson]

Does accept zip files.

Anyway, the simulation seems to work fine now.
If you rum it for longer (maybe 100uS or 200uS), the output voltage will settle down to what it should be.

It started high because your V3 pulses started high, i.e. you set V1=20, V2=0 in the settings for V3.

If you swap those settings, then the output voltage will start at 0 and rise to the correct value.

Here's the final result for V1=0 V2=20 Print step = 1us. Hope I got it right. You're a life saver! Thanks a lot!



The output varies a lot. Is that fine? Thanks again.
Going to implement the same for soft switching circuit.

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The circuit for soft switching. Not sure if the output is right