Solving Compensator Circuit for Synchronous Buck Converter (SBC)

[Ms FYP]

Hi All,

I'm working on my final year project on synchronous buck converter (SBC). In this project, i have to use an adaptive gate drive (AGD) and a compensator to switch on both of the SBC switches. The AGD is connected to the second switch (Low side switch) of the converter meanwhile for the compensator is connected to the 1st switch (high side switch). My problem is, I couldn't get a correct output for the compensator circuit.

I used type 3 compensator. The main objective here is to have a correct pulse/voltage to switch on the high side switch of the SBC. however, the highest voltage that I can achieve is only 0.2V instead of 5V (the threshold of the MOSFET).

Can anyone help or guide me in designing the compensator type 3 circuit for SBC ? FYI, I used PSPICE software to design and simulate the circuit..and I also attached the circuit that I have designed.

Really2 appreciate if u can share your ideas or correct me if there if I do it wrongly.

Thanks..

 

[berkeman]

Hi All,

I'm working on my final year project on synchronous buck converter (SBC). In this project, i have to use an adaptive gate drive (AGD) and a compensator to switch on both of the SBC switches. The AGD is connected to the second switch (Low side switch) of the converter meanwhile for the compensator is connected to the 1st switch (high side switch). My problem is, I couldn't get a correct output for the compensator circuit.

I used type 3 compensator. The main objective here is to have a correct pulse/voltage to switch on the high side switch of the SBC. however, the highest voltage that I can achieve is only 0.2V instead of 5V (the threshold of the MOSFET).

Can anyone help or guide me in designing the compensator type 3 circuit for SBC ? FYI, I used PSPICE software to design and simulate the circuit..and I also attached the circuit that I have designed.

Really2 appreciate if u can share your ideas or correct me if there if I do it wrongly.

Thanks..

Welcome to the PF. Are those Schottky diodes in series with the FET gates? Why are they there?

 

[Ms FYP]

hi berkeman,

thanks..hmm..im not using any schottky diodes in the circuit. i think the one that u saw is the GLIMIT component that looks like a diode.

thanks..

 

[berkeman]

hi berkeman,

thanks..hmm..im not using any schottky diodes in the circuit. i think the one that u saw is the GLIMIT component that looks like a diode.

thanks..

Ah, that's a relief. What's a GLIMIT component?

 

[Ms FYP]

It is a Gain block with limiter. Means, it increases the voltage from the compensator circuit with a specific gain that have been fixed. I used it to increase the output of the compensator to 5v but I only get 0.2v. And ya, before the block is there, i just manage to get 100-200mV only. So i suspect that, i have built the compensator incorrectly.

thanks..

 

[berkeman]

I don't see any oscillator sources in the circuit -- am I missing it/them?

 

[Ms FYP]

oh3...is it needed? because i have been consulted by my supervisor and he said that he wanted the SBC to be self-driven. that's why i didn't put it at the first place.

thanks..

 

[berkeman]

oh3...is it needed? because i have been consulted by my supervisor and he said that he wanted the SBC to be self-driven. that's why i didn't put it at the first place.

thanks..

But what is the SBC supposed to be synchronous to? I could be missing something elegant, but the circuit I see is all quiescent without any oscillations. The DC-DC topologies that I'm familiar with (including bucks) need an oscillator as the source, with comparisons being made against the (usually triangle wave for bucks) oscillator waveform.

As a side note, the better buck topologies that I've worked with also have in-line (high side) current sensing, for cycle-by-cycle current limit and/or regulation purposes...

 

[Ms FYP]

hi berkeman...

Many thanks for the replied. Before a compensator is built I designed the SBC only with AGD. The first switch (high side), it is been connected to the vpulse (pulse voltage source) and the circuit is work. So I should add an oscillations circuit together with the compensator is it? correct me if I understand it incorrectly.

thanks...

 

[berkeman]

hi berkeman...

Many thanks for the replied. Before a compensator is built I designed the SBC only with AGD. The first switch (high side), it is been connected to the vpulse (pulse voltage source) and the circuit is work. So I should add an oscillations circuit together with the compensator is it? correct me if I understand it incorrectly.

thanks...

To be honest, it sounds like you should spend some time learning the basics of buck DC-DC converter operation, before you look into ways to upgrade the basic topology to a synchronous (active lowside switch element) topology.

What resources have you used to learn the basics of buck DC-DC converter topolgies? What form of oscillator do most of them use (not a pulse oscillator typically)? How do they use this oscillator in the line and load regulation processes?

And what changes are typically used to convert a passive lowside switch (diode) to the actively switched lowside component? You have to know when to turn on the lowside switch...

 

[Ms FYP]

hi,

many thanks for the replied..yes..to be honest too, I'm actually a little bit confuse with both of the topologies..after my supervisor ask me to upgrade the SBC. Because at first I also thought it is impossible to control both of the switches without the oscillations sources..hmm..I guess I need to learn more and spend my time to correct what I have misunderstood...thank you very much for your comment and time..really appreciate it..

thanks...

 

[berkeman]

hi,

many thanks for the replied..yes..to be honest too, I'm actually a little bit confuse with both of the topologies..after my supervisor ask me to upgrade the SBC. Because at first I also thought it is impossible to control both of the switches without the oscillations sources..hmm..I guess I need to learn more and spend my time to correct what I have misunderstood...thank you very much for your comment and time..really appreciate it..

thanks...

Glad to help. What books are you using to learn about DC-DC topologies? Unfortunately my books are at work, and it's Friday night, so I can't post my favorite books until Monday morning (sorry). But hopefully you have some good references to help you with this question over the weekend.

You can also check the DC-DC converter IC manufacturer websites for application notes and schematics of their ICs for ideas. Look mainly for existing synchronous-capable ICs to see what they add in their IC circuit diagrams or block diagrams. They at least need to sense voltages or currents to know when to gate on the low-side switch, right?

 

[Ms FYP]

hi berkeman,

Thanks a lot for the suggestion..will do my research about it thoroughly..I'm using 'Power Electronic Circuits' from Issa Batarseh as my reference in learning the DC-DC topologies...erm..if it is okay with you, really appreciate that if you could recommend or suggest some other books that can help and guide me to understand more about this topologies.. sorry if I'm asking too much..

thank you for your time..

 

[berkeman]

Will post my main DC-DC books on Monday when I get back to the office. Fun subject, and very practical.

 

[berkeman]

Will post my main DC-DC books on Monday when I get back to the office. Fun subject, and very practical.

"DC-DC Switching Regulator Analysis" by Daniel M. Mitchell -- A good basic book about regulator design and analysis.

"High-Frequency Switching Power Supplies, Theory and Design", by George C. Chryssis -- A pretty good book and it has info on synchronous lowside switch elements for buck topology converters.

"Switchmode Power Supply Handbook" by Keith Billings -- A pretty good book, with more detail on some of the other topologies and thermal management.

 

[Ms FYP]

hi berkeman..

thanks a lot for the recommended books. i will definitely search for them at the library..

 

[mdjensen22]

Berkeman,
Do those address EMI at all?

 

[berkeman]

Berkeman,
Do those address EMI at all?

Good question! Yes, all 3 books have sections on EMI.