How to Drive a P-Channel FET in a Low Voltage Synchronous Buck Converter?

[bryce1]

I am trying to develop a circuit to convert 5-18Vdc, 100ma max to 0.4Vdc, 400ma. I am working on a synchronous buck converter design but I am finding limited resources on a discrete design. All of the buck converter IC's I found do not go low enough and have inadequate efficiency when they are even close. I have been toying with the idea of using the basic synchronous converter with a P and N channel fet. The problem is that I am trying to use a MSP430 microcontroller to drive the circuit (this means 3V peak PWM signal). What would you recommend for a gate driver for the upper P-channel fet? If you any other suggestions please put them in too. The load is a thermo electric cooler.

I am attaching a rough schematic for review.

Thanks

 

[The Glom]

Why not just go down the simplest route?

Use a large capacitance on the output, something in excess of 20,000uF. Then short across the capacitor with a 1 amp rectifier that has a .4v drop.

Put your load device across the capacitor and then adjust the input frequency until you achieve the desired current through the load. Or, get an exact load match by measuring across the shunt rectifier and adjusting the input until you get the minimum power dissipation in the rectifier.

You could probably run the front end with nothing but a voltage-regulated 555 timer.

 

[SwedishWings]

Did you ever figure out how to do this with discrete components?

I have exactly the same issue, and find no information on the subject...


Thanks,
Mike

 

[berkeman]

Did you ever figure out how to do this with discrete components?

I have exactly the same issue, and find no information on the subject...


Thanks,
Mike

What is the 0.4V output used for? I'm not familiar with applications in that voltage range.

 

[SwedishWings]

What is the 0.4V output used for? I'm not familiar with applications in that voltage range.

Good question, have no idea. Perhaps bryce1 can enlihgten us?

I'm not looking for 400mV solution, rather a discrete design for driving up and low side with break before make. It seems almost impossible to find some info on discrete solutions. It appears that gate driver chips and PWM controllers are used exclusivly.

Ideas anyone?

/Mike

 

[berkeman]

Good question, have no idea. Perhaps bryce1 can enlihgten us?

I'm not looking for 400mV solution, rather a discrete design for driving up and low side with break before make. It seems almost impossible to find some info on discrete solutions. It appears that gate driver chips and PWM controllers are used exclusivly.

Ideas anyone?

/Mike

This is a thread from 2004, so bryce1 probably isn't around anymore to give more info.

I'm not understanding what you want to do. You want to make a discrete buck converter? That's pretty straightforward. It gets more complicated if you want to add in the synchronous device in the flywheel position, instead of the traditional diode.

Could you describe in more detail what you want to build, and what it is used for?

 

[SwedishWings]

berkeman,

I did not observer that the original posting whas from 2004 :shy:

Yes, i want to remove the freewheeling diode in favor of a N-Channel MOSFET. The design is then basically just a synhrounous buck converter (as shown in the picture) driven by an mcu generated PWM.

I was hoping that someone had come up with a clever solution to do this without using a driver chip. Most driver chips don't like voltages over 20V, and is also rather expensive. Hence the search for a discrete confiuration.


Mike

 

[berkeman]

berkeman,

I did not observer that the original posting whas from 2004 :shy:

Yes, i want to remove the freewheeling diode in favor of a N-Channel MOSFET. The design is then basically just a synhrounous buck converter (as shown in the picture) driven by an mcu generated PWM.

I was hoping that someone had come up with a clever solution to do this without using a driver chip. Most driver chips don't like voltages over 20V, and is also rather expensive. Hence the search for a discrete confiuration.


Mike

To a first approximation, you just turn on the flywheel/freewheeling device when the main pass device is off. If you're running in discontinuous mode, you would want to turn off the flywheel device when its current dropped to zero.

You'll probably need to check out some books on DC-DC and switchmode power supply design. Company websites are only going to show solutions using their control ICs, not discrete designs. Out of the three books on my shelf on the subject, only Chryssis ("High-Frequency Switching Power Supplyies -- Theory and Design") discusses synchronous rectification.

 

[SwedishWings]

Thanks for your reply berkeman.

I guess this pretty much brings me back to square one again... Darn it, I really wanted to find a short-cut, without spending hours in trial and horror attempts!

By the way, many years ago some collegues of mine in Sweden put a lot of time in Echelon designs for "smart homes". You ever met a guy called Anders Ricknell? He was (at the time) my boss, and one of the frontiers using Echelon to communicate over the mains in Sweden. I remember they had a lot of problems with interference... :)

/Mike

 

[berkeman]

By the way, many years ago some collegues of mine in Sweden put a lot of time in Echelon designs for "smart homes". You ever met a guy called Anders Ricknell? He was (at the time) my boss, and one of the frontiers using Echelon to communicate over the mains in Sweden. I remember they had a lot of problems with interference... :)

The name doesn't ring a bell for me, but I don't work much with powerline or Smart Homes. Echelon's powerline transceiver technology has come a long way since the early days -- you should check out the website if you get a chance. And yeah, the AC power mains is a nasty place to be trying to communicate! But it can be done, if you do it right.