Matching a MOSFET to a synchronous driver

Preface:
In power electronic, many time reader will come accross with this synchronous MOSFET driver. The most useful characteristic about synchronous driver is that, it save the energy drop across conventional diode approach (to conduct right after mosfet stop conduct).

A conventional diode can 'eat' around 0.7V to few Volt if current passed through it, a metal processed schottky diode can reduce the voltage drop to lesser, some newer process allow very low forward bias potential. Supposed current flow at 10A, a 0.5V drop accross diode will result in 5Watt dissipate through the diode. 5Watt is good for many conventional big component, but definitely not too good for those small size SMD component, it will getting overheat due to limited surface area to dissipate the energy.

When the MOSFET getting cheaper and the current required by electronic device getting bigger in number, engineers/circuit designers begin to adding another MOSFET into the DC-DC driver, which known as synchronous regulator. Voltage drop accross a MOSFET can be less than 0.05V, depending on it on state resistance and current pass through it. Considered a MOSFET on-state resistance is 0.005Ohm, at current 10A, it theoritically(without considered switching loses) will only generate 0.5Watt heat, which can be withstand by SOIC8 SMD packaging or any other similar IC packaging.

Supposing one circuit designer design the power regulator for a notebook central processing unit, the processor required 1V-15A peak power, a synchronous buck allow >90% convertion efficiency from your power adaptor. The processor are expected to consume 15W maximum, we expect a waste of less than 1.5W for the power management unit. Using asynchronous buck one will expect to waste more than 3W, even with very good diode used. Which, is not good for any battery powered device. The advantage of synchronous buck become especially obvious when we are dealing with desktop CPU where it can consume up to or even >50A in today PC environment. Circuit designer are dealing with more phase buck to ensure stable power supply to the central processor unit.

One good thing about synchronous drived MOSFET is that, it can be both boost or buck driver, driving current in get a buck configuration, reverse it or driving current out, one can expect to get a boost regulator.

15 years ago, we can hardly, if not imposible, to see high frequency MOSFET driver available on the market. Primary is due to market demand, many consumer electronics do not required such a big power, even Pentium at the time use less than 15Watt, even a series linear regulator can handle that rate without causing exceed power waste. Morever, it use 3.3V or 2.7V or something close to that.

15 years ago, we are still dealing with those large size and large scale electronic in our TV, Monitor, Mini HiFi and so on. 15 years ago, TV repair service have high demand in Malaysia. One of the problem of those large size component is inductance on it lead. Don't overlook or neglect these inductance, it is the killer that limit the frequency response of a system. In today electronic, few Megahertz power switching become possible, driving high current is like driving a big truck, it could hardly achieve high speed.


Choosing the correct mosfet:
To choose a MOSFET that match your schematic involve many technical detail, I hope that reader have learned that before.

But to choose a MOSFET that match a driver, there are three important criteria, first the Vgs(on) must match with the driver's driving potential and within the permited range.

Second, the driver must be able to supply maximum current that able to charge the MOSFET gate up for full achieveable conductance in a very short time, usually recomanded less that 1% of the period of switching signal (in decades nano second).

Third, must assure the Turn On Delay and Turn Off Delay time of a MOSFET match the Turn Off Propagation Delay set in the mosfet driver. If using a mosfet with a Turn Off Delay very much time larger than the Turn Off Propagation Delay of the driver, cross conduction may occur when low side MOSFET just turn on. Causing switching lose in the turning off high side MOSFET.

Fourth, let me know if I have missed something important here.

Table 1: Timing diagram of a MOSFET driver

Table 2: Fraction of datasheet of MOSFET.

Avoiding crossed-conduction:
Please note that a MOSFET with Turn-On Delay Time larger than Turn-Off Delay Time is suitable for synchronous driver, for example, without taking account of the Turn Off Propagation Delay of the driver, if we turning off high side MOSFET, as the condition specified above, it take 13+10=23ns to fully turn off. At the same time(simultaneously), we turn on low side MOSFET, typically, it take 15+12=27ns to fully turn on, that mean 27-23=5ns, there are 5 ns both MOSFET are in non-conduct condition, thus no crossed conduction is possible to occur.

*Refer to table 1, please don't forget that there are a minumum of 10ns Turn Off/ON Propagation Delay intoduce by the MOSFET driver, so over all there will have 5+10=15ns minimum where both MOSFET non-conducting. One can also use a MOSFET which have Turn-On Delay Time slightly smaller than Turn-Off Delay Time, but make sure that the difference do not exceed Turn Off/ON Propagation Delay intoduce by the MOSFET driver. Or else crossed-conduction may occur where both MOSFET conduct at the same time, which can cause MOSFETs failure or overheat.

There are some MOSFET, specially designed to have Turn-On Delay Time much smaller than Turn-Off Delay Time, this type of MOSFET usually used in aplication where switching is not required or in MOSFET and DIODE type of DC-DC, usually in lower frequency. Longer Turn-Off Fall Time reduce the current lose transiently built up of higher potential in the inductor or wire of any circuit, which is hazard to most component and therefore making it more robust.

FEB2010
By: David

DIY SMD Rework Hot Pencil (for small component)

What is a SMD rework station? A picture worth a thounsand words.

I recently doing new purchasing research online about the SMD rework. The cheapest one will cost us around RM270, while some company selling it increadibly expansive, price can go up to thousand Ringgit Malaysia. Interesting enough, I come across with some DIY article about SMD Rework Pencil or even DIY rework station, what? This thing also can be DIY? The question is not correct, what can not be DIY? It just need correct knowledge and action.

The good thing about DIY is that we can make use some stuff that we already have. The material required is roughly as the picture above, aquarium pump 3.5L/min maximum (RM10-30, there are some more powerful unit can go up to 30L/min), guitar string (B2 will do, RM1-5), some big fuse (RM1-10) and a big transformer is required (I am using a 12V, 10A unit from Maplin Electronic, around RM40-50, of course you can also use a PC power suply 5V or 12V rail, provided it can output enough current). 12V is recomended as smaller current required to heat up the guitar string, 24V is the best but you may need twice the guitar string length compare to 12V supply, unless if it capable to source twice the current rate compare to 12V supply.

Remove the cap of big fuse by using hand saw, cut the cap as the picture below and you will be able to remove it. Inside the fuse there are some processed(salt removed to prevent corrosion) sand, the porpuse of the sand is to prevent explosion of fuse as air can instantly increase in volume when temperature suddenly increase during fuse blowing, can result in fuse burst or even explode. Remove the sand to make it empty.

Measure the resistance of the guitar string with some precise multimeter, in my case I am using 12V, 10A power source, the resistance must not smaller than 12/10=1.2Ohm, Ohm's Law, remember? Wind the guitar string at correct dimension corespond to the inner diameter size of the fuse used, as what is show in the picture below.

It is done, pretty job isn't it? I fail at first two string, what a waste!!!

Put the guitar string into the fuse with cap removed as the picture below.

Get some wire interconnector copper tube from your local electrical shop, this thing are used for clamping two wire together without the need of using soldering iron, for fast work generally, but the most important idea is, sometime solder lead just don't work especially in the few hundred dergree Celcius environment, example inside a hair blower.

Assemble the copper tube and ceramic tube together such as show in picture below, use some copper foil to wrap the fuse, if you can not found copper foil, aluminium foil or even tin also aplicaple. Alternatively, you can use back the original cap if you able to take it off without scratched it, I guess damp the fuse into some oil or WD40 will help you in removing the cap, take two adjustable spanar, adjust to correct opening as wide as the fuse body and apply force apart toward the direction of the cap.

It is time for first testing, fire time (generally, this thing also can be used as igniter), show time. It took ~1-2minutes to heat up the ceramic chamber (the ceramic (body of fuse) will somehow store energy and the chamber will getting hotter and hotter) before it ready to solder/unsolder any SMD component, quite satisfy with the result.

It is not done yet, please wait for part 2 to come up, once I am free, I will proceed to part two later if have any add on.

Ok, thank for reading, any question please mail to mail.altbattery@gmail.com, I will try to answer it if I am free, thank you.


Note: This is not a long term solution for any other use, the heating element (guitar string) can become oxidation and eventually broken after some period of use. There is however, this is a good solution if you only need it to solder some small component once a while. It is cheap, you can use your old PC unused power supply, then the whole thing will not cost you more than RM50 I guess, of course, in the sacrifice of some of your time, but it can be interesting when you see your own built unit work!

Update 11thFreb2010:
It is time to show some result, it take 10-20s to remove bigger ic or D-Pak transistor, I don't know will that spoil the ic, but usually when you need to remove a ic mean that it is already spoiled. You can increase the power of heat element, it will shorthen the time needed to remove the ic, but the heat element will blow like fuse easily, sooner or later. The solution is use thicker wire and wind more turn, that will distribute heat over a bigger wire surface area and thus increase the life of the heating element.

One can also built a circuit to control the temperature of the heating element within a range, but the problem is that it is so difficult, if not imposible, to get a electronic temperature sensor that withstand 1000 degree Celcius or above. You can of course put the sensor on the exhaust or ceramic, but that sacrifice transient response measurement, i.e. you get information slower that what happen on the heating element. As a result, it is hard to limit the maximum temperature of the heating element. I suggest using some sort of metal strip thermo-couple which can be placed as close as possible to the heating element, I shall experiment on this once I am free.

The board still in perfect condition after ic removed, ready for next ic installation.

Some modification, I found out the the heat will going up and melt the silicone hose, therefore, I add another segment of 'fuse', this segment allow air preheat and prevent any heat reaching the handle. The whole thing is glued together using Araldite Rapid Steel Epoxy, which can withstand high temperature.

Suggestion:

1. Built a on/off switch for heating element, on the handle of the Pencil.

2. Built in a thermocoupler, it gain is control by a potential-meter, and then make the control available on the handle itself.

3. Built a tyristor control unit that control the duty cycle of the input voltage 230V, thus control the power output by big transformer.

*using a DC PWM control may result in great lost of power in the rectifier and mosfet, as the current involved is high 8-10A. Unless if 24V source is used for maximum efficiency.

4. Use thicker nichrome wire for longer life.

*Sorry for my poor English language, there must be a lot of grammer mistake...
*Do not attempt to do this DIY if you do not know what you are doing, AC240V use in main power is hazardous. Safety First, use insulated AC to DC. Hot air can burn skin, please proceed carefully.

Update 12thMarch2010:
The nichrome wire from hair dryer, this one is better choice and last longer, I get it from a stop working hair dryer. It resistance gradually increase as the element getting hotter and it getting red, so you need a lower resistance. Carban steel (guitar string) as heating element won't last long, the heat will consume the carban and eventually it will become iron, which have much lower resistance, high current then will pass through the iron and it will blow like a fuse.

Going smaller, going SMD

Resistor/Capacitor:

SMD for reparing job and Alternative Tools or other project development use.

Space tight for custom circuit? Go smaller with SMD! SMD component can be soldered by using tweezers and normal soldering iron.

Price: RM0.20 per pieces SMD resistor/capacitor

Zener Diode:

6.2V Zener diode to protect digital i/o port that have risk to expose to higher voltage, example 24V. By design always perfect, but sometime wrong connection, ESD, transient current or component failure can let you down. Protect the port will avoid damaging i/o port which cost higher to replace the main ic.

Price: RM0.30 each

*Please mail to mail.altbattery@gmail.com for inquiry about value available, shipping and handling RM10 within Malaysia for poslaju, RM5 for normal parcel pos or RM1 for envelope post.