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
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