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Low Voltage Power MOSFETs

To address the needs of high frequency applications, the MOSFET gate charge plays a more important role than the RDS(on) in minimizing total power losses. To this purpose the STripFET technology is more suitable than the traditional planar or, more recent Trench technology offered by competitors. It has been properly optimized to obtain devices showing the best compromise between dynamic characteristics (strictly linked to the gate charge Qg) and the static ones (RDS(on)). A good example of an application is the high-efficiency DC-DC converter for motherboards and portable PCs.
LVMOS photo

Synchronous DC-DC Converter Requirements
The most popular topology for the DC-DC conversion stages used in Desktop and in Mobile computers is the synchronous buck regulator. The introduction of the latest microprocessors with low CPU core voltage and high currents has placed an increasingly heavy burden on these converters, resulting in demands for higher levels of efficiency. In this topology, which consists of two MOSFETs (Q1 and Q2) switched alternately by the control IC, the efficiency is determined by the total power losses of the MOSFETs. As you would expect such losses are impacted by the RDS(on) (conduction losses), however a large portion of the remainder is the result of losses depending on the gate charge (switching losses).

MOSFET Requirements
Taking into account that the duty cycle (D) determines the ratio between output and input voltage (D=Vout/Vin) for low D value (high input voltage) the power losses in Q1 are due mainly to the gate charge while in Q2 (Zero Voltage switching) to the RDS(on). However by increasing D the total power losses in both Q1 and Q2 MOSFETs are due to both RDS(on) and gate charge. The latest modelling techniques and the Mathcad simulation tools have been used for MOSFET designing in order to minimize the total losses and in this way increase the converter efficiency.

Competition Analysis
- Type RDS(on) @10V
(m Ohm) typ		 Qg@10V
(nC) typ		 Ciss / Coss / Crss
(pF) typ
ST STB70NF03L 7.5 35 1650 / 800 / 170
Best Competition HUF70132S 8.5 44 1650 / 850 / 200

Synchronous buck topology
Synchronous buck topology

"NF" Low Qg Series for Motherboards
The STripFET technology has been optimized in order to get the best compromise between RDS(on) and gate charge. This is of fundamental importance for the DC-DC converters for motherboards where the value of the duty cycle is in the range of 0.4 / 0.5. The STB70NF03L gives the best performance in terms of total power losses when used as both Q1 and Q2 in the buck topology shown on the right. In fact with this D in Q1 the amount of the total losses due to the RDS(on) (conduction losses) are comparable with the losses due to Qg (switching losses); the same way for the Q2 (synchronous rectifier) where RDS(on) is a key characteristic but Qg impacts the gate drive losses. In addition the STB36NF03L has also been developed to be used as Q1 as a cost effective solution.

MOSFETs for Motherboards
Type BV(DSS)
(V)		 Current
(A)		 RDS(on)
@10V
(m Ohm) typ		 Qg@10V
(nC) typ		 Ciss/Coss/Crss
(pF) typ		 Funct Pack
STB70NF03L 30 70 7.5 35 1650/800 /170 Q1&Q2 D²PAK

"NF" Low Qg Series for Mobile PCs
A typical portable computer operates from a four-cell Li-Ion battery pack which supplies 17V when fully charged. The challenge, therefore, is to design high-efficiency DC-DC converters capable of generating voltages down to 1.6V (new microprocessors) from an input of about 20V. Consequently this leads to a lower value of duty cycle than in a motherboard application. In order to reduce the total power losses, what is needed is a low gate charge for Q1 and low on-resistance for Q2. The STS12NF30L represents the best solution for Q2 whereas the STS9NF30L and the STS11NF30L are the right options for Q1.

"NF" Low Qg Series for Mobile PCs
Type BV(DSS)
(V)		 Current
(A)		 RDS(on)
@10V
(mOhm) typ		 Qg@10V
(nC) typ		 Ciss/ Coss/ Crss
(pF) typ		 Funct Pack
STS12NF30L 30 12 10 35 2400/590 /200 Q2 SO-8
STS9NF30L 30 9 24 8.5 750/400/75 Q1 SO-8
STS11NF30L 30 11 13.5 21 1650/800/170 Q1 SO-8

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