Fairchild RC5040, RC5042 MOSFET Selection, Two MOSFETs in Parallel, Thermal, Conditions1

APPLICATION NOTE AN42

MOSFET Selection

This application requires the use of N-channel, Logic Level Enhancement Mode Field Effect Transistors. The desired characteristics of these components are:

•Low Static Drain-Source On-Resistance RDS,ON < 37 mΩ (lower is better)

•Low gate drive voltage, VGS ≤ 4.5V

•Power package with low thermal resistance

•Drain current rating of 20A minimum

•Drain-Source voltage > 15V.

The on-resistance (RDS,ON) is the main parameter for MOS- FET selection. It determines the MOSFET’s power dissipa- tion, thus significantly affecting the efficiency of the converter. Several suitable MOSFETs are shown in Table 5.

Table 5. MOSFET Selection Table

			RDS,ON(mΩ)			Thermal
	Conditions1
Manufacturer & Model #			Typ.	Max.	Package	Resistance
Fuji	VGS = 4V	TJ = 25°C	25	37	TO-220	ΦJA = 75
2SK1388	ID = 17.5A
		TJ = 125°C	37	—
Siliconix	VGS = 4.5V	TJ = 25°C	16.5	20	SO-8	ΦJA = 50
SI4410DY	ID = 5A				(SMD)
		TJ = 125°C	28	34
National Semiconductor	VGS = 5V	TJ = 25°C	13	15	TO-220	ΦJA = 62.5
NDP706AL	ID = 40A					ΦJC = 1.5
NDP706AEL		TJ = 125°C	20	24
National Semiconductor	VGS = 4.5V	TJ = 25°C	31	40	TO-220	ΦJA = 62.5
	ID = 10A
NDP603AL		TJ = 125°C	42	54		ΦJC = 2.5
National Semiconductor	VGS = 5V	TJ = 25°C	22	25	TO-220	ΦJA = 62.5
	ID = 24A
NDP606AL		TJ = 125°C	33	40		ΦJC = 1.5
Motorola	VGS = 5V	TJ = 25°C	6	9	TO-263	ΦJA = 62.5
	ID = 37.5A
MTB75N03HDL		TJ = 125°C	9.3	14	(D2 PAK)	ΦJC = 1.0
Int. Rectifier	VGS = 5V	TJ = 25°C	—	28	TO-220	ΦJA = 62.5
	ID = 31A
IRLZ44		TJ = 125°C	—	46		ΦJC = 1.0
Int. Rectifier	VGS = 4.5V	TJ = 25°C	—	19	TO-220	ΦJA = 62.5
	ID = 28A
IRL3103S		TJ = 125°C		31		ΦJC = 1.0

Note:

1.RDS(ON) values at Tj = 125°C for most devices were extrapolated from the typical operating curves supplied by the manufac- turers and are approximations only.

Two MOSFETs in Parallel

We recommend two MOSFETs used in parallel instead of a single MOSFET. The following significant advantages are realized using two MOSFETs in parallel:

•Significant reduction of power dissipation. Maximum current of 14A with one MOSFET:

PMOSFET = (I2 RDS,ON)(Duty Cycle) =

(14)2(0.050*)(3.3+0.4)/(5+0.4-0.35) = 7.2 W

With two MOSFETs in parallel:

PMOSFET = (I2 RDS,ON)(Duty Cycle) =

(14/2)2(0.037*)(3.3+0.4)/(5+0.4-0.35) = 1.3W/FET

*Note: RDS,ON increases with temperature. Assume RDS,ON = 25mΩ at 25°C. RDS,ON can easily increase to 50mΩ at high temperature when using a single MOSFET. When using two MOSFETs in

parallel, the temperature effects should not cause the RDS,ON to rise above the listed maximum value of 37mΩ.

•No added heat sink required.

With the power dissipation down to around one watt and with MOSFETs mounted flat on the motherboard, no external heat sink is required. The junction-to-case

thermal resistance for the MOSFET package (TO-220) is typically at 2°C/W and the motherboard serves as an

excellent heat sink.

•Higher current capability.

With thermal management under control, this on-board DC-DC converter can deliver load currents up to 14.5A with no performance or reliability concerns.

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