Sub-Milliohm MOSFET's On-State Voltage Drop Is Only 27.9 mV at 50A

May 1, 2011 12:00 PM
Davis, Sam, PET -

With on-resistance less than 1 mΩ, the PI5101 MOSFET enables low-voltage, high-current applications for active ORing, hot swap power managers, and load switches.

THE MOSFET on-resistance race is reaching new levels with Picor's PI5101 that achieves sub-milliohm values. Ideal for low voltage, high-current power path management applications, the new µRDS(on)FET™ device combines a high-performance 5V, 360µΩ lateral N-Channel MOSFET with a thermally enhanced high density 4.1mm × 8mm × 2mm land-grid-array (LGA) package to enable performance in the footprint area of an industry standard SO-8 package. The package is fully compatible with industry-standard SMT assembly processes and most off-the-shelf MOSFET controller ICs. Table 1 lists the characteristics of this MOSFET.

The PI5101 is a companion product for Picor's Power Management IC controllers. When combined with a compatible Picor controller, the PI5101 enables on-state low power dissipation for applications such as active ORing, hot swap power managers, load switches and high current DC-DC converters.

A single PI5101's on-resistance is equivalent to that of up to six paralleled conventional SO-8 form factor MOSFETs. One PI5101 reduces the required board space by approximately 80% compared with the paralleled SO-8 approach. In fact, a single device would be much easier to use because multiple paralleled devices should be arranged so that all six devices draw approximately the same current. Plus, a single PI5101 has substantially less power dissipation than the six-pack of SO-8s.

The PI5101 exhibits an excellent figure-of-merit for R_DS(ON) × Q_G. In addition, gate resistance (R_G) and package inductance (L_DS), outperform conventional MOSFETS and enable very low loss operation.

DETERMINING OPERATING TEMPERATURE

You can determine the junction temperature of the PI5101 by means of a few simple steps. In applications such as low loss ORing diodes or circuit breakers where the MOSFET is normally on during steady state operation, MOSFET power dissipation is a function of total drain current and the MOSFET's on-resistance. Thus, the PI5101's power dissipation is:

P_D = I_D² × R_DS(ON) (1)

Where:

P_D = MOSFET power dissipation in watts

I_D = Drain Current in amperes

R_DS(ON) = MOSFET on-state resistance in ohms

R_DS(ON) is temperature dependent so for the worst case condition, so use the maximum rated value at the MOSFET maximum operating junction temperature. Fig. 1 shows normalized R _DS(ON) values over temperature for the PI5101. The PI5101's maximum R_DS(ON) at 25°C is 0.450 ×10^-3 ohms and will increase by 24% at 125°C junction temperature. Junction temperature rise is a function of power dissipation and thermal resistance. Therefore:

T_RISE = R_θJA × PD = R_θJA × I_D² × R_DS(ON)
(2)

Where:

T_RISE = Temperature rise in ºC

R_θJA = Junction-to-Ambient thermal resistance (40°C/W) in °C/W

Although this would require several calculation iterations to determine the final junction temperature, Fig. 2 and Fig. 3 simplify finding the final junction temperature. Fig. 2 is the MOSFET's junction temperature vs. conducted current at maximum R _DS(ON) of 0.450 mΩ and various ambient temperatures. Fig. 3 plots the junction temperature vs. drain current for the given PCB temperatures.

You can find the final junction temperature for a given drain current at a given ambient or PCB temperature. For example, assume that the MOSFET maximum drain current is 50A and maximum operating ambient temperature is 70°C. First, using Fig. 2, draw a vertical line from 50A to intersect the 70°C ambient temperature line. At this intersection draw a horizontal line toward the Y-axis (junction temperature). Fig. 2 shows that the junction temperature is 126°C with maximum R _DS(ON), at 50A load current and 70°C ambient temperature.

As a check, recalculate the junction temperature to confirm the plot results. R_DS(ON) is 0.450mΩ maximum at 25°C and increases as the junction temperature increases. Start from the final junction temperature, 126°C, and do the following:

Using the normalized values from Fig. 1, at 126°C, R _DS(ON) will increase by 24%, so the maximum RDS(ON) is:

R_DS(ON) = 0.450 × 10^-3 × 1.24 = 0.558 × 10^-3 Ω (3)

Maximum power dissipation is:

P_D = I_D² × R_DS(ON) = 50A² × 0.558 × 10^-3 Ω = 1.39 W
(4)

Maximum junction temperature is:

T_J(MAX) = 70°C + 40°C/W × (50A² × 0.558 × 10^-3 Ω)W =125.8 °C (5)

Voltage Drop = 0.558 × 10^-3 Ω × 50A =27.9 mV

ACTIVE ORING

In redundant power system architectures, as shown Fig. 4, the PI2001 provides high-speed active ORing control together with the PI5101 MOSFET. The PI5101 is fully enhanced at a V _GS of 4.5V, so the VC voltage need only 5.5V for a low voltage rail of 1V. An internal shunt regulator at the PI2001's VC input allows higher voltage inputs, provided that you add the appropriate series resistor and gate clamp circuit shown in the PI5101-EVAL1 evaluation board users guide.. Also, you can program undervoltage and overvoltage thresholds using external resistor voltage dividers. To ensure high availability, the PI2001 and PI5101 combination offers an extremely low power loss solution with fast dynamic response to fault conditions.

In normal steady state conditions, the PI2001's gate drive output turns the PI5101 on. If there is an input power source fault that causes reverse current flow, the PI2001 produces high-speed current turn-off with auto-reset after the fault clears. In addition, the PI2001 continually monitors the PI5101's drain-to-source voltage to detect fault conditions. If there is excessive forward current, reverse current, light load, overvoltage, undervoltage and overtemperature conditions the PI2001 outputs an active low fault flag.

LOAD SWITCH

Imagine a 50A, 1V load supplied by two redundant feeds. For the highest efficiency, the active ORing portion would need as many as six parallel MOSFET's for each feed. Plus, the designer may need short circuit protection. Previously, this would require many MOSFETs, a difficult layout to enable each individual MOSFET to current share the load, and multiple controllers to achieve the target design.

The PI5101, combined with the appropriate controller, can simplify this circuit with 1.8W per feed loss. Back-to-back PI5101 MOSFETs along with the Picor PI2002 Active ORing and load switch controller exhibits a typical R_DS(ON) of 0.720mΩ (Fig. 5).

The Picor PI2002 acts as both an active ORing controller and a load switch controller in one package. If the forward voltage drop across the series connected MOSFET's combined R_DS(ON) exceeds the forward trip point, the load switch PI5101 is turned off within 200ns typical. . If V_IN1 fails short circuit, the ORing PI5101 turns off within 200ns (typical), allowing the V_IN2 rail to continue to supply the load current, while the body diode of the PI5101 blocks. The ultra low R_DS(ON) of the PI5101 makes a high current back-to-back load switch/ORing configuration feasible, with a series total R_DS(ON) of less than 1 mΩ.

EVALUATION BOARD

A PI5101-EVAL1 evaluation board (Fig. 6) allows a designer to test the basic principles and operational characteristics of a low voltage high current active ORing function in a redundant power architecture. The board has two independent power source inputs, similar to a typical redundant power architecture in which two active ORing channels combine to form a redundant power output. Each channel contains a PI2001 controller and PI5101 MOSFET.

Optimized PCB layout and component placement enable the board to duplicate a realistic high density design for an embedded high side active ORing circuit intended for 3.3V bus applications up to 60A. It is a simple way to test the electrical and thermal performance of the PI5101 ultra-low R_DS(ON) MOSFET and PI2001 active ORing controller.

The board allows dynamic, steady-state testing of the PI5101 and the PI2001. Dynamic testing can be completed under various system-level fault conditions to measure fault response time to faults.

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