Custom mosfets help to simplify hot-swap design

Called ‘Linear FET’, Infineon’s new mosfets combine the safe-operating area (SOA) characteristics of older planar mosfets with the low on-resistance of modern trench FETs.

Trench FETs tend to be designed for switching, which means they are optimised for fast operation and are low on-resistance, among other parameters.

They are not optimised for the ability to pass current as well as drop voltage for a considerable amount of time, because they are only ever in the linear state – not fully ‘on’ – for nanoseconds, or a few microseconds at the most.

The metric describing the ability to carry voltage and current simultaneously is known as SOA – from expanse of the voltage against current graph that the device can operate in without damage (Figure 1).

Figure 1: ‘Hot-swap mosfets have to absorb power surges at plug-in, then run with low on-resistance

p28 Infineon LinearFet app web

On the right side is a trench FET, which contrasts with older planar mosfets (left) which can survive power dissipation for far longer. Holding off 48V for 10ms, the planar device can conduct over 4A compared with only 500mA for trench technology.

Hot swap

In a hot-swap application (Figure 2), the mosfet spends most of its life fully ‘on’ supplying power to the board, which requires low on-resistance, but its real job lasts only a fraction of a second when it limits current flowing into the board’s reservoir capacitors as the card it is on is plugged into a live equipment rack.

Figure 2: SOA diagrams of a planar (left) and a modern trench mosfet (right)

p28b Infineon planar and trench fet

Without this, the huge current surge – hundreds of amps – will cause a voltage dip within the rack, possibly corrupting data on other boards. As the mosfet limits current, it has to operate linearly – carrying both current and voltage.

“One way to tackle this problem is to use two devices in parallel where one device is selected according to the required SOA behaviour for the in-rush phase and the other with low-resistance for the continuous conduction phase,” says Infineon.

“This approach combines the benefit of both worlds. However, the bill of material is increased, and an additional control circuit is needed to switch between the two devices.”

Why the trench problem?

Power mosfets are large area devices, requiring current flow to be spread evenly across the semiconductor to avoid the development of hot-spots, which will damage the device if they get out of control.

“A more advanced trench mosfet has a narrower SOA because the pitch between trenches gets smaller and smaller,” Infineon application engineer Alan Huang told Electronics Weekly.

“As a result,” he said, “in certain conditions, SOA is no longer power‑limited but limited by thermal stability.”

All the time, the temperature coefficient of on-resistance is positive, any point getting too much current will get hotter, increase in resistance, and push part of its current to other parts of the device – naturally spreading current over the whole chip.

Where on-resistance has a negative coefficient, current concentrates, resulting in thermal run-away if allowed to happen for too long.

“Linear FET uses Infineon’s patented technology to distribute in-rush current more evenly around on the chip,” said Huang.

“This helps to spread heat around during the switching event, and thus avoids forming such a hot spot.”

Temperature point

There is a point on a mosfet’s transfer curve (red spots on Figure 3) below which the temperature coefficient is negative, risking run-away, and above which it is positive and safe. The red spot is the zero temperature coefficient (ZTC) point.

Figure 3: Typical transfer characteristis IPB017N10NS (left) and IPB017N10N5LF (right)

p28c Infineon trench fet and LinearFet transfer

The diagram shows Infineon’s conventional trench IPB017N10N5 on the left, which needs to be conducting over 400A before it is on safe ground, while on the right is the Linear FET equivalent (IPB017N10N5LF), where, without compromising the trench advantage of low on-resistance, Infineon has found a way to modify the trench process and move the ZTC point to only 30A.

“This is exactly the reason why the permitted SOA of the Linear FET is much larger and wider than that of the standard mosfet,” says Infineon. “Linear FET has many fewer chances to operate in the conditions which could run into thermal run-away.”

Comparing the SOAs of the two mosfets (Figure 4), the standard IPB017N10N5 can allow only 0.5A of 10ms current, whereas the Linear FET version can withstand up to 11.5A – a 20x improvement.

Figure 4: Safe operating area IPB017N10NS (left) and IPB017N10N5LF (right)

p 28d Infineon trench fet and LinearFet

“Therefore, with Linear FET, it is possible to charge a much larger capacitor bank in the same amount of time without damaging or destroying the mosfet.

“Alternatively, the same capacitor bank can be charged at higher currents in a shorter duration,” according to Infineon.

Linear FETs are available with 100V, 150V and 200V ratings with 56V/10ms safe currents spanning 5.6A to 10.2A and on-resistances between 1.7mΩ and 11mΩ.

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