PI2211 datasheet(9/26 Pages) VICOR

9 / 26 page

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Picor Corporation · picorpower.com

PI2211

Rev 1.1, Page 9 of 26

PI2211

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PI2211 Application/Theory of Operation

Maintaining a MOSFET within its SOA boundary:

The PI2211 has a programmable digital model of a MOSFET

thermal response to transient and static loads that it uses to

predict a junction temperature rise, as a function of power,

for a given MOSFET. The equivalent analog model is shown in

Figure 2. It consists of two RC stages to emulate the total

thermal equivalent of the junction-to-case and case-to-

ambient characteristics of the MOSFET and its package. In the

model, the case-to-ambient characteristics are fixed while the

junction-to-case can be tuned to match the published data

for a specific MOSFET by two programming resistors; RSOAT

and RSOAR.

Figure 2 - Simplified representation of the PI2211 thermal

model.

The RSOAT resistor controls the time constant (TauP) of the SOA

junction-to-case model. This resistor programs the model to

adhere to the manufacturer’s transient thermal impedance

graph of the junction-to-case response to "single pulse"

power changes, as well as the extended SOA curves beyond

the DC area limit.

This instantaneous power calculation

refreshes in less than 50µs and predicts junction temperature

rise within the 1ms extended SOA MOSFET curves so the

PI2211 will protect the MOSFET from prolonged heating with

excessive static loads and hot-spotting from transient loads.

The RSOAR resistor programs the model with the RJ-C of the

MOSFET. Scaled by the ratio of the junction-to-case/case-to-

ambient thermal impedances (RJ-C/RC-A), referenced to the

fixed internal 60°C/W RC-A of the PI2211.

The RSOAS resistor programs the magnitude of the calculated

current through the MOSFET and the power it is dissipating.

All three of these resistors have a maximum value 20kΩ and a

1.30kΩ minimum value. Values outside of these ranges will

not stop the PI2211 from working, but will force the internal

references to either their minimum or maximum values. See

the Recommended Design Steps section for more details on

calculating the required SOA programming resistors.

Figure 3 - The range of PI2211 RθJ-C thermal impedance adjustment at 1W.

0.01

0.1

1E-05

0.0001

0.001

0.01

0.1

Time

.5°C/W, Tau = 1ms

.5°C/W, Tau = 15ms

7.5°C/W, Tau = 1ms

7.5°C/W, Tau = 15ms

RθJ-C =

0.5°C/W

Tau= 15ms

Tau= 1ms

RθJ-C =

7.5°C/W

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