A bias resistor (Rbias) is required if Vaux is higher
than 15V. Rbias should be connected between the
VC pin and Vaux.
source. At the same time the energy stored at the
drain side of the internal MOSFET will be released
and produce a voltage higher than the load voltage.
This event will create a high voltage difference
between the drain and source of the MOSFET. To
reduce the magnitude of the ringing voltage, add a
ceramic capacitor very close to the source that can
react to the voltage ringing frequency and another
capacitor close to the drain. Recommended values
for the ceramic capacitors are 1µF, refer to C5 and
C7 in Figure 24.
Minimize the resistor value for low Vaux voltage
levels to avoid a voltage drop that may reduce the
VC voltage lower than required to drive the gate of
the internal MOSFET.
Select the value of Rbias using the following
equations:
Vauxmin −VCclamp
Rbias =
Slave:
ICmax
For a high current application where one PI2121 can
not handle the total load current, multiple PI2121’s
can be paralleled in a master / slave configuration to
support the total current per input. In the Master /
Slave mode, one PI2121 is configured as the master
and the rest are configured as slaves. The slave
Rbias maximum power dissipation:
2
(Vauxmax −VCclamp
)
PdRbias
Where:
=
Rbias
Vauxmin : Vaux minimum voltage
Vauxmax : Vaux maximum voltage
VCClamp : Controller clamp voltage, 15.5V
(
) pin of the master unit will act as an output
SL
driving the units configured in slave mode. The
SL
pins of the slave units will act as inputs under the
control of the master.
ICmax : Controller maximum bias current, use
Tie the BK pin to VC to configure the unit in slave
mode.
4.2mA
Example: Vaux 20V to 30V
Power dissipation:
In Active ORing circuits the MOSFET is always on in
steady state operation and the power dissipation is
derived from the total source current and the on-
state resistance of the internal MOSFET.
Vauxmin −VCclamp
20V −15.5V
4.2mA
Rbias =
=
=1.07KΩ
ICmax
2
(30V −15.5V)2
(Vauxmax −VCclamp
)
PdRbias
=
=
=196mW
The PI2121 internal MOSFET power dissipation can
be calculated with the following equation:
Rbias
1.07KΩ
PdMOSFET = Is2 ∗Rds(on)
Where:
Internal N-Channel MOSFET BVdss:
The PI2121’s internal N-Channel MOSFET
breakdown voltage (BVdss) is rated for 8V at 25°C
and will degrade at -40°C to 7.75V, refer to Figure
10. In an application when the MOSFET is turned
off due to a reverse fault, the series parasitic
elements in the circuit may contribute to the
MOSFET being exposed to a voltage higher than its
voltage rating.
Is
: Source Current
Rds(on) : MOSFET on-state resistance
Note:
Calculate with Rds(on) at maximum MOSFET
temperature because Rds(on) is temperature
dependent. Refer to figure 11 for normalized
Rds(on) values over temperature. PI2121 nominal
Rds(on) at 25°C is 1.5mΩ and will increase by 40%
at 125°C junction temperature.
In Active ORing applications when one of the input
power sources is shorted, a large reverse current is
sourced from the circuit output through the
MOSFET. Depending on the output impedance of
the system, the reverse current may reach over 60A
in some conditions before the MOSFET is turned off.
Such high current conditions will store energy even
in a small parasitic element. For example: a 1nH
parasitic inductance with 60A reverse current will
generate 1.8µJ (½Li2). When the MOSFET is turned
off, the stored energy will be released and produce a
high negative voltage ringing at the MOSFET
The Junction Temperature rise is a function of power
dissipation and thermal resistance.
Trise= Rth ∗PdMOSFET = Rth ∗Is2 ∗Rds(on),
JA
JA
Where:
RthJA : Junction-to-Ambient thermal resistance
(54°C/Watt)(3)
Picor Corporation • picorpower.com
PI2121
Rev. 1.0 Page 14 of 21
VICOR [ VICOR CORPORATION ]
