User Guide
11 of 46
Rev. 1.0
2022-03-09
48V Battery Switch Reference Design
R 48V BATT Switch10
System and functional description
parallel to the power switch. The
2ED4820-EM
gate driver channel B is driving a pair of
IAUZ40N10S5N130
MOSFETs to turn the prechargarging path ON and OFF. The precharge current is limited by a 10
W
power
resistor.
At the beginning of the charge pulse the power resistor will see the whole battery voltage. The initial power
dissipation at 48V therefore is as big as:
P=
V
2
R
=
(48V)
2
10 Ω
=230.4 W
However, while the capacitor gets charged, the voltage across the resistor will decay exponentially. In spite of
the large initial power surge the energy of the charging pulse is limited and the resistor will not even get hot.
Nevertheless, a power resistor with high pulse current capability has to be used to withstand the high inrush
current. Wire wound resistors usually can handle a multiple of their DC power rating for short pulses and are
well suited for this application.
The time needed to charge the DC link capacitor is specified by the time constant
𝜏
=
𝑅𝐶
of the RC circuit
formed by the precharge resistor and the DC link capacitor. After
5
𝜏
the voltage at the capacitor will have
reached 99.3 % of the supply voltage and the capacitor can be considered fully charged. Assuming a 10 mF DC-
link capacitor we get a precharge time of
𝑡
𝑝𝑟𝑒𝑐ℎ𝑎𝑟𝑔𝑒
= 5
𝜏
= 5 ∗
𝑅𝐶
= 5 ∗ 10 Ω ∗ 10
𝑚𝐹
= 500
𝑚𝑠
The energy that will be dissipated in the resistor during that precharge pulse is the same as the energy stored in
the capacitor and can be calculated as
𝐸
=
1
2
𝐶𝑉
2
=
1
2
∗ 10
𝑚𝐹
∗
(48
𝑉
)
2
= 11.52
𝐽
According to the specification the resistor can handle five times the rated power for five seconds. This would
amount to 5 * 7 W * 5 s = 175 J and should give us enough headroom for charging even bigger DC-link
capacitors.
A problem however might occur when there is a short circuit or a low ohmic load at the output of the switch
and the precharge switch is engaged. In that case the dissipated power would quickly exceed the resistor rating
and the resistor could be damaged. Therefore, the precharge resistor is monitored by the microcontroller.
Whenever channel B is switched on, the voltage across the resistor is measured and the dissipated power is
calculated. If the power exceeds 5 W the microcontroller starts integrating the power over time to get the
energy in the resistor. As soon as it exceeds 100 J, channel B is switched off.
2.2.4
MOSFET gate driver
The
2ED4820-EM
is a gate driver designed for high current 48 V automotive applications, with two powerful
gate outputs that allow to drive many MOSFETs in parallel. It supports the back-to-back configuration, both
common source and common drain topology.
The gate driver generates the supply for the gate outputs with the help of on an integrated one-stage charge
pump with external pump and tank capacitors. Except for the supply and charge pump capacitors there is little
external circuitry needed.
For shunt based current monitoring and overcurrent detection the
2ED4820-EM
contains a current sense
amplifier with programmable gain. The voltage drop at the shunt resistor is amplified and the amplified voltage
is provided at the CSO output. The current sense amplifier gain as well as the overcurrent threshold can be
configured by SPI.
