MIC4604 Datasheet, PDF (11/18 Pages) Micrel Semiconductor – 85V Half Bridge MOSFET Drivers with up to 16V Programmable Gate Drive

Micrel, Inc.
MIC4604
Rg is the series resistor (if any) between the driver IC and
the MOSFET. Rg_fet is the gate resistance of the
MOSFET. Rg_fet is usually listed in the power MOSFET’s
specifications. The ESR of capacitor C_B and the resistance
of the connecting etch can be ignored since they are much
less than Ron and Rg_fet.
The effective capacitances of Cgd and Cgs are difficult to
calculate because they vary non-linearly with Id, Vgs, and
Vds. Fortunately, most power MOSFET specifications
include a typical graph of total gate charge versus Vgs.
Figure 7 shows a typical gate charge curve for an arbitrary
power MOSFET. This chart shows that for a gate voltage
of 10V, the MOSFET requires about 23.5nC of charge.
The energy dissipated by the resistive components of the
gate drive circuit during turn-on is calculated as:
E
=
1
2
× Ciss × V_gs ²
but
Q = C×V
so
E = 1/2 × Qg × V_gs
Where
Ciss = total gate capacitance of the MOSFET
Gate Charge
10
V_DS = 50V
8 ^{ID = 6.9A}
Eq. 6
6
4
2
0
0
5 10 15 20 25
Qg - Total Gate Charge (nC)
Figure 7. Typical Gate Charge vs. VGS
The same energy is dissipated by Roff, Rg and Rg_fet
when the driver IC turns the MOSFET off. Assuming Ron
is approximately equal to Roff, the total energy and power
dissipated by the resistive drive elements is:
Edriver = Qg × Vgs
and
Pdriver = Qg × Vgs × fs
Eq. 7
Where:
E_driver = energy dissipated per switching cycle
P_driver = power dissipated per switching cycle
Qg = total gate charge at Vgs
Vgs = gate to source voltage on the MOSFET
fs = switching frequency of the gate drive circuit
The power dissipated inside the MIC4604 is equal to the
ratio of Ron and Roff to the external resistive losses in Rg
and Rg_fet. Letting Ron = Roff, the power dissipated in the
MIC4604 due to driving the external MOSFET is:
Pdiss _drive
= P_driver
Ron
Ron + Rg + Rg _ fet
Eq. 8
Supply Current Power Dissipation
Power is dissipated in the MIC4604 even if nothing is
being driven. The supply current is drawn by the bias for
the internal circuitry, the level shifting circuitry, and shoot-
through current in the output drivers. The supply current is
proportional to operating frequency and the VDD and VHB
voltages. The typical characteristic graphs show how
supply current varies with switching frequency and supply
voltage.
The power dissipated by the MIC4604 due to supply
current is
Pdiss _{sup ply} = VDD × IDD + VHB × IHB
Eq. 9
Total Power Dissipation and Thermal Considerations
Total power dissipation in the MIC4604 is equal to the
power dissipation caused by driving the external
MOSFETs, the supply current and the internal bootstrap
diode.
Pdiss _total = Pdiss_supply + Pdiss_drive + Pdiode _total Eq. 10
June 25, 2013
11
Revision 1.0