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MIC4604YMT Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC4604YMT Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 18 page  Micrel, Inc. MIC4604 June 25, 2013 11 Revision 1.0 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 CB 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: gs 2 gs 2 1 V Qg 1/2 E so V C Q but V Ciss E × × = × = × × = Eq. 6 Where Ciss = total gate capacitance of the MOSFET 10 8 6 4 2 0 0 5 10 15 20 25 Gate Charge Qg - Total Gate Charge (nC) VDS = 50V ID = 6.9A 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: fs V Qg P and V Qg E gs driver gs driver × × = × = Eq. 7 Where: Edriver = energy dissipated per switching cycle Pdriver = 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: fet _ Rg Rg Ron Ron P Pdiss driver drive + + = 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 IHB VHB IDD VDD Pdiss ply sup × + × = 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. total drive ply sup total Pdiode Pdiss Pdiss Pdiss + + = Eq. 10 |
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