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LT1162 Datasheet(PDF) 9 Page - Linear Technology |
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LT1162 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LT1160/LT1162 Power MOSFET Selection Since the LT1160 (or 1/2 LT1162) inherently protects the top and bottom MOSFETs from simultaneous conduction, there are no size or matching constraints. Therefore selec- tion can be made based on the operating voltage and RDS(ON) requirements. The MOSFET BVDSS should be greater than the HV and should be increased to approxi- mately (2)(HV) in harsh environments with frequent fault conditions. For the LT1160 maximum operating HV supply of 60V, the MOSFET BVDSS should be from 60V to 100V. The MOSFET RDS(ON) is specified at TJ = 25°C and is generally chosen based on the operating efficiency re- quired as long as the maximum MOSFET junction tem- perature is not exceeded. The dissipation while each MOSFET is on is given by: P = D(IDS) 2(1+ ∂)RDS(ON) Where D is the duty cycle and ∂ is the increase in RDS(ON) at the anticipated MOSFET junction temperature. From this equation the required RDS(ON) can be derived: R P DI DS ON DS ( ) = () +() 2 1 ∂ For example, if the MOSFET loss is to be limited to 2W when operating at 5A and a 90% duty cycle, the required RDS(ON) would be 0.089Ω/(1 + ∂). (1 + ∂) is given for each MOSFET in the form of a normalized RDS(ON) vs tempera- ture curve, but ∂ = 0.007/°C can be used as an approxima- tion for low voltage MOSFETs. Thus, if TA = 85°C and the available heat sinking has a thermal resistance of 20 °C/W, the MOSFET junction temperature will be 125 °C and ∂ = 0.007(125 – 25) = 0.7. This means that the required RDS(ON) of the MOSFET will be 0.089Ω/1.7 = 0.0523Ω, which can be satisfied by an IRFZ34 manufactured by International Rectifier. Transition losses result from the power dissipated in each MOSFET during the time it is transitioning from off to on, or from on to off. These losses are proportional to (f)(HV)2 and vary from insignificant to being a limiting factor on operating frequency in some high voltage applications. APPLICATIONS INFORMATION Paralleling MOSFETs When the above calculations result in a lower RDS(ON) than is economically feasible with a single MOSFET, two or more MOSFETs can be paralleled. The MOSFETs will inherently share the currents according to their RDS(ON) ratio as long as they are thermally connected (e.g., on a common heat sink). The LT1160 top and bottom drivers can each drive five power MOSFETs in parallel with only a small loss in switching speeds (see Typical Performance Characteristics). A low value resistor (10 Ω to 47Ω) in series with each individual MOSFET gate may be required to “decouple” each MOSFET from its neighbors to prevent high frequency oscillations (consult manufacturer’s rec- ommendations). If gate decoupling resistors are used the corresponding gate feedback pin can be connected to any one of the gates as shown in Figure 1. Driving multiple MOSFETs in parallel may restrict the operating frequency to prevent overdissipation in the LT1160 (see the following Gate Charge and Driver Dissi- pation). Gate Charge and Driver Dissipation A useful indicator of the load presented to the driver by a power MOSFET is the total gate charge QG, which includes the additional charge required by the gate-to-drain swing. QGisusuallyspecifiedforVGS=10VandVDS=0.8VDS(MAX). When the supply current is measured in a switching application, it will be larger than given by the DC electrical characteristics because of the additional supply current associated with sourcing the MOSFET gate charge: II dQ dt dQ dt SUPPLY DC G TOP G BOTTOM =+ + GATE DR GATE FB LT1160 RG* RG* *OPTIONAL 10 Ω 1160 F01 Figure 1. Paralleling MOSFETs |
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