Datasheet LT1160, LT1162 (Analog Devices) - 9

LT1160/LT1162
U U W U APPLICATIONS INFORMATION Power MOSFET Selection Paralleling MOSFETs
Since the LT1160 (or 1/2 LT1162) inherently protects the When the above calculations result in a lower RDS(ON) than top and bottom MOSFETs from simultaneous conduction, is economically feasible with a single MOSFET, two or there are no size or matching constraints. Therefore selec- more MOSFETs can be paralleled. The MOSFETs will tion can be made based on the operating voltage and inherently share the currents according to their RDS(ON) RDS(ON) requirements. The MOSFET BVDSS should be ratio as long as they are thermally connected (e.g., on a greater than the HV and should be increased to approxi- common heat sink). The LT1160 top and bottom drivers mately (2)(HV) in harsh environments with frequent fault can each drive five power MOSFETs in parallel with only a conditions. For the LT1160 maximum operating HV supply small loss in switching speeds (see Typical Performance of 60V, the MOSFET BVDSS should be from 60V to 100V. Characteristics). A low value resistor (10Ω to 47Ω) in The MOSFET R series with each individual MOSFET gate may be required DS(ON) is specified at TJ = 25°C and is generally chosen based on the operating efficiency re- to “decouple” each MOSFET from its neighbors to prevent quired as long as the maximum MOSFET junction tem- high frequency oscillations (consult manufacturer’s rec- perature is not exceeded. The dissipation while each ommendations). If gate decoupling resistors are used the MOSFET is on is given by: corresponding gate feedback pin can be connected to any one of the gates as shown in Figure 1. P = D(IDS)2(1+∂)RDS(ON) Driving multiple MOSFETs in parallel may restrict the Where D is the duty cycle and ∂ is the increase in RDS(ON) operating frequency to prevent overdissipation in the at the anticipated MOSFET junction temperature. From this LT1160 (see the following Gate Charge and Driver Dissi- equation the required RDS(ON) can be derived: pation). P RDS ON ( ) = GATE DR 2 D I ( ) (1 ∂) DS + LT1160 RG* RG* For example, if the MOSFET loss is to be limited to 2W GATE FB when operating at 5A and a 90% duty cycle, the required R *OPTIONAL 10Ω DS(ON) would be 0.089Ω/(1 + ∂). (1 + ∂) is given for each 1160 F01 MOSFET in the form of a normalized RDS(ON) vs tempera-
Figure 1. Paralleling MOSFETs
ture curve, but ∂ = 0.007/°C can be used as an approxima- tion for low voltage MOSFETs. Thus, if TA = 85°C and the
Gate Charge and Driver Dissipation
available heat sinking has a thermal resistance of 20°C/W, the MOSFET junction temperature will be 125°C and A useful indicator of the load presented to the driver by a ∂ = 0.007(125 – 25) = 0.7. This means that the required power MOSFET is the total gate charge QG, which includes R the additional charge required by the gate-to-drain swing. DS(ON) of the MOSFET will be 0.089Ω/1.7 = 0.0523Ω, which can be satisfied by an IRFZ34 manufactured by QG is usually specified for VGS = 10V and VDS = 0.8VDS(MAX). International Rectifier. When the supply current is measured in a switching application, it will be larger than given by the DC electrical Transition losses result from the power dissipated in each characteristics because of the additional supply current MOSFET during the time it is transitioning from off to on, associated with sourcing the MOSFET gate charge: or from on to off. These losses are proportional to (f)(HV)2 and vary from insignificant to being a limiting factor on ⎛ dQ ⎞ ⎛ dQ ⎞ operating frequency in some high voltage applications. I = I G G SUPPLY DC + ⎝⎜ dt ⎠⎟ + ⎝⎜ dt ⎠⎟ TOP BOTTOM 11602fb 9