Design and Implementation of On-board Electrical Power ... - OUFTI-1
Design and Implementation of On-board Electrical Power ... - OUFTI-1
Design and Implementation of On-board Electrical Power ... - OUFTI-1
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2. The switch is included.3. High expected efficiency (85% to 90%).4. <strong>Power</strong> PAD <strong>and</strong> thermal shutdown (the power PAD is a metallic area under the IC,that is soldered on the PCB for a better power dissipation).5. There is a s<strong>of</strong>t-start (this avoids a peak current call when the converter starts).The choice <strong>of</strong> the control IC mainly depends on the properties <strong>of</strong> its internal switch. Thisswitch must be able to:1. withst<strong>and</strong> a current equal to I max (when the switch is closed, the current is the sameas in the inductor).2. withst<strong>and</strong> a voltage <strong>of</strong> V out + V diode .3. dissipate the heat due to switching losses <strong>and</strong> ohmic resistance.This is verified below.Maximum current in the switchThe maximum current will be determined during the inductor design. The switch is ableto withst<strong>and</strong> 3.2A.Maximum voltage on the switchThe TPS61087 can be used for an output voltage up to 18.5V.Heat dissipationThe first version <strong>of</strong> the converter was designed with a slightly different model <strong>of</strong> control IC:the TPS61085 [3]. The internal circuitries <strong>of</strong> the TPS61087 <strong>and</strong> the TPS61085 are practicallyidentical but the package (or case) is different. The TPS61085 has a 8-pin TSSOP packagewhile the TPS61087 has a 10-pin QFN package.With an output able to deliver 420mA at 7.2V, the 7.2V converter deals with quite importantamounts <strong>of</strong> power for its size. An important part <strong>of</strong> the losses in the converter are localizedin the semiconductor devices. For V in = 3V <strong>and</strong> maximum load, the losses in the wholeconverter were measured at 550mW (first prototype) [1]. The losses in the diode alone caneasily be estimated once the average current in the inductor is known, i.e. P = I av V diode,2 D ′(where Iav is the average current in the inductor, V diode,2 is the voltage across the diode whenit is forward biased, <strong>and</strong> D’ is the complement <strong>of</strong> the duty cycle). For more precision, V diode (t)was measured <strong>and</strong> integrated to a Matlab code written to compute the ohmic diode losses.The above formula for P gives an ohmic loss <strong>of</strong> 225mW, which represents about 40% <strong>of</strong> thetotal losses. There are still about 330mW that are potentially dissipated in the control IC.57
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