It is necessary to design many different power supplies in order to supply all the active components in the NIBB, like op. amplifiers, current sensors, microcontroller, logic gates and so on. In particular, it is necessary:
1) 5V for the operational amplifiers and all analog circuitry 2) 1.2V for the microcontroller (low ripple required)
3) 3.3V for the microcontroller 4) [9-12] V for the Gate Drivers
In order to design any power supply, it is necessary to take into account the following features:
1) Topology Choice (Buck, Boost, Buck-Boost, Isolated, Non-Isolated)
2) Input Voltage Requirements: both the minimum and maximum voltage must not be exceeded
3) High efficiency: at least 90% in the working condition
4) Output current requirements: the maximum output current should be enough to supply all the components in full load 5) Low ripple
6) Input and Output Capacitor Design
Since the nominal voltage of the battery (12V) is used and all the other voltages are less than 12V, buck converters are employed.
In order to have a more reliable input voltage for each buck converter, an CLC filter (fig 3.11) was designed to reduce furtherly the output ripple.
The frequency of the CLC low pass filter is given by equation(40)
Fig 3.11 CLC Pi Filter
𝑓 = 1
2𝜋√𝐿𝐶
(40)
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From the previous chapter, it was confirmed that in DC-DC converters the noise in the output voltage and currents is mainly given by the switching frequency. Hence it is necessary impose an attenuation of at least -40/50dB at the switching frequencies.
A suitable combination is given by C=10uF and L=4.7uH. The bode diagram is showed in fig 3.11
Fig 3.11 Bode Diagram of PI Filter
Let’s test the filter by imposing to the nominal 12V battery a 1V ripple in LTSPICE. Simulation results are shown in Fig 3.12
Fig 3.13 Simulation Results of a 12V Voltage with 1V ripple(green) compared with the filtered waveform after CLC filter (blue)
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From the previous simulation results it is shown that 1V of ripple is greatly filtered, only less than 200mV remain.
L is a power inductor. Selection criterion of power inductor and capacitors are explained in chapter 2.
A suitable inductor is the SRU1048-R80Y, able to sustain up to 8A.
The full CLC filter in shown in fig 3.14
Since the 12V has reference to the GNDPWR and the 12V filtered to the GNDA instead, this filter decouples the two previous ground mentioned.
Fig 3.14 KiCad CLC Filter used to smooth the output voltage From this point onwards the 12V filtered as input to each buck converter is used instead of the nominal 12V.
5V
The 5V Power supply was designed taking into account that the LMP7715MF-NOBP consumes only 1.15 mA of supply currents.
To be sure to supply all the opamps it is necessary at least 50mA. In addition to the opamps there are the logic gate and the three INA currents sensors.
The P7805-2000-S , suitable for 5V offers:
• 8-36V as Input Range
• 5VOutput Voltage with 75mV Output Ripple
• 2A maximum output current
• 90% efficiency
• Very easy connections since only two capacitors are required
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Fig 3.15 KiCad 5V Power Supply
From Fig 5.5 a typical application circuit suggested by the datasheet is shown. The input capacitance has to be 22uF/50V while the output capacitance must be 22uF/10V
3V and 1.2V
These two voltages must be very precise since they have to be fed to the microcontroller. Another π filter is then required.
Ferrite Beads are employed since the current requirements are not so high as in the previous case ( power inductor not needed). Compared to standard inductors ferrite beads have:
1) High DC resistance ( 100-200 Ω) 2) Less Q
3) Capability to block high frequency noise with less demanding space ( SMD 0603 package quite common on the market)
Let’s test a CLC filter with a ferrite bead on LTSPICE and compare it to the previous filter in fig 3.12
The circuit is shown in fig 3.16, while the simulation results in fig 3.17
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Fig 3.16 LTSPice circuit representing two PI filers. Above, Pi filter designed with standard inductor, while the other designed with ferrite bead.
The BKP1608_HS101-T with 180Ω DC resistance is employed.
Fig 3.17 LTSPice results of circuit in fig 3.16 In blue, the PI filter with the ferrite bead, while in green the PI filter with the standard inductor
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From the simulation results in fig 3.17 it can be verified that the behaviour of the PI filter with the ferrite bead is similar to the one designed with the standard inductor. As expected from the proprieties of the ferrite bead, the quality factor Q is way less than the standard inductor.
Then, for the same reason explained before, the P78003-2000 is employed for the 3V output voltage .The full circuit is showed in fig 3.18
Fig 3.18 3.3V Buck Converter in KiCad with ferrite beads The ferrite beads with the bypass capacitors of 0.1uF in the
microcontroller (fig 4.42) create an CLC filter , filtering the 75mV output of the step-down.
The same applies for the 1.2V. The TSR_1-2412 is chosen and the output voltage is followed by the ferrite bead.( fig 3.19).
Fig 3.19 1.2V Output Step Down in KiCad
85 9V Driver
From the datasheet of the NCP51820 the sink current required is 1A while the minimum power supply is 9V.
Hence the P709-2000 is chosen, since it’s able to provide up to 2A and designed like the previous case.
Fig 3.20 9V Power Supplies Driver
The optimal Voltage would be 10V to be sure that the driver is ON , but due to the lack of devices in the market, 9V are used instead.
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