path: root/analog/alim/alim_design.txt
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authorPierre Prot2008-02-01 00:17:07 +0100
committerPierre Prot2008-02-01 00:17:07 +0100
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tree61dcee0d69c9196ae44a9c8e3b7adab7d8784e13 /analog/alim/alim_design.txt
parent730ec2b971458a86c7a2e90901159eaf7b57601d (diff)
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+The power supply board aims at :
+- Generating 5V power for electronics in the robot
+- Generating 5V power for servo motors (separate 5V to avoid perturbations of
+ the electronics by servomotors)
+- The power is taken on a battery (between 8V and 30V)
+- An external supply (Nestor) can be plugged on the board for charging the
+ battery while powering the robot. When Nestor voltage is present, the board
+ shall switch to it automatically.
+- A voltmeter shall give a visual and quickly understandable information on
+ battery state
+It is composed of the following elements :
+- 5V bucks
+- a switch controled by a switching circuitry for input power selection (Nestor
+ or Battery)
+- a SEPIC controler for battery charge
+- a 4-leds voltmeter
+*** 5V bucks ***
+- the bucks used are LM2678. They are compliant with the fllowing requierements :
+ * Input voltage : 8V to 40V.
+ * 120mOhm switcher allow up to 92% efficiency up to 5A
+ * 260 kHz operation
+ * simple design
+ * good power design, allow easy dissipation
+ * package handcratfly solderable
+- dc converting from 12V to 5V@4A allows a power dissipation of only 1.5W in
+ the buck component (see calculs table)
+- the feedback is made of 1% resistors which allows an overall precsion of
+ +/-3%. Good isn't it ?
+- a few capacitors are placed into the feedback loop as a provision for
+ stability improvement if needed. Normally, they shoudn't be needed, but only
+ tests will validate that fact.
+- according to methodology explained in the datasheet, we use a 22µH inductor.
+ It can stand a continuous 4A current and have a 60mOhm resistance. at 4A,
+ they will dissipate around 0.8W.
+- the output cap is 47µH, but shall probably be upgraded to 100µF
+- input cap is 10µF/50V. It is the most critical capacitor, because it shall
+ stand great dI/dT, so a ceramic type is chosen for it aibility to react very
+ well to high frequencies
+- we use 4 40V/1A schottky diodes in parallel for cost reasons and dissipation.
+ I guess a single well-sized diode would have been better for efficiency, but
+ global cost would be higher and dissipation problems would not be kind. They
+ are expected to dissipate the most when input voltage is the highest. At 25V,
+ 4A in output, they should stand a 0.8A current each, which will produce
+ something like a 0.6V forward voltage. Thus, they would dissipate around 0.5W
+ each.
+*** Power input ***
+- battery input as well as nestor input are protected against overvoltage and
+ reverse pluggin with a transil coupled to a polyswitch.
+ - In case of a default, the transil is passing and the polyswitch is expected
+ to blow off (it as capable to rearm itself).
+ - Anyway, in the case where the transil is stressed before it breaks, it may
+ (seldom) blow as an open circuit : in that case, if the polyswitch has not
+ be blown (which is supposed to be the case, otherwise, the transil doesn't
+ breaks), and the overvoltage will apply on the supply rail. So => avoid
+ problems : don't try to apply overvoltages
+- when a voltage is present on nestor input, it enlightens LED_NES_PRE
+- the power selection is achieved by a relay driven by the selection circuitry.
+ The relay switches either V_nestor, nor V_battery on the signal V_alim.
+- an input tank capacitor is implemented. We used 2 aluminium 470µF/63V
+ electrolytic capacitors which can be upgraded to 1000µF each for a little
+ overcost.
+ - For max consumption (40W on each output), they allow approx 0.5ms which is
+ clearly not enough for saving power during the relay switch time (estimated
+ to 5ms).
+ - For a 1A output current on 5V_dirty, we suppose anyway that the transparecy
+ time given by these tank capacitors will be long enough for ensuring no
+ voltage drop to output
+- the tank capacitors are connected to V_alim through a soft start circuitry.
+ - It aims at preventing from harmfull surge charging current which may happen
+ on power-on.
+ - This is done by a MOS whose gate is driven at off state by a capacitor.
+ - When power is quickly applied, the capacitor is slowly charged. When gate
+ voltages reaches linearity zone, the current begins to flow and charge
+ smoothly the tank capacitor.
+ - After the gate capacitor is fully charged, the soft-start circuitry looks
+ for the tank like a very small resistor.
+*** Power selection control ***
+- this circuitry checks nestor voltage to decide wheather the board is supplied
+ by nestor or by the battery
+- It is power by an independant 5V linear regulator powered by V_nestor. It is
+ expected to be +/-2% precise and produces the +5V voltage.
+- A comparators checks V_nestor to a reference derived from +5V. The threshold
+ is set 11.0V for 12V operation and 22.0V for 24V operation. The time constant
+ is set between 0.5 and 0.8ms.
+- upon the comparator trips low (nestor drops under the threshold), it shorts
+ to GND a timer capacitor, which cuts the relay command. Thus, the relay
+ releases and selects V_bat for powering the board
+- when nestor goes up, the comparator's output becomes high impedance (through
+ the diode), the timer capacitor can charge smoothly through R14 resistor.
+ When it reached the thresholds of the buffer gate, the relay switches on and
+ selects V_nestor as power supply for the board.
+- the timer allows thus :
+ - a short switching time to battery when V_nestor drops
+ - a long switching time to nestor when nestor arrives. This is done to be
+ sure nestor voltage is valid for sufficient time to be really present when
+ we switch it.
+- the led LED_NES_ACT is enlightened when nestor powering is activated
+- for avoiding the linear regulator to heat too much, the relay is powered by
+ when P5V_NUM when it is present. This is done by closing a P-MOSFET with
+ P5V_NUM voltage. If this voltage is absent (in case of a startup on nestor
+ without a battery), the P-MOS allows +5V to power the relay solenoïd until
+ nestor powers on the board and P5V_NUM takes the hand to poer the relay.
+- the diode D16 is used to protect the regulator from power arrival via its
+ output. Indeed, when V_nestor is not present, P5V_NUM takes back to +5V
+ trough the P-MOS body diode and could break the regulator if it's input was
+ 0V.
+*** Led voltmeter ***
+- The led voltmeter checks battery voltage. It is implemented around 4
+ comparators packaged in the MAX969.
+- Battery voltage is fed to a divider bridge with a filtering capacitor, and
+ then fed to the 4 comparators.
+- Each comparator has a different voltage reference. The references are
+ generated by a multiple divider bridge powered by the 1.2V reference included
+ into the MAX969.
+- An hysteresis is set to 1% of battery voltage
+- The thresholds are calculated with steps of about 4% of battery range
+- See calculs.xls for detailed threshold calculation.
+*** Charge controler ***
+- The battery charge control is ensured by a SEPIC converter. This topology
+ allow DC conversion from an input voltage hich can be either greater or lower
+ than the output voltage. It is opposed to bucks (step-down) and boost
+ (step-up) topology, and similar to flyback, but with simplified switching (no
+ snubber problems)
+- We use this converter from 10V to 30V input to a 10V to 28V current-limited
+ output.
+- Normally, SEPIC are built with a boost-type controler. For cost and
+ availability reasons (it's been hard to find a boost regulator matching our
+ requierments), we use a buck controler which is a bit modified.
+ - The normal loopback reaction of a buck is acted on the high-side MOS : the
+ lower the output voltage (sensed by the feedback pin), the higher the 'ON'
+ duty cycle on the MOS (that will result as a growth in inductor current
+ which will compensate the fall of output voltage).
+ - In a SEPIC (as in a boost or a flyback), the MOS is low-side (source
+ conected to GND, inductor on the drain). It shall react as the high-side
+ MOS of a buck ('ON' duty cycle augmentation upon output voltage fall).
+ - For availability reasons (we didn't find boost controlers with high enough
+ output voltage, high enough current, small enough price and package), we
+ will then use a Buck controler with external MOS, and we will use the
+ high-side gate drive output for driving our SEPIC low-side MOS.
+ - The MOS is sized according to the following constraints :
+ - it shall stand a Vds equal to input voltage + output voltage (due to the
+ fact that the transformer's primary voltage is always equal to
+ transformer's secondary voltage). Takin a few margin, if we want 30V
+ input and 30V output, we need at least a 60V MOS. For security, we take a
+ 100V.
+ - it shall have the lowest Rdson. The MOS chosen has a 30mOhm Rdson, which
+ will result in a 30mW dissipation at 1A charge current
+ - solderability (handcraft), small package, price.
+- The unloaded output voltage is limited to the final charging voltage fitted
+ to the battery : we have chosen 14V (28V for a 28V battery). This voltage is
+ set by a divider bridge, as the "normal" working mode of the buck controler
+ implies. See calcul.xls for detailed calculation.
+- The current is limited by a current sense sending proportional positive
+ feedback to the buck controler upon overcurrent condition.
+ - a shunt resistor accumulates the current into the transformer secondary.
+ This current is exactly equal (in mean value) to battery charge current.
+ V_shunt is the voltage accumulated on the shunt resistor ; it is negative
+ (below GND).
+ - a low-pass filter is used to get more or less the mean value of V_shunt :
+ V_shunt_f
+ - a negative amplifier amplifies V_shunt_f relatively to a reference voltage
+ of 100mV. The gain is about 3. It's output is equal to : V_out=(|V_shunt_f|
+ + 100mV) * 3 + 100mV
+ - a diode is inserted at the right output of this amplifier. Then, when
+ amplifier's output is intended to be lower than V_fb=0.7V, it is inactive
+ on buck's feedback. It it is intended to be greater than V_fb, the diode is
+ passing and the positive feedback action makes the controler to lower duty
+ cycle, reducing inductor current. The diode is inserted in the feedback
+ loop of the amplifier so that when it works in linear region, the diode's
+ drop is compensated.
+ - the threshold voltage on V_shunt_f over which the limitation occurs is
+ equal to : V_shunt_f_th=(0.7 - 0.1) / 3 - 0.1
+ - for a 0.22 ohm resistor, it represents a charging current of about 400mA.
+- See calcul.xls table for detailed (and maybe up-to-date) calculus and values
+- The SEPIC is compensated (which means "the system frequency response is
+ corrected for ensuring the closed-loop stability) by a few capacitors in the
+ feedback loop. These caps are defined by nose metrics and a kind of black
+ magics that I don't master enough for explaining it in a comprehensive way.
+ It may work properly... or not, we will see upon real testsand after the help
+ of simulation