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Gibbot Board v4.1 Charge Test Board

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ajgriesemer edited this page Apr 4, 2014 · 99 revisions

Because of the complexity of the battery charger and management circuitry the design was implemented iteratively. For the first iteration of the design we are designing a battery charger circuit that is flexible to be able to charge 1-8 Li-ion cells in series. To allow for this flexibility a larger 1206 surface mount resistors package is used for ease of replacement.

##Controller The LTC4000 is a charge controller that interfaces with a DC/DC Buck Converter to regulate the voltage input to the main board and the charging circuit. The controller changes the voltage level coming out of the buck converter based on the following three conditions:

  • The main board and the magnets must have a high enough input voltage and current
  • The batteries have appropriate current and voltage during the charge cycle
  • The batteries do not charge if temperature is not in a safe range

###Operation The LTC4000 battery charger has 4 regulation loops for

  • input current control
  • battery current control when the battery is charging and trickle charging
  • battery voltage control
  • output voltage control.

Threshold values for each of these regulation loops are set using multiple resistor divider networks.

####Input Current The input current regulation loop ensures that battery charger current plus the main board load current do not exceed a given value. During normal operation the charge circuit will draw 4A and the load will draw 1A max so the input current limit was set to 5A. This regulation loop serves as an added layer of safety and should not control the voltage level during normal conditions.

The input current limit is set by a sense resistor between the IN and CLN pins and a resistor between pin IL and GND. For this board the sense resistor used is a 5mOhm, 1% accuracy, 3W, metal element sense resistor. A maximum input current of 5A is then set using a 10k resistor between pin IL and GND according to the formula below from the "Input Current Limit Setting and Monitoring" section on page 16 of the LTC4000 datasheet

\mathrm{R_{IL} = \frac{I_{LIM} \cdot R_{IS}}{2.5uA}} = \frac{5A\cdot5mOhm}{2.56uA}=10k

The datasheet recommends a 10nF filter capacitor between IIMON and GND. The voltage drop across the current sensor can also be read as an output on the IIMON pin. This output is connected to a through-hole pad on this iteration of the board and should be connected to an ADC pin of the PIC in future iterations.

####Charge Current The charge current loop monitors the current flowing into the batteries. During the constant current phase of charging this regulation loop will control the voltage level on the DC/DC converter.

The charge current limit is set by a sense resistor between the CSP and CSN pins and a resistor between pin CL and GND. The sense resistor used is a 5mOhm, 1% accuracy, 3W, metal element sense resistor. A maximum charge current of 4A is then set using a 8k resistor between pin CL and GND according to the formula below from the "Charge Current Limit Setting and Monitoring" section on page 17 of the LTC4000 datasheet.

R_{CL} = \frac{I_{CLIM} \cdot R_{CS}}{2.5uA} =\frac{4A\cdot5mOhm}{2.5uA}=8k

The datasheet recommends a 10nF filter capacitor between IBMON and GND. The voltage drop across the current sensor can also be read as an output on the IBMON pin. This output is connected to a through-hole pad on this iteration of the board and should be connected to an ADC pin of the PIC in future iterations.

####Battery Voltage The battery voltage loop monitors the voltage applied across the batteries. After the constant current phase of charging has charged the batteries up to the float voltage of 3.6V per cell the charging will move into constant voltage mode and the voltage loop will take control of the voltage level on the DC/DC converter.

For 18650 LiFePO4 batteries the voltage float level is 3.6 times the number of cells in series. The float voltage limit is set by a resistor divider from BAT to FBG with the junction connected to BFB. R_BFB2, the resistor between FBG and BFB, will have the datasheet recommended value of 133k. The value of R_BFB1 is set using the formula

R_{BFB1} = \left ( \frac{V_{FLOAT}}{1.135} - 1 \right ) \cdot 133kOhm

Cells V_FLOAT R_BFB1
1 3.6 289k
2 7.2 711k
3 10.8 1.13M
4 14.4 1.55M
5 18.0 1.98M
6 21.6 2.40M
7 25.2 2.82M
8 28.8 3.24M
9 32.4 3.66M
10 36.0 4.09M

####Output Voltage

When the battery is not charging the output voltage loop controls the voltage from the DC/DC buck converter. The output voltage needs to be at least 105% of the float voltage. Setting the output voltage also sets the "instant on" output voltage. If the output voltage drops below the instant on voltage (because of a heavily discharged or shorted battery) the controller supplements the battery output voltage using the buck converter to ensure the output voltage remains constant.

The output voltage limit is set by a resistor divider from CSP to FBG with the junction connected to OFB. R_OFB2, the resistor between FBG and OFB, will have the datasheet recommended value of 127k. The value of R_OFB1 is set using the formula

R_{OFB1} = \left ( \frac{V_{OUT}}{1.193} - 1 \right) \cdot 127kOhm

Cells V_OUT R_OFB1
1 3.8 275k
2 7.6 677k
3 11.3 1.08M
4 15.1 1.48M
5 18.9 1.88M
6 22.7 2.29M
7 26.5 2.69M
8 30.2 3.09M
9 34.0 3.49M
10 37.8 3.90M

####Test Setup

Cells R_BFB1 R_OFB1 V_{in} V_{float} V_inston
1 3.8 275k
2 7.6 677k
3 11.3 1.08M
4 15.1 1.48M
5 18.9 1.88M
6 22.7 2.29M
7 26.5 2.69M
8 30.2 3.09M
9 34.0 3.49M
10 37.8 3.90M

Vin is the minimum Vin required to charge the circuit. It is calculated from the LT1074 datasheet based on the equation for duty cycle and the limitation that the maximum duty cycle is 90%. ![V_{IN} = \frac{V_O + V_f}{90%} + V_{SW}]

Where: ![V_{SW}] = Switching voltage of LT1074 ~ 2V ![V_{f}] = Diode forward voltage = 0.48 for PDS760

####Input Voltage The input voltage pin VM measures the input voltage through a voltage divider. If the input voltage level drops below a given threshold the controller will pull the RST pin low. If this pin were connected to the EN of a buck converter it disable the buck converter and consequently cutting power to the batteries and VOUT. An LED was connected to the RST pin so that instead of shutting down the buck converter, the LED provides a visual indicator if the voltage is too low. The voltage is set by a voltage divider from the input voltage source to GND with VM connected at the junction. The value of R2, the resistor from VM to GND is set to 100k according to the datasheet. ![R_{VM1} = \frac{R_VM1 \cdot 1.193V}{V_{MIN}-1.193V}](Equation maker not working)

####MOSFETs Two P-Channel MOSFETs control the flow of current from the input and to the batteries. A MOSFET between IID and CSP controls the flow of power from the input source. A MOSFET between CSN and BAT controls the power flowing in and out of the batteries.

Input Power Control The parameters defining the MOSFET choice are:

  • The maximum drain to source voltage is 103.61.05 = 37.8V
  • The drain to source current is 5A
  • An acceptable temperature rise is 30 C during operation

We chose Vishay's SUM110P06-07L which can tolerate 60V D-S. If V_GS = 4.5V, R_DS(on) = 8.8mOhm at ambient temperature. When the MOSFET is passing 5A it will be dissipating 0.22W. With an \mathrm{R_{\Theta JA} } of 40 C/W the junction temperature would only rise by about 9 C.

Battery Power Control The BGATE MOSFET has slightly looser restrictions than the IGATE MOSFET:

  • The maximum drain to source voltage is 10*3.6 = 36V
  • The drain to source current is 4A

However, to simplify the design, the same SUM110P06-07L MOSFET will be used in both locations. This could be reconsidered in future designs to save space--move from a D2PAK to a DPAK MOSFET--or to reduce the RDS_ON on the BGATE MOSFET.

####Temperature Monitor The charge operating range for the batteries is from -10C to 70C according to the Tenergy 18650 datasheet. The resistor is connected between pins NTC and GND. Using a 5% 10k NTC thermister from Vishay the resistance value at -10C is 55k and at 70C is 1.75k. A 1% resistor R1 in series with the NTC thermister reduces the effect of the change in resistance and a 1% resistor R2 between BIAS and NTC completes the resistor divider. According to the "Battery Temperature Qualified Charging" section of the LTC4000 datasheet on p22:

R_3 = \frac{R_{NTC cold}- R_{NTC hot}}{2.461}= \frac{55k - 1.75k}{2.461} = 21.6k

R_D = 0.219 \cdot R_{NTC cold} - 1.219 \cdot R_{NTC hot} = 0.219 \cdot 55k - 1.219 \cdot 1.75k = 9.91k

####Charge Termination The charger is set to finish charging when, in constant voltage mode, the charge current drops below the C/X value. This is setup by tying TMR to BIAS to disable the timer functionality. To set the charge termination value to 0.5A with a resistor between CX and GND. The value of the resistor is calculated using the following formula using R_{CS} = 5mOhm from above.

R_{CX} = \frac{I_CX \cdot R_CS + 0.5mV}{0.25 uA} = \frac{0.5A \cdot 5mOhm + 0.5mV}{0.25uA} = 12k

##Input Buck Converter The input DC/DC buck converter uses the LT1074 from the Gibbot v3 board. The specs for the buck converter circuit are:

  • Max input voltage: 43V
  • Max throughput current: 5A
  • Output voltage range: 3.8V - 37.8

The minimum input voltage provided to the buck converter to produce a certain voltage for the battery charger is given by the following equation derived from equation 1 in the LT1074 Design Manual:

\mathrm{V_{IN} =\frac{ V_{OUT} - V_{f}}{DC_{MAX} }+ V_{SW}}

where

  • V_f = Diode forward voltage, 0.5V for PDS760
  • V_SW ~= 2V
  • V_OUT is the voltage required by the charger, which is V_FLOAT
  • DC_MAX is the maximum duty cycle of the buck converter which is 90-93% according to the LT1074 datasheet

for example, in the case where 10 x 18650 cells are used V_FLOAT = 37.8V from the table above

\mathrm{V_{IN} =\frac{ 37.8V - 0.5V}{0.9 }+2V} = 43.4V

Component Selection

####Inductor The minimum inductance required is calculated by the following formula:

L_{MIN} = \frac{V_{IN}\cdot ((V_{IN}-V_{SW})-V_{OUT})}{2\cdot f\cdot (V_{IN}-V_{SW})\cdot (I_M - I_{OUT})}

where:

  • V_SW ~= 2V
  • f = switching frequency, 100kHz typical
  • I_M = max current, 5.5A for LT1074

L_{MIN} = \frac{37.8V\cdot ((48V-2V)-37.8V)}{2\cdot 100,000Hz\cdot (48V-2V)\cdot (5.5A - 5A)} = 67 uH

The RMS current is equal to the output current, the peak current is equal to

I_P = I_O + \frac{V_O (V_I - V_O)}{2\cdot L\cdot f\cdot V_I} = 5A + \frac{37.8V(48V - 37.8V)}{2\cdot 68uH\cdot 100kH\cdot 48V} = 5.59A

An inductor that fits these criteria is the Bourns 2200LL-820-H-RC

The formula given in the design manual for calculating core loss in an inductor requires information on the inductor core material that is not provided in the inductor's datasheet.

####Capacitors The capacitors is selected based only on the rated voltage and the maximum ripple current (which depends on ESR). According to the LT1074 Design Manual, the RMS current through the capacitor for a desired output current can be calculated as follows: I_{AC,RMS} = I_{OUT}\sqrt{\frac{V_{OUT}(V_{IN}-V_{OUT})}{(V_{IN})^{2}}}

In the worst case scenario where Vin = 2*Vout, I_{AC,RMS} = I_{OUT} so the RMS current rating of the capacitor must be greater than or equal to the output current (5A). The capacitor maximum rated voltage must also be higher than the maximum input voltage (43V).

For a single capacitor that meets these specs the size increases dramatically and the seated height of any capacitors available is around 1". This is more than the 0.8" we have allotted. Instead, a bank of multiple capacitors in parallel will be used to distribute the ripple current load.

A bank of 2 x PCV1H680MCL1GS 50V, 2.6A capacitors will be used for both input and output smoothing.

Power Dissipation Power dissipation in the capacitor will be P = (I_{AC,RMS})^2 \cdot ESR The ESR of the PCV1H680MCL1GS is 29mOhms so the power dissipated in each capacitor will be:

P = (2.5)^2 \cdot 0.029 = 0.18W

####Output catch diode

  • Maximum reverse voltage must be >= Vin Max
  • Diode rated current should be 1.5 to 2 times output current according to Output Catch Diode section on page 19 of the LT1074 Design Manual. The output current value used for this calculation is the 5A continuous current needed for charging.

We will use PDS760 based on the following specs:

  • DC reverse voltage = 60V
  • Maximum Rated Current = 7A
  • Forward voltage at 5A ~= .5V

Power Dissipation At 5A, assuming a diode forward voltage drop of 0.5V, the continuous power dissipation is 2.5W. Assuming a thermal coefficient from the junction to the case (\mathrm{R_{\Theta}_{JC}}) of 2 °C/W

\mathrm{T=R_{\Theta}_{JC}\cdot P}

\mathrm{T=2 \frac{\circ C}{W}\cdot 2.5 W}

the temperature difference between the junction and the case will be about 5°C. The thermal coefficient from the junction to ambient air is not given. However, current will only be applied with a duty cycle of (Vin-Vout)/Vin. Heat sinking will probably be necessary.

####Resistor Divider R2 in the resistor divider network is set to 2.21k, the value of R1 is determined from the equation below derived from equation 19 in the LT1074 Design Manual

R_1 = (V_{OUT} - V_{REF}) k\Omega

V_{OUT} should be higher than the maximum V_{OUT} required by the charge controller.

###Board Issues

  • The anode of the LEDs connected to CHRG and FLT should be pulled up to a higher voltage input, they are currently connected to GND.
  • The reset pin should be connected to an LED, through a resistor, with the anode of the LED connected to a higher voltage input.

###Last Second Additions The board has a set of test footprints for a few different varieties of JST board-to-wire connectors. The connectors on the board are:

The board also has a simple circuit for powering the LEDs on the steel wall.

http://www.codecogs.com/latex/eqneditor.php

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