QwikiCharger

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Coming Soon - Tired of RF noise from those pesky switching power supplies? Don't feel like shelling out the big bucks for a Linear? Have high peak current requiremenst? Look no further! Our handy-dandy-one-of-a-kind intermittent charger circuit is just the answer for you! Read on to learn how the magic happens.

While off the shelf power supplies are more than adequate for many purposes, from time to time they fall flat... er, not flat, as the case may be. In our case, we require an extremely quiet, high voltage (100V), high peak current (10A-15A) intermittent power supply. The 60kHz RF noise put out by our switching power supplies was larger than we could accept, and linear power supplies are expensive don't always have stellar dynamic response. The solution: batteries.

This article describes what it takes to put together a battery power system including the charge circuitry, beginning with a quick introduction to Sealed Lead-Acid (SLA) batteries followed by a detailed discussion of the charge circuitry. The punch line: the battery system in it's final version will provide ~100V with ~70A peak current (although we don't go near this!) with a total capacity of 7Ah. Read on if you are interested.

1 Operating Instructions
- 1.1 The Relay Box
- 1.2 The Charger Box
2 Charging Sealed Lead-Acid Batteries
3 The Charge Circuit
- 3.1 Theory of Operation
- 3.2 Design Equations
4 Relay Box
5 Files
6 References

Operating Instructions

The battery power supply system is divided into two boxes, a switching box and a charging box, and the two are functionally independent and can be used separately when needed. The charger is fully automatic and so no adjustment need be made once the internal voltage dividers have been set.

The Relay Box

The relay box is used to switch the batteries between the charger and the power out terminals. The relay box is controlled either manually by the two front panel switches or through the TTL control. When the TTL is high, the batteries are connected to the charger outputs, and when the TTL is low or not connected, the power outputs are energized. The switches override the TTL input. The charge switch has priority in that if both switches are in the manual override position, the relay box will connect the batteries to the charger outputs.

The relay box is responsible for setting the polarization of the batteries correctly for the mode that is being used. The negative terminal of the positive battery pack is always grounded (to the case and the earth connection) while the negative terminal of the negative pack is grounded during charging, and the positive terminal is grounded during power output. The relay box requires +15V to operate the relays.

The Charger Box

The charger box requires 60V @3.2A input, and the negative input should be at earth ground when connected to the relay box as the relay box ground and charger box ground will be connected when the two are connected. Either one or both of the charger channels can be connected, either to the relay box or directly to the battery packs (the cable from the battery packs will fit either box).

Charging Sealed Lead-Acid Batteries

There is a lot of information to be found on how to charge lead acid batteries, so I won't go into great detail here. A great source of technical information can be found in the Sealed Lead Acid Handbook. The methods document is particularly useful in our case. A bit of nomenclature: the capacity of a SLA is defined by C, that is, a 7Ah battery has C=7. While the units don't work out, we will refer to currents (both charging and discharging) as fractions of C. Thus a discharge current of C/7 is 1A for a 7Ah battery. We have chosen to work with 7Ah 12V batteries and so numbers will be given both in terms of the battery capacity and in parentheses for our specific setup.parentheses

As with any electrically recharged battery, (see flow battery) the SLAB is restored by forcing an current through the battery in the reverse direction. This is accomplished by applying a voltage across the battery that is greater than the chemical cell potential. In the case of a SLA the cell potential is ~2.1V. The lifetime of the batteries is highly dependent on the level and timing of the charging potentials applied. Above a cell potential of ~2.4V, gas will form at the electrodes which will cause the batteries to deteriorate if the potential is maintained for a long period. SLA's are internally vented and will not deteriorate if these voltages are reached for short periods of time. Open lead acid batteries will release this gas (Hydrogen) depleting the electrolyte in the process, and thus water must be added to these batteries from time to time to maintain the capacity. The battery can be held indefinitely at 2.28 V/cell- trickle charging- which will maintain the battery at 100% capacity without degrading it in any way.

We adopt a variation of the two step charge cycle which is recommended to charge the batteries as rapidly as possible without reducing the operating life time. The cycle begins by charging the battery at constant current $I m a x$ to a fixed overcharge voltage, $V 12 =$ 2.375V (14.25V/battery). The charger then switches to hold mode, where the charger will provide a current of at least $I H$ and will keep the voltage at or above $V F = 13.6$ V, providing up to $I m a x$ to do so. If the battery draws more current than $I m a x$ and the voltage drops below $.9 V F$ , the charger returns to the initial rapid charge state. Typical currents in the initial charge phase range from C/5 to C/3.3, but it is a good idea to glance at the battery data sheet. We have chosen C/4.2 (1.66A). The holding current needs to be small so as not to overcharge the battery. We chose C/600 (~12mA). All of the set points are temperature dependent(see below). The following is a typical charge cycle:

Rapid charge: Current $I m a x$ charges the battery until $V 12$ is reached
Holding: Initially current $I H$ is applied until the battery voltage drops to $V F$ where the voltage is pinned by a current up to $I m a x$ . The current will drop as the battery charges until $I H$ is reached at which point the voltage will float to maintain the current $I H$ .

The Charge Circuit

The charge circuit.

The internal block diagram of the UC3906, from the datasheet.

It is surprisingly straightforward to build a circuit to accomplish the two-mode charge described in the previous section, thanks to the UC3906 SLA charger IC. All that it takes to get up and running is a voltage sense divider, a current sense resistor, a pass transistor (pair), and hold current circuit, and a few additional resistors and capacitors... easy as $π$ !

Theory of Operation

The charge circuit is divided into tree functional groups, the hold current circuit, the pass transistor, and the voltage sense network. The UC3906 provides most of the active components and the control logic to switch between charging modes and also provides a temperature compensated voltage reference to ensure that the voltage set-points properly track the battery temperature, provided that the charge IC and battery are maintained at approximately the same temperature. The reference voltage $V R E F$ is 2.3V at 23C and has a temperature coefficient of -3.5mV/C.

Hold current circuit

The simplest block is the hold current circuit. This circuit provides current $I H$ at all times provided that the input-battery voltage difference is sufficiently large. Initially this current was provided by a circuit involving the current sense amplifier on the UC3906, however, this was found to kill the IC, often rather spectacularly. (While the current through the chip was within spec, I believe that the total power was not, although no total power figure is given in the datasheet!) As a result, the hold current is provided by the simple current source consisting of $Q 3$ , $R H o l d$ , $R e$ , and $D 1$ . The circuit operates as follows: $Q 3$ will conduct in steady state when the is 1.2V across the base emitter junction ( $Q 3$ is an epitaxial transistor, hence the large drop). The potentiometer $R H o l d$ is used to set the base voltage, $Q 3$ will then turn on and conduct current through $R e$ to establish the 1.2V drop. The diode is in place to ensure that the battery never drives current into the network. While the network is fairly robust, it is sensitive to the input voltage. The output voltage will fluctuate by 100 $μ$ A per volt of input voltage fluctuation, and so should not be a serious concern provided that the input voltage is around 60V. A total voltage difference of 4.2V between the supply and the battery is required to source this current.

Pass transistor circuit

The pass transistor circuit provides the main charging current during the initial rapid charge phase and provides current above $I H$ when necessary to hold the battery voltage to $V F$ . This circuit consists of four elements, $Q 1, Q 2, R s m$ , and $R s r c$ . During the rapid charge state, the voltage amplifier tries to provide maximal base drive to the pass transistor pair. $R s m$ , the current sense resistor, provides feedback to the main pass transistor pair through the current limit amplifier, which maintains 250mV across the pass transistor by shunding the base drive of the internal transistor. In the holding mode, feedback is provided only throught the voltage sense circuit to drive the base of the internal transistor to maintain $V F$ .

A pair of pass transistors is used because the internal transistor can only sink 25mA of current, which is insufficient base drive for 1.6A of pass current. The PNP 2N3906 has been included to provide the required base drive. The second pass transistor limits the minimum power supply-battery voltage difference to 1V because of the .7V base emitter drop on Q1 and the 25mV across the sense transistor. The resistor $R s r c$ is needed when working with high voltages to minimize dissipation on the IC.

Voltage sense network

The voltage sense network serves two purposes, the first is the set the switch-points between the two modes of operation, and the second is to provide feedback through the voltage sense input. The on-board sense comparator is responsible for switching between the charging modes. One input to the sense comparator is the voltage sense input pin and the other some multiplier of the internal reference voltage, depending on the charging mode (see below)

Voltage feedback is provided by the switched voltage divider network consisting of $R A$ , $R C$ , and $R D$ . In the initial rapid charge mode, the state level control transistor shunts to ground. The voltage sense input is then the voltage over the divider consisting of $R A$ and the parallel combination of $R D$ and $R C$ . In the rapid charge mode, the voltage amplifier saturates at maximum drive and the the battery voltage increases until the voltage input reaches .95 $V R E F$ , when the sense comparator causes the charge state to switch to the hold current mode.

In the hold current mode, the state level control pin floats, and the voltage divider is simply $R A$ and $R C$ . The voltage amplifier provides base drive to the pass transistor when the voltage sense input is at or below $V R E F$ . If the voltage input drops below .9 $V R E F$ the comparator again switches modes, returning to the rapid charge mode.

Additional Details

There are several additional details which are important for understanding the circuit. The first is that the UC3906 has a maximum supply voltage of 45V. As we charge four batteries in series we use a supply voltage of 60V. In order to overcome this, we tap across two of the four batteries in the pack, this tap provides the power to the charger circuit. The voltage monitoring is then only across two of the batteries, but as the batteries will always be used in the pack and we have chosen a hold current instead of a hold voltage, all of the cells will equilibrate. The resistor $R B$ is used to compensate for the current used by the IC and the voltage sense network. $R c m p$ and $C c m p$ are compensation to prevent oscillation. Finally, it is necessary to tie the overcharge terminate and charge enable pins high for proper operation.

Design Equations

These design equations are lifted pretty much straight from the data sheets, but I'll go through them anyway. I'll use the following definitions:

$I m a x$ = maximum current output at any time
$I H$ = hold current
$I B$ = voltage divider bias current
$V 12$ = switch from rapid to hold mode when reached
$V F$ = minimum voltage during holding current mode
$V B A T$ = the voltage between the + terminal and the tap (i.e. the voltage across two batteries.

Voltage Sense Network

The first step is to choose the approximate current to flow in the voltage divider network, the datasheet recommends 10 $μ$ A to 100 $μ$ A. Since $R C$ will always have approximately $V R E F$ across it, we find

$R_C=\frac{V_{REF}}{I_B}$ .

Once $R C$ is set, $V F$ is given by the voltage divider created by $R C$ and $R A$ . The voltage in the middle of the divider is

$V_{in}=V_{Bat}\frac{R_C}{R_A+R_C}$

This network provides the feedback which will maintain the battery at $V F$ during the holding current stage. The voltage sense OpAmp compares $V i n$ to $V R E F$ , and so we choose $R A$ so that $V i n = V R E F$ when the battery is at $V F$ , i.e.

$R_A=R_C\left(\frac{V_F}{V_{REF}}-1\right).$

We next choose $R D$ to set the switch point $V 12$ to change from rapid-charge to hold-current mode. As the switching is done by the sense comparator, the switching will take place when $V i n = .95 V R E F$ . In the rapid charge mode, the charge state pin (pin 10) is grounded, so $V i n$ is set by the divider including $R D$ ,

$V_{in}=V_{Bat}\frac{R_CR_D}{R_CR_D+R_AR_C+R_AR_D}.$

Because we want switching to occur at $V i n = .95 V R E F$ , we find

$R_D=\frac{R_AR_C}{\left(\frac{V_{12}}{.95V_{REF}}-1\right)R_C-R_A}.$

Hold Current Circuit

The component values in the hold circuit are not particularly critical as considerable adjustment is possible. The sense resistor $R e$ should be chosen so that the voltage drop across it due to $I H$ is smaller than the minimum powersupply-battery voltage difference, and that the resistor can dissipate the power. (Note that the voltage drop is fixed) The potentiometer should be at least a few turns to allow sufficient accuracy to set the hold current, and the total resistance should be small enough to ensure that changes in base drive won't significantly effect the set point.

Pass Transistor Circuit

The most important component in this circuit is the sense resistor, $R s m$ The UC3906 sets the charge current by maintaining a .250V drop across this resistor, so the resistor value is given by

$R_{sm}=\frac{.25V}{I_{max}}.$

It is also necessary to choose the source resistor $R s r c$ which is responsible for limiting the on-chip current dissipation. The resistor must be chosen to be much larger than the equivalent 12 $Ω$ series resitance to dissipate the power, but also small enough so to ensure sufficient base drive to the transistor pair, as set by the charge current and the minimum $H F E$ of the two drive transistors.

Compensation network

This is actually needed (I once disconnected it and low and behold, she started to oscillate) but I didn't do a calculation. The values are pulled from the application note. If you have a problem with these values, it shouldn't be too difficult to determine a network with the required properties to mitigate the loop poles.

Relay Box

The relay box is responsible for switching between power supply mode and charge mode. Initially, we had though of charging between each shot, but eventually we decided to charge only periodically when it became evident that the batteries will last at least for several days between charges. The circuit can be used either on a TTL control or manually. The default state is in the power supply mode. 4PDT relays were chosen to ensure that all the connections for each battery switch simultaneously, thus ensuring that a battery is never shorted. In our actual implementation, we use ttl controlled Crydom SSR's external to the box to switch between shots. These SSR's are used to prolong the battery life due to the considerable quiescent current drawn by the APEX driver boards.

The relay board

Files

Schematic Files
- Charger Board Schematic
- Relay Board Schematic
PCB files
- Charger Board PCB
- Relay Board PCB

References

Datasheets
Application Note: U-104 Improved Charging Methods for Lead-Acid Batteries Using the UC3906
Battery information
- Panasonic Sealed Lead Acid Handbook
- SLA charging methods

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