Buck-boost converter with dual-mode control for battery charging

A voltage controlled flyback converter used as a battery charging circuit avoids the power dissipation of the more conventional series-type regulator-type charging circuits but cannot be readily utilized because of the high gain in the feedback loop due to the battery voltage, which causes instabilities therein when it is operated in a continuous current mode. These instabilities are avoided in a flyback converter used for battery charging by using current control techniques to control current on the primary side of the converter during high-rate battery charging. During low-rate charging, normal voltage feedback from an output current sensing resistor is used as long as the converter operates in a discontinuous mode.

TECHNICAL FIELD 
This invention relates to battery chargers and, more particularly, to a 
battery charger using a buck-boost type flyback converter. 
BACKGROUND OF THE INVENTION 
It is customary to recharge a battery with a high rate of charge when the 
battery is discharged, and later reduce that rate of charge when the 
battery voltage approaches its rated value. A buck-boost type flyback 
converter is a highly desirable charging circuit because, at high rates of 
charge, its power dissipation is considerably lower than that of the 
conventional series type charging circuit utilizing series impedance 
regulation techniques. 
Due to differing current demands of the charging battery, regulation 
control techniques used for low-rate charging are not necessarily suitable 
for use at high-rate charging. For example, with voltage regulation 
techniques sensing a small voltage across a resistor in series with the 
charging current, the sensed battery voltage tends to be so large that it 
is a significant multiplier of the loop feedback gain. A flyback converter 
is unstable with a high voltage gain in a voltage feedback loop in a 
continuous current mode of operation having high duty cycles. Hence, the 
voltage regulation technique is not satisfactory for high-rate charging. 
Hence, while a conventional voltage regulated flyback converter is more 
efficient than the more conventional series impedance-type regulators, it 
cannot be readily utilized because of instabilities induced by high gain 
in the feedback loop due to the sizable battery voltage, and also at high 
duty cycles; especially during high-rate charging when current in the 
induction elements of the circuit is essentially continuous. 
SUMMARY OF THE INVENTION 
Therefore, these instabilities are avoided in a regulation control for a 
flyback converter circuit used for battery charging by utilizing different 
methods of regulation during high and low rates of charging. During 
high-rate charging, current control techniques responsive to current 
sensed on the primary side of the converter power transformer are used to 
regulate charging when current in the inductive elements of the circuit is 
essentially continuous. During low-rate charging, when current in the 
circuit's induction elements is essentially discontinuous, voltage 
feedback techniques, responsive to a voltage across a current sensing 
resistor on the secondary side of the converter, are utilized to regulate 
the battery charging. 
The control circuit is responsive to the battery voltage and causes a high 
rate of charge when the battery voltage is low and a low rate when it is 
high. This causes the control circuit to use primary current regulation 
with high charging rates and continuous conduction in inductive elements 
at low battery voltages, and to switch to a voltage regulation technique 
at high battery voltages where such current is discontinuous. Hence, the 
voltage regulation technique is not used whenever current conduction is 
continuous.

DETAILED DESCRIPTION 
The reserve energy system shown in FIG. 1 is designed to supply energy from 
a battery to a bus 101 and load 102 normally energized by rectified 
commercial AC, upon failure of that AC source. When the AC source is 
functioning, charging current is supplied via lead 150 to negative rail or 
bus 105 to both charge and float a battery 103. Positive and negative 
voltage rails 104 and 105 are connected to the positive and negative 
terminals 106 and 107 coupled to load 102. When the AC source fails or 
drops below a preset rectified threshold level, the reserve energy system 
connects the battery 103 to the load 102. If the battery discharges to an 
undesirable level (i.e., low voltage), it, too, is disconnected from the 
rail 101; and a restart is not permitted until normal AC power is 
restored. The battery 103, when discharged, is charged by a DC-to-DC 
flyback converter, or charging circuit 110, which provides a high-rate 
charge until the battery reaches 85% of its voltage capacity, after which 
further charging continues at a low rate of charge. 
The peak AC voltage of the secondary winding 160 of a transformer 161 
coupled to an AC source is sensed by an AC voltage sensing circuit 120. 
When the AC fails, sensing circuit 120 supplies a signal to a logic 
circuit 122. The logic circuit 122 when further signaled by DC voltage 
sense circuit 121 responding to a low DC bus voltage supplies, in turn, a 
drive signal to a relay coil driver circuit 123, which drives a relay coil 
125 through a relay contact 118. Coil 125 controls the open/shut condition 
of relay contact 126. 
A DC voltage sense circuit 121 compares the bus or rail 101 voltage against 
a threshold reference value. During normal operation, both AC and DC 
voltages are high; and relay coil 125 is unactivated with contact 126 
open. Both the AC voltage sense and DC voltage sense supply a high-state 
drive signal to the logic circuit 122, which controls the coil drive 
circuit to leave coil 125 unenergized. If the DC voltage of the bus is 
satisfactory, and the AC fails, the logic circuit 122 receives the first 
signal but does not supply any output until a signal is received from the 
DC voltage sense circuit signifying that the DC bus voltage has fallen 
below the threshold level of circuit 121. The logic circuit 122, in 
response to the AC and DC sense signals, supplies a signal to drive 
circuit 123 to activate relay coil 125 and close the relay contact 126, 
thereby connecting battery 103 to load 102. Thus, if both the AC voltage 
and DC voltage go low, in that particular sequence, the relay coil 125 is 
activated; and with contact 126 closed, the battery is connected to load 
102. 
The ability of the coil drive circuit 123 to energize the relay coil 125 is 
dependent upon the relay contact 118 being closed. Both relay contact 118 
and relay contact 117 are controlled in response to a take-out controller 
115. Ihe take-out controller is responsible for determining when the 
battery 103 should be connected to the charger circuit 110 in order to 
restore the battery charge to a desired level, and for disconnecting the 
battery from the load when its charge is depleted. 
The DC-to-DC converter charger 110 is powered by DC voltage from bus 101 
via auxiliary rails 111 and 112, and is coupled to charge the battery 103 
through a take-out relay contact 117 responsive to take-out controller 
115. 
The take-out controller is operative to disconnect the charging circuit 110 
and the load 102 from the battery 103 when the battery voltage is low, and 
to disable relay coil 125 by opening contact 118, which decouples coil 
driver 123 and assures the opening of contact 126. Hence, both battery 103 
and charger 110 can be readily disconnected from the load as needed. 
Charging circuit 110 supplies a charging current to recharge battery 103 
when it has discharged, and includes a capability to charge it at two 
different charging rates, depending upon the level of battery discharge. 
It is embodied as a flyback-type converter circuit having a feedback 
arrangement with enhanced stability to permit a full range of recharging 
of the battery. 
The charger circuit for the battery is shown in schematic form in FIG. 2 
and comprises a flyback-type DC-to-DC converter. It is energized by the DC 
rail or bus voltage applied at input terminals 201 and 202. This voltage 
is applied, through an input filter comprising capacitor 203 and inductor 
204 and power switch 215, to the primary winding 205 of power transformer 
210. Zener diode 206 supplies a fixed bias voltage to power the regulator 
control circuit 220. 
Power switch 215, shown herein as a power FET, is periodically biased 
conducting by the driver circuit 214 in response to control signals from 
the regulation control circuit 220. Current flow through power switch 215 
is reflected by the voltage on capacitor 209 which, in turn, is discharged 
through resistor 208. The voltage on capacitor 209 is applied to a current 
comparator 221, included within the regulation control circuit 220. 
Comparator 221 supplies a signal to drive circuit 214 to control a duty 
cycle of power switch 215 during high-rate charging of the battery. This 
control signal is generated as a primary side current limit mode of 
operation. 
The chopped DC current, applied to primary winding 205, is periodically 
stored as energy in the core of transformer 210 when the power switch is 
conducting. As is apparent from the polarity notation shown in FIG. 2 for 
windings 205 and 211, and the orientation of rectifier diode 212, the 
converter is connected in a flyback mode. Hence, as soon as power switch 
215 is nonconducting, energy stored in the core of transformer 210 flows 
as current through rectifying diode 212 to output terminal 213 and 214 
coupled through the take-out relay contact 250 to the battery to be 
charged. A capacitor 218 is included to filter the output current which 
charges the battery. A return current flows through the take-out relay 
contact 250, as discussed above, to a parallel circuit path, including one 
path comprising a series array of three diodes 224, 225, and 226, and a 
second path, including a series-parallel connection of resistor 236, 
resistor 237 and thermistor 235 for temperature change compensation of the 
parallel path and a third path comprising resistor 231 and the base 
emitter junction of transistor 230. 
Voltage regulation of the battery charging is controlled in response to an 
error amplifier 242, coupled to respond to a voltage drop across the 
parallel circuit path, namely resistor 237. A reference voltage is 
supplied by the voltage reference source 240, via resistor 245 to the 
inverting input of error amplifier 242. 
In the embodiment shown herein, the voltage divider comprising the 
resistors 245 and 290 reduces a 4-volt output of the voltage reference 
source 240 to essentially a one volt reference at the inverting input of 
error amplifier 242 during the voltage regulation operating mode of the 
converter. 
A comparator circuit 264 is connected via lead 269 to monitor an output 
voltage of the converter as supplied to the battery terminals at output 
terminals 213 and 214. This voltage is compared with the reference voltage 
of source 240, and the comparator output is coupled via diode 265 to the 
inverting input of error amplifier 242. 
As discussed above, error amplifier 242 normally compares the voltage 
across resistor 237 with the reference voltage. It generates an error 
signal from this comparison, which is coupled by photo-coupler 248 to the 
regulation control circuitry 220 on the primary side of the converter, and 
specifically interacts with a very low gain amplifier 222 therein to 
achieve a voltage regulation mode of operation. By using a low gain 
amplifier, nonlinearities in the opto-isolator 248 are not unduly 
amplified. 
Since the voltage regulation mode of operation is only acceptable when the 
battery is at or near its rated voltage, the comparator 264 is connected 
with its output, coupled via diode 265, to significantly change the 
reference voltage at the inverting input of error amplifier 242, to a high 
enough value so that a voltage drop across the series-connected diodes 
224, 225 and 226 limits the maximum permissible voltage across the 
resistor 237. Hence the voltage regulation control is no longer able to 
function, and the regulation function is taken over by the primary current 
limit control. This may be readily appreciated from the following 
description of the operation of the converter. 
With the battery substantially discharged, the charging circuit operates in 
a high-rate current regulation mode until the battery voltage reaches a 
predetermined threshold value. With an operative voltage applied to error 
amplifier 242, regulation control now changes from primary side current 
regulation to output voltage regulation. The voltage regulation control 
includes a voltage reference source 240, energized by the voltage drop 
across zener diode 241. Reference source 240 supplies a first level 
reference voltage to the inverting input of voltage error amplifier 242 
which compares this voltage to the voltage drop across the parallel 
circuit path to a first level reference voltage. This reference voltage, 
as applied to error amplifier 242, is determined by a voltage divider 
comprising resistors 245, 246 and 290 and, as indicated above, by the 
output state of operational amplifier or comparator 264. This first level 
reference voltage is compared with the voltage drop across the 
series-connected path comprising resistor 237 and of the parallel path 
comprising resistor 236 and thermistor 235 included for temperature 
compensation. The resulting error voltage output of amplifier 242 is 
utilized to regulate the voltage across the parallel circuit path at a 
one-volt value; and, hence, institute and control the voltage-regulated 
low-rate charging mode. The output of amplifier 242 is coupled through 
opto-isolator circuit 248 to a voltage amplifier or comparator included in 
the regulator control circuit 220, and which is operative to control a 
duty cycle of power switch 215 to achieve the desired voltage regulation 
control of the low-rate charging. 
When the output voltage of the converter, which is identical to the battery 
voltage at terminals 213 and 214 drops below a critical value, comparator 
264 having its inverting input coupled to this voltage via lead 265 
changes state and its changed state output coupled to the inverting input 
of error amplifier 242 significantly changes the value of the reference 
voltage thereat. Amplifier 242 is now inoperative to function as a voltage 
regulation control, and hence the power switch current in the primary 
portion of the converter increases until the primary current limit mode of 
operation gains ascendancy. The comparator 264 includes a feedback circuit 
comprising diode 267 and resistor 295 which is included to provide a wide 
hysteresis in the responsiveness of the inverting input to the sensed 
output voltage, so as to determine the two switchover points. 
As shown in FIG. 2, the parallel circuit path includes the resistor 231 and 
the base emitter junction of transistor 230. This transistor 230 is biased 
conducting when the voltage across the parallel circuit path exceeds a 
certain threshold. The resulting current flow through light emitting diode 
280 may be utilized to generate an indication that the battery is being 
charged. 
Should the battery be accidently disconnected from the output terminals, it 
is possible that the voltage regulation circuit in attempting to regulate 
a voltage across the parallel circuit path could cause the voltage at 
output terminals 213 and 214 to rise to a dangerous high voltage level. A 
breakdown diode 301 is included to operate in limiting the maximum output 
voltage. Diode 301 breaks down when a voltage threshold is reached and 
permits current flow through resistors 302 and 303. The current through 
resistor 303 is coupled via diode 304 to the cathode of diode 305. This 
supplies a current to the parallel circuit path permitting the error 
amplifier to respond and attempt to regulate the voltage drop caused by 
this current as if it were a normal operating voltage drop. Since the 
diode 301 does not break down with normal voltage outputs, normal 
regulation is permitted, but should the output voltage rise when the 
battery is disconnected the break down of diode 301 and the current flow 
through diode 304 is utilized to prevent the output voltage from reaching 
dangerous levels.