GB1578922A - Battery charging equipment - Google Patents

Description

(54) BATTERY CHARGING EQUIPMENT (71) We, CONTROLOGY TECH NIQUES LIMITED, a British Company of 18 Colvilles Place, Kelvin, East Kilbride, G75 OTF, formuly of 12 Carron Place, Kelvin, East Kilbride, Glasgow G75 OTF, Great Britain, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to charging equipment for electric storage batteries.

Electric storage batteries are commonly used as standby electric supply means for other equipment which is normally supplied from a mains supply, the storage batteries being automatically connected in the event of a failure in the mains supply. In such circumstances it is necessary that the batteries are continuously maintained in optimum condition and this may be achieved by utilising charging equipment incorporating circuits for achieving automatic control of the charge current supplied and automatic monitoring of the battery voltage.

According to the present invention there is provided charging equipment for electric storage batteries, said equipment comprising input terminals for connection to an A. C. mains voltage supply, a high frequency convertor connected to said input terminals for increasing the frequency to the kilo Hertz range, a high frequency transformer connected to the output of the high frequency convertor, and a rectifier connected to the output of the transformer the rectifier output having output terminals for connection to a battery which is to be charged.

Preferably, the high frequency convertor provides an A. C. signal which has a frequency of up to 1000 times that of the A. C. voltage supply.

Conveniently, the high frequency convertor incorporates an AC/DC convertor in series with a DC/AC convertor, the DC/AC convertor incorporating transistors connected in push-pull and driven by drive circuitry from a clock oscillator.

Conveniently also, the operation of the drive circuitry is modified in the event of excess temperature being attained by the aforesaid transistors, excess transistor collector current being attained by said transistors, and either excess or less than a preset voltage being attained at the output of the AC/DC convertor.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a block diagram of charging equipment according to the present invention; Figure 2 and 3 together illustrate one form of circuit which the block diagram of Figure 1 may take; and Figure 4 is a modification of a detail in Figure 2.

In figure 1 an A. C. signal at power frequency (i.e. in the range 5-500 Hz) is applied to input terminals 10 forming part of a high frequency convertor 11. The input A. C. signal is frequency increased by the convertor 11 (to within the range 1-200 KHz) and applied to a high frequency transformer 12. A rectifier 13 is connected to the output of the transformer 12 and incorporates terminals 14 to which is connected a battery 15 which is to be charged.

The current supplied to the battery 15 and the voltage there across are monitored by a circuit 16 and a feedback signal is applied via an isolating feedback device 17 to control the operation of the convertor Il. The device 17 preferably incorporates photo emissive and photo sensitive elements.

The convertor 11 may take any form but conveniently it incorporates an AC/DC convertor 20 in series with a DC/AC convertor 21 incorporating power transistors connected in push-pull and controlled from a clock oscillator 22 by drive circuitry 23. Control is achieved by varying the period of the individual half cycle pulses produced in the circuitry 23 by a generator circuit 24. Normally the circuit 24 provides a series of drive pulses in accordance with the signal received from the feedback device 17 but this may be overridden to reduce the power output of the equipment by signals from protectionmonitoring devices on any one of paths 25A, 25B 25C or 25D. The output of generator 24 feeds a drive control logic unit 27 which prevents simultaneous conduction of the two push-pull transistors and which feeds signal shaping drive circuits 28 for the transistors of convertor 21.

Signal path 25A emanates from convertor 20 and provides a signal in the event that the supplied mains voltage or its converted DC value exceeds a predetermined maximum value; the path 25B emanates from a control power supply 29 within the convertor 20 and produces a signal in the event that the supplied mains voltage or its converted DC value is less than a predetermined minimum value; path 25C emanates from convertor 21 and provides a signal in the event that the temperature of the transistors exceeds a preset value; and path 25D emanates from convertor 21 and provides a signal in the event that the transistor collector current exceeds a predetermined value.

The drive circuit 28 initially (i.e. at switchon) receives its D. C. power from the d. c.

power supply 29 forming part of AC/DC convertor 20 but a connection 28A is provided whereby d. c. power may be derived subsequently from either the frequency convertor 21 or the transformer 12 as will be explained hereafter.

One form of circuit which the block diagram of Figure 1 may take is shown in simplified form in Figures 2 & 3. In this circuit the filter forming part of the convertor 20 is not shown; it is provided for eliminating radio frequency noise. The AC mains input is applied to terminals 10 which supply a diode rectifier D5 to D8 with output smoothing capacitor C4 and the D. C.

output produced is fed to the frequency convertor 21 which is in the form of transistors VT6 and VT7 connected to the primary winding of transformer Tl which is equivalent to block 12 of figure 1. The primary winding has a centre tap which is connected to the positive side of the D. C.

supply from capacitor C4 and the transistors Vr6 and VP are connected in a common emitter configuration to the negative side of the same supply.

Transformer Tl has its secondary winding connected to paralleled diode rectifiers with a centre tap connection to waveform filtering chokes and capacitors forming block 13 of Figure 1.

The control power supply 29 incorporates a diode rectifier Dl to D4 which is fed from the terminals 10 through series resistance and capacitance components Rl, R2, C2, C3 and the output waveform is smoothed by a capacitor C5.

The drive circuit 28 for transistors VT6 and VT7 incorporates transistors VT3, VT4 and VT5 to drive VT6 and a similar circuit (not shown) to drive VT7. The base of transistor VT3 is driven by the drive logic circuit 27 and D. C. power is supplied to the collector of VT3 from the positive terminal of capacitor C5 through a diode D29 and from the rectified output of a tertiary winding forming part of transformer Tl. A regulated D. C. power supply 29 is derived from that produced by the capacitor C5 by way of resistors R7, R4, zener diode Zl, capacitor C7 and transistor VT1. This regulated D. C. supply 29 is used to feed the logic circuitry 27, the period generator 24 and the clock oscillator 22 and its output feed incorporates a positive temperature coefficient resistor B2 whereby when the temperature of the transistors VT6 and VT7 exceeds a pre-set value the output of the charging equipment is reduced.

The period generator 24 uses CMOS type logic devices and incorporates a gate ICID which receives the output from the the clock oscillator 22 and from an overvoltage monitoring unit (conveniently in the form of a Schmidt trigger circuit). In the event that an over-voltage does occur the clock signals are prevented from issuing from the gate ICID. The output of gate ICID feeds a first D-type bistable IC3A which produces controlled duration pulses at its Q output in accordance with the output of a two-input NOR gate IC4A connected to the reset terminal of IC3A.

The clock oscillator output has an approximate 5:1 mark-to-space ratio and the positive going edge of the clock signal clocks bistable IC3A to provide a logic '0' output at the Q output and a logic I output at the Q output. NOR gate IC4A is connected to the Q output through an invertor IC2A and is therefore enabled and the second input of gate IC4A receives signals from three sources, the first of which to provide a logic 0 level after IC4A is enabled switches gate IC4A and puts a logic level on the reset terminal of IC3A; this resets IC3A and discontinues the '0' logic level at the Q output. The second input (i.e. the 'b' input) of the IC4A is connected to the output of gate ICID by way of capacitor C15 and diode DIO I to the Q output of bistable IC3A by way of capacitor C16 and diode DIO2, and to a feedback circuit by way of resistor R20 and capacitor CIOI.

The feedback circuit receives a regulated d. c. voltage from power supply 29 and this voltage is applied to the collector of a photo transistor OC1 forming part of the isolating feedback device 17. Positive temperature coefficient resistor B2 forms part of the collector circuit and is mounted on the heat sinks of the two push-pull transistors VT6 and VT7. Potentiometer RV2 is connectedbetween the emitter of OC1 and resistor R3 which carries the actual collector currents of the push-pull transistors RV6 and RV7.

Because of the opposite polarities involved when the current level in the collectors of the push-pull transistors VT6 and VT7 reaches the demand level set by the battery as monitored by the opto-coupler a logic 0 signal is applied to input b of gate IC4A for resetting the bistable IC3A.

The maximum current demand occurs when the opto-coupler transistor (OC1) switches 'ON' (i.e. is in saturation). The positive voltage fed to RV2 is then approximately that at the junction of R10 and B2. B2 is the positive temperature coefficient resistor which has a sharp knee in its resistance/temperature characteristic at a selected temperature. B2 is mounted on the main power transistor heat sinks so that when these reach the selected temperature the resistance of B2 increases and the voltage at the junction of R10 and B2 falls, hence reducing the permissible maximum current drawn by the push-pull transistors.

A similar technique is used to protect against low voltage on the logic supply due to low supply voltage or component failure.

If the voltage across C7 falls below limits, ZI is not held above its zener knee. The current through Zl is insufficient to hold VTI in conduction and VT1 comes out of conduction reducing the voltage at the junction of B2 and R10.

C13 is charged by leakage current on the OC1 secondary transistor and provides "soft start" at switch on and effective filtering or ripple from the opto-coupler secondary with a small value capacitor.

When the main high voltage transistors VT6 and VT7 switch on at each half cycle, there is a short duration surge in their collector currents due to circuit parasitic capacitance and allied effects. When the command signal to input 'b' of IC4A is sensed this negative parasitic current surge is gated out by the positive voltage pulse applied via ClS and 16 and the relative charge time constants of ClS and C16 and COOL.

The Q output of bistable IC3A is gated by NOR gates ICSA and ICSB to be routed alternately to the bases of the two transistors VT6 and VT7. The gates ICSA and ICSB are fed with anti-phase equal duration pulses derived from the Q and Q outputs of a second bistable IC3B which is fed from the output of gate ICID.

Additionally, the gates ICSA and ICSB are fed with simultaneous conduction prevention pulses derived from resistor chains connected in parallel with the transistors VT6 and VT7.

When the charging eqipment is initially switched on the output is increased gradually over a period of seconds. This is achieved by a combination of three features; firstly a negative temperature coefficient resistor (not shown) in the leads connected to terminals 10; secondly the presence of capacitors C2 and C3 charging capacitor C5 in the power supply 29; and thirdly the output of the feedback device 17 is designed to have a long time constant.

The D. C. supply through diode D29 is sufficient to start the convertor 21 and, to provide for improved efficiency of switching on and off the transistors VT6 and VT7 thereafter, the output of the tertiary transformer winding reverse biasses the diode D29 and provides an alternative D. C. supply which is of variable amplitude, according to the average level of collector current in the transistors VT6 and VT7.

Current drawn from the tertiary winding passes through a resistor R41 and this provides a voltage signal which is compared with a signal developed across the resistor R3 and a comparator (VT2) modifies the base drive signal applied to transistor VT6 to maintain optimum base drive output to VT6 and VT7. The current through resistor R3 is the instantaneous collector current of the two transistors VT6 and VT7. (No circulating current between the two diode rectifiers exists due in part to the presence of the two capacitors, C2, C3 which are balanced). This feedback controlling effect of the transistors VT6 and VT7 on the drive circuit 28 is denoted in Figure 1 by a connection 28B. A PNP transistor VT5 removes base charges from transistor VT6 at switch off and so assists in speeding up the collector current zero being achieved by transistor VT6. Furthermore VT5 holds VT6 firmly off during non-conducting periods by shunting the collector-base leakage current directly to the negative terminal of the D. C. supply from capacitor C5.

In a modification of a detail of Figure 2, the transformer tertiary winding may be replaced in function by a current transformer CT connected as indicated in Figure 4. Thus the output of the CT provides base drive currents to the two transistors VT6 and VT7 which are in proportion to the instantaneous collect currents of these transistors. This arrangement permits the drive transistors to act as activated switches with low losses which is advantageous.

It will now be appreciated that the embodiment described utilises a driven and regulated frequency convertor with two high voltage switching transistors with a centre-tapped transformer configuration which permits components of minimum size and weight to be used. The battery circuit is isolated from the A. C. supply circuit by virtue of the transformer Tl and the feedback device 17 which is in the form of an electronic opto-coupler and in this way the number and size of transforming devices is minimised.

Control is achieved in each half cycle of operation by adjusting the pulse width from the generator 24 to provide the demanded level of transistor current. This provides fast and accurate current limitation on each half cycle automatically and allows for storage time and different equipment values between the two sections of circuitry. Thus there is less need for stringent component matching and transformer saturation is automatically avoided.

WHAT WE CLAIM IS: 1. Charging equipment for electric storage batteries, said equipment comprising input terminals for connection to an A. C. mains voltage supply, a high frequency convertor connected to said input terminals for increasing the frequency to the kilo Hertz range, a high frequency transformer connected to the output of the high frequency convertor, and a rectifier connected to the output of the transformer, the rectifier output having output terminals for connection to a battery which is to be charged.

2. Charging equipment as claimed in Claim 1, wherein said high frequency convertor incorporates an AC/DC convertor in series with a DC/AC convertor, the operation of the DC/AC convertor being under the control of drive circuitry.

3. Charging equipment as claimed in Claim 2, wherein said drive circuitry is responsive to a signal from means monitoring the output of said rectifier, when the circuitry is in use.

4. Charging equipment as claimed in Claim 3, wherein said DC/AC convertor incorporates transistors connected in pushpull and said drive circuitry drives said transistors, the operation of said drive circuitry being modified in the event of excess transistor collector current or excess temperature being attained by said transistors.

5. Charging equipment for electric storage batteries substantially as hereinbefore described by way of example with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. act as activated switches with low losses which is advantageous. It will now be appreciated that the embodiment described utilises a driven and regulated frequency convertor with two high voltage switching transistors with a centre-tapped transformer configuration which permits components of minimum size and weight to be used. The battery circuit is isolated from the A. C. supply circuit by virtue of the transformer Tl and the feedback device 17 which is in the form of an electronic opto-coupler and in this way the number and size of transforming devices is minimised. Control is achieved in each half cycle of operation by adjusting the pulse width from the generator 24 to provide the demanded level of transistor current. This provides fast and accurate current limitation on each half cycle automatically and allows for storage time and different equipment values between the two sections of circuitry. Thus there is less need for stringent component matching and transformer saturation is automatically avoided. WHAT WE CLAIM IS:

1. Charging equipment for electric storage batteries, said equipment comprising input terminals for connection to an A. C. mains voltage supply, a high frequency convertor connected to said input terminals for increasing the frequency to the kilo Hertz range, a high frequency transformer connected to the output of the high frequency convertor, and a rectifier connected to the output of the transformer, the rectifier output having output terminals for connection to a battery which is to be charged.

2. Charging equipment as claimed in Claim 1, wherein said high frequency convertor incorporates an AC/DC convertor in series with a DC/AC convertor, the operation of the DC/AC convertor being under the control of drive circuitry.

3. Charging equipment as claimed in Claim 2, wherein said drive circuitry is responsive to a signal from means monitoring the output of said rectifier, when the circuitry is in use.

4. Charging equipment as claimed in Claim 3, wherein said DC/AC convertor incorporates transistors connected in pushpull and said drive circuitry drives said transistors, the operation of said drive circuitry being modified in the event of excess transistor collector current or excess temperature being attained by said transistors.

5. Charging equipment for electric storage batteries substantially as hereinbefore described by way of example with reference to the accompanying drawings.